psv reaction forces and direction

PSV Reaction Forces and Direction Miyamoto Member

Registered: 09/13/11 Posts: 69 Loc: Brazil

Hi all,

I know this issue is an old discussion in this forum, but I have a lot of doubts yet. I read a lot of thread but no one solved my problem. I didn't find threads considering open discharge system for liquids. So, I'm here again. I'll be glad if someone can answer any question.

First, my system consists in a single PSV (PRV to be more exact) in a pipeline with water and Open Discharge System.

My question is:

I contacted some vendors and everybody said me the same: "For open discharge system, the pressure at exit pipe is almost atmospheric". I think CAESAR II uses ASME B31.1 equation for force calculations:

F1 = (W*V1)/gc + (P1 - Pa)*A1

Being, P1 the static pressure at exit pipe, and this value is almost 1 kgf/cm2, the second term will be zero.

So,

F1 = (W*V1)/gc

In other words this force may be calculated only with mass flow vs fluid sonic velocity?

Someone knows any equation that I can calculate pressure at exit pipe and fluid sonic velocity for liquid? All equations that I saw is specific for vapor/gases.

Vendors said me also, for liquids the PSV don't "pop". Is this right?

For conclude, the only reaction force that acts in this system is F1 and this force is applied in last elbow of pipe exit?

Thanks in advance,

Miyamoto

Top Reply Quote Quick Reply Quick Quote Notify Email Post#50313 - 08/18/12 01:40 AM Re: PSV Reaction Forces and Direction [Re: Miyamoto] mariog Member

For liquid, the flow does not accelerate to sonic speed in the exit section, so in your case exit_speed=[vol flow-rate]/[exit area]

http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=newreply&Board=1&Number=50312&what=showflat&fpart=1

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http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=50313#Post50313

Registered: 09/29/07 Posts: 333 Loc: Romania

You must know the mass flowrate and density and calculate the volumetric flowrate- anyway this info is specific to PRV "process" calculation.

You may evaluate dP (difference between pressure in the protected equipment and pressure in exit section) by Bernoulli; an approximate form is:exit_speed=SQRT(2*dP/Density)

Just as a little more advanced topic: to calculate dP seems to be very simple, however the "trick" here is based on the fact a correct "process" calculation already considers the actual dP when evaluate the actual flowrate- so when we know the actual flowrate we know also dP and by calculation we count also the energy lost through PSV/PRV. I would add that a process calculation is not focused on the "actual flowrate" but to a conservative procedure to select PSV/PRV orifice and the last step- a calculation with actual orifice isn't performed under normal circumstances... eventually your calculation is strong linked to the quality of that process calculation. So isn't so simple, but the conclusion is you may go ahead based on the process calculation!

You may review this post where is discussed a result from Cheresources. For your case that "D" is the diameter of exit section.

Top Reply Quote Quick Reply Quick Quote Notify Email Post#50321 - 08/18/12 04:19 PM Re: PSV Reaction Forces and Direction [Re: Miyamoto] mariog Member


About the explanation on Pressure discharge in the end of PSV liquid piping, I should have explained it better- even it is not a "big issue".

Rigorously speaking, Bernoulli equation has to include also the pressure loss through PSV and also pressure loss through inlet piping and discharge piping of PSV (for the last ones their values are limited by constructive rules).

By PSV Liquid Sizing procedure, the difference between pressure in the protected equipment and pressure in discharge section (let’s say it is "DP") is the basis of the calculation of the PSV minimum required orifice area. That means that the basis of PSV orifice dimensioning is the idea that PSV shall consume almost all that DP. It would follow that the discharge pressure is near "zero"- as your Vendor said.

However, the calculation of PSV orifice includes coefficients to dimension the orifice a little bit larger than necessary, and in the end, it is selected a "commercial" (standardized) PSV orifice valve bigger than the calculated one. Consequently, PSV will be a "hydraulic resistance" with a value less than is required and that means also the flow-rate will be greater that it was counted as "required to relief" the equipment.Normally, the calculation does not consider "actual values", since the calculation goal is to dimension the PSV and this is achieved by the calculation algorithm. However, if you want to see how much is the pressure discharge, you have to






repeat the hydraulic calculation with the actual orifice, actual PSV coefficients, actual piping configuration, liquid viscosity, etc and the result of that hydraulic calculation will show also the actual flow-rate released. In my opinion, this is far exceeding the stress engineer duty and it’s likely that "process" department will not be supporting you for a calculation that is "unnecessary" for them...

I would underline that the above written ideas are specific to liquid PSVs. For gas or steam, a supplementary issue is the fact the fluid accelerates to critical speed and density is also changed, so a calculation of "exit pressure" must follow other rules than a simple hydraulic calculation specific to liquids.

Top Reply Quote Quick Reply Quick Quote Notify Email Post#50334 - 08/20/12 08:00 AM Re: PSV Reaction Forces and Direction [Re: Miyamoto] Miyamoto

Member


mariog,

Thank you very much. You explanation was very clear.

