Electromagnetic Filtering of Magnetite from Steam Boiler Condensate

By Joel Meissner layer increases in thickness and a coarser porous scale

Magnetite formation within a steam boiler begins to form on top of the thin layer (see illustration
system is costly to plant managers as a result of below).
increased maintenance and lost production time. Does
your steam plant have problems with Magnetite?
Talking with pulp and paper mill managers has yielded
surprising responses to that question. While some
understand the complexities of water chemistry and the
resulting formation of Magnetite along with the
difficulties these particles present, others have stated
If left undisturbed, the growth rate decreases.
“We don’t have a problem with Magnetite; in fact it
There are at least two mechanisms which cause the
helps our system.”
disruption of this coarser layer. The first is chemical in
So is Magnetite a problem or not? It is true that a nature. The porous layer allows dissolved oxygen in the
thin film of Magnetite along the water side of a steam water to diffuse through the pores and creates
boiler tube is beneficial as it passivates the tube surface conditions for superheated boiling below the surface of
and aids in the prevention of corrosion. Chemical the scale. The pressure from the expansion of the
additives are often added to the makeup water upon bubbles in these areas will weaken and fracture the
commissioning a new boiler to accelerate the formation Magnetite film.
of this protective layer. An additional benefit comes
from the fact that the film acts to improve thermal
transfer efficiency.
Magnetite Formation
To fully understand the benefit of Magnetite
let’s first look at how Magnetite is formed within a
steam boiler system. Magnetite is a product of the iron
oxide called Hematite. It is formed on the steam side of
the boiler tubes under pressure and high temperature in Courtesy of “The Protection Effectiveness of Magnetite
the presence of water and can be expressed by the Layers in Relation to Boiler Corrosion.” W.M.M.
following equation. Huijbregts, A. Snel (1)
Fe + 4 Fe2O3 → 3 Fe3O4 The second mechanism is mechanical in nature
as the Magnetite and the steel tube have significantly
So if a Magnetite film protects the boiler tubes different coefficients of expansion. When the boiler is
from corrosion and increases thermal transfer shut down for maintenance or production downtime the
efficiency, why is Magnetite formation a problem? The tube walls cool and contract. The strain developed
Magnetite film is actually comprised of two layers. The between the tube and the Magnetite layer will fracture
inner layer, directly in contact with the boiler tubes, is the scale. In either case, the result is that the coarse scale
dense, compact and continuous, providing excellent breaks free from the surface and enters into the water
corrosion protection. The outer layer is less dense, condensate system. The surface of the boiler tube where
porous, and loosely bound to the inner layer. Each of the scale broke free is now subjected to accelerated
these layers will continue to increase in thickness due to corrosion. Once the protective layer of Magnetite is
water diffusion in the outer porous layer, and latticed compromised, highly caustic ferric chloride within the
diffusion in the inner layer. Quite simply, over time the
water system will enter into these microscopic pores and are introduced by feedwater, chemical treatment
begin to corrode the steel tubing. (2) precipitates, and soluble salts, are removed through the
Magnetite Scale Problems regular use of blowdowns. In essence, the particulate
loaded water is blown out of the system and replaced
Within the boiler itself, the biggest problems
with cleaner make-up water.
caused by scale is overheating leading to the rupture of
boiler tubes. While the thin dense film of Magnetite is Inefficient blowdown may lead to carryover of
thermally conductive, thicker porous layers create a particulate laden boiler water into the steam system
more ceramic-like insulating layer which limits the which leads to the formation of solid deposits. But
thermal transfer. It is estimated that a scale thickness of excessive blowdown wastes energy, water, and
1/16” will increase fuel consumption by as much as expensive treatment chemicals. Blowdown rates
12.5%. This phenomenon leads to premature failure typically will range from 4% to as high as 20% when
through softening, bulging and eventual rupturing of the makeup water contains high solids content.
