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#40425 - 01/24/11 11:37 PM PSV Closed system Force

Nalibsyah[1]

Member

Registered:01/24/11 Posts: 56 Loc: AbuDhabi

Dear All,

there is some issue that i need to clarify in this forum since i see that many expert in Piping StressEngineering and COde Committe are visited this forum. The issue is about Force on PSV Closed system. As what i know that Force that show up when PSV in closed system begin to ring is including inImpact Force so it is a dynamic event, but still we can use the quasi static method to analyze it by usethe DLF factor. As long as i know this event also not a static condition because the pressure will change rapidly whenthe PSV rst ring. The problem of my concern is for this PSV Closed system i usually used and consider not only the rstimpact/unbalanced forced in rst elbow or obstruction but also in the second elbow and otherdownstream this PSV. I also read in Peng book, that he mentioned that the event when PSV in closed system ring stillconsider as Non static condition, so from here i can take the conclusion there will be no Balancedforce, it means Impact force will occurs in each elbow or obstruction but in opposite direction. We can measure each of this Force if we know the opening time of the PSV and multiply it with the

length of each pipe section to get the Max. Unbalanced force in each elbow or obstruction.

length of each pipe section to get the Max. Unbalanced force in each elbow or obstruction. But here in my new assignement i found a different method for client method, where Forcedownstream of PSV (other than force on rst elbow) are consider to be balanced each other. I also found this method in one Engineering company that become a trade mark for PipingEngineering. So for all Expert and Collegues here, i ask for your advice, amybe opinion about this Issue. Hope that it can give another value for all of us here. Thanks.

With Regards Nalibsyah

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#40429 - 01/25/11 04:22 AM Re: PSV Closed system Force [Re: Nalibsyah]

stressguy81[2]

Member

Registered:07/03/08 Posts: 71 Loc: India

As per API 520 Part 2, Cl. 4.4.2 "

Pressure-relief devices that relieve under steady-state ow conditions into a closed system usuallydo not transfer large forces and bending moments to the inlet system, since changes in pressure andvelocity within the closed system components are small. ..... A complex time history analysis of the piping system may be required to obtain the reaction forces and associated moments that are transferred to the inlet piping system."

As you said in general practice, the momentum component is imposed as a reaction force at PSV. generally divided as two cases,

Case1: When PSV pops up (Force imposed at PSV) and

Case1: When PSV pops up (Force imposed at PSV) and Case2: When there is established ow (When the forces cancel each other)

You might nd this topic interesting

http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=show at&Number=11775#Post11775 http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=show at&Number=24685#Post24685

I case of having a long run reaction forces can be imposed at the rst elbow at a different case, ex: F1at PSV and F2 at rst elbow in the establised ow condition, conservatively.

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#40430 - 01/25/11 06:35 AM Re: PSV Closed system Force [Re: stressguy81]

mariog[3]

Member

Registered:09/29/07 Posts: 525 Loc:Romania

The only accident with PSVs in closed systems I know it is one in which the PSV was wrong chosen andwas chattering for years before piping was broken. It was a fatigue damage "assisted" by poor supports-this was the conclusion of investigations. In my opinion it would have been avoided by common senseprocess/piping experience but also by eld operators feedback.

Reversing the point of view, you can see in eld PSV arrangements that cannot be quali ed by todaycalculations but have had a satisfactorily service for long time.

I think any calculation for PSV in closed systems is good because offers the opportunity to review therelated piping and helps providing stiff supports. For this point of view the calculation must beencouraged.

In the same time, I’m in doubt that 99% of these calculations would simulate what it’s happening in

In the same time, I’m in doubt that 99% of these calculations would simulate what it’s happening ineld with PSV in a closed system. That's why I don't want to say one approach is wrong and another one

is good.

In addition, when the Client/Company regulations are hard (and usually it is the case, because it seemsto be o hot topic!) you have to comply with.

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#40431 - 01/25/11 07:33 AM Re: PSV Closed system Force [Re: mariog]

danb[4] Member

Registered:04/22/05 Posts: 1174 Loc: ...

I agree with mariog and stressguy81.

I'd like to add the following.