Due relief is very slowly for liquid, I can't consider "pop condition" right? So, what is the direction of these forces? I understand should have a reaction force against pipe direction acting on valve and another in last elbow of the system. Please, look at draft attached.

At node 10 only F2 act (DLF x F1)

At node 20 all forces are balanced.

At node 30, vertical force is balanced and horizontal force act at the elbow.

Is my understand right?

Regards,

Miyamoto

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Top Reply Quote Quick Reply Quick Quote Notify Email Post#50346 - 08/20/12 11:20 AM Re: PSV Reaction Forces and Direction [Re: Miyamoto] Miyamoto Member


Forgot to mention, CAESAR II uses flow velocity at orifice to calculate Thrust Force. So, may I use this velocity as v1 (exit velocity at Point 1)?

Top Reply Quote Quick Reply Quick Quote Notify Email Post#50355 - 08/21/12 01:32 PM Re: PSV Reaction Forces and Direction [Re: Miyamoto] mariog Member


The "reaction force" formula (as is given in B31.1, for example) is linked to the "free jet" assumption. So the horizontal force in node 30 is a steady-state force as a consequence of the third law of dynamics.

At node 10 you may consider a similar horizontal force; however that means you consider there is a free-jet downstream PSV/PRV.Velocity is [v-orifice]=[vol flow-rate]/[orifice-area], multiplying by mass-flow-rate gives a force, etc.

Top Reply Quote Quick Reply Quick Quote Notify Email Post#50362 - 08/22/12 01:49 AM Re: PSV Reaction Forces and Direction [Re: Miyamoto] danb Member

Registered: 04/22/05 Posts: 1059 Loc: ...

One side note. If is liquid, the dowstream line need to go down, not up._________________________Dan



danb,

I need downstream up because this line will discharge in a open tank that is a little bit tall.

May I have any problems with downstream up?

Regards,

Miyamoto


The line need to be self-draining.



























In your case you need to place the psv at an elevation above the tank, then to go down with the discharge line._________________________Dan



danb,

As mentioned in API 520, I can install a manual drainage at discharge line.

Regards,

Miyamoto

Edited by Miyamoto (08/23/12 06:50 AM) Top Reply Quote Quick Reply Quick Quote Notify Email Post#50394 - 08/23/12 07:04 AM Re: PSV Reaction Forces and Direction [Re: Miyamoto] danb Member


Then you do not need the tank._________________________Dan

Top Reply Quote Quick Reply Quick Quote Notify Email Post#50620 - 09/05/12 10:46 AM Re: PSV Reaction Forces and Direction [Re: Miyamoto] Ltorrado Member

Registered: 10/19/10 Posts: 10 Loc: Metairie, LA

I would like to open this topic back up regarding the proper method of perfmorming a "static analysis" on relief valve discharge piping. We have an ongoing discussion at work on how to do it. My opinion is that, when dealing with a discharge pipe that has multiple turns before it discharges either to atmosphere or to a larger relief header, all the vector forces should NOT be applied at the same time but instead as different vectors AT different points in time (i.e F1 at t1, F2 at t2, etc.). Of course, assuming F1 = F2 = F3 if cross sectional area stays constant throughout.

So let's say you have Miyamoto's piping but relief being passed is gas. I agree the fluid will be traveling at extremely high velocities, but unless the spools are very short, the metal will NOT feel these at the same time (even if it's only miliseconds). But yet I see everyone's standard practice is to apply them all at once at every turn in direction as one single vector force (F1 at t1, t2, t3, etc.).

There is one post in particular from Loren Brown that I believe backs my reasoning up, see below:
















"For an open system, if you have more than one bend in your vent stack then apply this force at each bend under a separate load vector.

For a closed system you would apply this force on bends on each “long” leg of pipe. The only way to truly figure out which pipe leg is short enough to ignore the PSV force is to run the force/time profile through Caesar II's DLF generator in the dynamics module, but then you might as well perform this analysis dynamically. For short pipes the duration of the unbalanced PSV force is small and this shifts the DLF peak to the right (higher frequency) which at some point is past the majority of your piping system natural frequencies of interest. But if you are going to do this statically you might simply take the nine longest pipe legs and apply your force to each bend corresponding to these longest legs. This would be the "brute force" approach, not really an approach based on physics.

You have 9 different force vectors to choose from so apply your PSV force under a different force vector for each bend because we want to only examine the effect on one bend at a time. Then set up separate OPE cases that include your different force vectors."

