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
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
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
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
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
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
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
- 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
Tel: 1-336-217-3822 | Fax: 1-336-668-4452