PURPOSE OF DISCHARGE PIPE FOR PSV 2

[sager880]

Dears,

I fall into interesting inquiry regarding the need to have discharge pipe for spring loaded PSV to atm. it is clear the need for discharge line for safety of personal, however, my question is regarding if the set point pressure is as low as 0.4 barg and the fluid is air with 30-40 degree C. is it also mandatory to have discharge pipe for this case, or for this case its not needed. please let me know if there are any other reasons that i didn't pay attention to.

excuse my question as I have search for an answer for this case but with no luck.

Thank you in advance.

BR,
Sager

 

[LittleInch]

I don't believe it is "mandatory", but as you note yourself "it is clear the need for discharge line for safety of personnel". There is also an increased risk of debris or damage to the inside of the valve ( birds, rodents, sand etc)

They can be very short, but unless they are a long way from anyone working or walking past, then they need to be guided to a safe location. The one below could be even shorter, but directs air to a safe place.

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

 

[MortenA]

Its quite common (if your discharged gas is flammable) that there is a 3m distance from the point of exit (pointing upwards) to the ground. Since most PSV discharge at an angle to the inlet i guess thats one reason why the tail pipe is needed.

 

[vladrath]

You really do want some sort of bend in the discharge pipe. The tail pipe should also preferentially have a weather cover on it. Putting some discharge pipe on the PSV helps to reduce debris which could enter the device. It can also help to keep anyone from sticking a screw driver or something into the valve.

I don't know why anyone would do such a thing, but I wouldn't put it beyond the scope or reality. I've heard that in a bygone era at a plant I've been at that it is possible there was a leaking 'PSV' in steam service and the maintenance guy would use a screw driver to 'help' seat the valve.

 

[The Obturator]

If using a tailpipe like the one pictured, be careful not to put all of it's weight on the valve outlet. With full nozzle designs, it has been shown that outlet piping can distort the valve seating leading to leakage. Of course, tailpipe should be at least size of valve outlet (expect some built up back pressure, in your case negligible). As an alternate thought, a well known bursting disc manufacturer was once supplying outlet bags which simply slipped over the SRV outlet !

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[sager880]

Thank you. I believe that answers make sense to me in regards weathering and the possibilities of damaging PSV internals parts.

Thank you

 

[georgeverghese]

Short as this discharge pipe may be, also check that you've met the max permissible backpressure in this line, equivalent to 10% of PSV set press at max process design relief load. This should also include the additional loss due to the sudden exit to atm.

If there is some risk of moisture accumulating at the low point of the discharge line, also add a normally open 1/2inch or 3/4inch valve at the BOP, else a weephole may do just as well.