NRV positioning

[sb00]

Hi Guys,
I have a set of PIDs with me and it seems the designer has put NRV before discharge valve in all pumps. Dont u think this can be a problem due to thermal expansion of the liquid trapped in between the NRV and the discharge valve (when the discharge valve is closed)? Actually, I have seen many gaskets rupturing due to thermal expansion in my previous refinery experience. It not only eats away time of manpower constrained plants but also adds fire hazards. What dya say?
-SB

 

[joerd]

This is usually done in the case of parallel pumps (one operating, one standby), so the NRV can be serviced without interrupting the process.
As long as the distances are short, which they should be, thermal expansion should be limited.

Cheers,
Joerd

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[saxon]

sb00, The location of the NRV or check valve is always placed between the pump Disch. nozzle and the pump discharge block va. The reason is simple, blocking out the check valve for servicing with out having to take the entire process line off line.

saxon

 

[sb00]

oh yes, why didnt it click to me?
Thnx guys!

 

[sur63]

sboo,
saxon has outlined the reason why check valves/NRv's are upstream of the discharge valve. If thermal expansion is a cause of concern - will depend on the fluid, a bypass around the check valve will help during the situation when the discharge block valve is closed.Whenever the discharge block valve is closed "keep the NRV bypass valve open". This would take care of your thermal expansion concerns. Having said this, in case your system has a standby pump with auto cut in feature,take care to ensure that operating procedures ensure that NRV bypass is kept close during normal operation.
hope this helps.

 

[sb00]

thnx saxon...ur inputs will surely help me resolve my problems.

 

[ihg]

Further to comment on bypasses - on standby pump systems it can be highly desirable to have the standby pump at operating temperature (eg. hotoil heating circuits). A small air-operated valve around the checkvalve is a handy solution.
Similarly, on cryogenic systems it can be VERY desirable to ensure that the trapped volume between checkvalve and isolating valve can relieve back to the pump side. A heavily spring-loaded checkvalve in reverse can fix it.