boiler tubes. Even before a catastrophic rupture occurs, An example would be a boiler operating at
loss of heat transfer efficiency, reduced flow, and 50,000 lb/hr at a pressure of 125 psig with a blowdown
complete plugging of the tubes will lead to reduced heat content of 330 Btu/lb. If the continuous blowdown
performance and increased cost to maintain temperatures system is set at 5% of the maximum boiler rating, then
and flow. the blowdown flow would be about 2500 lb/hr
In summary, Magnetite scale can cause containing 825,000 Btu. If the boiler runs at a typical
efficiency of around 80%, and the heat requires about
 thermal damage
1050 cu-ft / hr of natural gas, then the loss of energy is
 increased cost to maintain performance approximately $42,000 per year based on 8000 hours of
 increased cost to clean and repair tubes operation at $5 per 1000 cu-ft. (3)
 unscheduled downtime Any reduction in blowdown contributes to water
and fuel savings. When uniform concentrations are
 reduced working life of a boiler
maintained at or near maximum permissible levels in the
While excessive Magnetite scale not only boiler water, savings result in several areas, including
damages heat transfer surfaces within the boiler, the makeup water demand, cost of processing water, cost of
shedding of this scale creates numerous downstream blowdown water waste treatment, fuel consumption, and
problems in the form of fouling. Fouling is the chemical treatment requirements. These savings are
restriction of flow in piping and equipment. Process noticeably greater where makeup water quality is poor,
equipment, filters, sand beds, screens, needle valves, and where heat recovery equipment is nonexistent or
steam traps are susceptible to reduced performance and inefficient, and where operating conditions are
in some cases complete blockage from Magnetite frequently changed.
particles. This fouling directly affects both the
Removal of Magnetite within the condensate system
equipment availability and cost of operations. Corrosion
is therefore beneficial for numerous reasons.
products and impurities can accumulate in the secondary
side of steam generators, causing accelerated corrosion,  Reduced blowdown frequency
steam flow disruption and heat transfer loss.  Reduced reintroduction of Magnetite particles
Components are cleaned either mechanically, such as by into the steam cycle
scrubbing and grinding, or chemically through acid  Reduced fouling of screens, filters, heat
etching. exchangers,
Magnetite Control  Improved heat transfer efficiency
 Improved efficiency of heating fuel
One method of controlling excessive build-up of
 Reduced dependence on chemical additives
Magnetite particulates is the process of boiler
 Reduced downtime
blowdown. It is used to control boiler water parameters
to minimize the accumulation of scale, corrosion,  Reduced maintenance
carryover and other problems. Suspended solids which Filtering Alternatives
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 There are several mechanical devices available to Originally, electromagnetic filters (EMFs) were
filter the condensate return and prevent Magnetite intended to remove Magnetite (Fe3O4), which is
particles from reentering the steam boiler. Several present in most boiler condensate and in nuclear power
considerations must be given when choosing a systems, but they have also been proven effective in
condensate filter including pressure, temperature, flow removing weakly magnetic species such as Hematite
rate, and certainly the startup costs as well as on-going (Fe2O3) and copper. Other materials such as cobalt,
costs. The options include cartridge filters, ion exchange nickel, and chromium, which form spinel crystals or
condensate polishers, mixed bed condensate polishers, ferrites, can also be efficiently removed.
sand filters, precoat filters, reverse osmosis, and Modern EMFs are extremely efficient. They can
electromagnetic filters. easily remove over 95% of the Magnetite present in a
The most common technology utilized within the condensate stream, and depending on the conditions in
paper industry is the ion exchange condensate polisher. a particular application, they can remove over 90% of
The units are robust and if well maintained and under total iron and well over 50% of the copper found in
proper operating conditions provide low Magnetite typical boiler condensates.
concentrations at the outlet although they are limited to
certain flow conditions. The mixed bed polishers are
suitable for power generation stations but the condensate
temperatures at paper mills are too great for available
anion resin grades. These two types polishers are usually
preceded by some other form of filtration to remove the
larger solids to prevent fouling. Sand filters are another
low-cost option although they quickly become fouled
with particulates reducing the flow rate and requiring
them to be rebuilt frequently.
Cartridge filters are frequently used since the
implementation cost is relatively low and the filter media
has improved over the years to handle the higher
temperatures in the paper mill condensate lines. While
this is a simple well understood technological solution,
the cost of filter cartridges and regular maintenance
offset the initial savings. Shortly after a boiler start-up
EMF units employ an electromagnetic solenoid
operation, the particulates which shed from the boiler
wrapped around a vessel which is filled with a stainless
tubes will quickly cause fouling of the cartridges.
steel matrix. When the magnet is energized, the matrix in
Reverse osmosis is an effective method of removing the vessel will attract and hold the Magnetite particles
both particulates and dissolved solids. It works by without affecting the flow of the condensate lines. Once
passing the feedwater through a semipermeable the pressure differential begins to increase, the filter
membrane by pressurizing the inlet side sufficiently to controller automatically degausses the filter media and
overcome the osmotic pressure. The downside to these back-flushes itself much like a swimming pool filter.
