Why is pressure drop tailpipe important

[rojans]

The API code states that the pressure drop of the tailpipe should not exceed 10% of the set pressure.
Why is this? I want to understand what happens when the 10% is exceeded.

Is there a file of website where relieve valve are described how they work and why pressure drop is important?

 

[oil198]

It is simply due to chattering issues. As you build up pressure in the discharge line of a PSV, forces are applied on the valve disc causing it to close. When the force is removed the valve opens. So open, close, open, close i.e. chattering. I am not sure how the 10% came about but that is recommended to avoid this type of a cycle.

 

[rojans]

someone told me it was related to garantee capacity. is that true?

why is chattering prevention important? IF the valve fails, wouldn't it fail open? At least the pressure is released; so overpressure is not possible.

 

[Latexman]

In the simplistic, ideal world of relief sizing, excessive pressure drop in the inlet gives chattering, and excessive pressure drop in the outlet gives reduced capacity.

Good luck,
Latexman

 

[dcasto]

Agreed with Latexman. PSV's are basically differential pressure driven on capacity. I will allow the pressure drop to exceed 10% if the the differential pressure is still at sonic flow and is "choked", for example, a gas relief valve at set at 1000 psig and a 150 psi drop in a vent line would be acceptable. This is very rare, but I've seen it on some big valves going to vent stacks far away.

Excessive drop in inlet lines also means that the system is overpressured by the amount of the drop. Ie a relief valve set at 1000 psig and has a 40 psi drop on the inlet means the line is at 1000 when the valve is starting to open and 1140 when fully open. You can overcome this by setting the PSV at 950, but best to just get the inlet line larger or shorter.

 

[JoeWong88]

rojan,

The API code states that the pressure drop of the tailpipe should not exceed 10% of the set pressure.

Something to clarify first...

The built-up back pressure should not exceed 10% of the set pressure at 10% allowable overpressure for a conventional pressure relief valve (PSV) application.

It is not only the pressure drop of tail pipe. It include the superimposed back pressure, pressure drop on the discharge system including tail pipe.

It is not applicable to all type of PSV. The "10% rule" only applicable to conventional type PSV.

Why is this? I want to understand what happens when the 10% is exceeded.

someone told me it was related to garantee capacity. is that true?

Overpressure of protected vessel, reduced capacity & instability lead to damage of PSV. Read more in "Several Impact of Backpressure on Conventional PRV"

why is chattering prevention important?

Chattering refers to rapid and cyclic opening and closure of PSV disc in contact with seat. It damage the seat and lead to leakage and potential failure of PSV.