better piping and expansion joint design

7/27/2019 Better Piping and Expansion Joint Design

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.Better piping andexpansion joint designThe requirement that metal bellowse x p a n s i ~ n joints in high temperaturepiping systems be designed for piping

h y d r ~ s t a t i c test levels unnecessarilypenalizes expansion joint operationand increases costsD; J. Peterson, Pathway Bellows, Inc., Oak Ridge, Tenn.

HIGH TEMPERATURE GAS piping systems in refineriesan d petrochemical complexes incorporate a variety of components in addition to the piping. These components involvea variety of materials other than basic piping materials.Also, the various components frequently operate at temperatures below the process temperature. Nevertheless, systemdesigners frequently write common hydrostatic test pressuresp:::cifications for the entire system regardless of these differ-ences in piping an d component materials an d operating temperatures.Generally, the material used as the basis for this commonsystem hydrostatic test pressure calculation is the piping material. This is the material of largest usage and often the material with the lowest allowable strength at the design temperature.

As a result, some component designers are forced to over-design their equipment, adding unnecessarily to equipmentweight an d cost. In the case of metal bellows expansionjoints (Fig. 1) this practice no t only adds cost, bu t also canimpair operation by compromising other specification re-quirements such as low reaction forces, maximum fatiguelife, stiffness an d overall length.

Metal bellows expansion joints are unique among pipingsystem components. Seldom are other components designedno t only for adequate strength or rigidity, but also for accept-ing deflections with relative low resistance forces or moments. Logic tells us that a bellows' strength increases asthickness increases. However, allowable deflection increasesas thickness decreases. This apparent dichotomy is not withoutsolution, and a balancing act between strength an d "weakness" will produce a "best-fit" metal bellows expansionjoint design.

Another unique characteristic of metal bellows expansionjoints that must be dealt with is column instability or

Fig. 1-Typical FCCU gas duct system."squirm" (Fig. 2). This potential failure mode is a functionof the bellows' length, spring rate, diameter and workingpressure. Th e stiffer the bellows, the higher the pressure atwhich squirm will occur. Unfortunately, a stiffer bellows isless able to accept deflection.

I f a metal bellows expansion joint must meet the hydro-Hydrocarbon Processing, January 1991 89

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Fig. 2-"Squirm" damage due to improper design.

static test pressure requirements of the high temperature piping system without allowance for material or temperaturedifferences, it is severely penalized because no account canbe taken for the significant difference that usually exists between the hot and cold allowable strengths of the materials.

Table 1 shows the resultant bellows and pipe thickness in aho t wall pipe system designed to contain 45 psig gas at1,300F. This is typical of the conditions found in a fluid catalyic cracking unit. Bellows are generally left uninsulated inthese systems, an d often a thermal barrier exists between themedia and the bellows, resulting in the bellows operating atwell below 1 000F. This temperature reduction is easily calculated, an d field measurements performed at many refin-ery installations confirm that the bellows actually do operateat 1 000F or less if left uninsulated externally.

Table 2 shows the bellows thicknesses that result from thehydrostatic test pressure option selected for the specification.The bellows design which results from the hydrotest pressuredictated by the piping design is 3.13 times heavier than thedesign that is based upon the real operating temperature ofthe bellows.

Substantial penalties are paid for the decision to imposethe pipe hydrotest pressure on the expansion joint design.Th e spring rate increases by a factor of 28, and the cycle lifeis reduced by a factor of 317. To maintain the same cycle lifewith the heavier wall design, many more convolutions wouldhave to be added to the bellows. Sometimes this results in abellows design that is so long that it is marginal for columnsquirm at hydrostatic test or for operating conditions. Whilethe heavy wall bellows may satisfy the piping hydrostatic testrequirements, it is no t the best design for the application.Table 2 also shows how expansion joint hardware (Fig. 3)designed to restrain the pressure thrust of the expansion be-90 Hydrocarbon Processing, January 1991

TABLE 1Comparison of allowable stress and hydrostatic test pressure of a typical duct material andan unreinforced, 50 in. diameter, D, expansion joint operating at a media temperature of1 300F and a pressure, P, of 45 psig. Bellows pitch is 1.875 in. Convolution height is 2 in.

T3D4 Stainless steel IBOOH Bellowsduct material materialCold allowble stress, S,, at 70F 18,800 psi 16,200 psigHot allowable stress, Sh, at 1 300F-MediatemperatureHot allowable stress, Sh. at 1 000F-Bellowsoperating temperatureMinimum calculated thickness, T. for 1300Fdesign temperature-T= PDI2Sh for dueling.-Standards of the expansion Joint Manufacturers Assn. (EJMA) for bellows design.Temperature adjusted hydrates! pressure (45 (1.5)(Sh IS,)) at 1300FMinimum calculated thickness for 1000F bellowsoperating temperatureTemperature adjusted hydrates! pressure (45 (1.5)(Sh ISc)) at 1 000

TABLE 2

3,700 psiN/A

0.304 in.343 psig

N/AN/A

4,700 psi14,400 psi

0.125 in. '233 psig0.048 in., 76 psig

Data showing the impact of selection of design temperature and system hydrostatic testpressure, P1, on bellows performance. The performance information is based upon imposinga ixed movement on bellows of the same convolution count operating at Table 1conditions.The bellows effective area, A, is 2,124 in2.Recommendedbellowsthickness