filters is that they cannot handle pressures greater than These filters will handle the flow rate and water
1000 psig, electrical usage costs are very high, and temperatures of a paper mill condensate line without any
significant amounts (20-30%) of energy laden further maintenance for up to 40 years of continuous
condensate are dumped into the waste stream. operation. One huge advantage is that the units will
Electromagnetic Filtration extract particles down to submicron sizes without any
Another solution is electromagnetic filtration or EMF fouling of the bed. Once the units are installed, they run
as it is commonly known. Electromagnetic filtration is a and maintain themselves. There are two types of EMF
simple, cost-effective way to remove suspended, units available, a ball matrix and a filamentary matrix.
magnetically susceptible material from process streams.
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 The ball matrix filter available from the Milhous deteriorate, small particles may actually be introduced
Company in the United States was originally developed into the condensate if they are not captured in the
in the 1960’s by Siemens. Babcock & Wilcox purchased screens. Replacing the media requires breaking down the
the technology from Siemens and spent considerable pressure vessel to access the screens whereas a ball
resources to reengineer it to maximize the efficiency of matrix vessel never needs replacement of the media.
the filter. In 2009, Milhous Company then purchased the Filamentary matrix EMF units require an additional air
technology and today offers six different sizes for accumulator tank and high pressure air for flushing
varying applications. making it slightly more complex than the ball matrix
The matrix material is comprised of type 430 filters.
stainless steel balls and has a void volume of 40%. The Electromagnetic filters work by intensifying the
efficiency of a ball matrix filter is not affected by the magnetic force within the matrix to capture the magnetic
micron size of the material to be filtered. They can particles. Several factors influence the ability of the
remove suspended material from 20 microns down to matrix to attract and hold these particles. The greatest
0.08 microns in size and when used in a system where factor is the percentage of Magnetite or strongly
the suspended material is predominantly Magnetite, they magnetic species within the condensate stream.
are independent of the flow rate through the filter up to Secondly, the flowrate is another variable which greatly
the maximum design flow rate. affects the performance of the filter as the magnetic
A ball matrix filter is flushed by degaussing the force must overcome the dynamic force of the flowing
magnet and the bed of balls and the flow is reversed to liquid which tends to wash the particles out of the
matrix. The greater the velocity, the greater the magnetic
force must be in order to hold the weakly magnetic or
paramagnetic particles such as Hematite.
Removing weakly magnetic or paramagnetic
particles with an EMF may sound paradoxical but it does
occur and is quite predictable. Weakly magnetic
particles will agglomerate with strongly magnetic
particles such as Magnetite and will be captured by the
magnetic force of the filter media. Particles such as
copper, chromium, and nickel on the other hand will
combine chemically with Magnetite to form spinel
crystals which are strongly magnetic and removed very
efficiently as long as the Magnetite concentrations are
significant enough. Elemental copper in some
Milhous Company EMF
condensate streams will plate around the Magnetite
particles making it relatively easy to remove it from the
upset the bed and tumble the balls which knock the fluid. Data has shown that a ball matrix filter will
corrosion products from their surfaces. This tumbling remove over 60% of copper in the stream if over 50% of
occurs within 15 seconds and is largely responsible for the iron present is in the form of Magnetite.
the tremendous bed-life of the matrix which will last for Tests have shown that the Milhous Company filter is
approximately 40 years. the most effective overall. It operates at significantly
The second type of electromagnetic filter available is lower power consumption, is more efficient in removing
the filamentary matrix filter produced by Sala of up to 95% of all magnetic particles, and removes as
Sweden. The matrix is composed of either expanded much as 50% of paramagnetic particles such as copper.
metal, or wire mesh and compressed steel wool. The The Milhous filter was originally developed for use
media within these filters will last for several years within the nuclear power industry and met the rigorous
before requiring replacement unlike the 40 year lifespan requirements for continuous maintenance-free operation.
of the ball matrix filters. As the steel wool begins to It is the only design to offer a truly permanent matrix

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and consistent efficiency for the life of the equipment. through the outlet line. This action is schematically
Because of the unique ability of this matrix to be shown in the diagram above. The entire cleaning process
thoroughly cleaned each time the filter is flushed, the is completed in approximately 2 minutes. The matrix
matrix will never have to be replaced or chemically balls are designed for the life of the filter and never need
cleaned to restore filtration efficiency. replacement.
