dynamic stress analysis-an article

8/16/2019 Dynamic Stress Analysis-An Article

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PIPING STRESS ANALYSIS A BIT

DYNAMIC…

Objective

The objective of pipe stress analysis is to ensure safety against failure of the Piping System by

verifying the structural integrity against the loading conditions, both external and internal,

expected to occur during the lifetime of the system in the plant. This is to be undertaken with themost economic considerations.

*Ensure that the stresses in the piping components in the system are within the allowable limits.

* Solve dynamic problems developed due to mechanical vibration, acoustic vibration, fluid

hammer, pulsation, relief valves etc.

* Solve the problems associated due to higher or lower operating temperature such as

b) !o""le loading on the connected e#uipment

c) $ipe displacements

d) %oads and moments on the supporting structures.

The steps inv!ved in the st"ess #n#!$sis c#n be !isted #s % %

* &dentify the potential loads that the piping system would encounter during the life of the

plant.

* 'elate each of these loads to the stresses and strains developed.

* (et the cumulative effect of the potential loads in the system.

* )ecide the allowable limits.The system can withstand without failure.

* fter the system is designed, to ensure that the stresses are within the safe limits.

T$pes & !#ds

ll the merican code for $ressure $iping classify the loads mainly into three types . .

* S'st#ined L#ds( Those due to forces present during normal operation

* Occ#sin#! L#ds( Those present during rare intervals of operations


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* Disp!#ceent L#ds( Those due to displacement of pipe

LOADS ON PIPING

P*RPOSE O+ DYNAMIC ANALYSIS

• )ynamic analysis is re#uired whenever there is any dynamic load acting on piping

system.

• )ynamic loads are those which changes #uickly with time like earth#uake, fluid hammer,vibration, relief valve etc. s a result, piping system may not have time to fully react tothe applied load before it changes + system become unbalanced i.e. sum of forces +

moments on system are not "ero. )ue to these unbalanced forces, piping system moves

according to

- *

These induced system reaction are not e#ual to applied loads + may be much higher or muchlower.

TYPES O+ DYNAMIC ANALYSIS

• There are different types of dynamic loads acting


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• odel nalysis is used for !atural re#uency /heck.

EART,-*A.E / SEISMIC RESPONSE SPECTR*M ANALYSIS

(round motion associated with seismic event is supplied as displacement, velocity oracceleration response spectra. The assumption is that all the supports move with the defined

ground motion + the piping system 0catches up1 to the support, it is this inertia effect which

loads the system.

• &nput data re#uired for Seismic response spectrum analysis are

2. Suggested )amping 3in 4 of critical5 values 64, 6.74 or 8.74..

6. /ut9off fre#uency

8. Site spectrum generally Time period v:s cceleration or re#uency v:s cceleration

spectra 3see table 25.

• )ifferent Steps involved in Seismic response spectrum analysis inputting


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Spect"' De&initin(

Earth#uake loads are defined by defining one or more response spectra + applying them in a

specified direction over part or all of the piping system.

0% Gene"#te Respnse Spect"' T#b!e( 'esponse spectrum table values can be entereddirectly or built + stored as a file for use by /ES' &&. &t must first be defined in the Spectrum

)efinition page.

1% Spect"' L#d C#ses( %oad case consist of simultaneously applied spectra. Each spectrumin the shock case is assigned a direction + factor. The factor is used to modify the magnitude of

the shock.


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2% St#tic / d$n#ic Cbin#tin( Each shock case produces an output report listing disp.,

forces, moments + stresses. The sum of sustained + occasional stresses is compared to theoccasional allowable stresses and operating case is combined with dynamic case to check the

no""le loads and restraint loads. This combination is provided through the Static : )ynamic

combination page.

EART,-*A.E / SEISMIC3RESPONSE SPECTR*M) ANALYSIS


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%umped masses + Snubbers &f there are any lumped masses in piping system then we have to

enter that in dynamic inputting otherwise left blank.

lso if we re#uire snubbers in our piping system then they must be entered in dynamic inputting

otherwise left blank.

