# Calculate gas velocity from separator

What would be the best way to calculate velocity of gas from a three phase separator? I know that operating temperature is 75F, pressure 1000 psia, pipe diameter 20 inch, and pipe length to the point where velocity needs to be known is 45 inch. I'm having some trouble determining pressure drop over such a small distance and was wondering if there is a better way I could work this out.

• The length shouldn't affect the normal calculation methods you use for pressure drop unless there's something unique about your system that you haven't shared. Alternatively, there should be velocity guidelines for ensuring that the gas doesn't entrain liquid. – J. Ari Dec 3 '19 at 17:51
• Can I work out pressure drop without knowing flow rate or linear velocity? I'm trying to determine if I currently have enough information to approximate the gas velocity, equations for pressure drop seem to require flow rate and to calculate flow rate don't I require a pressure drop value based on what I can see from, for example, the weymouth equation? – Brian Dec 4 '19 at 9:08
• Do you have any mass balance info on the separator, so that way you could back calculate the flowrate? – J. Ari Dec 4 '19 at 13:52
• Perhaps some information here can help you - petrowiki.org/Separator_sizing – J. Ari Dec 4 '19 at 14:04