Ignoring the calculation method of the actual thrust loads (which I calculate based on Process Engineering's computer modeled fluid conditions at discharge of PSV and at downstream points and also applying a conservative DLF of 2.0), what do the experts think is the correct way of doing this static analysis? Shouldn't it be like this (in Miyamoto's example):

L1 = W+P1+T1 (OPE)L2 = W+P1+T1+F1 (OPE)L3 = W+P1+T1+F2 (OPE)L4 = W+P1+T1+F3 (OPE)L5 = W+P1+T1+F4 (OPE)L6 = W+P1 (SUS)L7 = L1-L6 (EXP)L8 = L2-L1 (OCC) segregated effect of F1L9 = L3-L1 (OCC) segregated effect of F2L10 = L4-L1 (OCC) segregated effect of F3L11 = L5-L1 (OCC) segregated effect of F4L12= L6+L8 (OCC) use Scalar Combination MethodL13= L6+L9 (OCC) scalar combinationL14= L6+L10 (OCC) scalar combinationL15= L6+L11 (OCC) scalar combination

Thanks.

Top Reply Quote Quick Reply Quick Quote Notify Email Post#50636 - 09/06/12 04:39 AM Re: PSV Reaction Forces and Direction [Re: Ltorrado] mariog The reaction force as a result of a free jet existence is "physics" and nobody can






Member


deny it.The assumption F1 = F2 = F3= reaction force of "free jet" as describing the "traveling wave" is not "physics", is just an assumption based on our limited knowledge about the real phenomenon- this is my opinion.Of course, nothing wrong to be conservative; the only question is how we can realize when we are too conservative... But when it works with a reasonable piping layout , will be OK.

Best regards.

Top Reply Quote Quick Reply Quick Quote Notify Email Post#50641 - 09/06/12 07:47 AM Re: PSV Reaction Forces and Direction [Re: Miyamoto] Ltorrado Member


As I feared, we are steering away from the static analysis method topic and more into the fluid dynamics and physics of the phenomenon. I always say begin doing analysis very conservatively, and if you encounter problems, only then begin to step away from conservatism and more into "reality".

Now when the gas exits the relief valve it will lose most of its pressure due to frictional losses in the initial length of the discharge piping. As the gas travels down the piping it will lose pressure, which will lower the density, and in turn will increase velocity. So the assumption of F1 = F2 = F3 is not "real" because as the flow reaches higher velocities, the thrust load will increase. That is why I tell our Process Engineers to provide us with the maximum velocity exhibited in the discharge piping which in a closed system is usually right before entering the larger diameter relief header.

As far as the DLF goes, I begin with a conservative 2.0 and only in situations where I am forced to reduce the thrust loads due to problems do I start looking at PSV opening times, piping periods, etc. to calculate an approximate DLF. And correct me if I'm wrong, but doesn't the DLF stem from the traveling pressure wave due to the unbalanced system pressures?

But what I am more interested really is the way of applying these loads in CAESAR if you are not performing the dynamic analysis. I have not found much guidance in COADE's literature. The best advice I've encountered is Loren's method I quoted above.

Regards.

Edited by Ltorrado (09/06/12 07:48 AM) Top Reply Quote Quick Reply Quick Quote Notify Email Post#50644 - 09/06/12 09:06 AM Re: PSV Reaction Forces and Direction [Re: Ltorrado] mariog Member

It seems you are interested in the implementation of these loads in CAESAR static analysis.Mr. Loren Brown has a post where he detailed the procedure:












http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=2148

About flow dynamics you've mentioned. IMO the gas that exits the relief valve already lost most of its pressure due to frictional losses in PSV. Is not a rule the fluid will accelerate so much in the initial length of the discharge piping, it will accelerate in the end of system where the boundary conditions offers conditions for such acceleration to critical speed. As you said, some software is available for steady state calculation. About the transient calculation for gases exiting the PSV- well... this is an endless discussion.



Is anyone that saw this paper: "Flow Reaction Forces upon Blowdown of Safety Valves"?

What formula use?

In addition it mention a tee piece at the outlet but this is not a common layout, even it sound quite interesting.

Regards,_________________________Dan

Top Reply Quote Quick Reply Quick Quote Notify Email Post#52538 - 01/18/13 05:09 AM Re: PSV Reaction Forces and Direction [Re: Miyamoto] SJ Member

Registered: 11/14/09 Posts: 163 Loc: India

Dan,

The reasoning behind using a tee at the outlet end is to nullify the forces which are of equal maginitude.

I,too, was naive on this issue till I found such layouts in my current organisation.

Seems, interesting...huh..._________________________Keep Smiling

SJ


Registered: 04/22/05

SJ,

There are also some PSV with two discharge nozzles at 180 degree.

My question is: Why is not a standard layout if the advantage exists? Still, it is
















Posts: 1059 Loc: ...

not common._________________________Dan

Top Reply Quote Quick Reply Quick Quote Notify Email Post#52584 - 01/21/13 08:02 AM Re: PSV Reaction Forces and Direction [Re: danb] Ltorrado Member


I've only seen the dual-outlet type tail pipes a couple of times. It is definitely not standard practice.

Although how much of a benefit would it really be? It would seem that you would still experience the impact force due to the momentum of the fluid hitting the end before shooting out the sides. It would also seem that you'd need some kind of clamp or something to dampen the vibrations from a possible imbalance upon the fluid exits to atmosphere. Only in theory it would cancel the forces out perfectly.

