Flexibility Analysis of Piping

7/26/2019 Flexibility Analysis of Piping

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Flexibility Analysis of Piping (Part-1)Posted by Ankit Chugh on 10:30 AM2 Comments

Purpose of Stress Analysis

The designer after careful consideration, prepares most economic layout he can

undertake ithin parameters a!ailable to him only to find that further alterations,

dictated by "tress #ngineer ill be re$uired%

&t is the stress analyst's function to

a( )ecide on hich set amount of of conditions go!ern the that must be pro!ided in

fle*ibility layout, and

b( To establish, by one method or another, that the re$uired fle*ibility has been pro!ided

in layout%+nder the heading a(, there are number of criteria defining the minimum acceptable

fle*ibility and these fall into to main categories:

i( Ma*imum alloable stress range in the pipe

ii( The limiting !alues of forces amd moments hich piping is permitted to impose on

connected e$uipment%

The fle*ibility re$uired in those cases here the piping reaction on connected e$uipment

go!erns, in!ariably o!errides that re$uired to satisfy the ma*imum stress range

condition%+nder the heading b(, the stress analyst, ha!ing decided hich criteria applies, has the

choice of:

i( Accepting a layout based on past e*perience,

ii( Analysing a layout by an appro*imate method, and

iii( Performing comprehensi!e stress analysis%

Code and Regulation

The stress engineer-s first charge is to ensure the compliance ith all applicable code

regulations, both national and local, apart from satisfying other conditions re$uired% .or

a particular, contract, the piping specification/design specification ill state% hich code

is to be used for the purpose of design and in case of anomaly, hich document is to take

precedence% Code sets forth engineering re$uirements deemed necessary for safe design

and construction of pressure piping% hile safety is the basic consideration, this factor

alone ill not necessarily go!ern the final specification for any piping system% Code is not
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a design hand book% &t does not do aay ith the need of designer or for competent

engineering udgement%

The code re$uirements for design are stated in terms of basic design principles and

formulas% These are supplemented as necessary ith specific re$uirements to assure

uniform application of principles and to guide selection and application of piping

elements% The code prohibits design and pratices knon to be unsafe and contains

arnings here caution, but not prohibition, is arranted%

"ome codes and standards in present use are:

American 2ational Code for Pressure Piping A2"& 31%

4n global basis this is tne most idely used code and compliance ith its re$uirements

ill almost certainly be accepted as dem 5nstrating the structural integrity of a piping

system% &t has its origins in a document issued in 1637 as 8An American Tentati!e

"tandard Code for Pressure Piping9% &n order to keep the code abreast of current

de!elopments in the !arious fields of #ngineering and Technology, se!eral re!isions and

supplements and ne editions ere published since the first 8Americal "tandard Code

for Pressure piping9 A"A 31%1appeared in 16; ?as transmission and distribution piping

31%6 uilding ser!ices piping

31%11 "hirry transportation piping systems

Code Requirements

(A) Design Pressure:

The design pressure of each component in a piping system shall not be less than the

pressure at the most se!ere condition of coincident internal or e*ternal pressure and

temperature minimum or ma*imum(, e*pected during ser!ice% The most se!ere

condition is that hich results in the greatest re$uired component thickness and the

highest component rating%

(B) Design Temperature


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The design pressure of each component in a piping system is the temperature at hich,

under coincident pressure, the greatest thickness or highest component rating is

re$uired%

i( +ninsulated Metallic Components:

.or fluid temperatures belo 100. 3> C(, the component temperature shall be taken

as the fluid temperature%

.or fluid temperature 100 . 3> C( and abo!e, unless a loer a!erage all temperature

is indicated by test or heat transfer calculations, the temperature for uninsulated

component shall be no less than folloing%

a( @al!es, pipe, lapped end elding fittings and other components ha!ing all thickness

comparable to that of pipe: 67 of fluid temperature

b( .langesB e*cept lap oint( including those on fittings and !al!es: 60 of fluid

temperature

c( ap oint flanges :>7 of fluid temperature d(Blting:>0t of fluid temperature%

ii( #*ternalli &nsulated Piping:

The component design temperature shall be fluid temperature unless calculations, tests

or ser!ices e*perience based on measurement supports use of another temperature%

hen the piping system is heated or cooled by tracing or acketing, this effect shall be

considered in establishing component design temperature%

iii( &nternally &nsulated Piping:

The component design temperature shall be based on heat transfer calculations%Scope of Code Rules

et us consider !arious aspects of design of piping system hich must be dealt ith by

any code orthy of such a description and hich are of importance to stress engineer in

e*ercise of his duties% #!ery such piping code ill contain recommendations, or

mandatory re$uirements on the folloing design topics%

a( The thickness of pipe to ithstand internal pressure

b( The thickness of pipe to ithstand e*ternal pressurec( =einforcement re$uirement of branch connection

d( Minimum fle*ibility re$uirements for e*ternal e*pansion

e( Alloable stresses for !arious piping materials

&t is the matter of stress analysist demonstrating compliance ith the re$uirement

coming under heading of c(, D( and e(% e shall no consider the abo!e three topics in

turn to see ho these affect the piping system%

(c) Reinforcement requirement of branch connection


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hen a pipe hich is subected to an internal pressure has a hole cutin it for branch

connectiDns, a disc of material hich ould normally be carrying tensile stresses in the

