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Natural Gas-Fired Engine
Lube
Oil Filtration
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Natural gas is the preferred fuel for large stationary engine applications.
It is plentiful, clean-burning and a relatively good BTU-value. The lubricating
oil and the filters used on these engines have different performance requirements
than those found on diesel or gasoline-fired engines.
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A Stationary Natural Gas-Fired Engine-Compressor Package
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The primary challenge in designing oil filters for gas-fired engines is
sulfur consumption.Commercial-grade natural gas is considered “sweet” if
it contains less than 20 grains of sulfur (mg/m3).A 1200 horsepower
engine, running on 150 CFM of pipeline-quality fuel gas, can consume over
a quarter-pound of sulfur per day! Engines running on wellhead or digester
gas, can ingest fifty times this amount of sulfur in “sour” gas applications.
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Many Gas-Fired Engines Run on Wellhead Gas in Remote Locations
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Leading manufacturers of gas-fired engines, such as Waukesha
Engine, have developed reliable methods of combating the detrimental effects
of high-sulfur fuels. Good metallurgy in the upper engine is of critical
importance, particularly with regard to the valve guides, crown and piston
rings. High-ash lubricating oils are recommended. Often, the water-jacket
temperature is elevated for problem-fuel sites (up to 235oF).
Sometimes, oil sump capacity is increased by adding a large oil filter housing
to the system. Depth-type filters, such as those made by Filtration Systems,
Inc., are the filter of choice for gas-fired engines, because of their large
dirt-holding capacity, good value and water-removing properties.
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Note the Large Oil Filter on the Side of this Engine
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In the engine’s combustion chamber, the elemental sulfur
in the fuel gets oxidized to become sulfur dioxide (SO2). That combines with water (H2O, a
by-product of combustion), to form sulfurous acid (H2SO3). This, in turn, reacts with oxygen (O2)
to form sulfuric acid (H2SO4). A significant amount of these impurities blow
past the piston rings and accumulates in the oil sump.
These acids attack the additive packages in lube oil,
and over time, diminish the lubricity of the oil. Gas engine lubricating
oils should have a high alkalinity reserve, with a “TBN” (Total Base Number)
of 7 to 13. Frequent oil analysis will show trends in the oil’s properties
as they approach Condemning Limits. One measure of the acidification of
the oil is its “TAN” or Total Acid Number. Condemning Limits for TAN Levels
in lube oil are typically a 2.0 rise over the new oil value (per ASTM
D-664) and/or a 50% decrease in the new oil TBN (per ASTM D-2896).
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Filtration System’s depth filters contain a large amount of excelsior
wood fibers, which absorb an appreciable amount of water out of lube oil.
By reducing the water content in the oil, they prevent the creation of acids,
and keep the oil “sweet”.
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Excelsior Wood Fibers
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Many filter manufacturers only measure particle-removal
properties of their filters. These
are measured by the filter’s “Beta-Ratio” micron rating. While particulate removal is important, it is
often over-stressed by filter manufacturers playing the “numbers game”. Filters should do more than remove particulates;
they should also “condition” the oil by removing semi-solids (sludge &
varnish), water and acids. This
is of particular concern in gas-fired engine lube filter applications. Filtration System’s sock filters offer the best performance value
in the industry for gas-fired engine lube oil filtration.
The table below shows the various types of filters used
by various manufacturers of gas-fired engines:
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Most engine and manufacturers
recommend 15-micron filtration to protect their bearings and prevent abnormal
wear on components. Many filter manufacturers try to sell the advantages
of tighter filtration into such applications. The trade-off for high-efficiency filters is
a double-whammy – increased cost per filter element, and shorter service
life. There are five-micron particles
floating on your eyeball right now. There
are no demonstrable benefits from removing such microscopic fines from
today’s high-detergent oils. Indeed,
there is such a high percentage (over 95%) of gas-fired engines that have
run continuously from the 1950’s and 1960’s, using nominally-rated depth-type
sock filters, that it is impossible to determine their half-life!
Filtration Systems manufactures a complete
line of depth-type, pleated paper and pleated synthetic oil filter elements,
and centrifuges:
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The following table shows the different lube oil filter
micron ratings for various manufacturers of gas-fired engines
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The following test procedures are used to measure
the various performance parameters of lube oil filters:
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There are two basic types of lube oil filters, full-flow
and bypass:
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Integral Engine-Compressor. Note the Large Full-Flow Depth-Type Oil Filter
in the Foreground
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There are two types of media used in oil filters,
surface and depth:
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There are five types of full-flow media:
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The characteristics of these various media types are
listed below:
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The following ISO Cleanliness Chart shows the relative
performance of Filtration System’s sock, pleated paper and pleated fiberglass
filters:
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Water-in-oil is either “free” or “dissolved”, and
is usually measured in parts-per-million (ppm). Filtration Systems, Inc.
has a Karl Fischer titration device in its lab for quantitatively measuring
the concentration of water in oil.
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Water not only reduces an oils lubricity, it also
promotes the creation of acids, which can damage engine components.
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Each Filtration System’s depth-type filter removes
several ounces of water from lube oil, which is the secret to long oil
life and extended drain intervals:
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The following graphs succinctly show the relative
trade-off’s of using various filter technologies in terms of flow characteristics,
service life, cost and vessel size: |
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