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# PRD Reaction Forces4

## PRD Reaction Forces

(OP)
API 520 gives an equation to determine the force at the point of discharge of a pressure-relieving device (PRD). I understand that but intuitively it seems like there would be something going on at the valve itself. I've attached a drawing of a segment of piping that includes 4 PRD's for reference. So when deciding how to support this you would obviously consider it's weight and the reaction force at the outlet. To keep it simple let's not consider temperature changes, wind, or seismic. Would you throw in some inertial force (or fluid flow or whatever) at the valves themselves or anywhere else? If so, how would you calculate? A reference explaining it would be great if it's too involved to describe here. I'm familiar with the concept of a control volume in fluid mechanics and it seems like that may contain the answer but I'm not sure I have enough data or a big enough brain to apply it.

Oh, and I'm sure many of you are going to have reasons why this is not a very good layout. That's fine, feel free to expound. We've actually got someone marking up the drawing as I type this but my fundamental question remains the same.

### RE: PRD Reaction Forces

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: PRD Reaction Forces

(OP)
David, actually they exacerbated the problem by cutting the slant the wrong way on the exhaust pipe. If they would have cut it opposite of how it is the reaction force would have been more in line with the valve thus reducing the moment. Who knows, maybe it would have been enough to save the pipe and support. Maybe not.

Anyway, I'm hijacking my own thread. I get that there is a reaction force at the tip of the exhaust. If you look at the model I attached, you will see that the PRD's are a long way from the exhaust. Technically, API 520 says the equation is for one elbow and a vertical exhaust pipe, so maybe I'm already in no-man's land. What I'm wondering is, if you have a support (or two) near a pressure relief valve and you have adequately supported the exhaust pipe which is quite a bit further away. Does that support (one near the PRV) feel any extra load when the valve opens?

Like I said, intuitively it seems like it would but I can't find anything that talks about that. Hoping some experts here can give me a sense of what is going on.

### RE: PRD Reaction Forces

zdas04, can you repost that picture but without added text? It's a great picture we can use for our company internal relief valve course, as an example of incorrect supporting installation.

### RE: PRD Reaction Forces

(OP)
I'm just curious, XL83NL, are you referring to the slant being incorrect or is there something else you see?

### RE: PRD Reaction Forces

Not sure if the slant is incorrect - I was interested only in the picture because of the strutcutral steel that was torn. But if there are more things incorrect, Id like to hear (Im not PSV expert by any means so keep me informed)

### RE: PRD Reaction Forces

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: PRD Reaction Forces

dozer,
I've spent the morning looking at videos of gathering system blow downs (I have several that I took myself and found more on YouTube). If the slant of the cut mattered, then the blowdown plume would be skewed from the pipeline centerline. I can't find a single case where the plume was any different with or without a bevel on the end of the pipe. The area of the opening (normal to the cut surface) is larger than the pipe cross sectional area by the square root of 2 if the angle is 45°, but the plume doesn't seem to be modified from the pipe diameter and centerline until several pipe diameters past the end of the pipe. If the force were normal to the cut face, then you would see an offset to the pipe centerline and you never do.

One of the secondary (maybe tertiary) forces in this scenario is friction, and there is no friction where there is no pipe, but I don't believe that the friction on the pointy side of the pipe is enough to skew the force profile to be normal to the cut face, or really to skew it in any measurable way. The dP due to friction in the entire tail pipe is a fraction of a psi.

If I still had wells to play with I'd set this experiment up and measure the lateral force, but those days are far in my past (14 years on September 1). I'm betting that just connecting a fish scale between the pipe and a support strut and opening a ball valve would show that the lateral force is the same regardless of the slope angle of the bevel.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: PRD Reaction Forces

There is a force on the valves due to the high velocity flow, but in your case it is matched by the near identical force on the attached elbow pointing down. So long as the flange & pipe between the valve and the elbow is strong enough then these forces cancel out. If the valve was simply venting horizontally then you would have a considerable force acting horizontally on the valve.

The issue comes at the end point of the vent where there is no balancing force other than that supplied by the support.

In the photo I'm working on the basis that the support was only attached by a U bolt to the vertical leg of the vent and has sheared off and also been pushed over. If the beam was instead under the horizontal bit then it should have taken the load.

The brown valve just beyond the white one has sheared off completely! and by the look of it was also U bolted to the beam.

your pipe isometric view doesn't show any supports, but so long as your long horizontal bit is supported properly then there should be relatively low forces or movements.

