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The Canadian Pipe Stress Analysis Design Manual for Owners, Engineers and Contractors

The Canadian Pipe Stress Analysis Design Manual for Owners, Engineers and Contractors for a premium piping engineering & full-service pipe design and pipeline / pipe stress analysis services across Canada & globally. Using CAESAR II and pipe stress calculations as per API, ASME B31.3, B31.1, B31.8, B31.4, CSA Z662.


The Canadian Pipe Stress Analysis Design Manual for Owners, Engineers and Contractors

Index:

1. GENERAL

2. DRAWING DISTRIBUTION AND PROCEDURES

3. INITIAL PIPING STUDIES

4. STRESS RELIEVED VESSELS AND PIPING

5. REVIEW OF CRITICAL PIPING

5.1 Pumps

5.2 Compressors

5.3 Turbines

5. 4 Airfans

5.5 Heaters

5.6 Buried Piping

5.7 Cryogenic & Low Temperature Piping

6. STANDARDIZATION OF APPROACH TO PIPING PROBLEMS

6.1 Allowable Pipe Spans

6.2 Allowable Pipe Overhang

6.3 Pipe Guide Spacing

6. 4 Instrument Strong Back Flexibility

6.5 In-Line Pumps

6.6 Expansion Loop Design

6.7 Pipe Anchors

6.8 Stacked Exchangers

6.9 Off Plot Pipeways

7. MISCELLANEOUS AND SPECIAL PROBLEMS

7.01 Slug Flow

7.02 Mitered Elbows

7.03 Tee Connections

7.04 Injection Connections

7.05 Heater Coil Decoking

7.06 Catalyst Regeneration

7.07 Reformer Furnace Pigtail Design

7.08 Cold Spring

7.09 Blowdown Systems

7.10 Field Checkout

7.11 Soot Blowers

7.12 Settlement and Frost Heave

7.13 Ambient Temperature Effect on Bare Piping

7.14 Control Valve Piping

7.15 Hydrotest of Large Low Pressure Piping

7.16 Pipe Supports

7.17 Tank Field Piping

7.18 Steam Trace and Steam Trap Piping

7.19 Plastic Piping

7.20 Rotations, Reactions and Stresses at Nozzle Connections to Vessels

7.21 Bowing of Pipe

7.22 Compressor Bottle Support

7.23 Tank Nozzle Movements Due to Pressure and Temperature

8. PIPING STRESS ANALYSIS WORK CHECK LIST


1. GENERAL


1.1 This Design Guide is intended to aid stress personnel / Piping Stress Engineer in following approved procedures and techniques to complete their work (Pipe Stress Analysis) on an assigned project.


1 .2 Although it is recommended that the standards be followed closely, individual thought and sound engineering judge­ment must be used at all times.


1.3 In reviewing piping isometrics, models or drawings, the Pipe Stress Analysis Engineer should keep in mind that the aesthetic de­sign of the piping systems is the responsibility of the piping design groups and therefore he should review them from a stress and support standpoint only. Exceptions to the above should only be made when a situation ridiculously improper or a large economic saving is involved, keeping in mind lost time in making revisions and their affect on schedules.


1 .4 All piping systems reviewed by the Piping Stress Analysis Group shall be considered for all the "Design Conditions" as listed under Section 301 of the Code for Pressure Piping ANSI B31 .3, latest revision, or other applicable codes. As a general rule most computer analyses of piping should include only the effects of thermal expansion, restraints and effects of support, anchor and terminal movements. Effects of dead load on a well supported system are generally small. Other effects are to be studied by special calculations only when engineering judgement deems them to be possibly severe.


2. DRAWING DISTRIBUTION & PROCEDURES


The following normal procedures may be adjusted for particular projects or office locations to suit the special conditions and requirements of those projects and locations.

2.1 The assigned Piping Stress Analysis Engineer shall confer with the Pressure Vessel Job Supervisor and indicate his preference of draw­ings which should be distributed to him. These drawings should generally be plot plans, P&ID's, paving and grading, underground piping, pipe way stanchions, line designation tables, basic data, flow diagrams, piping drawings and piping isometrics. When vessel drawings and structural drawings are included, the filing of drawings becomes a major problem. In fact, much filing would be avoided if P&ID's and paving grading drawings were not included. This judgment is left to each individual.


2.2 The routing of piping isometrics between the Plant Design Group and the stress group has been standardized to increase efficiency of all groups concerned and to reduce the amount of paper handling. Isometrics will be referred to as iso's in further discussions. The presently adopted procedure for iso distribution on modelled jobs is:


a) After isometrics are drawn up and checked within the Plant Design Group and are ready to issue for construction, a print of each together with a transmittal list shall be sent to the Pipe Stress Analysis Engineer one week before date to be issued for construction.


b) The Piping Stress Analysis Engineer then places a design data stamp on all iso's except those which can be approved for stress by inspection without specific design data. The stamped iso's should then be filled in with the necessary design data from piping specifications and line design tables. An efficient and acceptable method of recording the expansion temperatures is to prepare a list of maximum "exp" temperatures for each particular service as shown in the Line Designation Table, i.e.:


IA (instr. air)--------100°F

UA (util. air)--------100°F

N (nitrogen)--------100°F

DW (drinking water)--------100°F

PW (potable water)--------100°F

RW (raw water)--------100°F

CW (cooling water)--------120°F

LS (low press. steam)----40# sat stm temp.

MS (med press. steam)-150# sat stm temp

HS (high press. steam)-600# sat stm temp


But process lines require individual temperature assignment from the line tables.

Likewise, a list can be prepared for pipe specifications which are repeated often that are of carbon steel and the same schedule. Alloy spec.'s and their schedules should be specially listed for ease of identification.


c) The iso's are then reviewed at the models and passed by judgment as much as possible, leaving only a few to verify by computer calculation. All iso's passed by inspection should be marked up with support designations during the review of each iso. This in general will be the most efficient operation except where a group of iso's must be immediately released by the Plant Design Group for prompt delivery to the fabricator to meet a schedule. After all the iso's listed on a particular transmittal have been reviewed, those which can be field-supported, or require no supports, or which can be supported by wholly standard support details, are indicated on the transmittal and the blue print of the iso itself with the designations FTS, NS or STD respectively. The Plant Design Group can stamp the original iso's accordingly without need of their passing through the pipe support groups. Technicians, will be retained by The Plant Design Group for the purpose of assigning proper designations to the "STD" supports required on every iso. This should expedite the preparation of iso's to be issued for construction on the Rev. 0 issue. All other iso's are checked off on the original transmittal as being approved for stress with an engineered support designation ES except where a flexibility change or calculation is needed. The symbol HFS indicating "Hold For Stress" will be tagged on the transmittal opposite the iso involved. Two copies of the trans­mittal with the above notations should then be given to the Plant Design Supervisor.

d) All iso's as they are approved by the Pipe Stress Analysis Engineer, should be initialed on the tracing by the Pipe Stress Analysis Engineer or his designated alternate. Where iso's require a calculation, the tracing should be detained by the Plant Design Group until the Piping Stress Analysis Engineer finalizes his study of them. The Piping Stress Analysis Engineer should assign the highest priority to finalizing these iso's.

e) When iso's are verified as satisfactory by calculation, the Plant Design Group should be immediately notified for its release. And if iso's require a revision, the print should be marked up with the required change and a copy of the print should be given to the plant Design Group. After the iso revisions have been made, a new print should be again issued to the Pipe Stress Analysis Engineer for final review. If the iso is correct the Pipe Stress Analysis Engineer will initial the tracing as approved.

f) All prints marked up by the Piping Stress Analysis Engineer with the support require­ment symbol £S are then turned over to the Support Group. If iso's are stamped for review of critical support details, the pipe support designer must return the iso and support details to the Piping Stress Analysis Engineer who, upon approval of the detail, initials the stamped area on the iso.

g) The Support Group then adds the "PS" numbers and locations to the iso tracing and initials the tracing. The tracing is then returned to the Plant Design Supervisor for issue.

h) If after an iso is issued for construction, the Plant Design Group makes a revision to the piping, it is the responsibility of the Plant Design Supervisor to stop the support group from further work on the iso and reclaim the print marked up by the Pipe Stress Analysis Engineer. The Piping Supervisor then reissues the iso and the originally marked up print to the Pipe Stress Analysis Engineer who reviews the iso for further approval and support mark-up. Where piping revisions are judged insignificant by the Plant Design Supervisors, (i.e. not affecting flexibility or support of the system) the iso is then just reissued for construction, by-passing the Stress Group.

i) If piping isometric numbers are revised by the Plant Design Group, a cross reference list of new numbers versus old numbers must be provided to the Stress Group to keep records straight. To keep better control of iso's marked up by the Stress Group, the Plant Design and Support Groups should also keep a check list of iso's received.

j) The stress markups are then kept in alphabetical and numerical order in special long binders by the Ripe Support Group for reference.

k) When the job is complete the marked isometrics are returned to the Piping Stress Analysis Engineer who keeps them close at hand for approximately 1 year, then files them in storage.


2.3 A sepia of all orthographic drawings of piping on-plot or off­ plot should be issued to the project Pipe Stress Analysis Engineer prior to being issued for construction. The sepia shall be stamped and distributed per owner's standards upon stress review completion. The Piping Stress Analysis Engineer shall convert sepias of the piping drawings into stress STR drawings and maintain a drawing control of all STR drawings per Owner's standards.


3. INITIAL PIPING STUDIES


3.1 Study preliminary plot plan and pipe way layouts for troublesome arrangements.


a) Indicate pump placements which will aid in achieving flexible piping arrangements. Avoid placing pumps directly opposite connecting equipment.

b) Estimate the number and position of pipe way expansion loops for steam, condensate and other long, high-temperature systems.

c) Keep movements in steam lines to generally 4 inches or a maximum of 6 inches by judicious number and location of loops. Determine the loop size to help in positioning the header in the pipe way to avoid large overhangs or the necessity of auxiliary means of supporting loops. Design /rests of loops as early as possible and give exact layout to Plant Design Group. Expansion movements, insulation thickness, effect of cold spring and extra clearance should all be included. Generally keep a minimum of 1^ to 2" extra clearance from adjacent piping or other obstructions for worst case of design temperatures or differential pipe movements.