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Dear All,

there is some issue that i need to clarify in this forum since i see that many










http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=dosearch&Searchpage=1&topic=0

expert in Piping Stress Engineering 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 firing is including in Impact Force so it is a dynamic event, but still we can use the quasi static method to analyze it by use the DLF factor.As long as i know this event also not a static condition because the pressure will change rapidly when the PSV first firing.The problem of my concern is for this PSV Closed system i usually used and consider not only the first impact/unbalanced forced in first elbow or obstruction but also in the second elbow and other downstream this PSV.I also read in Peng book, that he mentioned that the event when PSV in closed system firing still consider as Non static condition, so from here i can take the conclusion there will be no Balanced force, 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.But here in my new assignement i found a different method for client method, where Force downstream of PSV (other than force on first elbow) are consider to be balanced each other. I also found this method in one Engineering company that become a trade mark for Piping Engineering.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 RegardsNalibsyah

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Post#40429 - 01/25/11 04:22 AM Re: PSV Closed system Force [Re: Nalibsyah] stressguy81 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 flow conditions into a closed system usually do not transfer large forces and bending moments to the inlet system, since changes in pressure and velocity within the closed system components are small......A complex time history analysis of the piping system may be required toobtain 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 Case2: When there is established flow (When the forces cancel each other)

You might find this topic interesting

http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=11775#Post11775http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=24685#Post24685

I case of having a long run reaction forces can be imposed at the first elbow at a different case, ex: F1 at PSV and F2 at first elbow in the establised flow condition, conservatively.

Top Reply Quote Quick Reply Quick Quote Notify Email Post#40430 - 01/25/11 06:35 AM Re: PSV Closed system Force [Re: stressguy81] mariog Member


The only accident with PSVs in closed systems I know it is one in which the PSV was wrong chosen and was 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 sense process/piping experience but also by field operators feedback.

Reversing the point of view, you can see in field PSV arrangements that cannot be qualified by today calculations 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 the related piping and helps providing stiff supports. For this point of view the calculation must be encouraged.

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


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 elbows equal with w x v (flow times velocity)

Regards,_________________________Dan

Top Reply Quote Quick Reply Quick Quote Notify Email Post#40434 - 01/25/11 07:46 AM Re: PSV Closed system Force [Re: danb] MoverZ Member

Registered: 11/22/06 Posts: 978 Loc: Hants, UK

A further warning about Mach speed ..... it cannot in most cases be exceeded in a PSV body due to choking. 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 well in excess of Mach speed and thus impossible. A reduction to a realistic mass flow rate should give better force results.

Top Reply Quote Quick Reply Quick Quote Notify Email Post#40435 - 01/25/11 08:17 AM Re: PSV Closed system Force [Re: MoverZ] danb Member


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

Top Reply Quote Quick Reply Quick Quote Notify Email Post#40440 - 01/25/11 09:56 AM Re: PSV Closed system Force [Re: MoverZ] mariog Member


Dear MoverZ

You sayQuote: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.

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 reactive force. This is the force that API counts as:

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

where

- mass flow rate must be the actual value (it is greater than the designed flow rate, because the actual PSV orifice is larger than minimum required!)- jet_velocity is the critical speed when the jet gas flow has Mach=1 feature (is counted in Fluid Mechanics as jet_velocity= sqrt(2*R*k*T/ ((k+1)*M)), where notations are as in API, R is the universal perfect-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 coefficient is sqrt(2*R), in SI units sqrt(2*8314.5)=129Obviously, the formula is based on the "choked" condition i.e. Mach=1 and this is taken into consideration 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 flow is 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 flow (Mach=1). You can see the same result in some articles, but the fluid mechanics model is more complicated there.

My best regards.

Attachments Choked_flow_pressure.pdf (321 downloads)

Top Reply Quote Quick Reply Quick Quote Notify Email Post#40441 - 01/25/11 10:10 AM Re: PSV Closed system Force [Re: mariog] MoverZ Member

Registered: 11/22/06 Posts: 978 Loc: Hants, UK

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






Top Reply Quote Quick Reply Quick Quote Notify Email Post#40449 - 01/25/11 03:56 PM Re: PSV Closed system Force [Re: danb] mariog Member


Dan,

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 software calculation decided to have 24" size line downstream of the 2"x4" piloted PSV. So 24" wasn't the subheader size, it was the size connection to the subheader. The model was a visual aggression and the calculation was "worse than nothing".

Top Reply Quote Quick Reply Quick Quote Notify Email Post#40455 - 01/26/11 05:40 AM Re: PSV Closed system Force [Re: mariog] danb Member


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

I was talking about flow induced forces. Decent problems require decent solutions, sort of "better than nothing". For complex problems, there are other solutions and I am not one of the specialists that can solve them.

