6 replies to this topic

[mosayef]

hi all,

 

i need to perform sizing calculation of Pressure Relief Valve (PRV) for Fire Water System. According to NFPA 20, if 121% of shuttoff pressure + max static pressure exceeds the rated system components, PRV shall be installed. in my case, the pressure is above the max allowable pressure for the  fittings (around 250 psig), so i shall design the PRV.

 

i once design the PSV for gas application (Blocked discharge / fire  governing case ). but i have confussion to size the PRV for my fire water system (1000 gpm). should i design PRV based on API 520 ? or NFPA 20 ? because in table 2-20 NFPA 20 (1999 edition). it is stated that the minimum  relief valve size for 1000 gpm is 4 inch. i dont understand, because usually i calculate the area of orifice which is in square inch, not in inch.  could you please guide me ?

 

if i decide to calculate based in API 520 , i need to determine the back pressure (which is pressure drop from my fire water pump to fire hydrant , isn't it ? ) i need a balanced below PRV because if my back pressure is 85 psig, it is less than 30% of my set presure (250 psig). am i right ?

 

 

please enlighten me

Edited by mosayef, 18 August 2015 - 02:23 AM.

[fallah]

mosayef,

 

The PRV used in fire water system, on discharge of engine driven pumps to prevent over pressure due to over speed, is different from API 520 sizing basis. If NFPA 20 is to be followed, there would be pre sized PRV's based on mentioned pumps rated capacity which can be used in your application. Then no need to size the required PRV based on API 520...

[mosayef]

dear Fallah,

 

thanks for your reply. but honestly i still dont understand if i follow  NFPA 20.....

i attach table 2-20 from NFPA 20 here : http://oi60.tinypic.com/dm5xqd.jpg

 

from the table we can see that the relief valve size and the relief valve discharge size. both of them are in Inch. could you please tell me what does it mean ? because i usually design PSV, and i calculate the orifice area which is in square inch. not i inch. and how about the set pressure ? shall i set the PRV on the shutt off (churn) pressure of the pump ? or should i follow another criteria ?

 

i have tried to read but i can find brief explanation in NFPA 20

 

 

please help me

Edited by mosayef, 18 August 2015 - 08:27 PM.

[shan]

The listed PRV dimensions are PRV body nominal diameters not PRV orifice areas.

Edited by shan, 19 August 2015 - 06:52 AM.

[fallah]

mosayef,

 

Mentioned sizes in table 2-20 of NFPA 20 are inlet/outlet lines sizes (nominal size in inch) of selected PSV for specific fire water pump.

 

The PSV should be set based on relevant fire water company's standard and normally a little bit above the pump's rated shut off pressure...

Edited by fallah, 19 August 2015 - 10:32 AM.

[mosayef]

thanks Fallah & Shan,

i thought that if we want to install PRV, we have to determine the PSV orifice area like in gas application. ok, so inlet/outlet flange size is enough te determine the PRV

Edited by mosayef, 21 August 2015 - 03:45 AM.

[shan]

Fire water properties are well established comparing varieties of oil and gas.  Therefore, PRV sizes can be determined by a single factor pump rating.