4% Cnt"! P#"#ete"s( These parameters describe how the analysis will be conducted.


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5% Adv#nced P#"#ete"s( These parameters are rarely to be changed by the user.


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DYNAMIC O*TP*T( )ifferent things to be checked in )ynamic ;utput includes

!atural fre#uencies + odes mass normalised &n this we check whether our natural fre#. is

more than the /ut9off fre#uency or not. &f not then we have to take the corrective measures.

0% St"esses( &n this we will compare the sum of sustained + occasional stresses with theoccasional allowable stresses .


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1% Rest"#int s'#"$( &n this we will check the no""le loads and restraint loads in operating

and sustained plus occasional case .


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or a >?@ Elbow 3A - >?5,

where B - density of the li#uid 3lb:ftC5

D - Delocity of vapor 3ft:sec5

- rea of cross9section 3ft5

g - gravitational constant 386.6 ft:sec5

The Stresses and forces caused by the impact of slug will be 2.7 F 6.? times the static force.

/onsidering )% - 2.7 for impact loading ,

Rest"#int App!ic#tin

'estraints designed for slug flow will allow movement due to normal operation conditionsGhowever the design will be primarily for excessive movement due to impact the slug. ll changes


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of pipe run direction will re#uire restraint e.g. the loop shall be restrained as shown. &n case of

any negative effect of the restraint in thermal case, sufficient gap may be introduced.

Standard guides and anchors are normally inade#uate for restraining slug flow. Special design is

re#uired.ree standing structural 0T1 supports are also inade#uate for restraining slug flow. These

supports are usually #uite tall and tend to offer little resistance to hori"ontal loads. /onse#uently,

even though they are not overstresses from the effects of the load, they do not sufficiently

dampen the impact and the resultant displacement of the pipe at the restraint point can be aserious problem.

S!'7 +!8 An#!$sis 9 St#tic Methd 3'sin7 CAESAR II)


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Inp't P"ced'"e

&n the absence of process parameters, it is preferable to use the density of vapor for stress

calculation as well as to find out the minimum fre#uency of the system. However, the hydrostaticload may be used for the design of supports. This is applicable wherein hanger selection is not

re#uired. Systems which have hanger design re#uirement, it is advisable to refer all relevant process data and find out the e#uivalent density of the fluid which is more realistic approach

L#d C#ses t be b'i!t 'p

S!'7 +!8 An#!$sis 9 Respnse Spect"# Methd 3D$n#ic #n#!$sis 'sin7 CAESAR II

!ormally, Slug force is treated as an impulsive force. This method is based on the assumptionthat the slug traverses the elbow and then suddenly drops to "ero again. &t is also assumed that

the slug is formed across the pipe full cross section. The duration of slug is calculated based onthe length of slug and stream velocity. This results in short duration impulsive loads on the pipe.

The slug si"e is calculated based on the length of the pipe before the elbow + li#uid volume

fraction i.e. %ength of li#uid slug - %ength of pipe * %i#uid volume fraction

)uration of slug is calculated as,

The slug periodicity is calculated as,


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orce on the elbow due to fluid flow is calculated as,

axial - B D 329 cosI 5

orthogonal - B D sinI

Jhere

axial orce on elbow in axial direction 3 ! 5

orthogonal orce on elbow in orthogonal direction 3 ! 5

B )ensity of li#uid 3 Kg: mC 5

&nternal cross9sectional area of pipe in 3 m5

D Stream velocity 3 m:Sec 5

I ngle of bend 3 )egree 5

&t is assumed that the elbow is subjected to force due to li#uid slug and drops to a smaller value

based on the density of gas, after some duration as the li#uid traverses the elbow. This load istreated as a rectangular pulse load with the duration and periodicity calculated as above. The no.

of cycles are given as per maximum limit of /ES'9&& package.

!atural fre#uencies of the piping system is calculated for the few modes up to fre#uency cut9offof 88 H".

S!'7 +!8 An#!$sis 9 D$n#ic Methd 3'sin7 CAESAR II)

Respnse Spect"# 9 Inp't


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dynamic stress analysis-an article

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