PRV overpressure due to external leakage 4

[Mechaneer16]

Hi All

Pressure Relief Valve failure due to internal leakage is a common scenario. However, for external leakage i.e. if the leakage occurs through the external seals of the PSV, could it lead to overpressure? Some cartridge type relief valves are installed with O-rings on the outside diameter. Also, a leak can occur from the inlet flange gasket for flanged PSVs. If the external seals leak, would that cause pressure drop and inlet loss for the relief valve and cause overpressure? My understanding is due to the pressure drop through the leak (depends on leak size etc.), the relief valve would not open until the system pressure is considerably above the set pressure. Could anyone confirm this statement?
Is there any resource/ publication that discusses these scenarios? Thanks for your help.

 

[LittleInch]

I've read that three times and that big block of text makes no sense to me.

A sketch might help but IME there are very few leaks of any significance but mainly I haven't got a clue what your issue is.

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

 

[Mechaneer16]

Hi LittleInch

Thanks for your reply. Please see attached sketch of a relief valve (just as an example). If there is a leak through the O-ring (marked in red), that would cause a pressure drop. For this pressure drop (similar to inlet loss), the relief valve won't lift at the set pressure and the system pressure has to rise above the set to lift the relief valve. Would that be correct to say?

 

[Latexman]

Yes, but I think the leak would have to be quite large for the effect to be significant.

Good Luck,
Latexman

 

[Mechaneer16]

Thanks @Latexman for the reply.
Is there any reference that I can consult to calculate how much pressure drop can occur. Such as an API standard or similar.

 

[Latexman]

Idk.

Good Luck,
Latexman

 

[GBTorpenhow]

Any leak of this sort would be smaller than the flow when the PSV is open. If the DP at flowing conditions doesn't cause overpressure, how could a small seat leak create overpressure? How could bleeding fluid out of the protected system ever cause overpressure?

 

[Mechaneer16]

@GBTorpenhow
The leak is externally i.e. through the body seal of the relief valve. Please see the image I have attached in the above comment. This would mean there is a pressure drop occuring through the leak which will also cause dynamic loss. So, just like inlet loss scenario, this loss will also be added on top of the system pressure before the relief valve is lifted. Is this wrong to say?

 

[The Obturator]

This internal (not external) leak), is not part of the main valve nozzle flow path. It will not be leaking with any significant volume to make a difference. The valve will open once set pressure is reached, leak or no leak. In any case, if the valve is experiencing a leak from either between the seats, or this lower sealing position, you would try to remedy the situation ASAP would you not? Have you asked the valve manufacturer of your concern?


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Mechaneer16]

Thanks @TheObturator.
The valve will lift at set pressure but say any upstream leak drops the pressure by 5 bar (valve set pressure is say 300 bar) then the system pressure would rise to 305 bar before the valve lifts right?
Yes, I agree this leak will be fixed asap ideally but sometimes slow leaks may be left unnoticed and I am trying to visualise the scenarios when it can cause overpressure in the system.

 

[LittleInch]

Mechaneer16,

You seem to looking for problems which don't exist.

Pressure Relief Valve failure due to internal leakage is a common scenario. Eh? What information have you been getting. "Internal leakage" is what they are supposed to do so it isn't a failure. If you operate too close to the set point or they don't re-seat they can weep a bit, but that isn't a "failure". The vast majority sit there doing nothing for years on end. Provide some evidence of this.

However, for external leakage i.e. if the leakage occurs through the external seals of the PSV, could it lead to overpressure? No - not possible. Any leakage would have to be more than the relief rate and this is just not a concern.

Some cartridge type relief valves are installed with O-rings on the outside diameter. Also, a leak can occur from the inlet flange gasket for flanged PSVs. If the external seals leak, would that cause pressure drop and inlet loss for the relief valve and cause overpressure? - There might be a very small pressure drop in the inlet line, but this is limited to 3% at MAX relief flow so it would not cause overpressure.

My understanding is due to the pressure drop through the leak (depends on leak size etc.), the relief valve would not open until the system pressure is considerably above the set pressure. Could anyone confirm this statement? - No one can confirm this statement because it's not correct. IMHO. At the very very very worst case situation, the pressure drop in the inlet line would be <3% of the set point so "considerably above" the set pressure is not correct. Also to get to that pressure drop you would need to be leaking out the relief rate so there would be no need for the relief valve to open (!)

Is there any resource/ publication that discusses these scenarios? - No because they are not credible

" I am trying to visualise the scenarios when it can cause overpressure in the system." - You are dramatically over thinking this and getting yourself into a complete spin over something which is neither credible or realistic.



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

 

[Latexman]

The backpressure on top of the disc, caused by the "leak" flowing through the exhaust port, outlet pipe, and fittings, is what will increase the opening pressure above the set pressure. That's why I said, "but I think the leak would have to be quite large for the effect to be significant." In fact, if the leak ever got that large, I think heads would roll. Figure it out yourself. Assume a leak = 1% of the capacity of the PSV. Then, estimate the backpressure. If the backpressure at capacity was at the limit of 10% of set prtessure for a conventional PSV, at 1% of capacity the back opressure would be 0.001% [10*(0.01)[sup]2[/sup]] of set pressure. Negligible!

Good Luck,
Latexman

 

[GBTorpenhow]

@GBTorpenhow
The leak is externally i.e. through the body seal of the relief valve. Please see the image I have attached in the above comment. This would mean there is a pressure drop occuring through the leak which will also cause dynamic loss. So, just like inlet loss scenario, this loss will also be added on top of the system pressure before the relief valve is lifted. Is this wrong to say?

Seat leak or seal leak a leak from the inlet of the PSV to the exhaust or atmosphere has the same result: the leak path will be in parallel with the main flow path when the valve opens, not in series. The impact of any such leak would be insignificant.

 

[georgeverghese]

Pls note that higher inlet pressure drop doesnt mean the RV lifts at a higher pressure. It means the RV will chatter/simmer a bit more as it lifts open.