_________________________Dan

Top Reply Quote Quick Reply Quick Quote Notify Email Post#40462 - 01/26/11 08:52 AM Re: PSV Closed system Force [Re: danb] mariog Member


Dan,

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

The point is I have no evidence such criteria is a decent one in closed system except the connection with subheader/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/their criteria 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 decent criteria to evaluate roughly the magnitude of forces in a closed system. I accept also their position.
















Best regards.

Top Reply Quote Quick Reply Quick Quote Notify Email Post#43155 - 06/02/11 08:40 AM Re: PSV Closed system Force [Re: Nalibsyah] Tengku_Syahdilan

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 first pop up there will be a travelling wave downstream the PSV discharge or RV discharge. This load is not balanced each other. This force will have the same history shape throught out the system but the arriving time is different in each point. This is why in API 520 as stress guy taken above mention : A complex time history analysis of the piping system may be required toobtain the reaction forces and associated moments that are transferred to the inlet piping system."

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 and suddenly opened, there is a period of transient flow until the steady state discharge condition is reached.

and in Para II.2.3.2 :

......"Relief Valves discharging into an enclosed piping system create a momentary unbalanced forces which act on the piping system during the first few milliseconds following relief valve lift. The pressure waves travelling through the piping system following rapid opening of the safety valve will cause bending moments in the safety valve discharge piping and the reminder of the piping system. In such a case, the designer must compute the magnitude 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 on each leg (elbow or other obstruction) downstream the RV or PSV discharge as a result of the traveling wave or transient condition.

So what i mean is that we also have to considered the effect of this transient load during pop not only at the first 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 upward and horisontal at the body of the Rv or PSV.....i also have see some one post this kind of pictures in other thread about PSV too).

Apply the load at n elbow/obstruction and n+1 elbow/obstruction, but because it is quite complicated and we will not know the limit of this travelling wave unless we perform simulation (maybe with Boss fluid) 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 upward, and horizontal at valve body), and the result the pump downstream of this RV system having a trouble. so we fix it by apply and considered the transient effect that not considered by previous company and now have been 3 years the pump still run smoothly.

_________________________Tengku_Syahdilan"From Failure we Learn"



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I am looking for any book, paper or code to know a bit more how to calculate the transient loads during the discharge of safety valves, specially in closed discharge systems. Could you recommend m any?

Thanks in advance_________________________Carletes

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Post#3648 - 09/02/05 01:14 PM Re: Safety valve discharge loads Sun Wee I would like to recommend following;











Member

Registered: 12/20/99 Posts: 75 Loc: Calgary,Canada

1) Analysis of Power Plant Safety and Relief Valve Vent Stacks, Transaction of the ASME, 1975, G.S.Liao2) ASME B31.1 Appendix II3) Time-Dependent Pipe Forces Caused by Blowdown and Flow Stoppage, F.J.Moody, Transaction of the ASME 19734) Transient Analysis of water Slug Discharge in PWR safety/Relief Valve Piping, D.A.Van Duyne, ASME 19815) Flow of Fluids through Valves, Fittings, and Pipe, CRANE, 1985

Other member may give better idea._________________________Sun Wee

Top Reply Quote Quick Reply Quick Quote Notify Email Post#3649 - 09/02/05 03:01 PM Re: Safety valve discharge loads John Breen Member

Registered: 03/09/00 Posts: 482 Loc: Pittsburgh, PA (& Texas)

Hello Carletes,

Sun Wee has it covered. I can only add American Petroleum Institute Standard RP-520.

If you want to really get serious about the topic seek out the DIERS publications:http://www.aiche.org/diers/

Regards, John._________________________John Breen

Top Reply Quote Quick Reply Quick Quote Notify Email Post#3650 - 09/04/05 10:01 PM Re: Safety valve discharge loads anindya stress

Member

Registered: 04/12/04 Posts: 493 Loc: London, UK

I would like to add one more to the list:

Steam Flow through safety valve vent pipes by Brandmaier and Knebel- Journal of Fluids engineering-June 1976.

Regards_________________________anindya

Top Reply Quote Quick Reply Quick Quote Notify Email Post#3651 - 09/21/05 04:39 PM Re: Safety valve discharge loads tubecomp Member

Registered: 05/27/04

Thank you all.

I think I have got all the papers you have suggested (except Transient Analysis of water Slug Discharge in PWR safety/Relief Valve Piping), but I still don't see it clear how to get the transient forces in the discharge of my closed system. I
















Posts: 42 Loc: spain

would desire a document with any "example" about how to apply all those hard differential equations to a real system (perhaps I am dreaming..) Do you know any?

regards,_________________________Carletes

Top Reply Quote Quick Reply Quick Quote Notify Email Post#3652 - 09/22/05 02:54 AM Re: Safety valve discharge loads SUPERPIPER

Member

Registered: 08/13/03 Posts: 404 Loc: Europe

Look in the back of the piping codes at the section marked 'how to guess your way through common analytical problems'You'll probably find out there how to do trunnions and pipe attachments as well.

