5 replies to this topic

[sanganipratik]

Hi friends,

 

Case: I have a gas swivel seal leak line of 3/4" which is connected to the LP flare header and the pipe line is fitted with an orifce and downstream to orifice is a flowmeter.

Under normal operation there shouldnt be any leaks and if there is it should be very minor @acceptable range.

 

If the seal fails, then the line see a an upstream gas pressure of 170 barg, which is the gas swivel operating pressure. An orfice is selected, inorder to safe guard the downstream LP header which operates at 1.5 barg.

 

Now the question is the significance of the choke flow/ crtical pressue. As obviously i see that under leakage condition the orifice sees a choked flow @ 97 barg critical pressure. So should I design my downstream piping and the flowmeter for pressure of 97 barg or 1.5 barg.

 

Your suggestion is appreciated.

Pratik

[shan]

1.5 barg because downstream piping back pressure is not determined by the orifice critical pressure.

[sanganipratik]

Can you pls elaborate, as if i specify my operating conditions of the downstream fitings & instruments as 1.5 barg, what haapens when the choked flow/critical pressure is reached at the orifice?

[shan]

The orifice outlet pressure is determined by the backpressure of downstream system not critical pressure.  Please think about filling air into your car flat tires.  The pressure out of air nozzle is determined by the tire pressure (backpressure of downstream system) not the air compressor pressure.  Your critical pressure may be reached only in the case of blocked outlet.     

[sanganipratik]

Thanks Shan.

So under these conditions, how can I calculate Choke Flow ? I know the critical pressure and the dia of the orifice which i calculated using normal/operating flowrate..

[shan]

If the nozzle outlet backpressure is lower than the critical pressure, flow through the nozzle will be constant choke flow and will not impacted by the backpressure.