3.2 Review preliminary alloy piping isometrics or layouts by inspec­tion for material commitment. Generally this is done to avoid large differences between material commitment and final purchase of alloy pipe and fittings required; therefore, an exact analysis should not be made. Retain the preliminary study for comparison with the final iso to be issued-for-construction At this time many iso's can still be passed for stress by inspection, but it is recommended that piping to pumps, compressors and possibly heaters, exchangers or reactors when high reactions are suspected, should be run as a formal calculation on the computer.


4. STRESS RELIEVED VESSELS AND PIPING


4.1 The Pressure Vessel Job Supervisor will provide a list of all stress relieved vessels on the job and all established dates from the fabricator for stress relief of each particular vessel. These dates will be marked on tags put on the vessel models by the vessel department. Normally the model should be completed and "checked" a minimum of (6) weeks ahead of the stress relief date. This gives the Pipe Stress Analysis Engineer and support group (2) weeks to complete their work and get details sent to the fabricator(4) weeks prior to actual stress relief.


4.2 It is very important that the Plant Design Supervisor remind all his designers that the piping should not be revised thereafter. If the change must be made, the revision has to be coordinated with the vessel fabricator immediately to avoid serious problems such as re-stress relieving and delay in delivery.


4.3 Piping requiring stress relief generally is drawn up and issued to the shop together with the required pipe supports which are to be welded on and stress relieved with the pipe. Occasionally, support details are held up for one reason or another and fail to reach the shop in time. The supports must then be welded to the pipe in the field. Welding of supports to stress relieved piping in the field is to be avoided. The stress relief kits are not only costly in themselves (sometimes amounting to several hundred dollars) but require many manhours for their installation, application and removal. Stress relief must still be applied where process reasons dictate (i.e. stress corrosion or other), but for P1 material, non-pressure parts or external attachments are not required by A.N.S.I. Code to be stress relieved as long as the throat of the attachment fillet does not exceed 3/4".For any questions regarding welding of supports to stress re­lieved pipe refer to the general welding instructions for pipe supports.


5. REVIEW OF CRITICAL PIPING


The following equipment 6 conditions involving critical piping require special treatment, and are briefly discussed within each classification.


5.I PUMPS


5.101 Pumps, turbines and compressors have common sources of concern. The greatest concern is for keeping proper alignment of the pumps and compressors in relation to their turbine or motor drivers. Improper alignment causes hot bearings with resulting wear and/or serious vibration. Reactions to the cast steel nozzle and casing structure is generally of secondary concern. Whenever the casings are made of cast iron the allowable loadings should be reduced 25%.


5.102 Acceptable loadings on most centrifugal and rotary pumps which are base, frame, flange or centerline mounted, are shown in owner's standards. When the loadings are higher than permissible every effort should be made to meet the allowable loadings by increasing the flexibility of the piping system rather than employing expansion joints.


5.103 Owner's standards (k sheets) shows some common configura­tions of pump piping. The tables accompanying the various figures show the maximum operating temperature of the system without overstressing the pipe. When the maximum allowable temperature is greater than 150°F, the system is OK for 300°F steam out or steam tracing.


5.104 Piping reactions on in-line, deep well, vertical frame mounted, reciprocating pumps, heavy barrel type, or other specialized pumps must be reviewed on an individual basis. The primary rule regarding any piping system to pumps is that the allowable stress of the pipe at the nozzle must not be exceeded, and that reactions in lbs should generally not exceed 150 x the nozzle diameter in inches or that permitted by the pump manufacturer in loadings published on his vendor prints, or by agreement, or per specifications.


5.105 In-line pumps should be capable of withstanding equivalent pipe allowable stress based on the minimum nozzle size and re­duced to material allowable stress for the cast body. These pumps should be supported by the adjoining piping only, except, where the horsepower of the pump exceeds 75H.P.,the pump itself should also be supported on a pier. See owner's standards Generally, none of these supports re­quire bolting, in fact, if the pump can slide it provides relief for thermal expansion. (Refer Par. 6.05) .


5.106 Deep well pumps generally have a cylindrical plate steel casing which is flanged and bolted to a concrete founda­tion. Loadings to nozzles of this type of equipment are limited to the allowable stresses of the pipe and/or casing.


5.107 Pump piping can be designed to twice the normal allowable stress as per owner's standards when considering steam-out or upset steam trace temperatures. When the pump and/or piping is being steamed out, the pump is not running and therefore misalignment does not dictate.


5.108 Pressure rating of pumps is indicative of ability of pump casing and supports to withstand piping reactions. As the pump pressure rating is increased, it is built more sturdily; it has heavier walls, weighs more and is more stable with sturdier supports. Naturally, therefore, it can withstand higher piping reactions.


5.109 Where pumps are top suction and/or top discharge, the only manner of removing eccentric loads on the pumps would be from beams above. For pumps handling hot materials the piping should be spring supported to beams above. Therefore, for ease of supporting pump piping in this case, the pump should be located under the stanchion struts, (i.e. those running parallel to and on each side of the pipe way).


5-110 Whenever possible the pump suction lines should be supported to a concrete pad extension of the pump foundation. Where this is not possible, beams should be embedded in the foundation and projected out the sides or front far enough to support the piping under the vertical riser. In the case of plants located in regions of frost heave, these beams must adequately clear the maximum estimated heave of the area slab. Where differential vertical expansion of the pump versus the piping permits, the supports discussed above should be solid, sliding type supports. Spring sup­ports should only be used when this vertical differential expansion is high or questionable.


5.2 COMPRESSORS

The types of compressors usually found in refineries and chemical plants are as follows:

Paragraph

Centrifugal, Rotary and Screw 5.21

Reciprocating 5.22

In-Line 5.23

Blowers and Fans (Below 1 psig EAP) 5.24


The allowable loadings, methods of calculating them, types of support, and piping design considerations for each of the above compressor types, are discussed individually in the paragraphs noted.


5.21 Centrifugal, Rotary and Screw Type Compressors Allowable loads on centrifugal compressors shall be covered by Owner Standard specifications. These specifi­cations shall state that the equipment shall be designed to withstand the following external loadings:

Vertical Component

The allowable vertical reaction from combined forces, and mo­ments due to all piping connections, or to any one piping connection (either upward or downward) at any support point shall be at least one half the dead weight reaction of the compressor at the support point.

Horizontal Transverse Component

The allowable horizontal reaction from combined forces and moments due to all piping connections, or to any indivi­dual piping connection, in a horizontal transverse direction at any support point shall be at least one third the total dead weight reaction of the compressor at the support point.

Axial Component

The allowable axial force from combined axial forces piping connections, or axial force of any one piping connection, in an axial direction on the compressor casing be at least one-sixth the compressor weight.


a) For calculation preparation set up the individual systems connected to the compressor casing and support structures as indicated in owner's standards, or by some other equivalent system. To avoid moment restraints, all restraints used should be simple couples. The layout of the problem and the subsequent computer run should be based on the coordinate system as shown in the Standards.

b) Generally, centrifugal compressors are not sources of serious vibration and therefore, the piping is given only a cursory review for resonance. Large frameworks of free standing pipe or large overhangs should be snubbed to prevent large amplitude vibration.

c) Piping to centrifugal compressors need not have a machined spool piece to makeup the last connection to the compressor. For years the construction department has displayed the capability to mate flanges by bringing misaligned piping into proper position by the heat and quench method. How­ever, where cold spring is employed the field should be instructed carefully as to the proper procedure to produce the results desired.


5.22 Reciprocating Compressors

Piping reactions on reciprocating compressors are not cri­tical from the standpoint of misalignment of equipment, but due to piping vibrations, the piping stresses should not crowd the allowable stress range. Although higher stresses can be allowed at the nozzle than for centrifugal compressors, it is not unreasonable to keep axial and shear forces within those shown in owner's standards, and as a conservative rule, keep stresses to within twice those permitted for turbines in the same standard.


a) Generally there is no need to combine pipe system loadings for reciprocating compressors as was required for centrifugal compressors since piping is usually small and reactions are negligible relative to the sturdy equipment. In fact, most piping systems to this type of equipment can be reviewed by inspection.


b) Vibration is a rather serious problem within piping to recip­rocating compressors. The piping generally should be guided, held down and possibly restrained with hydraulic type vibration snubbers when unsupported lengths or spans fall into the range of the first or second harmonic of the compressor operating frequency.


c) Pulsating compressor discharge requires that special cylindrical "bottles" be designed to prevent surge vibra­tion. These bottles are often large diameter and heavy. Therefore, to reduce the possibility of a fatigue failure between the discharge bottle nozzle and the cylinder head nozzle, the dead load of the bottle should be supported by elastic supports as described in paragraph 7.22. Sometimes the compressor manufacturer recommends a wedge type solid support. These have been widely used but don't allow any room for error of installation. The wedges have to be adjusted when the compressor is at operating tempera­ture. For upset temperatures the wedge type may be danger­ous since no further expansion can be absorbed. Owner Refinery Division practice usually avoids using wedge supports. Suction bottles can utilize solid supports since suction temperatures vary negligibly.


5.23 In-Line Compressors

Misalignment of in-line compressors obviously is no problem, since their driver is bolted to their casing. Permissible loadings on their nozzles can approach the allowable of the piping system, but should be reduced by the allowable stress for the cast material of the equipment when nozzle and pipe thicknesses are comparable. Whether the in-line compressor is supported or not depends on ability of piping to support it. The analyst must be sure to take vibration into consideration.


5.24 Blowers and Fans

Due to the possible light weight construction of this type of equipment the allowable nozzle load tables should not be used. The vendors prints should be examined for clues rela­tive to strength and manner of supports, and/or other pertinent data. If no allowable loads are published, the intake and/or discharge lines might require impregnated cloth or neoprene expansion joints. This type of joint is banded on to the exterior of the adjacent pipes with suitable small gap between the pipe elements. Generally, the sheet should have a slight circumferential bulge between bands to absorb tensile movements. Generally, piping or ducts to blowers and fans are large and thin walled, requiring direct routing. These may require expansion joints made of rubber or stainless steel and be rectangular, oval or circular in shape. Allowable loadings on this type of equipment are based on engineering judgement, since allowables are not usually published or known by the manufacturer.


