2 replies to this topic

[ashleyg]

Greetings All,

I would like to size and select a pressure relief valve for an application but first need to determine the relief flow capacity for the valve. The worst case scenario is for nitrogen gas at 25 deg C exiting a HP cylinder valve which has an upstream pressure of 25000 kPag and downstream pressure of 7000 kPag. The cylinder valve size has an ID of 4mm (OD 20mm) but reduces after the cylinder valve fitting to 12mm ID. I have used two methods for calculating the flow, one is the ISA 75.01.01-2000 calculations which takes into account inlet and outlet pipe fittings and the other is a Cv calculator from the Swagelok website. Both methods give me different results, and I am unsure which method I should use to calculate the flow. At this stage I am inclined to take the worst case scenario which is that calculated from the Swagelok calculation.

Out of interest, when do you use the ISA calculations for no fittings vs with fittings considering all field applications will have pipe fittings attached?

Cheers, Ashley G.  Maximum_Flow_Calculation.doc   73.5KB   394 downloads

[proinwv]

Ashley,

The flow capacity of a relief valve should always be stated by the manufacturer. They will test each model and determine its capacity at various overpressures.

The engineer must determine the flow rate, pressures, fluid properties etc. and select the proper rv based on this and the manufacturer.

PAUL

[pleckner]

Go to Fisher Controls and download their Control Valve Sizing Handbook. Follow the bouncing ball.

I always use the ISA valve sizing method. This is pretty well industry standard. It isn't just any fitting we are concerned with here. It is reducers/expanders. If you have a reducer on both sides of this valve, then you use the equations that take these into account. If there are only elbows, tees and manual block valves, don't worry about them.