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
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.