5.25 To reduce operating reactions from piping to compressors the most generally used methods are to employ cold spring or by increasing the flexibility of the piping. Expansion joints are virtually forbidden since they suffer from vibration fatigue.

If a system is to be cold sprung it should follow the rules of the ANSI B31-3. The cold spring should be located at a convenient place in the system, generally a flanged connec­tion or a field weld. See owner's standards for cold spring notations.

It is important that no rotation at the welded joint is per­mitted to assure that proper counter moments are built into the system. Instructions on this procedure should be sent to the field for critical systems. Where cold spring is in­effective or impractical, the piping should be rerouted to improve its flexibility.


5.3 TURBINES

Centrifugal turbines with pedestal, base or flange mountings, are the only types considered herein.


5.31 Flange mounted steam turbines are used as in-line pump dri­vers and are therefore not misaligned by piping reactions. Piping stresses can approach the maximum piping allowable except where cast iron casings are encountered, then the stresses should be reviewed considering the lower allowable stress of cast iron.


5.32 Piping reactions on pedestal and base mounted centrifugal turbines are governed by two conditions. First, if the tur­bine is a pump driver and is single stage, the allowable loadings as noted in owner's standards should apply. Secondly, where the turbine is multi-staged or is used as the driver to a compressor, the allowable loads will be in accordance with the Owner Standard Turbine Drive Specification as previously described under Centrifugal Compressors.


5.33 For preparation of calculations to verify loading conditions on the turbine, use the procedure as outlined under paragraph 5.21(a) for centrifugal compressors.


5.4 AIRFANS

Airfan heat exchangers have gained widespread popularity and use over the last several years. At least three major problems confront the Piping Stress Analysis Engineer.


5.41 First, where the inlet and outlet header boxes have two or more nozzles per unit, a difference in expansion exists bet­ween it and the attached pipe header. For years many such units have been connected together using only fitting makeup with no apparent ill effects. (Very similar to cylindri­cal exchangers being connected by their nozzles being bolted together directly.) Therefore, a practical standard is need­ed for determining when additional flexibility is required and how to compute it. Owner's standards suggests that fitting makeup is tolerable until the difference in horizontal expan­sion between the nozzles of the pipe header and header box exceeds 1/16". This applies to either the inlet or discharge sides but not when several units are joined together and the inlet and discharge nozzles are at the same end. Where the expansion difference exceeds 1/16" use the formula indicated to compute length "Q " required between manifolds.


5.42 Second, overall expansion of the pipe header joining several airfan units together must be accommodated by allowing the header box to slide on its clip supports within the unit side- channel supports. Normally the gap between each end of the header box and the support channel should be 5/16" or more. This is now generally accepted and appears in Owner speci­fications issued to manufacturers who are to bid on the jobs. Where more than 5/16" movement is required, the pipe header can be cold sprung, as shown on owner's standards, pulling the units together as much as 5/16", whereby the permissible expansion can be increased from 2 x 5/16" or 5/8" at each end of the units to 2 x 5/8" or 1 1/4" total for the overall length of all units connected together.


5.43 Thirdly, where inlet and outlet piping are at the same end of the airfan units, extra flexibility of the outlet piping is generally required and should be routed as shown on owner's standards or in some equivalent manner. External piping loads affecting the equipment nozzles additionally should conform approximately to those loadings published by each manufacturer.

Another manner in which difference in expansion between inlet and outlet pipe headers can be absorbed is by requesting the airfan manufacturer to supply horizontally split header boxes that slip individually to absorb the difference in movements. This method would generally permit fitting makeup between the pipe header and the header boxes for both inlet and outlet connections even though both are located at the same end of the airfan.


5.5 HEATERS

Early in the design of a plant, specifications are drawn up and material requisitions are prepared regarding the types of heaters to be used. It is at this stage that the stress group should confer with the project engineers regarding support requirements of external piping to the heaters. The material requisition should state that it will be the responsibility of the heater manufacturer to provide adequate platform framing or other means to accommodate all external piping loads of the inlet and/or outlet piping.


5.51 A preliminary piping load estimate should be sent to the selected manufacturer for completion of his platform design. Unless this is done at an early stage, it might prove costly to arrange for piping to be supported to the heater after the design and/or fabrication is completed.


5-52 In general, piping to the heaters should first be studied for inherent flexibility without alteration of heater inter­nal supports or openings into the heater. If the proposed piping is either overstressed or creates unacceptable, high reactions on the heater nozzles, then either the piping should be rerouted to produce a desired flexibility or the heater manufacturer should be requested to absorb some reasonable lateral movement of the heater tubes. This movement may re­quire some alteration of the tube support castings on hori­zontal, rectangular (box type) heaters and some possible enlargement of the openings to either the horizontal or ver­tical (cylindrical) heaters.


5.53 When a horizontal, rectangular heater is being used, the radiant and convection section tubing is generally anchored (axially only) at the front of the heater with allowable loadings indicated. Where the manufacturer does not indicate an anchor, he should be requested to add an anchor to all nozzles and submit their allowable reactions. It is better to have the piping anchored and the movements therefore con­trolled rather than to let systems float and be in doubt as to ultimate movements. In some cases, such as heaters used in ammonia plants, the heater tubes are anchored internally, whereby large movements are indicated at the nozzle and are imposed on the external piping. By judicious location of equipment these movements can be counteracted by expansion of external piping.


5.54 Cylindrical heaters (axis vertical) have their tube coils running vertically. They can be supported either at the top or the bottom of the tubes. The tubes are guided periodi­cally to the heater shell. The inlet and outlet nozzles generally hang free, being supported to the adjacent tube through the 180° return bend at one of the ends. Therefore, these tubes can be moved laterally in a horizontal plane, to relieve external piping stresses and reactions if necessary. But the manufacturer must be agreeable to the particular re­lief movements requested. If the piping is amply flexible, no modifications are necessary by the heater manufacturer, but the reactions on the nozzle must be reasonable. These allowable loadings as indicated on their drawings generally are 500 to 1000 pounds.


5.55 When considering the design of piping to cylindrical heaters the location of the tube supports can be critical. When top supported, with inlet and outlet nozzles at the bottom of the heater, large vertical movements occur and are im­posed on the external piping below. This may require costly additional pipe for flexibility and the use of expensive constant load spring supports.


5.56 If the tubes are top supported with inlet and outlet nozzles at the top, then the external piping can be supported to the platforms or shell at the same level as the tube supports. This would reduce the need for constant load spring supports but external piping flexibility is still required between the heater and other equipment or the pipe way. When the tubes are supported at the bottom and the nozzles are at either the bottom or the top, the need for external piping flexibility or constant load spring supports can both be mini­mized.


5.57 Additional care must be used when considering 2 phase flow in heater piping. The inlet will generally be 100% liquid at .50 to .85 specific gravity but the material in the outlet will vary from the inlet liquid density to a nearly 100% vapor flow. This creates special support problems and the differential load must be minimized on connecting piping by pre-setting springs for an intermediate load condition.


5.6 BURIED PIPING

Buried piping, regardless of depth of burial or soil in which it is buried, has the tendency to expand or contract with temper­ature changes whether from flow temperatures or surrounding soil temperature changes. The total change in length it undergoes depends on the restraint of the soil both from friction and pas­sive resistance.


5-61 Computing Growth of Buried Pipe

A reasonable approach to calculating buried pipe movements is based on resistance to movement from soil friction in a rect­angular load pattern as shown in owner's standards. This has been found to be slightly unconservative by roughly 20% since cyclic expansion and cooling tend to increase end movements. The choice of a proper coefficient of soil friction is of great importance since the value can vary from .4 to greater than 1.0.


5.62 Results of Jacking Tests

From Jacking tests made by P.G.S-E. Co. (see Sept. 1933 issue of "Western Gas") on 37,_4" length of 22" pipe with 2'-6" of cover (assume average cohesion less soil) shewed a soil friction of 0.40 psi or closely a co-efficient of friction of 0.4.


5.63 Method of Restraining Expansion of Buried Piping

At corners (right angle turns) of buried piping systems, large expansions might cause a failure at the elbow, due to restricted flexibility, or similarly at branch connec­tion of underground header.

For small temperature changes the system can be fully restrained to prevent the above failures. Methods of providing full restraint are by anchoring the pipe with concrete blocks which encircle the pipe or by dead men with struts attached to the pipe. (owner's standards) Also the line can be fully restrained using very large bends in the pipe through the principle of hoop compression. (See owner's standards.)


5.64 Stress Analysis of Buried Piping Systems


5.641 General

As in above ground piping systems, thermal expansion stresses are induced in buried piping systems when the temperature of the systems changes. However, the thermal stress condition of buried piping systems is much more complex than that of above ground piping systems due to restriction of the piping movement by the surrounding soil. The stress level in the pipe depends on the temperature change, pipe size, piping configuration, soil characteristics, depth of burial, skin friction, operating pressure, etc. For a long straight buried pipe under temperature change, the thermal expansion of the middle portion is completely restrained by the soil friction and only the end portions, generally a few hundred feet long, show some movement. See the Sample Problem, as herein after referred to, of owner's standards. The length of the end portions, which expand under partial restraint, and the resulting end movements may be calculated by the formula shown in owner's standards and is shown on Page 2 of the Sample Problem. Buried piping systems under temperature change may be moved laterally near the bends and branch connec­tions. It is assumed that the pipe moves against a soil spring and the maximum spring force is equal to the passive soil resistance. A buried piping system may be analyzed for thermal expansion efforts to include soil friction and soil resistance by the piping flexibility program ME632 or ME 10


5.642 Input Data Preparation


a) Dimensions of Calculation Model

After determining the length of the partially restrained portions of the buried pipe system, the calculation model can be set up as shown on Page 4 of the Sample Problem. As can be seen, only 8001 of the 5000' run of the complete system, as shown on Page 1 of the Sample Problem is included in the model since the remainder is totally restrained. To achieve the 3.^9" deflec­tion of Data Point 33 either the anchor at Data Point 80 can be moved in the "-X" direction or an equivalent rate of expansion can be applied to the 800' length to produce the same result. Actually, the length of the partially restrained run of pipe as calculated by Owner's Design Guide does not include the soil resistance on the pipe at right angles to the main run as shown by Data Points 8 through 33. A more accurate result may be obtained by a rerun with a new partially restrained length, Data Point 33 through 80, including the lateral soil resistance on Data Points 2 through 33.


b) Soil Resistance "Springs"

A buried piping system under temperature change moves against a soil spring force (subgrade reaction) which has a limiting value equal to the passive soil resistance. It is found from tests that the buried pipe moves against the soil a certain amount or displacement before developing a maximum passive soil resistance. This displacement depends on the soil property and the depth of the burial. From the Foundation Engineering Handbook, the displacement is about 0.05 H for sand and 0.10 H for clay, where H is the depth of the burial to the bottom of the pipe in inches. In the absence of the subgrade reaction data for the jobsite, the displacement of 0.03 H has been used as the necessary movement to develop a passive state and it is used generally for conservatism. The soil spring constant Kg is calculated as follows;


Ks=passive soil resistance/0.03 H


The soil springs are treated as translational restraints with a flexibility of KA #/in. from Ks x length of pipe affected. The restraints are spaced such that the passive soil resistance on the pipe is adequately represented. In general a closer restraint spacing is required for the area where high stresses and movements are anticipated. However, the spacing should not be closer than two times the pipe diameter. Since the soil resistance must not be higher than the passive soil resistance on the pipe, the analysis shall be carried out by a trial- and-error method. More than one computer run may be needed to obtain a satisfactory answer. The forces of the soil spring restraints from the computer result must be below the passive soil resistance. Otherwise, the soil spring restraints must be changed to the restraints with constant force whose magnitude is equal to the passive soil resistance. Another computer run with the new restraints should be made until no restraint reactions from computer result are significantly higher than the passive soil resistance.