I'm with Dave on the angle of the vent. I can't see any reason to modify the force angle. If you had put a 12.5 degree elbow then that's different, but not a cut angle like that.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: PRD Reaction Forces

LittleInch,
If you zoom in on the brown valve, it looks like it was either unsupported or the support was removed early in the process (by the other valve bending it away maybe?) and the spool piece between the PSV and the block valve is bent 90°. It is still connected.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: PRD Reaction Forces

ok, that was some ductile piece of pipe!

The support when you zoom in seems to have two dents / U bolts which match up to the location of the vents. So support seems to be incorrect as they were just supporting it int he vertical plane instead of under the pipe.

I've seen another one like this supplied by BI, but can't find it where the spool up to the valve had been sheared off by the reaction force. (might have been the same place or the same designer....)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: PRD Reaction Forces

I think that it was the brown relief valve that vented. The U-bolt clamp slipped and the vent pipe moved down causing rotation around the point where the pipe leaves the ground to connect to the relief valve. This rotation pushed over the pipe support beam. You can see the ground is scoured at the exit of the brown vent pipe.

### RE: PRD Reaction Forces

Having a picture (or iso) of the original situation for the photo shown above would really help. I have a hard time understanding what happened, and what caused it.

### RE: PRD Reaction Forces

XL83NL,
Sorry, you are on your own for that one. I found the picture in a government publication on the accident report, but I don't have a link back to the report anymore.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: PRD Reaction Forces

(OP)
Well, I guess this thread has been officially hijacked. At least it's an interesting hijack. Thanks Littleinch for your reply.

### RE: PRD Reaction Forces

Not certain I believe a spool piece like that could be bent 90 degrees and not fail.

Any chance they replaced it with an elbow immediately after the initial incident to keep the system live while a real solution was engineered?

Agree that the scoured ground indicates the brown valve has been venting with the outlet in it's photographed position.

### RE: PRD Reaction Forces

I agree, XL83NL, something(s) looks off in the photo.

### RE: PRD Reaction Forces

jgKRI,
The engineer that would purposely put their PSV outflow in a location that guaranteed that rocks, gravel, and dirt would have to become projectiles is not an engineer that I would ever want to work around. I don't believe that that pipe started in that orientation.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: PRD Reaction Forces

I remember looking at that incident when it first came out on CCPS (or wherever). That elbow was bent in the incident. It probably happened in a fraction of a second. Like David, I can't find it now though. Yep, the supports should have been UNDER the horizontal (initially) PSV outlet pipes.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: PRD Reaction Forces

The orientation of the support columns also contributed to failure. If rotated 90 degrees the column flanges would be oriented to be stronger in bending.

### RE: PRD Reaction Forces

My recollection is there is a full report/presentation out there. I saw it at one time. I don't think I have it now. I suspect it can be found if someone really wants it. And, yes, column rotation was a finding.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: PRD Reaction Forces

Ok, ok. I'm interested in this topic as well as topicstarter is and I would like to find a gist. Picture is really great. It is so cute that I'm ready to print and sleep hugging it. But topic has not been answered. Is or is not there any force that shall be taken into consideration in bends, relief valves etc?

I have encountered PSVs in high pressure polyethylene plant that had set pressure >2kbarg. Contractor provided for Operator a calculation record and stated that during relief forces in bends rise:
- in PSVs up to 5.8kN
- in bends downstream of PSVs up to 8.8kN
Links to some extractions - one, two. I'm not sure this calculation should be believed, but anyway. There is a real problem as a min I have encountered. Operator was anxious about this case and demanded from Contractor an evidence because in the past Operator had an accident with PSV in steam service - during relief downstream pipe was torn away and injured a worker. Contractor provided a calculation record that forces inside (not exhaust) pipes were calculated and taken into account.

Can anybody provide a clear statement and a proof? If forces shall or should be taken into account than can links be provided to documents to calculate forces inside pipes?

### RE: PRD Reaction Forces

#### Quote (zdas)

The engineer that would purposely put their PSV outflow in a location that guaranteed that rocks, gravel, and dirt would have to become projectiles is not an engineer that I would ever want to work around. I don't believe that that pipe started in that orientation.

But you believe that:

A) that spool piece could bend (what appears to be) exactly 90 degrees, with a very tight bend radius and no support and not fail

AND

B) that spool piece could be bent exactly 90 degrees without (what appears to be) any significant deflection of the rest of the stack

Seems pretty unlikely to me.

