supporting near PSVs 3

[B.L.Smith]

Dear friends,

Open discharge PSV needs special supporting for withstanding forces due to fluid relieving. Does close discharge PSV need supports for withstanding this force(unbalanced force)? Is this force negligible in the close discharge PSV?

 

[BigInch]

No, it's not negligible, it's zero.
When the PSV is closed the support must hold the dead weight plus any other simultaneously acting loads.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[B.L.Smith]

Thanks BigInch,

Are not "any other simultaneously acting loads" relieving loads?

 

[BigInch]

When the PSV IS OPEN, yes, of course, the PSV's dead load, thermal loads, wind loads and all other loads that exist at the same time the PSV is open must all be supported.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[C2it]

Open dischage PSV's create sustained forces when operating, usually only out of balance at the open end and last elbow.

Closed system PSV's have a very short term out of balance force which can be calculated in accordance with API RP 520. Once flow is established all fluid forces are usually balanced. That said, the initial kick reaction should never be ignored.

 

[BigInch]

So then, there is no load transferred from thrust at the top of the PSV discharge, down the PSV discharge stack pipe, around the elbow, through the PSV, down the riser, now in axial compression and bending, through the reducing tee and finally into and through the mainline and on to the concrete pad below the PSV riser? And I was going to make it XXS.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[C2it]

Big Inch,

A litle hint of sarcasm there ?

My statement assumes that the simplest arrangement exists .... PSV outlet is horizontal, outlet pipe is vertical and open ended. Thus the out of balance is in the vertical direction and acts at the elbow due to eflux thrust.

Clear now I hope.

 

[BigInch]

Sarcasm? I hope not. I've recently taken a communications and cultural sensitivity class where they say to present ideas using words like "different" rather than "better" and to ask nice questions instead of saying "That's rubbish, do it my way or hit the highway." I'm trying to suggest other perspectives, without being insistant. Be happy. You are one of the first benefactors of that course at E-Tips. I would feel especially talented, if it is true that I can make such nice questions sound like sarcasm if I wanted to, however that was not the intent.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[B.L.Smith]

I read "Piping Stress Analysis" by Prof. Taha Aldoss. In this presentation following sentences mention:
"Other than the normal consideration for designing pipe, there are no specific guideline for the design of closed systems".

I think it means in closed discharge systems we don't need supports for withstanding pressure thrust due to valve opening. Please introduce any reference against this statement If any.

 

[C2it]

Try common sense, and a bit of engineering. Every action has a reaction, in a closed system that reaction is a short term impulse.

Quote from API RP 520 pt 2:

Section 5.5
The reaction forces and stresses that originate in the downstream piping as a result of the release of a pressure-relief device are typically not signifcant once flow is established and has reached steady state conditions, due to small changes in pressure and velocity within the closed system components.

However, large forces may result if there are sudden pipe expansions within the system or as a result of unsteady flow conditions during the initial activation of the relief device.

Additionally, large reaction forces can be created at elbows as a result of two-phase fluid flow in the slug flow regime.

Ignore this at your peril.

 

[BigInch]

A relief valve opening is roughly equivalent to a jet engine. I don't see how you can say that the forces only exist during initial opening of the valve. The thrust is generated by escaping mass x velocity. If the valve is open, there's thrust and the equal and opposite forces to contend with until the valve closes again.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[DSB123]

C2it,
Agree with you entirely. However unfortunately there are many people out there (Stress Engineers included) whom beleive that when an RV discharges into a closed system (such as an RV discharging into a flare header) that the only force to consider is the force at the RV inself under the initial lift. They do not consider "transient" loads through the system at bends or tees!!!! Big mistake!!!! But this is the norm from my experiences.

 

[B.L.Smith]

Thanks freinds for your kind help