Swirl filter

What is a swirl filter?

A swirl filter is a continuously self-cleaning bypass filter that protects analysers from liquid and particulates in gas samples entering the analyser. It is also called a coalescer. In normal operation, components of interest flow through the filter element to the analyser. Swirl or bypass filters remove small micron and submicron size particles from both liquid and vapour samples. Small pore membrane filter elements are used in the filter. These filter elements have uniform pore size for good surface retention of particles. Standard filter elements have a 0.5 micron pore size. Other pore sizes are available. Maintenance of the filter elements is easy due to the bolted or cap screwed construction of the housing of the filter.

How does a swirl filter work? 

The swirl action of the flow inside the housing of the swirl filter separates fluids of different densities in gas sample streams. These gas streams can contain liquids and particles which will be separated from the sample stream in the swirl filter. Tangential entry of bypass flow creates a swirling action that keeps the filter element clean at the upstream side of the filter element, particles will return to process. The Swirl filter was developed to remove small micron and submicron size particles from both liquid and gas. Although coalescing swirl filters are well suited to remove the bulk of liquids in a gas stream, they have difficulty removing all of it.

A coalescer collects liquid on the downstream side of the filter media, where it’s almost impossible to prevent it from becoming re-entrained in the sample stream. Each size filter has a minimum and maximum flow requirement where the swirl effect is most effective. The swirl filter proved to be an excellent analyser filter by several large chemical and petroleum companies. By installing a swirl filter as a bypass filter, it will replace those existing filters that tend to plug shortly after being placed in service.

Gas sampling 

The swirl filter is specifically to protect gas analysers from particles and liquids entrained in a sample stream. The swirl filter separates the liquids and particles from the gas at the upstream side of the membrane making it much more effective for analyser protection. It’s quite common to be called a swirl filter, however, technically it’s a Membrane Separator and a very effective particulate filter. It may, however, load very quickly with particulates because it has much less surface area than a conventional filter media. For the fastest sample response time, you should always have a swirl filter installed at the inlet of a gas analyser.

Liquid Sampling

Swirl filters are used for separate immiscible liquids, most commonly to remove free water from refined hydrocarbon products such as gasoline, kerosene, diesel, etc. It is possible for the filter in dirty streams containing iron sulphide, iron rust, catalyst and other fine particles to remove this from liquid streams.

 Swirl filter applications

  • Gas and liquid Sampling Systems 
  • Gas Chromatograph Process Analysers
  • Sensitive equipment and instrumentation
  • Ideal for quality control applications, because of its small internal volume provides a fast sample transport time

Swirl filter vs Particle filter

The biggest difference between a swirl filter and a particulate filtration is that next to particle filtering the Swirl filter works like a coalescer. Coalescing is the process of separation of two 'phases' fluids with a different density, for example the removal of water aerosols and droplets from a gas. Because the volume of the Swirl filter is smaller compared to a particle filter and volume after the filter element is very small, the response time through the swirl filter is less. A three-port cartridge particle filter with the correct cartridge (filter element) can remove liquid from gas as well in bypass flow. Liquid will be drained to a bottom connection that is part of the bypass flow return while the clean gas sample flows to the analyser. This type of filter has a lot of volume downstream of the filter element compare to a swirl filter.

How to select a swirl filter?

  • To select a swirl filter, let us first begin by breaking it down by what is being sampled, gas or liquid, learn the differences between all filters and find out which is the most appropriate to install in your sampling system.
  • Check application compliance, or to remove particles and water droplets in vapour streams or to remove entrained water from liquid hydrocarbon or oil streams or removal of iron sulphide and other small particles in liquid sample streams
  • Most swirl filters are available in different corrosion resistant materials. Select what housing material, wetted material (316 SS, NACE compliant), sealing material, membrane type, process connection, maximum pressure and temperature rating, minimum bypass flow rate, internal volume and filter element size options (micron) will be needed to remove particles from the liquid or gas.
  • Each filter type and size has a minimum and maximum flow range. Choose the size of the filter housing that fits the bypass flow required for sample system and the filter to work proper
  • Choose the filter pore size based on the smallest particles to be filtered out of the sample

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