A vent scrubber is a two-phase separator designed to remove the liquid from the relieving fluids before the gas is flared or vented. The liquid is returned to the process. Design of vent scrubbers is covered in two-phase separators. A vent scrubber is sized as a standard two-phase separator with a liquid droplet size in the gas of 300-600 microns. The operating pressure depends on the pressure drop in the vent piping between the scrubber and the outlet to atmosphere. A reasonable range for operating pressure on a vent scrubber is 25 to 75 psig. For design pressure, if air migrates into the scrubber, forming an explosive mixture in the scrubber, and the mixture ignites due to lightning or static electricity, then the vessel will withstand the resulting detonation if the design pressure is 150 psig or higher. The liquid retention time can be short, but the scrubber needs to have liquid capacity to retain all the liquid produced while the shutdown valve is closing. The MMS has a requirement of 45 sec maximum for platform shutdown valves to close, so 1 min is a reasonable retention time.
