May/June 2012 | Volume 31, No. 3

Selecting Porous Metal Filter Media
By Mark Willingham, Purolator Advanced Filtration


GKN AS grade of sintered metal powder. (Courtesy of GKN Sinter Metals-Filters)

Sintered fiber metal felt media.
(Photo courtesy of Bekaert)

Most companies in fluid processing industries such as chemical processing, pharmaceutical, food & beverage, oil refining and others, utilize some type of filtration equipment in their process. These filters are typically used to remove unwanted particulates, or additives/catalysts from the process stream. In many cases, the operating conditions of the process require the use of porous metal filters. Although much higher in cost than "disposable" filters, porous metal filters are often the only choice for use due to these extreme conditions. Common reasons for the use of porous metal filters are high temperatures and/or operating pressures, and chemical incompatibility with typical disposable filters.

There are a variety of metallic filter types available, each with its own features, benefits, and unique performance capabilities. Among the available media, the most commonly used in the process industry are sintered metal powder, sintered fiber metal felt, and woven wire cloth (in both un-sintered and sintered form).

Sintered metal powder
This media is available in micron ratings ranging from .1µ - 150µ. The media has a relatively low porosity, which can result in high differential pressures in some applications. This same low porosity does, however, make this media extremely durable and capable of use in extremely demanding applications. It is available in a variety of alloys, including 300 series stainless steel, nickel based alloys, titanium and others. The media is most often used in tubular form, and is commonly used in both liquid and gas filtration applications. Due to the surface filtration nature of the media, it has proven to be cleanable in situ via backwash/back pulse systems.

Sintered metal powder is typically made from water atomized powder metal, which is subsequently sieved to specific powder sizes to yield the desired filtration grade. The powder is then compressed into tubes or sheets, and then sintered in vacuum or inert gas high temperature furnaces.

As a result of its low porosity, sintered powder metal exhibits very high clean differential pressure as compared to other porous metal media. In applications where a minimal clean pressure drop is required, this may require a significant number of filter elements. One option to minimize the number of elements required is the use of an asymmetrical media such the AS grade media from GKN Sinter Metals-Filters. This media utilizes a very thin membrane (~ 200µ thick) to achieve the filtration rating on top of a substrate of coarser powder metal. The result is a pressure drop of up to 5x lower than comparable symmetrical media, which in turn can minimize the number of elements required for an application.


Sintered wire cloth media Courtesy of Purolator Advanced Filtration

Sintered fiber metal felt
Sintered fiber metal felt media is available in micron ratings as low as 2.5µ, has a very high porosity, and thus provides a very low-pressure drop. The media can be fabricated into tubular form, or pleated to provide increased filter area. The media can be configured as a depth media, and has a much higher dirt holding capacity than either sintered metal powder or woven wire cloth. It can also be configured to function as a surface media, and can be efficiently backwashed or back pulsed. This media is available in a variety of alloys, including those noted above for sintered metal powder.

Sintered fiber metal felt is typically made from rod drawn wires as small as 2µ in diameters. These wires are then cut to the length, and precision air or water laid into a multi-layered un-sintered mat. The mat is then furnace sintered in a manner similar to that used for sintered powder metal media. The result is a permanently sintered web of fibers yielding the desired porosity, permeability and filtration removal rating.

This media has a tortuous pore path, making it an excellent media for applications such as polymer filtration, where it has become the media of choice due to its unique gel removal capabilities and high dirt holding capacity.

Woven wire cloth
Woven wire cloth is perhaps the oldest and most geometrically simple form of porous metal media. Available in micron ratings as low as 8µ absolute, and in a variety of alloys, including those noted above for sintered metal powder and sintered fiber metal felt. The media has a relatively high porosity, and thus a low-pressure drop. Unlike sintered metal powder or sintered fiber metal felt, which both have tortuous pore paths; woven wire cloth has a virtually straight line pore structure. Multiple layers of woven wire cloth can be sintered together to enhance strength and durability.

Wire cloth can be woven in many different patterns, each resulting in its own unique pore geometry, porosity, filtration rating and mechanical strength. Generally speaking, plain square weave - whether woven in market grade or as bolting cloth - possesses the highest permeability at a given micron rating. Plain Dutch, twilled Dutch, and reverse Dutch weaves are much stronger than plain square weaves, but have much higher clean differential pressure drop as well. When selecting a specific wire cloth for an application, all of these things must be taken into consideration. Woven wire cloth can be fabricated into cylinders, cones, discs, and many other shapes, and can also be pleated to provide more effective surface area.

Mechanical properties comparison
Due to the significant differences in the pore geometry, thickness, and porosity of these three media, their performance will differ greatly in most applications:

  • Sintered metal powder media is characterized as having a broad pore size distribution and therefore is capable of removing many particles smaller than the absolute rating of the media. In most cases this media functions as a surface type media, but in applications with a large particle size distribution it may also capture some particles within the matrix of pores beneath the surface. Due to its comparatively low porosity, it has the highest clean pressure drop of the three media (.5 psi at 10 gpm/ft2 of water flow).
  • Sintered fiber metal felt is also characterized as having a broad pore size distribution and thus the ability to also capture particles smaller than its absolute rating. This media is perhaps the ultimate porous metal depth media, with a very high porosity, which allows for the highest dirt holding capacity of all of the three. Due to its very high porosity and permeability, it has the lowest clean pressure drop of the three media (.03 psi at 10 gpm/ft2 of water flow).
  • Woven wire cloth is characterized as having a very narrow pore size distribution, thereby having a sharp particle removal cut-off. In most cases, wire cloth is used as a surface type media, which may limit its on-stream life if measures such as pleating the media to provide more effective area aren't taken. Due to its relatively high porosity and permeability, it has a relatively low clean pressure drop of the three media (.06 psi at 10 gpm/ft2 of water flow).

Flow rates and pressure drops noted for each media are based on 20 micron absolute media.

Many factors should be considered when selecting the optimum porous metal media for an application. Among them are the required particle removal rating, desired clean pressure drop, clean-ability of the filter in situ and externally, mechanical strength of the media, and of course the initial cost and cost of operation/maintenance.


Mark Willingham is Director of Sales & Marketing for Purolator Advanced Filtration. He has 30 years of experience in the field of porous metal filter products for applications in the oil & gas, chemical processing, nuclear power generation, polymer, and general industrial markets.

For more information contact:
Purolator Advanced?Filtration
Tel: 1-336-217-3822 | Fax: 1-336-668-4452
Email: mark.willingham@purolator-facet.com
Website: www.purolator-facet.com