Fuel gas knockout drum is sized for minimum surge time of 2 minutes between 19.5 barg to 16.5 barg for compressor change over.
I would like to calculate the surge volume. How?
Answers
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16/10/2014
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A:
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Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
You take the mass flow to the compressor for your 2 minute residence time and divide it by the change in density. For illustration purposes say the flow is 100 kg/min and the density at 19.5 bar is 5 kg/cu m and at 16.5 bar is 4kg/cu m. The volume for hold up you need then is 100kg/min*2min/(5-4)kg/cu m = 200 cu m. My numbers are all made up so use your actually gas densities and flow rates in the calculation. You also need to add some volume if there is going to be a liquid level in the bottom of the vessel.
Unless your gas flow rate is really small, my guess is that you are going to end up with a pretty big vessel. Also if I understand what you are doing it sounds like you are running your compressor at an artificially high discharge pressure to provide this surge capacity. You are compressing to 19.5 bar, but the system only needs 16.5 bar. This approach wastes a lot of energy.
A more common approach would be to use some other sort of gas source for such a period. If natural gas is available it would be easiest. If natural gas is not available then people will often have a vaporizer to vaporize LPG into the fuel system.
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