5.7 CRYOGENIC AND LOW TEMPERATURE PIPING

Cryogenic piping is understood generally to include the range of operating temperatures from (-150 F) to absolute zero (-459.4 F). Cryogenic piping is more critical than normal refrigeration and other low temperature piping for several reasons. Greater care in design Is required to prevent water vapor from entering the insulating media where it would freeze and cause an insulation breakdown. Special anchors and supports also are required to prevent low temperatures from affecting carbon steel support beams and causing brittle fracture. The pipe stress analysis engineer's responsibility covers thermal construction and design basis for supports, guides and anchors, etc. Project engineering shall specify the insulation and vapor barrier requirements.


5.71 Support Design

Special saddles have been designed within the stress group for cradling the insulating media. It was found that for 24" pipe containing LNG (liquid methane at -258°f) a 6" thickness of polyurethane (density 2 lbs/cu ft) or foam glass is usually required for insulation. At support points, a higher density of the polyurethane has been used instead of low density polyurethane or foamglass because of better abrasion and shear resistance. The limit of 1% deformation under dead load is a reasonable criteria to determine the proper density of the poly­urethane block. A recent installation required 7#/cu ft density for a 24" pipe and supports spaced up to 26*apart. The cost of polyurethane increases with density, so it is suggested that a practical minimum be arrived at. See owner's standards for a recommended saddle design. Saddle supports have been either clamped to the insulating media or cemented to the insulating block with polyurethane elastomer, both have been found to work satisfactorily. Likewise, the special support block of insulation between the saddle support and the pipe has satisfactorily been cemented to the pipe itself to ensure movement of the support with the piping system. Until feedback from operating plants or engineering design proves otherwise, all support blocks should be cemented to the pipe with Adiprene or its equivalent, #2050 Adhesive (Polyurethane Elastomer), by CFR Division of the Upjohn Company or other suitable compound. The above adhesive has been tested to -423 F (liquid hydrogen) by the research division of one of the aircraft companies and found to maintain its adhesive qualities at those low temperatures.


5.72 Anchor and Guide Details

Wherever it is felt that below freezing temperatures can affect support, guide or anchor members constructed of carbon steel stressed to values of 5000 psi or greater, special details should be provided to insure that the structural members are not detrimentally affected by those temperatures. Special carbon steels or alloy steels should be used having proper impact value where tempera­tures dictate. See owner's standards for recommended anchor and guide designs.


5.73 Reduction of Friction at Support Surfaces

Whenever the anchor forces or frictional forces at supports might prove detrimental to the system's design, special sliding or roller supports should be provided. Teflon slide plates bonded to the under surface of the pipe saddle support channel have been successfully used. These slide plates bear on a similar slide plate bonded to a metal plate which is tack welded to the support beam. The overall thickness of the two slide plates commonly is 7/16" total. Their usefulness does have temperature limitations which vary with each manufacturer.


5.74 Flexibility Design of the Piping System

The materials used in Cryogenic piping systems increase in strength as the system gets colder and brittle fracture is avoided by the proper selection of special materials of construction. Therefore, conservatively, the same allowable bending stress is permitted as if the system was at 100°F. To absorb the contraction of the piping system, the first consideration should be to use expansion loops or offsets. Where this is not possible, bellows type expansion joints should be utilized in tandem within a minimum offset in the piping. The use of the bellows type in direct extension or compression should be avoided but are not prohibited. Bellows expansion joints must be very carefully protected from icing up and ultimately being crushed. This is their main draw­back. Other methods of absorbing the systems contraction are as follows:


a) Jacketed piping with internal axial expansion joints. This may require expansion joints periodically in the external jacket pipe, if the system has long runs. This system incorporates insulation in the jacket space.


b) In very special cases, the line might be prestressed to absorb contraction. This requires no expansion joints but suffers from large expansion forces and requires very special installation procedures. The use of hydraulic jacks or liquid gas cooldown might be employed.


6. STANDARDIZATION OF APPROACH TO PIPING PROBLEMS


6.01 Allowable Pipe Spans


a) The spans in owner's standards are limited by longitudinal bending stress or a midspan deflection which has proven accept­able from past experience, whichever governs. Although the spans are the maximum allowable, they are limited to a practical span for general pipe way use of 20 to 25 feet. These and other limitations are explained in the Standard itself. Alos, read "Use of Standard Weight Spans"


b) Where it is impractical or very costly to install special stanchions for support of small line branches from the pipeway headers or the support of long runs of very small piping, consideration should be given to supporting the lines from a single large diameter header by cantilevered structural members welded on, or by a trapeze beam hung between larger lines. Normally, the supporting of pipe to any other piping system is not a good policy and should be generally avoided.


c) Occasionally groups of very small diameter piping, such as chemical injection lines, can be banded together whereby the moment of inertia of the group as a whole reduces the bending stress or deflection of the system to a permissible amount.


6.02 Allowable Pipe Overhang

At turning points of pipe way stanchions, the supported piping systems have varying lengths of pipe overrunning the last support beam and rise up or turn down to join similar "overhangs" of piping from the adjacent pipe way at right angles to the first one. These overhangs within certain limitations are permissible without support. But, when the overhang is such that stress or deflection limitations are exceeded (See owner's standards) then, the overhang requires a special support. Dummy legs welded to the piping elbow and extended until it crosses the next stanchion beam is the most commonly used method of supporting the overhang. Essentially, it supports the system by extending the pipe as a "beam" across two supports. (See owner's standards for dummy legs required.) Where the dummy leg becomes too long, special beams should be added to the stanchion to support the overhang.


6.03 Pipe Guide Spacing

Pipe guides are used for several purposes. They keep lines essentially straight for good general appearance, or they prevent buckling due to high axial loads from friction or expansion loop forces. Guides can also be used to react against lateral line connections thereby con­stituting an anchor for the branch pipe. When anchoring branch piping by this method the guides are placed on the main header at the beams on adjacent stanchion column lines. The lateral reaction is taken by "beam" action of the 20' to 25' pipe span. Under high loads the stress or deflection of the pipe should be checked.


a) Guide spacing varies for the different areas of application. On vessels, guide spacing is reduced from those permitted in on-plot or off-plot piping. This is due to higher wind loads with in­creased elevation and load limitations of the various guide details used. See the pipe support manual for these allowable spacings.


b) On-plot and off-plot guide spacings could be essentially the same except that within a guide range for any pipe size, it is pre­ferable to use the low side of the range for on-plot pipe ways and to use the high side of the range for off-plot pipe ways. The reason for this is that on-plot piping, being more critical in nature due to branch connections, should have a more conservative design.


c) The suggested guide space ranges are:

Line Size Guide Space Range

2" 40' - 50'

3" 40' - 50'

4" 40' - 60'

6" 60' - 80'

8" 80' - 100'

10" 100' - 120'

12" 120' - 150'

14" 120' - 150'

16" 150' - 200'

18" 150' - 200'

20" 200' Max.

24" 200' Max.


The guide space ranges are a general rule and in situations where high axial loads exist these guide spacings should be reduced, after checking for buckling in column action.


6.04 Instrument Strong Back Flexibility

a) During normal operation instrument strong backs heat up with the attached vessel and since no differential expansion exists between the two there is no flexibility problem. But, if some faulty operation develops within the instruments, the block valves at the vessel nozzles can be shut and the instruments removed for repair. The strong back at this time cools down to ambient temperatures. At this time there is a differential expansion that exists between the strong back and the vessel. Unless the nozzles or the offsets in the piping to the strong back are flexible enough a failure could occur in the vessel nozzle or in the strong back proper and connect­ing instruments. owner's standards has been developed to give the Piping Designer a reasonable approach in providing flexibility in the system before it is reviewed by the Stress Group. These systems should be approved by the Stress Group by checking with the above standard.


b ) Support of Instrument Strong Back

Where long strong backs are offset and "Christmas Trees" are hung from vessel nozzles there is a need of supporting these assemblies to the vessel shell or platforms. Generally this is done by inspection without taking time to go into lengthy calculations. If in doubt, add a support, always taking notice of affects of differential expansion between supports and nozzle connections.


6.05 In-Line Pumps

As a general rule, in-line pumps exceeding 75 HP should be supported on a foundation regardless of whether the piping is supported separately or not. On pumps of this size, base flanges may or may not be provided, but this need not dictate that flanged pumps be bolted down. If sliding is required, provide base plates and either eliminate bolting or add notes to pertinent drawings or isometrics to adjust nuts hand tight. Sleeves may possibly be used to assure that nuts will not bear tightly on flanges. The pipe stress analysis engineer should note that holes are to be oversized or slotted to allow for movement required. Pumps smaller than 75 H.P. may be supported to the adjacent piping. See owner's standards for suggested support techniques.


