Questions regarding the installation of refinery flares
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I saw a project that featured hydrocarbon flares with three different pressure levels (0.4/0.1/0.05 MPa gauge) as well as an acidic gas flare at 0.05 MPa gauge. On the system diagram, however, the three hydrocarbon flares appeared to merge together before reaching the flare separator tanks and water seal tanks – first they combined into one stream, and then that stream proceeded to two separate sets of flare separator tanks, water seal tanks, and flare nozzles. As a result, I assume the backpressure for all three pressure levels is the same. How is it then possible to reflect the difference in pressure between these three levels in terms of operation?
The back pressure should be different. It should be the height of the water seal that is used to distinguish pressures.
I checked, and the height of the water seal can be 400/600/800/1200 mm – the variation is quite small. How is it possible to achieve such a high back pressure (0.4 MPag)?
Back pressure is not the actual pressure, but rather the accumulated pressure, referring to the maximum pressure that the emission source can withstand. For a torch system, the terminal pressure is the same; the backpressure at the starting point is reflected in the relationship between pipe diameter and flow rate.
Agree with the opinion on floor 4. It is not possible to use a water seal to regulate the system pressure; different pressure levels are set in order to prevent, in the event of an accident when various devices release gas through flare systems simultaneously, gases from different pressure sources from mixing with each other and preventing some of the devices from being able to release their gas. The pressure near the water seal tank is mainly determined by the backpressure from the downstream side, and the difference in water seal height is not significant.