Ehoop direction is remo!ed and some alternati!e path must be pro!ided for loads

hich ere originally carried !ia: the disc% Most of the code for design of piping system

adopts the area replacement or compensation approach hereby ithBn a specified

distance from the edge of the hole, an additional area of material is pro!ided e$ual to the

area of

material remo!ed%

The replaced material may take a form of a doubler pad or of one of the proprietar y

forged fittings e%g% eldolet etc%( depending on ser!ice re$uirements% The notion is

illustrated in the sketches of fig% 1 for the case of simple pipe replacement%

4ccasionally, reinforcement has to branch intersections hich is to cater for hich arise

from thermal e*pansion effects% the stress engineer shall specify the same if by piping

specification for pressure purpose%

&n !arious codes, the sketches of fig% 1 appear as single draing at section AA, shoing

the cross section of the material to be replaced and boundary ithin hich the

replacement material must be located% here the all of the pipe is thicker than the

minimum re$uired for internal pressure, credit may be taken for e*cess material hen


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calculating replacement material but alays ithin boundary sets in fig%


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The minimum thickness re$uired for both 10N and ;N header from the basic e$uation:

The minimum thickness re$uired for both 10N and ;N header from the basic

e$uation:

The nominal thickness: Th, J 0%


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Here the cross section of fillet eld is neglected% .or more conser!ati!e calculation, this

area also may be considered%

$inimum %le#i!ility Requirements

GA pipe' is erected at ambient temperature, say beteen ;ff . >00 ., in different climates %O00.


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Stress range reduction factor

hen "is greater than the calculated !alue of "' the difference beteen them may beadded to the term 0%


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%igure *After application of the inplane bending moments Ml, the bend or branch remains in theoriginal plane% ut hen outplane bending moment Mo is applied, the bend or branchconnection goes out of original plane% The torsional moment about a*is of pipe is denoted byMt%

%le#i!ility (+uided Cantileer $ethod)

"uppose e ha!e < !essels Tl and T


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2o further suppose that e!ery thing is carbon steel and the !essel Tl has its

temperature raised to 370 .% hen the !al!e is opened, there ill be an e*pansion

beteen centres of Tl and T< and that ill beQ

To absorb the abo!e e*pansion, one of the folloing to things may happen%

1( Pipe ill dent the !essel at to noIIles as shon in figure


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ith the abo!e configuration for the piping, as point mo!es by V out to 1, it is able to

bend the leg C into ne position &C% The onger leg is easier to bend use to the

e*pansion of C%

e ill calculate the minimum length 1 re$uired forC to absorb the e*pansiond% As per

the elastic theory ?uided cantili!er method(Q


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2o if the stress range is 1D000 psi and considering oungs modulus of elasticity at

ambient temperature !alue of carbon steel,

"o hen locating the e$uipment T


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ength in inches

&f the same length of pipe is subected to torsion, the

rotation of one end relati!e to other is gi!en by,

W Angle of tist in radiansT Tortion moment, lb/inch ength in inch? Modulus of rigidity, &b/inch( #lectric poer generation station%

] ?eothermal heating systems

] Central district heating systems%

31%< Process piping1666( petroleum refineries

] chemical

] pharmaceutical

] te*tile

] paper

] cryogenic plants

31%; pipeline transportation system for li$%

166>( hydro carbons and other li$uids

31%7 =efrigeration piping for refrigerants and scondey coolants

166 ?as transportation and distribution piping%

31%6 uilding ser!ices piping

166D( industrial

] institutional / commercial

] public buildings

31%11 "lurry transportation piping system

=166>( a$ueous slurries

16


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16OD 31%3 as published to combine re$% of 3D%D and published as chemical

plant and petroleum refinery piping%

Addenda upto 16>0%

)ec 16O> American national standards committee

31 as reorganiIed as asme code for pressure piping, 31 committee% All addenda andne addition as de!eloped as A2"& / A"M# 31

16>0 2e edition A2"& / A"M# 31%3

Published

16>1 Code for cryogenic piping 3D%10( as ready appro!ed% Addenda of 31%3

16>0 as published to cater for 3D%10

16>; Chapter for cryogenic piping added%

16>O 2e edition 31%3


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To be determined by heat transfer calculation limitation of calculated stresses due to

sustained load and displacement strains%

a( &nternal pressure stresses:

The selected pipe thickness ^ T

The mill tolerance 17

.urness butt elded # J 0%D

#lectric fusion elded # J 0%67

double butt%(

100 radio graphed # J 1%0

Coefficient from table 30;%1%1 t _ )/D

t ) / D d ^ inside dia% ma*%(

function of material and design temperature 0%; to 0%O

Thic/ness Cal

Pipe Bends

And at side all on bend center line & J 1%0

The ^ thickness re$ is at the midspan%


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$iter Bends

Angular offset more than 3 deg are re$% to be checked

Branch Reinforcement


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.igure


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.le*ibility and "tress &ntensification .actors


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0ongitudinalStresses (S0))ue to pressure, eight and other sustained loading, sl, shall not e*ceed sn for

calculation of sl% ill be based on nominal thickness ^ mechanical, corrosion anderosion alloance%Allo-a!le Displacement Stress Sh"A J` 1%


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%le#i!ility in Tortion

&n!ending


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?uided cantile!er method


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Flexibility Analysis of Piping