The principal items of construction that should be present in a good liquid-gas separator are the same regardless of the over-al1 shape or configuration of the vessel. some of these features are itemized as follows:
1. A centrifugal inlet device where the primary separation of the liquid and gas is made.
2. A large settling section of sufficient length or height to al1ow liquid droplets to settle out of the gas stream with adequate surge room for slugs of liquid.
3. A mist extractor or eliminator near the gas outlet that will coalesce small particles of liquid that will not settle out by gravity.
4. Adequate controls consisting of level control, liquid dump valve, gas back pressure valve, safety relief valve, pressure gauge, level gauge, instrument gas regulator and piping.
It has been found that the bulk of the gas-liquid separation takes place in the inlet centrifugal separating section. Here the incoming stream is spun around the walls of a smal1 cylinder or usually the walls of the vessel in the case of a vertical or spherical separator. This subjects the fluids to a centrifugal force up to five hundred times the force of gravity. This action stops the horizontal motion of the free liquid entrained in the gas stream and forces the liquid droplets together, where they will fall to the bottom of the separator in the settling section.
The settling section is necessary to allow the turbulence of the fluid stream to subside and allow the liquid droplets to fal1 to the bottom of the vessel, due to the difference in the gravity between the liquid and gas phases. A large open space in the vessel has been found adequate for this purpose. Introduction of special quieting plates or baffles with narrow openings only complicates the internal construction of the separator and provides places for sand, sludge, paraffin, et cetera, to collect and eventually plug the vessel and stop the flow. It has been found that the separation of liquid and gas using the centrifugal inlet feature and a large open settling section will produce a more stable liquid product, which can be obtained in atmospheric or low pressure storage tanks. Minute scrubbing of the gas phase by use of internal baffling or plates may produce more liquid to be discharged from the separator, but it will not be a stable product, since light ends will be entrained in it, and, therefore, more vapor losses will be incurred from the storage system.
Sufficient surge room should be allowed in the settling section to handle slugs of liquid without carry over to the gas outlet. This can be accomplished to some extent by the placement of the liquid leve1 control in the separator which in turn determines the liquid level. The amount of surge room required is often difficult, if not impossible, to determine based on well test or flowing data. In most cases the separator size used for a particular application is often a compromise between initial cost and possible surging requirements.
Another major item required to affect good and complete liquid-gas separation is a mist eliminator or extractor near the gas outlet. Small liquid droplets that will not settle out of the gas stream, due to little or no gravity difference between them and the gas phase, will be entrained and pass out of the separator with the gas. This can be almost eliminated by passing the gas through a mist eliminator near the gas outlet which has a large surface impingement area. The small liquid droplets will hit the surfaces, coalesce and collect and form larger droplets which will then drain by gravity back to the liquid section in the bottom of the vessel. It is believed that the stainless steel woven wire.
mist eliminator is the most efficient type, and has been proven by removing up to 99.9% or more of the entrained liquids from the gas steam. This type offers the greatest surface area for the collection of liquid droplets per unit volume as compared to vane types, ceramic packing or other configurations. The vane type mist eliminators do have their application in areas where there is entrained solid material in the gas phase that may collect and plug a wire mesh type mist eliminator.