Check valve on flare sub headers 1

[nRGee]

I came across a flare header design where check valve is installed on one sub header (size 20 inch).

Is it common to have check valves on flare sub headers? can someone throw more light on this?

Thank you

Raghu N

 

[Latexman]

If this is downstream of a relief device, it may not comply with Code.

Good Luck,
Latexman

 

[shvet]

Obstructions of discharge pipe is hazardous because of risk of choked flow (Mach = 1). It is a good industry practice flow area of discharge pipe should be no less than that of discharge fitting of a relief device.

API 520-2-2015
8.3.1. ... the flow area in the outlet isolation valve should be equal to or greater than the outlet area of the PRV ... Check valves shall not be installed in PRD inlet or outlet lines since these devices are normally closed and the check valve can become stuck in the closed position or fail in a manner causing an obstruction in the PRD path.

ISO 4126-9-2008
7.4 The outlet line shall be as short as practicable and have a bore at least as large as the valve outlet.

Guess 1
Separation of subheader and main header due to low pressure or temperature rating of subheader. Check valve prevents backflow of too hot relief gas from main header to subheader.
Guess 2
Prevention of misopening of conventional spring PSV. Check valve prevents rise of pressure in subheader in case of too high pressure of relief in main header.
Guess 3
Prevention of backflow of relief gas to a vessel that breaths to subheader.

Note that any check valve can not guarantee prevention of separation of pressure in pipe segments due to design of sealing element. If backflow of gas to be prevented it is a good industry practice to provide 2 check valves in series having dedicated design of a sealing element e.g. spring-loaded or swing oriented upward.