Determining PCV Setting for Nitrogen Flow

[RJB32482]

Hey,
I am interested in how you determine the flow rate of nitrogen into an atmospheric bag from a line that has a PCV in it reducing the pressure. So for an instance, the nitrogen line pressure for our plant is 80 psig and we are going to put a PCV in the line reducing the pressure to 3 psig. So to find the flow rate of nitrogen through the line to the bag, I would:
1. Know the pressure drop of 3 psig.
2. Measure the length of pipe from that pressure control valve to the bag.
3. Using compressible fluid pressure drop equations, find the flow rate of nitrogen through the pipe.

Is this correct? If not, could someone show me how to perform this calculation?

Thanks

 

[sailoday28]

What is the expected pressure and temp just upstream of the control valve?
Where does the 3psig occur, at the valve outlet or just upstream of the bag?
If choking occurs in the valve and the piping can be considered well insulated, the the Fanno line approach should be used.

 

[RJB32482]

Thanks for the reply.
The 3 psig will occur right after the control valve (if sized right by the Cv value correct?).
How can you tell if the flow is choked? What is the Fanno line approach?

Thanks

 

[sailoday28]

For a typical valve with a pressure ratio of (80+14.7)/(3+14.7) choking will occur. (Exception, is if the valve disc were a porus material, yielding a tortus path thru the disc.)
Note: in the pressure ratio, I have assumed that the 80psi is stagnation.

The Fanno line is for adiabatic flow, typically using a fixed ID pipe.
Fanno line, solves equations of continuity, energy (with stagnation enthalpy constant) and momentum (with friction losses taken into account)
See a good compressible flow text for use of Fanno line.
Regards

 

[RJB32482]

What pressure ratio is minimum for choked flow? Is there anything in the Crane Valve Manual on Fluid Flow or the Cameron Book on Fluid Flow?

Thanks