Check should be done for pop forces and if the velocities are close to 1 mach, apply forces at elbowsequal with w x v ( ow times velocity)

Regards,

_________________________ Dan

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#40434 - 01/25/11 07:46 AM Re: PSV Closed system Force [Re: danb]

MoverZ[5]

Member

A further warning about Mach speed ..... it cannot in most cases be exceeded in a PSV body due tochoking. Since velocity is not directly addressed in the equations given, the formulas in API RP 520 used

to calculate reaction forces can give incredible results. If you check the associated velocity it may be

Member

Registered:11/22/06 Posts: 1189Loc: Hants,UK

to calculate reaction forces can give incredible results. If you check the associated velocity it may bewell in excess of Mach speed and thus impossible. A reduction to a realistic mass ow rate should givebetter force results.

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#40435 - 01/25/11 08:17 AM Re: PSV Closed system Force [Re: MoverZ]

danb[6] Member

Registered:04/22/05 Posts: 1174Loc: ...

Indeed, and this is in line with Norsok P-001 limiting criteria ro x v2 less than 200 000.

However a rough formula w x v = ro x v2 x A is quite reasonable. (e.g. for a 8" line and a ro x v2= 200 000,force will be 7534 N) Not big, not small, but will lead to a lot of guides and/or stops.

_________________________ Dan

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#40440 - 01/25/11 09:56 AM Re: PSV Closed system Force [Re: MoverZ]

mariog[7]

Member

Registered:

09/29/07

Dear MoverZ

You sayQuote:

Since velocity is not directly addressed in the equations given, the formulas in API RP 520 used to

Posts: 525 Loc:Romania

calculate reaction forces can give incredible results.

Well, I cannot blame the API formulas. Maybe the people applying API formulas.

API formulas are based on "free jet" approach. If a free jet is released in atmosphere or in a large volume, the piping system will receive a reactiveforce. This is the force that API counts as:

Reactive_Force= [mass ow-rate]*[jet_velocity]+ [p_jet]*[area_jet]

where

- mass ow rate must be the actual value (it is greater than the designed ow rate, because the actualPSV ori ce is larger than minimum required!) - jet_velocity is the critical speed when the jet gas ow has Mach=1 feature (is counted in FluidMechanics as jet_velocity= sqrt(2*R*k*T/ ((k+1)*M)), where notations are as in API, R is the universalperfect-gas constant , in SI is R=8314.5 J/kg mol/K. - p_jet is the gauge pressure in the released jet - area_jet is the internal area of piping at the point where the jet is released

This is exactly the API formula, where the numerical coef cient is sqrt(2*R), in SI unitssqrt(2*8314.5)=129 Obviously, the formula is based on the "choked" condition i.e. Mach=1 and this is taken intoconsideration by counting jet_velocity= sqrt(2*R*k*T/ ((k+1)*M)

A possible source of errors may be the term [p_jet]*[area_jet], because it seems that "p" in chocked owis somehow out of common engineering perception and API does not give details on the subject.

I reattach a paper showing a simple way to evaluate pressure in isentropic choked ow (Mach=1). You

I reattach a paper showing a simple way to evaluate pressure in isentropic choked ow (Mach=1). Youcan see the same result in some articles, but the uid mechanics model is more complicated there.

My best regards.

Attachments

Choked_ ow_pressure.pdf[8] (494 downloads)

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#40441 - 01/25/11 10:10 AM Re: PSV Closed system Force [Re: mariog]

MoverZ[9]

Member

Registered:11/22/06 Posts: 1189Loc: Hants,UK

Thanks for the uid mechanics lesson Mariog, I am well aware of theory. My note was a warning since Ihave had occasion to check calculations where a small PSV apparently attracted a huge force, dueexactly to the error I outlined. People do make mistakes and far too many 'engineers' apply equationsblindly, because all too often they have found an unchecked and non-validated Excel spreadsheetsolution on some dodgy company drive.

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#40449 - 01/25/11 03:56 PM Re: PSV Closed system Force [Re: danb]

mariog[10]

Member

Dan,

I think your example may be written as "if there is a free-jet exiting 8" piping area with (choked)

Member

Registered:09/29/07 Posts: 525 Loc:Romania

I think your example may be written as "if there is a free-jet exiting 8" piping area with (choked)parameters complying with rho*v^2=200000 kg/(ms^2), the reaction force would be 7534 N".

For a closed system I would say it is a "better than nothing" criteria.

