I have a refrigeration circuit with three stage compressorThere is an orifice in the bypass line. The line is the feed to the comressor suction KOD. I would like to know the purpose of the orifice.

I would say its a pressure equlisation line - so that you can open the main block valve with a high dP, thats also why there is a dP across the valve. The orifice is to limit the flow across the by-pass when the dP is high.

Since the line is used to purge and prussure up the system you need to control the flowrate.  Most people use an expensive globe valve sized with a proper orifice to obtain a specified flowrate.  Using a properly sized restrictive orifice downstream of a gate vale or ball vale saves money and allows for resizing if the flowrateneeds to be changed in the future.

As a side note, our engineering standards didn't allow purging or pressuring up ethylene systems any faster than 25,000 pounds per hour.  A 1/2" orifice in the 2" or 3" lines worked fine.

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