6.06 Expansion Loop Design

The design of expansion loops for pipe ways or any pipe system has been programmed to produce a book of "Loop Tables". These tables enable a piping stress analysis engineer to closely design by inspection a loop to any desired stress or reaction force. A complete description of the method used to arrive at a design is found within the Owner's Design Guide.


a) A design pad (form 149) is available for recording all pertinent information regarding the design and location of the expansion loop. To arrive at the minimum sized expansion loop required, the maximum allowable stress for the piping system has to be determined from the limitations in the code on the material at the operating temperature. The actual size of the expansion loop is equal to or greater than the minimum loop size to fit properly on a supporting media. The spring constant and the resulting bending force within the system are then tabulated on the design pad for reference.


6.07 Pipe Anchors

The anchors described herein are for above ground piping. Anchors are used to direct the expansions or contractions of piping systems and thereby prevent interferences with other piping or structures, and/or control reactions to attached equipment. The reactions at anchors are taken by support beams made of braced or unbraced struc­tural steel or precast concrete. These anchor reactions shall be placed on an ozalid of the pipe way, specifically reduced for use by the stress group, and a print of it passed on to the structural group for review of their stanchion design.


a) It is suggested that these anchor loads be calculated and faith­fully tabulated on a form for later reference. The client upon occasion has requested these loadings, therefore, the tabulation may be very important. The individual pipe stress analysis engineer may compute them one by one, as he comes to an anchor tentatively and compute all the loadings at once when the piping is finally completed. Standard calculation sheets are available for these anchor cal­culations (form 149).


b) The calculation sheet for expansion loop data and anchor force determination does not include a listing of every item for tab­ulation but covers key items for final summation to obtain the anchor force. It is suggested that the auxiliary sheet of pipe weights (form 188) will be used by the pipe stress analysis engineer to mentally add up incremental weights for a particular system under "Wt".The coefficient of friction to be commonly used for steel on steel shall be 0.2 unless special surfaces are applied or additional factor of safety is desired. Where piping is supported on round bars the coefficient of friction should be raised to 0.3.


c) Anchor loads for buried piping can be computed by formulas re­commended in the section on "Buried Piping". (Par.5.6)


d) When computing anchor loads for above ground piping, the loadings on each side of the anchor generally tend to balance out to some degree. In some cases a long run of piping will be anchored near the center of the run just to prevent gradual creeping of the system. The frictional force on each side of such an anchor may theoretically balance or cancel out. The load to assign to such an anchor should never be less than 25% of the frictional force from one side alone.


6. 08 Stacked Exchangers

a) When exchangers are stacked it is customary to use radial nozzles directly connecting the two channel sections and the two shells. The hotter shell expands more than its adjacent shell and tends to be constrained by the inter-connections. Some deformation of the nozzles takes place and when the temperature difference and resulting stresses are large enough they can cause a failure in service. This failure not only results in a plant shutdown but could be the cause of a disastrous fire or explosion.


b) Owner's standards has been established to give analysts a common approach in reviewing the problem. As can be seen, when the difference between mean temperatures of the adjoining shells is greater than 100 r some provision should be made to add flex­ibility to the nozzle connections. These studies should be made early in the job such that nozzle orientations can be corrected before fabrication is started. Nozzle and piping arrangements to improve flexibility are shown on the Standard.


c) To reduce movements of piping from exchangers leading into unit pipe ways, hot exchangers should be anchored at the support closest to the pipe way. The exchanger expansion tends to cancel the expansion of the connected piping and its affect on pipe way clear­ances. Where cooling water from below grade is connected to the channel end of the exchanger the exchanger should be anchored to the support closest to the channel end. For a dimensional guide see owner's standards.


6.09 Off Plot Pipeways


6.091 General

a) Prior to the design and layout of off plot pipeways the project piping stress analysis engineer should meet with the off plot project engineer to discuss and establish proper temperatures for the expansion de­sign of off plot piping. The temperature range shall be realistic, and it shall include reasonable ambient temperature variations at the job site, but unless the client insists, remote upset condi­tions should not be stipulated.


b) Review should be made with Project Engineer and the Client, regarding the use of steam-out. Usually off plot piping is not steamed out and is therefore not designed for that condition. Normal operating and maximum or upset design temperatures should be listed for all lines on the off plot line designation tables which should be completed prior to making the stress studies.


c) Methods of absorbing pipe expansion should be reviewed with the off plot project engineer to see if the client might have restrictions on the use of expansion joints or couplings, etc.


d) When the design conditions have been established and if no formal memorandum has been issued by the project engineer, the piping stress analysis engineer should prepare a memorandum covering all final decisions and issue it to both the Chief Pressure Vessel Engineer and the Off plot Project Engineer, who should be requested to transmit such information to the client for information and record.


6.092 Expansion Studies


a) The design approach to off plot piping should not be as strin­gent as that for on-plot piping, therefore systems should be designed up to the maximum stress allowed by ANSI code for the upset condition except where reactions dictate otherwise. Additional lengths may be required to nest loops or use common supports. In some cases where only few stress cycles may occur, Article 5~ 1 of ASME Section VIII, Div.2, Design based on Fatigue Analysis might be employed. This criteria allows up to 3 times the allowable stress intensity for secondary stresses, thereby permitting close to 60,000 psi for A106 or A53 GRB pipe materials.


b) The expansion review of off plot piping is essentially a clearance check of pipes as they move relative to one another or whether they interfere with the structural appurtenances of sleepers or stanchions. See owner's standards for other than 90 corner move­ments.


c) The tie-in temperature used as the calculation basis should consider the specific time of year for plant construction. Also care should be exercised to consider the clearances and stresses for both expansion and contraction of all adjacent piping systems.


d) Except in the vicinity of off plot pump manifolds or other equip­ment limiting reactions or stresses, the systems should be allowed to expand up to a practical limitation of 12" at a corner of the pipe way or at each leg of expansion loops. This means that ex­pansion loops should normally absorb up to 24" of expansion.


e) Support shoes for insulated piping in pipe ways now are ordered in two standard sizes, 18" 6- 30". The 18" shoe permits 6" and the 30" permits 12" of movement each way from their 4 with 3" of overhang for assurance that the system won't hang up on the support. This 3" overhang is a standard allowance to be used at any support after maximum movement of an insulated piping system. As an aid to the pipe support design group, the supports at which pipe expansions exceed 6" and 12" should be noted to assure that shoes of proper lengths are assigned to each support.


f ) Design of pipe guides and anchors is covered in paragraphs 6.03 and 6.07.


g ) Cold spring of systems should be avoided unless absolutely nec­essary to reduce reactions at equipment or provide necessary clearance. See owner's standards for method of noting cold spring.


h ) Branch piping from the off plot pipe ways leading into diked tank fields must be reviewed for restriction of lateral movement due to small clearance in the sleeve buried in the dike. Sometimes the pipe is just coated, wrapped and buried in the dike which there­fore permits negligible lateral movement. Anchors may therefore be required close to these branch connections to protect them against excessive lateral movement. Expansion of these branches whether from dike sleeves or pump manifolds can be allowed to deflect the headers laterally, therefore, the guides in the headers should be located far enough apart to keep reactions back to the pumps or sleeve seals to a reasonably low value. Axial movement of these branches is generally prevented by either the burial of the pipe in the dike or by a link seal between the sleeve and the pipe on the tank side of the dike. Piping within the diked area is described in paragraph 7.17 on "Tank Field Piping."


6.093 Pump Manifolds

Pump manifolds can be quite complicated and "tight" but when near ambient operating temperatures the expansion movements are usually small. Such movements can be directed away from the pumps if anchors and restraints are properly located. See owner's standards for an example of a properly anchored system. Offsets in the branches to the pumps should be avoided wherever possible. Normally no offsets are required in these branches on systems at temperatures of 150 F or less. Where temperatures exceed say 150 F, then offsets in the branches to the pumps may be required to improve flexi­bility and reduce reactions on the pumps. Where the suction or discharge lines leading to or from the pumps are eccentric by several feet from the pump centerline it may not only be required to support the overhang, but also restrain movement in the axial direction of the branch pipe. Several support-restraints of this type are shown in owner's standards.



7. MISCELLANEOUS AND SPECIAL PROBLEMS


7.01 Slug Flow


a) In two-phase gas-liquid flows where the phases are unevenly distributed and pass through a restriction, such as a valve or an expanded section, or a turn such as an elbow, there is a variable force exerted on the containing walls. This variable force creates impact loadings on the guides and supports of the system which must be adequately accounted for in their design. The magnitude of this variable force cannot be accurately evaluated due to the complexity of the flow. Therefore the design loads of the supports and guides should include an addi­tional design factor which might be classed as an impact factor.

In lieu of some definitely calculable factor it is suggested that this impact factor be 3.0 times the weight of a slug of liquid that might pass separately through the pipe as approxi­mated by the Project Process Engineer.


b) These variable flow conditions also can affect the system detrimentally by setting up severe vibrations. The system should be carefully reviewed with this in mind and if necessary hydraulic type shock absorbers should be utilized to prevent large amplitude vibrations. Within one of our recent refinery projects slug flow caused large diameter piping to vibrate continuously and though the amplitude was small (1/8" peak to peak) it resulted in a failure at a 30" diameter tee connection. The tee connection was reinforced considerably and a hydraulic damper was used to reduce the amplitude of vibration. Although guides and hydraulic struts can reduce slug flow effects, best results are obtained in reducing slug flow internally or routing pipe to permit a more uniform and smooth flow.7-02 Mitered Elbows


7.02 Mitered Elbows


a) Mitered elbows are used many times in low pressure piping systems for economy since the cost of welded or seamless elbows becomes prohibitive in the larger size pipes. Two of the drawbacks to mitered ells are high stress concentrations and poorer flow characteristics. As more pieces are used to make up a metered ell flexibility increases and stress intensifications decrease. In other words the miter with more pieces approaches the flexi­bility of a smooth elbow of the same radius.


b) For flexibility studies using some computer Programs the mitered ell must be replaced by an equivalent elbow with the same flexibi­lity. The method to be used in obtaining the equivalent elbow is shown in owner's standards. In most of the more recently developed programs the mitered elbow is handled automatically.


c) Before using miters in a flexibility calculation, they should be checked for permissible pressure by the formulas in paragraph 304.2.3 of the ANSI B31 -3 Code.


