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.
5. 4 Airfans
5.6 Buried Piping
6.5 In-Line Pumps
6.7 Pipe Anchors
7.01 Slug Flow
7.02 Mitered Elbows
7.03 Tee Connections
7.05 Heater Coil Decoking
7.08 Cold Spring
7.09 Blowdown Systems
7.10 Field Checkout
7.11 Soot Blowers
7.14 Control Valve Piping
7.16 Pipe Supports
7.17 Tank Field Piping
7.19 Plastic Piping
7.21 Bowing of Pipe
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
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.
5.101 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.
The types of compressors usually found in refineries and chemical plants are as follows:
Centrifugal, Rotary and Screw 5.21
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:
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.
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.
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.
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.
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.)
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
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.
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