Since electric power is the only consumable
associated with the filter, a great deal of effort was made International Paper has made it a
to design a series of filters that demand a minimum of corporate policy to include an
power. The result is that the Milhous units typically use electromagnetic filter in all their pulp and
a third of the power required of their competitor. paper mills.
Depending on which size unit is installed, the power
supply to energize the magnet coil will draw between 14 Long Term EMF Study Results
to 19 Amperes of DC power. When power consumption After 4 years of continuous operation in a pulp and
is considered over the 40 year life-span of the filter, the paper mill in the United States several observations were
Milhous company equipment has an overwhelming cost made by the mill operators about the Milhous EMF.
advantage over competing technologies. During that time over 2 billion liters (550,000,000
The Milhous EMF unit consists of a carbon steel gallons) of condensate had been filtered with almost 230
pressure vessel filled with small magnetizable stainless kilograms (500 pounds) of impurities having been
steel balls surrounded by a magnet coil and coil cooling removed from the steam generator feedwater. During
circuit. An outer carbon steel jacket and support one scheduled outage, the water sides of the steam
structure houses the filter, magnet coil, and cooling generator were inspected and found to be “whistle
circuit. The outer carbon steel jacket provides magnetic clean”. The maintenance engineer determined that the
shielding to eliminate stray magnetic fields for safety previous requirement for chemically cleaning of the
purposes. The power control module is fully tubes every two years could safely be extended up to six
programmable to allow for automatically degaussing and years. They also noted that the deaerating tank and other
backflushing the filter on either a predetermined components downstream of the EMF were almost
schedule or manually as needed. devoid of iron oxides and related impurities normally
During normal operation, the flow of process water deposited by the condensate. Analysis of water samples
is downward through the magnetized ball bed. The inlet taken prior to the scheduled outage showed that the filter
and outlet valves are open and the flush and bypass was removing an average of 96% of the magnetic iron
valves are closed. High field gradients in the interstices oxides. The facility reported that:
between the balls attract the magnetic particles which  They experienced increased boiler availability
collect on the balls. The pressure drop through the filter  There were fewer boiler cleanings required
bed is low and does not increase significantly as the  Softener maintenance had been reduced
filter becomes loaded.  They had a reduction in feedwater makeup
The automatic cleaning cycle consists of the  The unit had already paid for itself through
following steps: reduced maintenance and greater boiler
availability
 Opening the bypass valve
 Closing the process inlet and outlet valves Furthermore, they estimated that since the filter is
 Degaussing the filter bed using approximately 5 kilowatts of power with no other
consumables or labor required, it was costing less than 5
 Opening the process outlet and flush outlet
cents to purify 3,785 liters (1,000 gallons) of condensate.
valves.
(4)
With the upward flow of flush water into the filter, Chemical and mechanical cleaning of boilers and
the ball bed is fluidized and balls rise and fall in a heat exchangers to remove Magnetite is done on a
tumbling action. This tumbling action scrubs and frequent basis throughout the pulp and paper mill
dislodges the collected particles which are flushed industry. Since EMF provides a more cost effective and
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efficient method of performing this task, it appears that it
is an underutilized solution. Ion exchange columns still
cannot handle the high temperatures and cartridge filters
cannot sufficiently handle the large flows required to
efficiently filter out Magnetite from large condensate
streams. Electromagnetic filters generate no waste
products and can operate under much higher
temperatures and flow rates than traditional filters.
Minimal maintenance and low operating costs along
with a 40 year lifespan make them an attractive
alternative for steam plant operators. This proven
technology has yet to be fully recognized for the
economic advantages it provides.

Joel Meissner is the Program Manager at

Milhous Company and can be reached at
joel.meissner@milhous.com

REFERENCES
1. Huiijbregts, W.M.M., Snel, A., “The Protection
Effectiveness of Magnetite Layers in Relation to
Boiler Corrosion”. 5th International Congress on
Metallic Corrosion, Tokyo, 1972
2. McDonald, Alex, “Corrosion Processes in Water
Systems”, Austin, Steam Generation Systems, Inc.
2013
3. “Boiler Water – Problems and Solutions”,
http://www.pdhcenter.com/courses/m165/m165conte
nt.pdf, Aug. 12, 2013
4. Arnold, R.C., Csuhta, R.R., et. Al., “Electromagnetic
Filter – Four Years Paper Mill Service”, American
Power Conference, Chicago, Il, 1979