PS. I have a funny story with a PSV process issue in a big company that after 2 months of Olga softwarecalculation decided to have 24" size line downstream of the 2"x4" piloted PSV. So 24" wasn't thesubheader size, it was the size connection to the subheader. The model was a visual aggression and thecalculation was "worse than nothing".

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#40455 - 01/26/11 05:40 AM Re: PSV Closed system Force [Re: mariog]

danb[11]

Member

Registered:04/22/05 Posts: 1174Loc: ...

I think that I will not rephase as the purpose of this was more trivial.

I was talking about ow induced forces. Decent problems require decent solutions, sort of "better thannothing". For complex problems, there are other solutions and I am not one of the specialists that cansolve them.

_________________________ Dan

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#40462 - 01/26/11 08:52 AM Re: PSV Closed system Force [Re: danb]

mariog[12]

Member

Dan,

You were talking about ow induced forces in closed systems and I was talking on the fact rho*v^2

Member

Registered:09/29/07 Posts: 525 Loc:Romania

You were talking about ow induced forces in closed systems and I was talking on the fact rho*v^2transposed in "force criteria" would be a criteria for forces due to "free-jet" effect in open systems. Ithink also that rho*v^2 is a steady-state criteria and does not address to the "momentary, instantaneousforces that result when the valve rst opens".

The point is I have no evidence such criteria is a decent one in closed system except the connection withsubheader/header where a "free jet" may exist.

The only certitude I have is that every stress specialist and Company has the certitude that his/her/theircriteria is decent. I'm not in position to say they are right or not, so I accept their approach, criteria, etc.

It is interesting (for me) to see that the process people have the certitude they don't know such decentcriteria to evaluate roughly the magnitude of forces in a closed system. I accept also their position.

Best regards.

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#43155 - 06/02/11 08:40 AM Re: PSV Closed system Force [Re: Nalibsyah]

Tengku_Syahdilan[13]

Member

Registered: 12/26/09 Posts: 56 Loc: Indonesia

Stress guy and others, i think i need to clarify what asked. My concern is that when PSV rstpop up there will be a travelling wave downstream the PSV discharge or RV discharge. Thisload is not balanced each other. This force will have the same history shape throught out thesystem but the arriving time is different in each point. This is why in API 520 as stress guytaken above mention : A complex time history analysis of the piping system may be required to obtain the reaction forces and associated moments that are transferred to the inlet pipingsystem."

and in ASME B31.1 Non Mandatory App. II Para II.2.2.2 :

and in ASME B31.1 Non Mandatory App. II Para II.2.2.2 :

...."However when a safety valve discharge is connect to a relatively long run of pipe andsuddenly opened, there is a period of transient ow until the steady state dischargecondition is reached.

and in Para II.2.3.2 :

......"Relief Valves discharging into an enclosed piping system create a momentaryunbalanced forces which act on the piping system during the rst few milliseconds followingrelief valve lift. The pressure waves travelling through the piping system following rapidopening of the safety valve will cause bending moments in the safety valve discharge pipingand the reminder of the piping system. In such a case, the designer must compute themagnitude of the loads and perform approriate evaluation of their effects."

LC Peng book page 401-403 so talk about this, where there will be a net force impact oneach leg (elbow or other obstruction) downstream the RV or PSV discharge as a result of thetraveling wave or transient condition.

So what i mean is that we also have to considered the effect of this transient load during popnot only at the rst elbow ( i see in some company standard they apply this two condition,during steady state and pop up, but during pop up they only apply the force directly upwardand horisontal at the body of the Rv or PSV.....i also have see some one post this kind ofpictures in other thread about PSV too).

Apply the load at n elbow/obstruction and n+1 elbow/obstruction, but because it is quitecomplicated and we will not know the limit of this travelling wave unless we performsimulation (maybe with Boss uid) or do the time history analysis for better and realistic

result, then it is common to assume to apply until the third elbow/obstruction.

I have found a case where previous engineering company apply the usual method (apply

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I have found a case where previous engineering company apply the usual method (applyupward, and horizontal at valve body), and the result the pump downstream of this RVsystem having a trouble. so we x it by apply and considered the transient effect that notconsidered by previous company and now have been 3 years the pump still run smoothly.

_________________________ Tengku_Syahdilan "From Failure we Learn"

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