:p

(only joking)_________________________Best Regards T.J.N

Top Reply Quote Quick Reply Quick Quote Notify Email Post#3653 - 09/22/05 11:51 AM Re: Safety valve discharge loads tubecomp Member

Registered: 05/27/04 Posts: 42 Loc: spain

So, there is not any book, paper, etc where these problems are discussed from a more "peactical" point of view? Something like "differential equations applied to the real world"?

Regards_________________________Carletes

Top Reply Quote Quick Reply Quick Quote Notify Email Post#3654 - 09/23/05 02:01 AM Re: Safety valve discharge loads RS Member

Registered: 09/15/04 Posts: 81 Loc: South Africa, Johannesburg

Carletes, the formula in ASME B31.1 Appendix II is very straight forward and practical. I normally get the force from the RV supplier, when available. Dynamic analysis is described in detail in the Caesar Application Manual._________________________RegardsRanka

Top Reply Quote Quick Reply Quick Quote Notify Email Post#3655 - 09/23/05 09:52 AM Re: Safety valve discharge loads tubecomp Member

RS,

The problem is that the B31.1 formula is valid for steady discharge but there are





















Registered: 05/27/04 Posts: 42 Loc: spain

transient forces that must be evaluated for those systems whose discharge is not straight forward to the atmosphere. I suppose that a software of compressible flow transients will make it quite esay but when you don't have that...That/s my problem...

regards,_________________________Carletes

Top Reply Quote Quick Reply Quick Quote Notify Email Post#3656 - 09/23/05 10:44 AM Re: Safety valve discharge loads John C. Luf Member

Registered: 03/25/02 Posts: 1110 Loc: U.S.A.

anyone want to volunteer writing up a method for inclusion in the code, attend all the meetings, resolve all comments etc.? I thought not...

But in all seriousness this is a topic that should be covered but nobody has "volunteered" so we all are left wanting...._________________________Best Regards,

John C. Luf

Top Reply Quote Quick Reply Quick Quote Notify Email Post#3657 - 09/28/05 01:58 AM Re: Safety valve discharge loads RS Member

Registered: 09/15/04 Posts: 81 Loc: South Africa, Johannesburg

Yes Carletes, that is why we have Caesar and time history analysis. The time history analysis is suggested in the ASME B31.1 for the closed systems. You should use Caesar's Time History analysis to calculate stresses and forces in the piping system due to a short duration precalculated dynamic force.

Another analysis is the calculation of dynamic force due to a dynamic event. This is science in itself for which we "pipers" use approximate methods as one referenced in ASME Code. Dynamic piping stress analysis does not involve analysis of the fluido-dynamic phenomena induced by the process fluids flow. The phenomena is to be analysed and relevant dynamic loads are to be provided by the Process Department if approximate methods are not sufficient. There are specialised software packages for calculations of transients in fluids used by process guys._________________________RegardsRanka

Top Reply Quote Quick Reply Quick Quote Notify Email Post#3658 - 09/28/05 04:30 AM Re: Safety valve discharge loads sam Member

Registered:

You can read the following article . http://www.pipingdesign.com/articles/piping_surges.pdf

But, unless you have some knowledge about transient analysis, you will not be
















02/25/04 Posts: 463

able to run the softwares viz. pipenet-transient, AFT-Impulse in public domain correctly.

Many in the nuclear industry have dedicated their whole life in this field just to ensure plant safety. It is better not to trivialize this field. If you are really interested, go back to your engineering school once more for learning the same.

regards,

sam__________________________

Top Reply Quote Quick Reply Quick Quote Notify Email Post#3659 - 09/28/05 11:14 AM Re: Safety valve discharge loads Jackdliu Member

Registered: 09/21/05 Posts: 4 Loc: Calgary

Engineering design is about safety without waste of too much money.

The easiest solution for this problem is using the reaction force supplied by the safety valve manufacturer and taking 2 as DLF to run static analysis. The reason for this is that most of safety valve piping systems can be considered as one degree freedom system, in which the greatest DLF is 2.

Jack_________________________JDL

Top Reply Quote Quick Reply Quick Quote Notify Email Post#42071 - 03/31/11 05:09 PM Re: Safety valve discharge loads [Re: John C. Luf] danb Member


How about the fact that some huge companies consider POP forces while here in this forum nobody mention it?

Regards,_________________________Dan

Top Reply Quote Quick Reply Quick Quote Notify Email Post#42108 - 04/01/11 01:11 PM Re: Safety valve discharge loads [Re: danb] Crusader911

Member

Registered: 11/22/09 Posts: 25 Loc: Louisiana, USA

"We are but warriors for the working day..."

With the time I am given to do most analysis, I follow Jackdliu's method. However, many times I do not have the manufacturer's information, so I calculate a thrust load from the formula in API 520, put it in the model as a static load, and get on to the next job. Don't forget to double it, like he said.

