Evaluate a Relief Valve for liquids

[ngomdiaz]

Good Night
I ‘been ask to evaluate the change of diameter of a Pressure Relieve Valve, for a water pump system, We have two Vertical Centrifugal Pumps of various stages, one move by a electrical motor and the other by Diesel Engine, both suctions sea water and discharges to a industrial plant, this water is use for Fire Protection.
The pressure required for the system is 100 psig, the pump have a capacity of 3000gpm.
Someone told me that you need to install a PRV to protect the pipe for overpressure, and that valve have to be localized after the pump discharge and before a check valve, plus the system permit to discharge the exceed of water again to the sea
I first wanted to calculate the Area of the Orifice of the PRV installed so that I can check if the diameter is correct, I know that API 520 tells about calculate the PRV valve but also says discharge capacity is required, and I really don’t know how to calculate that discharge capacity
If I consider the worst scenario than the gate valve localized before the check valve, closes for a failure or a bad operation. And I believe that for a centrifugal pump the PRV don´t need to relieve the hole 3000gpm because this sort of pumps can operate with the discharge close for some time, I think I can estimate a discharge capacity less than 3000 gpm, but I don’t know how. Is there any formula or Code (ASME, API) that permit solve mi problem, maybe in fire pumps systems designs, etc
I really appreciate your help, I Know you guys can help me for taking the right decision… Thanks for your time…

 

[TD2K]

Get a copy of API 520 and 521 or the appropriate standards for your area. A pressure relief valve protects downstream piping or equipment against overpressure so you need to determine first can this pump overpressure downstream piping or equpment? If so then you can work out the sizing basis for the PSV.

NFPA 20, if it's applicable in your area or the corresponding standard, may also have requirements on the firewater pump.

 

[Pumpsonly]

The PRV used in the fire system is different from the API 520 sizing. If you are using NFPA20, there are pre-sized valves based on the pump rated capacity. Check out the web site of Cla-val , Singer , Tyco, OCV etc....and you will find the details. Typically you only need it on the engine driven unit to prevent over pressuring the down stream system in case the engine over speed.

 

[casflo]

Check your pipe design pressure that must be greater than the zero flow pump pressure with some margin for transient overpressures. Probably you don´t need to protect the pipe but you need to protect the pumps for minimum flow and this is the purpose of your actual PRV.
Check which is the minimum recirculation flow required by the pump supplier. If you don´t know for centrifugal pumps unless a 30% of the design flow (flow for the maximum pump eficiency) will be enough.
Regards
casflo