Natural Gas Filtration Applications

Natural gas is an abundant clean-burning fuel that provides an important part of the overall energy usage in the United States.  Gas (CH₄), also known as methane, undergoes a number of processes as it is transported from wellhead to end-user, and many of these steps require filtration of the product.  Impurities such as pipe scale, iron sulfide, hydrates, water, liquid hydrocarbons, sulfur products and carbon dioxide must be removed before it is deemed “pipeline quality”.

Gas-fired engine-compressor sets “gather” the natural gas from the wellheads, and pump it to a gas processing plant to make it marketable.

Notice the spider-webs of pipelines in the Gulf area, in western Texas, in the panhandle of Oklahoma and in western Pennsylvania.  Several pipelines come in from Canada.  Most of the gas is pumped to the upper Midwest and industrial sectors of the Northeast.

The natural gas is pumped in large underground pipelines, often 24” to 36” in diameter, at pressures of up to 3000 psig, traveling about 60 feet per second (approximately 40 miles per hour).

Most of the motive power comes from large stationary reciprocating engines that were built and installed in the latter half of the 20^th Century.  The engines are driven by the same natural gas that they are pumping.  Many of these engine manufacturers are no longer in business, but their engines continue to run.  In fact, with over 95% of the installed fleet still in operation, it is impossible to measure the half-life of these engines!

“Integral” Reciprocating Engine-Compressor

In the picture above, the horizontal cylinders are the compressors and the vertical cylinders drive the engine.  These engines use spark plugs and require shining a timing light on the flywheel to set the spark, much like the automobile engines of their day.  Note the large oil filter in the foreground of this photograph.

Many of the more recently-installed compressor stations are using turbine engines for their power.  Turbines have better fuel efficiency, but lack the turndown capacity of a bank of recip’s.

The oil filters on these engines are primarily either sock-type (depth filters) or pleated-paper-in-a-can (paper filters).  Again, Filtration Systems, Inc. manufactures both types.  Sock-type oil filters are the preferred technology for these old recip’s, because they hold an incredible amount of semi-solid contaminants (such as sludge and varnish), which would quickly blind-off a surface-type filter.  Usually, these engines will run for thousands of hours before the oil filters need changing.  Filtration Systems, Inc. also uses a special wood fiber in its sock filters that actually absorbs water and acids out of the oil that would otherwise attack the additive package in the lube oil. 

Because gas-fired stationary engines run continuously, even trace amounts of sulfur in the fuel gas can quickly acidify the oil, caused by combustion byproducts that “blow-by” the rings and build up in the oil sump.  Filtration Systems’ filters actually extend oil-drain intervals, often allowing these engines to run a whole heating season (4,000 hours) between shut-down and change-out.

Wellhead gas requires purification to make it “pipeline quality” for market.  These criteria include:

1,010 BTU +5%

<7 lbs. water vapor (H₂O) per 1000 Mcf

<1% Oxygen (O₂)

<4% Carbon Dioxide (CO₂)

<3% Nitrogen (N₂)

<20 grains total Sulfur (S) (mg/m³)

<1 grain Hydrogen Sulfide (H₂S)

Carbon dioxide and hydrogen sulfide are particularly corrosive to pipes and equipment.  Sour gas is “sweetened” in gas plants, which use amine as a solvent to strip these acidic chemicals out of the gas.

These amine units employ a reboiler in a closed-loop, once known as the “Giraler Process”.  Sour gas enters the bottom of the contactor and “lean” amine cascades down from the top of the tower through trays or packing. The amine strips the CO₂ &/or H₂S out of the gas, and sweet gas exits the top of the tower.

The amine exiting the bottom of the contactor is “rich” in acids, which are flared off as acid gas after separation in the reboiler tower.

Natural gas is often put into underground storage during the spring and summer months.  Storage fields can be salt caverns, aquifers or depleted oil fields.  Perfectly good pipeline-quality gas is pumped down-hole, and when it is withdrawn, it requires processing and filtration all over again!

These photographs were taken at an amine plant in southern Illinois.

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