7.03 Tee Connections


a) The history of failures in piping systems points to tee connections as being particularly vulnerable. Tee connec­tions have high hoop stress patterns around them which are non-uniform and involve stress raisers or intensification factors. Vibration causes cyclic stresses which may be low in magnitude, but can be troublesome when acting through medium to high frequency. When piping systems are studied by the computer flexibility program, care should be taken to always include the stress intensifica­tion factor at all tee connections. If the specific computer program doesn't have this capability, then add them manually to the output. Although no strict rule can be given regarding allowable stresses at tee connections in vibrating systems, good engineering judgment should dictate that the analyst use less than the maximum allowable stress.


b) The stress concentration factors for tee connections have been calculated by a computer program and the resulting values have been plotted on graphs for easy reference. The graphs are shown in owner's standards. They give stress factors for unreinforced tees, tees reinforced with pads both equal to the header thickness and 1.5 times the header thickness and for forged tees. It is suggested that all questionable stress levels at tee connections in flexibility calculations include the proper stress factor read from one of the graphs pro­vided. An accuracy of 2 decimal places is sufficient. When calling for a reinforcing pad the minimum width shall be 0.2 x branch outside diameter, which results in equal stress at both the crotch and the outside of the reinforc­ing pad.


7.04 Injection Connections

Whenever piping connections, involving injections of near ambient temperature fluids, are made into piping systems operating at elevated temperatures, say above 500 F, a critical stress condition exists at the nozzle connection whether reinforced or not. Failures have already been brought to our attention. To avoid the sudden transition of temperature change the small pipe should first enter a larger nozzle at a blind flange or weld cap attached to its end. This should be brought to the attention of the Project Engineer who in turn should locate these for review by the Project Stress Analyst. A standard will be developed to cover this problem.


7.05 Heater Coil Decoking


a) During normal operations of most heaters a layer of coke gradually builds up on the inner wall of heater tubes. As the thickness of the layer increases the firing rate must also be increased. This increase in firing rate results in an increase in the tube wall temperature and could eventually exceed the maximum allowable temperature for the stress level in the tube. To prevent this condition the coke must be removed periodically or whenever operating conditions indicate excessive coke build­up.


b) The coke is removed by using a steam-air or thermal decoking method. This method involves heating the tubes first and then passing steam through them at a specific mass velocity and then introducing a mixture of steam and air. The effluent is water quenched and discharged to the sewer, and the gas is vented from the quench drum stack. The temperature affecting the external decoking manifold and effluent piping during this time is generally 1000°F. Flue gas temperatures gener­ally reach 400 to 450°F.


c) Supports for piping to the heaters must be properly located and designed for the disengagement of both normal piping and the connection to decoking effluent or steam piping.


7.06 Catalyst Regeneration

Catalyst regeneration is required periodically in Catalytic Reformer Plants to reactivate the catalyst for efficient plant operation. At the time of regeneration the hydrocarbons are stripped or burned off of the catalyst and the entire system is made as free as possible of residual hydrocarbons. This is done by heating the system to normally greater than operating temperatures by nitrogen until a specific temperature level is reached and oxygen is then gradually added in small volumes further increasing the temperature. This burns off the hydrocarbons and is continued until a system gas analysis shows that little hydrocarbon remains. Regeneration tempera­tures usually read as high as 900 to I000°F for a period lasting several days.

Obviously these increased temperatures create an additional condition within the external piping system that must be accounted for in both piping flexibility and support.


7-07 Reformer Furnace Pigtail Design

The piping connections of both the inlet and outlet piping to the vertical tubes of Reformer Furnaces are subjected to both vertical expansions of the tubes and the horizontal expansions of the inlet and outlet collection headers. The large vertical expansions require that the outlet collection header be spring supported to move up and down with the furnace tubes. These furnace tubes are generally supported solidly near their base and expand upward several inches. The piping connections from the furnace tubes to these collection headers are called pigtails because of their design shape. Unfortunately there is a space limitation and these pigtails are somewhat restricted in their flexibility. In the analysis of any pigtail dead load stresses of the loop must be considered along with stress concentration fac­tors at tee connections.


7.08 Cold Spring


a) Cold Springing of piping systems originally was utilized to reduce stresses and reactions in piping systems, and to equalize somewhat the displacements of piping about a neutral axis, or reduce interferences.


b) The modern piping code no longer permits the reduction of stress, as such, but permits allowable stresses within a "stress range". Therefore, if a system is cold sprung, a certain portion of the stress range is already utilized and only the remainder is permitted for the expansion beyond the amount of cold spring.


c) Since cold springing is an additional operation for the field to complete, it is suggested that cold springing should only be requested where it is critical to the systems design. For example, piping to turbines or compressors might require cold spring to reduce reactions to an acceptable level. Generally, in pipe ways, the design of expansion loops should not involve cold springing although the loops may still be designed right up to the maximum allowable stress. Other systems between columns, exchangers, drums, or connections to other piping should not be cold sprung unless absolutely necessary to avoid the additional operation. It has been found that cold spring notations have been overlooked or cold springing has been improperly applied in the field, un­less great care has been taken to flag and describe the manner in which it is to be applied. If equipment must be protected by cold springing of its piping systems and the manner of procedure of cold springing is felt to be particularly important, the Pipe Stress Analysis Engineer should write step by step pro­cedures and send them to the Field Engineer in charge.


d) Typical cold spring notations are shown on owner's standards.


7.09 Blowdown Systems


a) Blowdown piping as a general rule operates at low pressures with medium to high temperatures (i.e. 300°F - 1000°F) and close to 100% vapor. The main headers are usually large diameter pipes up to as much as 4 or 5 feet in diameter. The systems become operative upon sudden release of vapors from safety relief valves and therefore are subject to sudden surges of gas flow. This tends to set up large am­plitude vibrations or shaking of the system. To protect against the piping from bouncing off supports or damaging adjacent equipment, hold down guides should be judiciously located through tout the system. The system should be amply anchored to direct its thermal movements and where movements are too large to absorb within the inherent offsets of the piping, loops, or offsets with tandem expansion joints, are recommended. Direct axial expansion joints are undesirable because of large anchor forces required to contain the system. There is no limit to the total expansion which the above devices can take except that sound engineering judgement shall be applied to limiting anchor forces, lengths of support saddles, and spacing required to other pipe or equip­ment.


b) Branch connections, expanding thermally between relief valve and the blowdown headers, may require the addition of flexible offsets to absorb such movements. The allowable stress of the pipe at the connection to the relief valve should be limited to prevent any distortion at the valve which would render it inoperative. As a rule of thumb the resultant bending stress at the connection should be kept below a maximum of 10,000 psi. Large weight reactions should be removed from the relief

valve by use of spring supports or equivalent. Welding of gussets from the valve discharge pipe to the valve inlet nozzle as a solid support is not recommended and shall not be used on systems exceeding 150°F.


c) When gas flow through blowdown systems has a velocity at tee connections above .2 Mach, the pipe wall for at least 5 diameters on each side of the tee connections should be in­creased in thickness to prevent cracking by ovaling vibrations. The branch pipe should likewise be increased in thickness for a short distance back from the tee.


d) The reactive forces resulting from the discharge of relief valves can be computed from the following formulas:

The reactive forces resulting from the discharge of relief valves

7.10 Field Checkout


a) Field Checking has become an important part of the Project Stress Analyst's responsibility. Errors in the Field due to omission or improper interpretation of design drawings have necessitated that critical piping be reviewed just prior to unit startup. A moan list should be developed at the field covering any items yet to be completed by the construction department (to cover possible omiss­ions) and to itemize in detail any corrections or modifications required on any support or piping installations where the design intent was not met. Exceptions may be made where the system as installed will function adequately and every effort should be made to avoid requesting corrections unless there is danger of failure of some component of the system. The moan list should become part of a report which is then given to the Job Superin­tendent and the Supervising Field Engineer.


7.11 Soot Blowers


a) In today's high performance steam generators, "Controlled Cleanliness" of horizontal and pendant tube surfaces must be maintained to assure proper heat absorption and optimum steam temperatures. Soot blowers are needed for the specific pur­pose of cleaning tubes in the convection section of heaters and boilers. The soot blowers are constructed of long hooded frames which support horizontal lances up to 24' long.

The lance (or female pipe) is extended into the heater or boiler convection section by retracting it from its internal "male" feeder pipe. The soot blower assembly is fed by air or steam at a flanged nozzle 15' or 20' out from the wall of the convection section. It is not rigidly held at this point but can be moved laterally a small amount (say 1" +) and even slightly rotated.


b) If several units are to be connected together by a common steam header, the above movements can normally be tolerated. The soot blower frame is supported at each end to the platform structures of the heater or boiler.


7.12 Settlement and Frost Heave


a) Differential settlement between pieces of equipment, or structures and equipment, can induce damaging reactions or stresses to both piping and the equipment to which it is attached.


b) It is essential that specific settlement or heave deflections are obtained from the structural department for critical locations such as around pumps, tanks, and at all vessels and columns. These deflections must then be incorporated into the design analysis of all affected piping. Where these deflections cannot be easily absorbed it may require that pipe supports be extended below the frost line or that piles be driven to prevent settlement.


c) To avoid special piles for foundations, pump piping may be supported to the pump foundation itself by extending a portion of the foundation under the piping. Also, a beam can be em­bedded into the pump foundation with a short section cantilevered out to support the eccentric pipe system. This cantilever section should be sufficiently above the grade slab so that anticipated frost heave will not affect it. Support lugs maybe cinch anchored into the side of the foundation. The method

of supporting the pump piping must therefore be agreed upon early in design stages of the plant.


d) At plant sites where frost heave is a problem, the support of piping manifolds alongside exchangers can be supported to structural members fastened to the sonotube or pier supports of the exchanger itself rather than provide deep separate foundations for the piping separately.


e) Where piping systems below grade are subject to settlement piled supports should be provided to prevent detrimental deflections of branch piping to pumps or other equipment. Deep burial of these headers are required in areas affected by frost heave.


