PRD Reaction Forces
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.
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
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
RE: PRD Reaction Forces
RE: PRD Reaction Forces
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
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
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
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
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
RE: PRD Reaction Forces
RE: PRD Reaction Forces
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
RE: PRD Reaction Forces
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
RE: PRD Reaction Forces
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
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: PRD Reaction Forces
RE: PRD Reaction Forces
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: PRD Reaction Forces
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
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
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
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
RE: PRD Reaction Forces
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
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 wasn't "in your face".
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
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
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
API520 does this for two phase flow.