The problem, of course, is that although you didn't have time to do anything more sophisticated, if something goes wrong the company that sues you will
















have plenty of money to pay some engineer who's never been outside the walls of a university to do some unbelievably impressive dynamic analysis that shows that the failure could have been anticipated if you had spent ten times the man-hours analyzing it.

If anyone has a better practical method I would love to hear it.

Top Reply Quote Quick Reply Quick Quote Notify Email Post#42119 - 04/03/11 01:51 PM Re: Safety valve discharge loads [Re: Crusader911] danb Member


Many engineers seems to be atracted by the dynamic analysis, but from my experience is quite unusual to be really required.

Regards,

P.S. And yes, at a rate of few hours per line, is anyone expecting full analysis?_________________________Dan

Top Reply Quote Quick Reply Quick Quote Notify Email Post#42235 - 04/06/11 10:29 AM Re: Safety valve discharge loads [Re: danb] Carter Member

Registered: 08/08/09 Posts: 21 Loc: Ontario, Canada

http://www.aft.com/products/impulse/

Carletes,

I have a similar concern regarding relief valve loading. From the instruction, it looks like the software AFT Impulse has this fuction. I have used AFT Fathom and others from AFT. They are good and easy to use.

Carter


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Hello Everyone,

I have been searching through this forum a discussion about PSV reaction force but unfortunately I can't find the specific one. If you do have a link please do send it to me.

My concern is PSV Reaction Force:

I am analysing an open system PSV directly above an equipment vessel by static analysis. I have referred to API RP520 for obtaining this force, what I'd like to understand is do we need to multiply this Force by 2 times for consideration of "dynamic load factor"? What is dynamic load factor and what part in the code can I read about it. By the way, I have read this in our project standard.

Thanks.

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Email Post#11775 - 06/20/07 10:48 AM Re: PSV Reaction Force [Re: ichigo] Loren Brown Member

Registered: 10/18/01 Posts: 281

pinoy,

The dynamic load factor (DLF) is a characteristic of the applied load shape (your PSV force versus time curve) which is plotted vs. system natural frequency. It is determined by a Time History integration of the dynamic equations of motion. For impulse loads, such as your PSV,

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the maximum DLF is usually less than or equal to 2.0 (which is why this value shows up in your project standard).

To perform your PSV analysis statically, multiply your PSV thrust force by 2.0 (check your API RP520 standard as it may already be applying this DLF=2.0 factor) and apply this at your bend midpoint downstream of the PSV in your vent stack. The direction of the force is opposite the direction of fluid flow in your vent stack.

For an open system, if you have more than one bend in your vent stack then apply this force at each bend under a separate load vector.

For a closed system you would apply this force on bends on each “long” leg of pipe. The only way to truly figure out which pipe leg is short enough to ignore the PSV force is to run the force/time profile through Caesar II's DLF generator in the dynamics module, but then you might as well perform this analysis dynamically. For short pipes the duration of the unbalanced PSV force is small and this shifts the DLF peak to the right (higher frequency) which at some point is past the majority of your piping system natural frequencies of interest. But if you are going to do this statically you might simply take the nine longest pipe legs and apply your force to each bend corresponding to these longest legs. This would be the "brute force" approach, not really an approach based on physics.

You have 9 different force vectors to choose from so apply your PSV force under a different force vector for each bend because we want to only examine the effect on one bend at a time. Then set up separate OPE cases that include your different force vectors.

The OPE load cases should be used to determine your equipment and restraint loads. Subtract your standard OPE case from each of these, then add that result to SUS to obtain OCC code stress (note some codes use the OPE+F1 load case directly for comparison to the allowable stress, so how you need to review the particular code you are using to determine the proper approach here). Here is an example of load cases for B31.1 or B31.3 assuming 3 PSV forces, F1, F2, and F3, each applied at a different bend:

L1 = W+P1+T1 (OPE)L2 = W+P1+T1+F1 (OPE)L3 = W+P1+T1+F2 (OPE)L4 = W+P1+T1+F3 (OPE)L5 = W+P1 (SUS)L6 = L1-L5 (EXP)L7 = L2-L1 (OCC) segregated effect of F1L8 = L3-L1 (OCC) segregated effect of F2L9 = L4-L1 (OCC) segregated effect of F3L10= L5+L7 (OCC) use Scalar Combination MethodL11= L5+L8 (OCC) scalar combinationL12= L5+L9 (OCC) scalar combination

Note that L10 through L12 are code compliance cases. L7 through L9 are interemediate load cases and not used for anything other than determining the stress results for L10 through L12._________________________Loren BrownDirector of Technical SupportCADWorx & Analysis SolutionsIntergraph Process, Power, & Marine12777 Jones Road, Ste. 480, Houston, TX 77070 USA

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Post#11794 - 06/21/07 07:29 AM Re: PSV Reaction Force [Re: Loren Brown] ichigo Member

Registered: 12/22/05 Posts: 51 Loc: USA-Philippines

Hello Loren,

Thanks for the quick reply, this is what I love about this forum... and thanks to COADE for making this possible.

