In this chapter, you will learn what filter separators are, why and where they are used,
the main types of filter separators, and the flow through a filter separator.
A filter separator is used to separate solids and liquids from natural gas.
Specifically, a filter separator removes impurities such as pipescale,
water, iron sulfide, liquid hydrocarbons,
compressor lube oil, and sulfur products from the natural gas.
A filter separator differs from a standard separating vessel and that it uses filter elements
to remove contaminants rather than using mechanical devices.
We're removing contaminants from the flow stream to meet quality and safety requirements set
by the purchaser, which must be removed before it can be sold.
The filter separator is installed where contaminants are present and must be removed.
This may be after compressors or prior to a dehydration unit or a delivery point.
In a dry gas field, this may be the only equipment required for separation.
Filter separators are used at tank batteries as well as
gathering sites and processing stations.
Every sector of our industry – upstream, midstream and downstream – use some type of filter separator.
Horizontal filter separators are common for gas conditioning. They can handle higher
concentrations of liquid and solids compared to a vertical filter separator.
However, vertical filter separators are ideal for when ground space is limited.
Inside the filter separator canister are filter elements – either replaceable socks or coalescing filters.
In a replaceable sock filter, the entire gas stream flows
through the sock filter trapping solids and condensing liquids.
Coalescing filters separate liquid particles from the gas stream.
Coalescing occurs when the entrapped liquid particles either combined with other nearby droplets or
are struck by additional droplets trying to go through the filter.
Coalescing filters are made of materials that repel many elements.
The filters fibrous surface will be coated with a very low energy polymer such as
fluoropolymer or a silicon.
This keeps the droplets from sticking to the filter and helps it shed the liquids.
Filter separators can have one or two stages, depending on the need of filtration.
This is not the same concept as when we've
talked about a vessel having two or three phases. On most production vessels
the number of level controls and the control valves can help determine if a
vessel is 2- or 3-phase; however, with filter separators this can be misleading
because there can be one or two separate chambers combined into one vessel shell.
These are called stages rather than phases.
In a two-stage vessel there is a
plate that divides the two stages and allows 2-phase separation to take
place in each side of the vessel. In a single-stage vessel there is only one
type of filter element used. This is also called a single-element filter separator.
Filter separator vessels can be high-pressure which is anything over 500 psi or low-pressure.
Which vessel is used will depend on the process conditions and application.
Pneumatic level switches are ideal for a two-stage filter
separator because they take up less space. The volume of liquids in the
vessel is relatively small and most of the space contains filter elements.
This leaves no room for the Gen II level controller or a trunnion assembly for mechanical operation.
In a horizontal filter separator,
a Kimray Gen II liquid level controller is an option
because of the available space in the sump chamber of the vessel.
This rendering shows the flow of natural gas
through a horizontal 2-stage filter separator. As natural gas enters the unit
the first stage of filter elements capture solid particles.
In this rendering, coalescing filters are used these allow any liquids present in the
gas stream to coalesce into larger droplets. The gas moves through the tubes
and into the second stage while the liquids drop out to the bottom of the tank.
In the second stage, the wire mesh of a mist extractor captures any
remaining liquid droplets. The resulting gas stream is then virtually free of all
particles no larger than the micron filter that's being used.
The liquid that has dropped out falls to the bottom of the vessel and is removed with a dump valve
controlled by either a liquid level switch or a Gen II liquid level controller.
The principles of separation are the same for a vertical
filter separator only the arrangement of components differ.