I came across the use of "velocity" check valves on a downhole hot water circulation line for a gas wellhead. I was told that they are installed to safeguard against any gas leaks into the circ system??.  I'm still not sure how that works? can someone familiar with this application kindly explain how does a "velocity" check valve function. How is it different from other check valves?

I'm not familar with the term, do you have a manufacturer and model #?

Is it also known as an "excess flow valve"?

You might find the following link helpful:  
www.psc.ca/manuals/arpvol2/electricalwireline/pressurecontrolequipment.html

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