7.13 Ambient Temperature Effect on Bare Piping


a) Empty piping in long pipe ways can be greatly affected by atmos­pheric temperature (ambient) changes. Stagnant systems in 100-110°F temperatures can reach effective wall temperatures of 130° to 140°F depending on pipe surface coloration or cover­ing. It is suggested that 130°F minimum be used for the high temperature design of systems affected by ambient changes only. For the contraction of systems below the tie-in temperature the basic design data of the locale should be reviewed to de­termine the minimum temperatures that the systems will be subjected to. It is very important that contraction from a tie-in temperature be considered when checking clearances, or designing expansion joints with limit stops or internal sleeves. Severe failures in systems have already occurred where this was not properly accounted for.


b) The tie-in temperature should realistically be chosen for the time of the year of installation and the locale of the plant. For example, if piping is to be installed in Alaska during the winter months the tie-in temperature might range from below zero to freezing (32°F), whereas piping installed in Canada in the winter months would range from 40° to 80°F. For long pipe ways this can result in a considerable difference in ex­pansion movements.


7.14 Control Valve Piping


a) Piping to control valves or let down valves are subject to vibration which sometimes reaches dangerous amplitudes or destructive frequencies. In general the connecting piping systems should be guided whenever possible to eliminate large amplitude vibrations. But, where sonic vibrations occur with high energy input, the pipe tends to oval or wave patterns develop circumferentially dictating that rigid attachments should be avoided since failure at points of rigidity on the pipe wall will generally occur.


b) It is the responsibility of the Control Systems Group to flag systems with those critical tendencies for special study and corrective design. When the pipe stress analysis engineer is confronted with this type of problem he should contact the Control Systems Specialist for the proper solution to the problem.


7.15 Hydrotest of Large Low Pressure Piping


a) The design of supports for large diameter piping systems can be greatly affected by whether or not the system will be filled with a liquid, since the filled weight can be many times the empty weight. Therefore in the early stages of plant design it is extremely important to get agreement with the client and our construction department on the basis of support design of large diameter piping systems normally handling gas flow. If a hydrotest is imperative then the structural group must design supports for liquid load. If the system will be air tested, or by similar alternate gas test, then all parties concerned must agree in writing in order to protect Owner's interest, and avoid design checking or modifications near job completion.


b) Ring girders or thick saddle plates may be required at support points for hydrotested systems.


7.16 Pipe Supports


a) When marking up piping isometrics or drawings for required supports, the list of Standard Support Symbols as shown on owner's standards should be utilized. This will help the support group to interpret the markings of each piping stress analysis engineer in a commonly understood fashion.


b) Elaborate, highly detailed, and non-standard supports should be avoided. Supports should be as simple as possible.


c) Some basic precepts on where, when and how to support piping are:


1) Avoid supporting one pipe to another except for small utility lines being routed to off plot facilities alongside a large line high above grade. Occasionally individual branch lines of small diameter are routed between the pipe ways and process vessels at an excessive span and at elevations in excess of 10' above grade. In cases like this, the support to a larger line nearby is acceptable.


2) Spring supports should be specified only when important to the safe or proper design of a piping system. A great number of spring supports are often rather casually de­signed into the piping in a plant. Upon closer inspection, however, it will usually be found that the system could be designed with solid supports. The use of shims in pre-­springing pipe will permit minor expansion movements.


3) Maintenance Supports should not be provided unless required by clients specifications. During plant shutdowns any system that is to be repaired can be temporarily shored up.


4) Piping to vertical vessels that are flanged at the vessel nozzle should be provided with a bracket support. This is more for installation and maintenance generally than for stress purposes. Non-flanged piping to vessels may not re­quire these bracket supports, stress permitting.


5) Where rigid guides or struts would restrict the free expansion of a piping system in such a way as to affect it detrimentally, the system should be guided with truck shock absorbers (see support group details of acceptable units) or in the case of large, critical piping the more specialized hydraulic cylinders should be installed. (i.e. Bergen, Grinnell, Barco, Marpak or equivalent types). Piping expansion occurs generally at a slow enough rate to permit the gradual adjustment within the hydraulic unit.


d) Spring Supports

l) Spring supports, when properly used, fulfill a very important need in the support of piping systems. However, they should not be used indiscriminately or as an easy solution for the support of piping which is affected by vertical expansions or other mechanical movements. For proper installation procedures for the construction department a structural owner's standards is available for their use.


2) Solid supports are usually practical when the support lug on the inlet or outlet piping is at the proper elevation to balance out vertical expansion of the equipment and its pedestal supports. In other cases as a general rule, it is better to make a calculation if it will prove that a solid support is acceptable. If calculation time does not exceed about four hours it is probably worth making the calculation in order to eliminate the spring supports.


3) When spring supports are used in pump systems the pipe stress analysis engineer must review the effect of the spring reaction on the system based on a spring preset reaction which has been calculated for a liquid filled system. Prior to start-up this reaction is applied to an empty pipe system. If the liquid weight portion of the reaction can't be tolerated by the pump or piping, the spring may have to be preset at some value between the full and half full pipe weight reactions. When the liquid weight affect is intolerable for even a 50% weight change, the piping will have to be rerouted to provide an acceptable design. See Specification M-504 for field installation instructions for spring supports and ensure that design of spring supported systems is consistent with the requirements of that specification.


4) When to Use Spring Supports

If vertical expansions or mechanical movements (imposed on a piping system restricted by solid type supports) result in intolerable stresses or reactions, then spring supports may be required. Spring supports permit the piping system’s flexibility to be used to absorb system movements within tolerable limits; they must be used on hot piping systems adjacent to pumps, turbines and compressors when solid supports cannot be tolerated. Wherever variable spring hangers are used, the piping stress analysis engineer must check to assure that the total variation in support effect does not result in harmful stresses and forces within the piping system. Otherwise constant spring supports or counter weight supports should be considered. Generally for non-critical systems, varia­tion of support force up to +. 25% and movements up to 3" may be allowed.


5) How to Avoid Using Spring Supports

If a support is not adjacent to a piece of rotating equipment or some other similarly delicate apparatus, a piping system subjected to 3/8" or less movement might well be shimmed at supports after the system has been completely welded in place or bolted up. If the flexibility of the piping permits, and the dead load of the pipe will not keep the expanded system on its supports, the use of nominal shims, from 1/8" up to 1/2" thick, should be utilized rather than specify spring supports. left up to the field forces and therefore it is the duty of the Pipe Stress Analysis Engineer during Field Check to review these connections.


b) Steam trace branches and condensate return lines are often banded together in plants located in freezing climates. Obviously differ­ential expansions of the steam and condensate headers may detrimentally affect these connections. Therefore, a horizontal loop must be extended from the smaller condensate branch before rejoining the steam line for banding together. The extended loop is separately insulated and is allowed to cantilever out unsupported except by the banding upon return to the steam line. Drip legs of steam headers have been routed directly to a stanchion beam or other structural member and clamped tightly to a fixed support permitting no movement at all. Standard drawing for drip leg details in owner's standards, has been revised to alert the field of this problem.


7.17 Tank Field Piping


a) The piping in tank fields is subject to several special design considerations such as:

  • Tank settlement

  • Earthquake movements

  • Containment within diked areas

b) Where tank settlement is a problem the first pipe support should be located, say, 20 feet away from the nozzle and be of an adjust­able type. Adjustable supports can be made up of wood block layers 1/2 inch or 1 inch thick that can be retracted as the tank settles. Where adjustable supports do not fit into the design, flexible couplings or joints can be used in a tandem arrangement. A tandem unit involves a length of pipe with a flexible connector at each end which absorbs deflections by angulating the unit at right angles to the axis of the pipe.


c) Earthquake movements can be accommodated by providing a pipe offset at the tank. This offset can be used for both settle­ment and earthquake movements. The routing of a long line without offsets directly connected to a tank nozzle should be avoided.


d) Piping routed between and anchored in dikes of a tank field generally requires either loops or offsets to absorb its expansion and contraction, even though only affected by ambient temperature changes. The burial of the pipe in the dike pro­vides sufficient restraint generally to anchor the pipe. Where sleeves are used, a link is generally provided between the pipe and the sleeve.


7.18 Steam Trace and Steam Trap Piping


a) Steam trace piping details are provided to the field by an Engineering owner's standards. A problem arises in the connect­ions from the steam headers to the steam traced pipe. The expansions of the steam header requires that the interconnecting branch pipe to the steam traced pipe be of sufficient flex­ibility to absorb the deflection without failure. Also the clearance of the branch pipe to other piping or structures must be considered. The location of these steam trace branches is.


b) On return to the main office, the Piping Stress Analysis Engineer should make a brief trip report to the Project Engineering Manager, and send copies of the report with the moan list attached to the Project Superintendent, the Supervising Field Engineer, the Unit Project Engineer, and the Chief Vessel-Stress Engineer. See owner's standards.


c) A typical check list of items for field review might include:


1) Clearances between piping systems or between critical piping and structural members or any equipment. This includes review of critical cold springing.

2) Sufficient overhang of pipe support shoes on beams to allow for maximum pipe movement.

3) Movement of piping as affecting instrument or electrical connections.

4) Spring supports adjusted to proper loadings and stops removed after hydro test of system.

5) Pipe anchors located and installed correctly.

6) Expansion joint assemblies installed properly including orientation of hinges or tie rods, if any. Sizing bars to be removed.

7) Critical Piping:

  • Steam lines, including Turbine piping.

  • Reactor piping.

  • Furnace Transfer lines.

  • Blowdown Systems.

  • Compressor Piping.

  • Pump Piping.

  • Hot process piping (generally over 350°F).

  • Cryogenic and refrigeration piping.

  • Steam trace connections.


7.19 Plastic Piping


a) Because of the need within refineries and chemical plants for piping to carry alkalies or acids, various metals are used in the fabrica­tion of special piping for this use. Some of the materials used are rubber lined or glass lined steel pipe, and solid plastic or reinforced plastic pipe, usually known as reinforced thermo-setting resin pipe, filament wound, either hand laid, bag molded, or cast. In the case of lined steel pipe, its flexibility and support are similar to unlined pipe. But, where plastic or reinforced plastic pipe is used the support and flexibility requirements should closely follow the recommendations of the specific manufacturer. As a guidance, refer to ASME Code Case N115-1.


b) It has been noted that different manufacturers of PVC (Poly Vinyl Chloride) pipe recommend different methods of supporting and restrain­ing the systems. The Pipe Stress Analysis Engineer is urged to consider expansion and and contraction forces and stresses in systems before agreeing to totally restrain the systems with thrust blocks as recommended by one of the plastic manufacturers. In fact, all manufacturers agree on the cemented joints as being equal to or better in strength than the pipe itself. Therefore allow the free expansion of the system normally with the suggestion that the field be notified to exercise great care in installing the cemented joints for complete adequacy. Also, the manufacturers allowable support spans should not be exceeded


c) Because of the considerably lower values of Young's modulus of elasticity (1.5 to 1.0 x 10^6) of the plastic materials, the pressure elongation of the pipe line may be a significant factor in the flexibility or displacement stress analysis of FRP pipes. To take this into consideration, an equivalent coefficient of expansion that will include the pressure strain effect should be used in the owner piping stress programs.