I agree that the outflow causing a spray of debris absolutely would happen in this configuration.. but if the engineer's choice was between creating a projectile stream and not being able to operate the plant or system this is attached too, maybe his hand was forced. I don't envy his position.

### RE: PRD Reaction Forces

(OP)
Shvet, thanks for trying to get this thread back on topic. The drawing you linked to with the internal forces is exactly what I'm asking about. That being an isometric drawing without any x, y, z force components listed makes it hard to tell which way some of the arrows are pointing although you can figure it out using common sense. Notice a lot of them nearly balance which is what LittleInch is saying. I do see some that don't seem to have an equal (or roughly equal) and opposite reaction. If that is correct then that is somewhat disconcerting to me because I don't know that I've ever seen that taken into account.

I know that the PSV failure is thrilling but if anyone has a reference for computing these forces that would be great.

### RE: PRD Reaction Forces

3
I found the full report, which contains better pictures.

https://www.phmsa.dot.gov/staticfiles/PHMSA/Pipeli...

The reverse shot shows quite clearly that the spool piece failed not just on the second stack, but on two more that are difficult to see in the first photo. Second two failures threw the valves and their vents clear.

### RE: PRD Reaction Forces

Spot on jgKRI! Upon further review of the original routing for situation of the photo's shown, there are some similarities between that and what the OP is posting; 4 reliev valves. However there are also differences involved, and I think you carefully all the possible scenario's of relief. And what it does to the system. Also, you have led all relief lines into 1 vent. Do a dynamic analysis and as kthe PRV vendor for input of the relief loads.

### RE: PRD Reaction Forces

jgKRI,
I'm not sure why you got in my face over this. The picture that you posted demonstrates that no one designed that PSV to blow along the ground, that is where it ended up after it failed. You should have done your research before you got mouthy.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: PRD Reaction Forces

You apparently didn't read my first post.

The question which I asked was whether or not it was possible that if that assembly was straight, the straight spool piece was replaced with an elbow to lay the vent on the ground so that the system could be operated while a repair or replacement to the supports was engineered.

I never thought, or implied, that the original design intent was that the vent would be anywhere near the ground.

I then did my research by finding the full report, which took about 20 seconds.

I was having what I thought was a civil difference of opinion. If you took that as aggressive, maybe you need to dial back the sensitivity a little.

### RE: PRD Reaction Forces

jgKRI,

#### Quote:

{Quote (zdas)}
The engineer that would purposely put their PSV outflow in a location that guaranteed that rocks, gravel, and dirt would have to become projectiles is not an engineer that I would ever want to work around. I don't believe that that pipe started in that orientation.{/quote}

But you believe that:

A) that spool piece could bend (what appears to be) exactly 90 degrees, with a very tight bend radius and no support and not fail

AND

B) that spool piece could be bent exactly 90 degrees without (what appears to be) any significant deflection of the rest of the stack

Seems pretty unlikely to me.

Still seems pretty judgmental to me. Just for your future reference saying "but you believe that ..." and "... seems pretty unlikely to me" will always come across as attacking and many people here will red flag that sequence out of hand. My "sensitivity" is dialed in pretty well. I'm here because I want to be, but that desire diminishes every time someone ascribes beliefs to me that I don't hold.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: PRD Reaction Forces

I asked a question, with the hope that someone who knows these systems better than I do would respond and I'd learn something.

My desire to work with an engineer who calls someone 'mouthy' for asking a question and then spending their own time searching for the answer when one isn't given, is approximately equal to your desire to work with an engineer who would put a PRD vent near and parallel to the ground.

I don't know how you've survived as an engineer if you interpret a request for information, a difference of opinion, or a request for clarification of an opinion as aggressive. Do yourself, and anyone who might dare ask a question, a favor and keep your condescension to yourself.

Getting back on topic:

Any reason why the horizontal sections of the vents are so long? Serviceability concerns for the valves, or something similar? Seems that shortening them would greatly reduce the moment created when they discharge, so I have to imagine they're that long for a reason.

### RE: PRD Reaction Forces

When calculating the PRV reaction forces take note if PRV flow calculation is conservative. If the calculation over estimates the PRV flow to made sure there is enough capacity then the discharge reaction forces will be under estimated.

API520 does this for two phase flow.

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