BellowsspringrateCycle lifeper ANSI831.3Appendix X

Crosssectionalarea of T3D4stainless steelstructural shaperequired torestrain pressurethrust duringhydrostatic test =AP1 /(1.5S,)Case 1For bellows actual operating temperature of 1000F, and hydrostatic test pressure of76 psig based on bellows material (see T a b ! ~ 1):0.048 in. I 1,559 lblin. I 6,660 5.7 in.2Case 2 far bellows design temperature of 1300F and hydrostatic test pressure of 233 psigbased on bellows mrteriai (see Table 1):0.125 in. 25,600 lb/in. 45 17.5 in.2Case 3 For bellows design temperature of 1 300 hydrostatic test pressure of 343 psig.Based on duct materai (see Table 1):0.150 in. 44,300 lb/in. 21 25.8 in.2Note-The above examples are for unreinforced bellows. By adding rootring reinforcement,thinner bellows would result for Cases 2 and 3. However, the basic problem of loss of bellowsperformance as hydrostatic test pressure increases is not avoided by adding bellows rein-forcement.


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Gimbaled expansion joint (GEJ)

PGllA:G:HEJ:GEJ:

Planar guideIntermediate anchorGuideHinge expansion jointGimbal expansion joint

lA: Intermediate anchorPG: Planar guideTUEJ: Tied universal expansion joint

Hinged expansion joint (HEJ)A

Fig. 3-Expansion joint hardware and supports.comes very massive if the pipe hydrotest pressure is imposed. Generally, the restraint hardware is a significant portion of an expansion joint cost. Since this hardware usuallyoperates at a temperature between ambient an d 1 000F, de-signing the hardware for the hydrostatic test pressure can bea needless waste of material. Massive hardware attachmentson high temperature piping also can be the cause of undesirable stress with subsequent cracking. It is well documentedthat large temperature differentials and damaging stress lev-els can result from the varying rate of heat-up of heavy support hardware an d the piping to which it is attached in hightemperature piping systems. Heavier is not always better.

Fortunately the concept of relaxing test requirementswhen they exceed their real purpose is not uncommon and isin fact allowed by the various codes. For example, hydrostatic testing of piping and vessel syst ems after installation isroutinely waived when it is obvious that the design of thesupport structures to carry the weight of the hydrostatic testwater is economically prohibitive.

ASME Code Sections II I an d VIII and the ANSI codesprovide relief for this metal bellows expansion joint designproblem of excessively high hydrostatic test pressure:(1) Section VIII, Division 1, Paragraph UG-99 (b) states:"Except as otherwise permitted in (a) above and (k ) below, vessels

designed for internal pressure shall be subjected to a hydrostatic testpressure which at every point in the vessel is at least equal to 1112times the maximum allowable working pressure to be marked on thevessel multiplied by the lowest ratio (for the materials of which thevessel is constructed) of the stress value S for the temperature on thevessel to the stress valueS for the design temperature (see UG-21 ).All loadings that may exist during this test shall be given consider-ation."(2) ASME Section III, Nuclear Code, Paragraph nd-

3649.4(2) entitled "Bellows ExpansionJoint Design," recog-the ratio of the modulus of elasticity at design temperaturevs. ambient in lieu of the ratio of the allowable stresses. Itstates:

"In the case of squirm tests, the equivalent cold service pressure isdefined as the Design Pressure multiplied by the ratio of E /Ehwhere E, and Eh are defined as the modulus of elasticit)' of he bel-lows material at room temperature and normal service temperature,respectively."

B c

(3 ) ANSI B31.3 Piping Code, 1987 Edition, Appendix X,entitled "Design Requirements for Expansion Joints" recog-nizes the effect on bellows instability when hydrostatic testpressures are adjusted for temperature. It states in Para.X3.2.3 entitled Leak Test:

"(a) Expansion joints shall be shop leak tested in accordance withpara. 345 except that if he test pressure adjusted for temperaure asstated in 345.4.2, will produce a membrane stress in excess of theyield strength or cause permanent dtformation or instability(squirm) of he bellows at the test temperature, he test ressure mabe reduced to the maximuyeuure Ji:Jf!.t will not excee ytel orcause instabilit .b) The expansion joint design shall be such that it will withstandtest pressure not less than 1. 5 times the design pressure during hy-drostatic testing or 1.1 times the design pressure during pneumatictesting.(c) Expansion joints designed to resist jmssure thrust shall not beprovided with arry additional axial restraint during the leak test.Moment restraint simulating piping n'gidity may be applied if nec-essary."When these three codes are taken together, they permit

the hydrotest of an expansion joint bellows to be as low asf. 5 times the design pressure. This prevents costly thicknessoverdesign just to meet a higher than necessary hydrostatictest pressure dictated by the piping material, and it reducesthe possibility of squirm or instability during test.

By differentiating between actual component needs in es-tablishing high temperature system hydrostatic test pres-sures, a designer can save significant costs an d provide opti-mum operating characteristics for the entire system.

The authorDavid J. Peterson is a product manager forPathway Bellows, Inc., Oak Ridge, Tenn. He re-ceived a BS degree in mechanical engineeringfrom the University of Minnesota, and prior tojoining Pathway in 1978, he worked for Texacoand Flexonics. Dur ing his 12-year career withPathway, he has held the positions of districtmanager, technical director, national sales manager, and marketing manager. He is currently re-sponsible for the metal expansion joint product group.

better piping and expansion joint design

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