7.20 Rotation, Reactions and Stresses at Nozzle Connections to Vessels


a) Most piping systems connected to shells of columns, exchangers drums and tanks are analyzed conservatively without consider­ing the rotational relief afforded at the nozzle connections. Generally, as long as the stresses in the piping and loadings on other attached equipment are within allowable limits, the systems as a whole is deemed acceptable. However, when the reactions on the shell nozzle appear high, then the Engineering Design Owner's Guide "Local Stresses in Cylindrical Shells due to External Loadings" may be used to approximate the vessel stresses due to the fixed end reactions. If this stress is too high, a calculation can be made employing the spring constant of the nozzle attachment whereby the reduced loadings on the vessel shell may be acceptable when compared to allowables in Owner's Guide.

If a system is obviously very tight, the spring constant "K" of any nozzle attachment should be evaluated from the owner's standards and incorporated in the calculation from the beginning.


b) If two vessels are interconnected by radial nozzles, such as stacked exchangers, and the shells are at different temperatures, the difference in longitudinal expansion must be absorbed mostly by a rotation of the joined nozzles at each shell connection, (i.e., nozzle rotation = differential expansion / total nozzle length). See owner's standards for condition requiring this evaluation and for the procedure to be followed.


c) The spring constants of owner's standards can also be used to find the deflection or rotation of pipe supports (i.e., cantilevers or brackets, etc.) attached to shells, due to the flexibility of the shell under the applied loads and moments.


d) Storage tanks present a unique problem involving rotation and deflection of shell nozzles close to the tank bottom during filling of the tank. These movements affect attached piping and should therefore be considered when locating external pipe supports or routing the pipe itself in a proper manner. See owner's standards for design criteria.


7.21 Bowing of Pipe


a) Bowing in piping systems is due to unequal heating of the pipe wall from side to side along its length. This type of bowing is unrelated to column instability from compressive axial loads. As one side of a pipe becomes hotter than the opposite, its longitudinal elements expand more than those of the colder side and bowing occurs.


b) Bowing may occur when:


1. Hot or cold fluid flows in partially filled pipes.

2. Sun's radiation heats the tops of large empty pipes laying close to the frozen ground.

3. Unequal surface heating of furnace tubes.

4. LNG in partially filled loading lines.

5. Channelling occurs in tubes filled with packing.

6. The burner flames in a furnace are not equally dis­tributed across the tube diameter.


c) If a piping system is not restrained and is considered weightless, bowing does not induce stresses in the pipe. Weight, friction and restraint, however, will induce stresses in the pipe, and the restraints may be subjected to very high reactions.


d) Bowing can usually be tolerated when it is of only short dura­tion. If bowing is considered detrimental, and it is not possible to improve the uniformity of the temperature in the pipe, then external restraints must be designed and provided.


7.22 Compressor Bottle Support


a) As explained in paragraph 5.22 (c), there is a need for an elastic support for compressor bottle to allow for vertical expansion downward from the cylinder support level. From design data for rubber bearing pads, a design procedure has been set up to properly size bearing pads. The pads can be placed between the support lug or saddle on the compressor bottle and the load adjustment plate underneath. This load adjustment plate is supported by four or more bolts embedded in a concrete pier. The plate is suspended about 3 inches above the top of the concrete pier to allow for tuning of the support to inhibit vibration.

The design procedure to size the bearing pad and the adjust­ment plate is explained herein and the rubber bearing pad physical data is shown on owner's standards. The support assembly detail is a structural standard. The size of the rubber bearing pad in the detail shall be determined by the Piping Stress Group by completing Form No. 70 of the Pressure Vessel Standards.

The suction bottles resting on the compressor cylinders need no support, except for eccentric or overhanging portions of the bottles, since the load is in compression. The discharge bottles hang from the cylinders putting tension into the nozzles which, under constant vibration, are more likely to fail. See Bearing Pad size calculation procedure below.


Bearing Pad size calculation procedure

Bottom Plate Thickness "T"

Design the bottom plate as a simple beam of cross-section "C" x "T" with supports spaced "F" apart and with a central concentrated load equal to "2R" (2 x (11) from above).


7.23 Tank Nozzle Movements Due to Pressure and Temperature


With the advent of larger diameter and taller storage tanks, a problem of shell deformation close to the bottom of tank due to product storage pressure has been magnified. Under pressure the tank wall will stretch and will move radially outward if unre­strained. At the juncture of this shell to the tank bottom the pressure creates a shear load which tends to stretch the tank bottom. This stretch is negligible compared to the shell radial distortion, therefore, the shell is nearly totally restrained at its juncture to the tank bottom plate. From this point a vertical section would show that the shell gradually follows an elastic curve to a point closely equal to 1.56x(Rt)^0.5 above the tank bottom where the radial deformation is equal to PR^2/Et nozzle on the tank located in this bulge area will exhibit both a downward rotation and an outward deflection. This results in a bending and shifting of the piping system connected to the nozzle which must be accommodated by its inherent flexibility considering all restraints acting on the piping system especially the location of the first pipe support adjacent to the tank. Where tank settlement is also involved adjustable supports or couplings can be employed as described in Section 7.17. When necessary to study the nozzle rotation effect on external piping to the tank refer to Engineering owner's standards for the conservative values of both rotation and deflection and input them into a flexibility calculation. Differences in expansion of the tank shell and tank bottom, which is reacted on by friction and may have minor buckling effects, are considered negligible.


8.PIPING STRESS ANALYSIS WORK CHECK LIST


The following list of work items is provided as a check list for Project Stress Analysts. It lists items essentially in their chronological sequence as they will occur on a project and is intended to draw attention to critical items, some of which must be reviewed and pre-planned at specific stages of a job in order to avoid delays and changes in other engineering work.


8.01 Design Data (Ref. Par. No.)

Obtain from the Project Engineer:

a) Basic design data for job site, i.e., Wind Loads, E. Q. Loading, Temperature Variations, etc.

b) Steam-out temp. - Project Engineer must issue memo to all Unit Engineers. (6.09)

c) Steam-trace temp. - Project Engineer must issue memo to all Unit Engineers. (7.18)

d) Steam-trace temp. -Project Engineer must issue memo to all Unit Engineers. (7.12)


8.02 Drawing Distribution

See that name is on Distribution of Documents, schedules, etc. for items required for stress work. (2.1)


8.03 Initial Piping Studies (3.0)


8.04 Alloy Piping (3.2.)

a) Give preliminary approval for material commitment.

b) Make final studies so that detailed supports can be issued for shop ISO's.


8.05 Stress Relieved Vessels & Piping (4.0)

a) Obtain dates from the Pressure Vessel Supervisor for scheduled shop stress relief of each vessel.

b) Complete stress studies of piping and send ISO's to Support Group six weeks ahead of scheduled |SU issued- for-construction date. Coordinate this with the Piping Supervisor.


8.06 Heaters (5.5)

The Piping Stress Analyst shall arrange a meeting with the Project Engineer 5- Heater Specialist and supply all infor­mation required on the heater bid specification, including the following:


a) Anchor nozzles - Yes or No.

b) Nozzle Movement - Amount and Direction.

c) Support of tubes - Top or Bottom -Effect on external piping.

d) Need for brackets on heater shell for pipe supports and plat­forms, etc. (Loads, details, etc.).


8.07 Compressors & Their Turbine Drivers (5.2 & 5.3)


a) Check with equipment specialists to assure that Mfrs. agree to our specified loading conditions, as related to equipment dead load.

b) Send piping ISO's with support locations to Mfr. with request for an analog vibration study for each reciprocating compressor.


8.08 Pumps (5.1)


a) Locate large bottom out pumps with respect to vessels to give best arrangements for flexible pipe configuration.

b) Avoid direct piping from equipment to pump.


8.09 Loops in Pipeways (6.06)


a) Place loops in headers to limit their expansion or affect on branches to turbines, pumps or compressors.

b) Locate off-plot pipeway loops as soon as possible as an aide to the Construction Dept, for field "fill-in" work. (6.09)

c) Locate and size anchors and loops for systems which are to be used during construction (steam and other utilities) when requested. (6.07)


8.10 Airfans (5.4)


a) Check with Project Engineer to assure that the specifications require a lateral movement tolerance of 5/16" min.


b) Acquaint pipe designers with flexibility requirements when several units are joined together.


8.11 Auxiliary Pipe Stanchions

Establish all additional auxiliary stanchions or special supports requiring piles or foundations, as soon as possible - when sufficient branch piping is modelled, so that field crews can complete pile driving operations and advance to later operations without concern for the need for additional piling in an area.


8.12 Tank Field Piping (7.17)


8.13 Field Checking (7.10)


8.14 Special Design Criteria


a) When complete thermal cycles within a piping system exceed 7000 and the expansion stress anywhere within the system exceeds 1.25 Sc, the overstressed section requires full examination in accordance with 336.5.1 (b) (2) (ANSI B31.3)


b) For piping in cold climates it is important to see that those constructed from carbon, low alloy and high alloy steels are not stressed higher than 6000 psi based on a combination of longitudinal stresses due to pressure, dead load and displacement strains. The operating pressure should be no greater than 15% of the maximum design pressure at that time nor should the temperature be below -50 F. If any of the above are exceeded an impact test is required . (See paragraph 323.2.2)

Read Also: The Canadian Piping Flexibility Stress Analysis Standard

LITTLE P.ENG. FOR ENGINEERS TRAINING / Engineering Consultant

3705 Fonda Way #18 Southeast Calgary, T2A 6G9

Canada

+1 (587) 802-4050

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