Keep up the good work!


Post#12035 - 07/04/07 04:45 AM Re: PSV Reaction Force [Re: ichigo] ajaykumar

Member

Registered: 06/23/07 Posts: 1

Hello Loren,

Thanks for your reply regarding PSV,I faced this problem while working on a project,I searched this form and got valuable information.

and thanks for Ianpinoy for asking this question.











Loc: AP,INDIA

with regardsajay


Post#16292 - 02/27/08 02:03 AM Re: PSV Reaction Force [Re: ajaykumar] supremo

Member

Registered: 06/18/07 Posts: 12 Loc: --

Mr. Brown,

Do we really need to consider the temperature for stress evaluation of thrust force?


Post#16317 - 02/27/08 11:40 AM Re: PSV Reaction Force [Re: Loren Brown] Dylan Member


Mr.Loren,

You wrote:

Php Code:L1 = W+P1+T1 (OPE)L2 = W+P1+T1+F1 (OPE)L3 = W+P1+T1+F2 (OPE)L4 = W+P1+T1+F3 (OPE)L5 = W+P1 (SUS)L6 = L1-L5 (EXP)L7 = L2-L1 (OCC) segregated effect of F1L8 = L3-L1 (OCC) segregated effect of F2L9 = L4-L1 (OCC) segregated effect of F3L10= L5+L7 (OCC) use Scalar Combination MethodL11= L5+L8 (OCC) scalar combinationL12= L5+L9 (OCC) scalar combination

Note that L10 through L12 are code compliance cases. L7 through L9 are interemediate load cases and not used for anything other than determining the stress results for L10 through L1

My question is this example a case for Non-Liniear bondary condition?(because you take F from subtracting it with Basic OPE) and which Paragraph of B31.1 or B31.3? Para for Occasional Loads?












Post#16333 - 02/28/08 12:53 AM Re: PSV Reaction Force [Re: Loren Brown] Dylan Member


Dear Mr.Loren,

I want to ask, if you have 2 pumps running together and have to analysis the worst case, Pump A & B ON (T1), Pump A on Pump B off(T2), Pump B on Pump A Off(T3) and Pump A B OFF(T4) with design temp -39/45 deg c and Amb Temp -39/39 deg C. You have 2 piping spec here with Spec1 P=12 bar, Spec2 P=230 bar. This system has 3 PSVF1,F2,F3. Boundary condition Liniear.

So Please can you give an example how to build the case for PSV in Load case editor?Because i confused, when T1 all 3 PSV are open, but when T2 it is only F1 and F3 will act, so does at T3 F2 and F3, for T4 it is only F3 act. I hope you understand my question.

Here i attached my example input for your review.

Attachments 264-B.O.T.P.System.zip (446 downloads)


Post#16345 - 02/28/08 12:08 PM Re: PSV Reaction Force [Re: Dylan] SUPERPIPER Member

Registered: 08/13/03 Posts: 404 Loc: Europe

People,

When doing load cases, try to think about all of the reasonable and possible real life situations which could happen in the system.

whatever about the accademics, if your scenarios are over or under cautious, your wasting your time.

Example:Some relief valves only open once at which you have to physically go and close it, therefore how can the reaction forces act on hot pipe? its impossible. (same as a Bursting Disk)











Why apply 3 valve reliefs at once? these things are quick and statistically, is it impossible for all 3 to open at exactly the same time?

So turn off the computer, get a pen, paper and a strong coffee and think about the design and operating regimes before pilling into caesar._________________________Best Regards T.J.N

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Email Post#27868 - 06/02/09 10:29 AM Re: PSV Reaction Force [Re: SUPERPIPER] Umair Member

Registered: 04/16/09 Posts: 13 Loc: Pakistan

Hello LaurenCould you please clarify as to what practical situation do these load cases refer to

L10= L5+L7 (OCC) use Scalar Combination MethodL11= L5+L8 (OCC) scalar combinationL12= L5+L9 (OCC) scalar combination


Post#27903 - 06/03/09 12:16 PM Re: PSV Reaction Force [Re: Umair] manu Member

Registered: 06/03/09 Posts: 28 Loc: india

Dear Umair

L5+L7 is the sum of principal stresses in the system. You must be knowing that principal stresses are different from secondary stresses in a way that the pipe may not fail if the secondary stresses exceeds the limiting stress value(they are self limiting). But it can fail if the sum of principal stresses exceeds the limiting value.now earthquake is a principal stress and so is the stress due to pipe wt(L5) hence they need to be added before they can be evaluated..

this is basic..asking Lauren for this is like asking God for coffee [:)]


Post#27925 - 06/03/09 11:57 PM Re: PSV Reaction Force [Re: manu] Umair Member

Thank you for the reply manu ..the metaphor used in the end was pretty logical
















Registered: 04/16/09 Posts: 13 Loc: Pakistan


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