The purpose of the gas separation section of the vessel is to condition the gas for final polishing by the mist extractor. From field experience it appears that if 100-micron drops are removed in this section, the mist extractor will not become flooded and will be able to perform its job of removing those drops between 10- and 100-micron diameter.
The gas capacity design equations in this section are all based on 100 micron removal. In some cases, this will give an overly conservative solution. The techniques used here can be easily modified for any drop size.
In this book we are addressing separators used in oil field facilities. These vessels usually require a gas separation section. There are special cases where the separator is designed to remove only very small quantities of liquid that could condense due to temperature or pressure changes in a stream of gas that has already passed through a separator and a mist extractor. These separators, commonly called “gas scrubbers,” could be designed for removal of droplets on the order of 500 microns without fear of flooding their mist extractors. Fuel gas scrubbers, compressor suction scrubbers, and contact tower inlet scrubbers are examples of vessels to which this might apply.
Flare or vent scrubbers are designed to keep large slugs of liquid from entering the atmosphere through the vent or relief systems. In vent systems the gas is discharged directly to the atmosphere and it is common to design the scrubbers for removal of 300- to 500-micron droplets in the gravity settling section. A mist extractor is not included because of the possibility that it might plug creating a safety hazard. In flare systems, where the gas is discharged through a flame, there is the possibility that
burning liquid droplets could fall to the ground before being consumed. It is still common to size the gravity settling section for 300- to 500 micron removal, which the API guideline for refinery flares indicates is adequate to assure against a falling flame. In critical locations, such as offshore platforms, many operators include a mist extractor as an extra precaution against a falling flame. If a mist extractor is used, it is necessary to provide safety relief protection around the mist extractor in the
event that it becomes plugged.