Wellhead And Sour Gas Can Cause Shortened Service Life For Gas Engines
All Caterpillar-Built Gas Engines
Caterpillar-built gas engines are being used more and more in gas compression units for petroleum applications. As a result, both users and dealers desire information on approved fuels for these engines. This information is necessary because many of the compressor engines burn wellhead gas, and the components of wellhead gas are different from location to location. Some of the components can cause shortened engine life.
Dry Natural Gas
Dry natural gas, also known as commercial pipeline natural gas, is a mixture of methane, ethane, propane and butane. The contents of propane and butane are less than 5 percent and 1 percent respectively.
The reference to “dry” is made because the gas has no liquid propane or liquid butane.
The heat content of dry natural gas may change from source to source. Caterpillar-built gas engines are adjusted at the factory with dry natural gas that has a low heat value of 33.72 kJ/I (905 Btu/ft3).
Field gas, also known as wellhead gas, is the gas available at the wellhead in a gas field.
The contents of wellhead gases change from location to location. As a result, the gas from one field may be acceptable for a Caterpillar-built engine while the gas from a different field may not. For this reason, a gas analysis is necessary to find if the fuel should be used.
Wellhead gases which have a minimum of 90 percent methane and ethane and have a remainder no heavier than butane can be used in low compression engines. However, many wellhead gases have some heavier hydrocarbons such as pentane, isobutane and other “gasolines”. These heavier hydrocarbons cause knock and other mixture problems, and they can have negative effects on an engine’s performance and service life.
Sour gas is gas that contains sulfur compounds such as hydrogen sulfide (H2S). Gases that have no sulfur compounds are known as sweet gases.
The use of gases that have hydrogen sulfide can damage the engine. Water vapor and sulfur oxides, which are products of combustion, chemically unite to make sulfurous and sulfuric acids. These acids destroy internal engine components such as oil coolers, valve guides, piston pin bushings, piston rings and cylinder liners. History has shown that oil coolers often are the first components affected by the acids.
Before sour gas is used to fuel an engine, the gas should be analyzed. If it has more than 0.1 percent by volume of hydrogen sulfide, the gas should be cleaned (scrubbed) to decrease the hydrogen sulfide content to below 0.1 percent.
When sour gas is used, steps should be taken to decrease the effects of sulfur compounds. Caterpillar’s recommendations are:
1. Keep the engine outlet coolant temperature between 88° and 93°C (190° to 200°F). A temperature increase of 8.3°C (15°F) across the engine is acceptable, but an increase of 5.6°C (10°F) is best. Lower jacket water temperatures permit water vapor and hydrogen sulfide to condense on the cylinder liners and make acids. Higher temperatures will decrease this condensation.
Generally, engines equipped with inlet-controlled cooling systems will keep the coolant in the correct temperature range. Engines equipped with outlet-controlled cooling systems may need added external controls to keep the coolant temperature within the acceptable range.
A set of thermostatically-controlled shutters on the engine coolant radiator or heat exchanger is the most effective device for controlling the engine temperature. Shutters should be considered for installations designed to operate on sour gas. Some field gathering units which are subjected to overcooling because of cold ambient temperatures, wind, rain, etc., may require an enclosure or a building for adequate temperature control.
A second method for increasing the coolant temperature is to install a temperature regulator with a higher setting. Such regulators are available from Caterpillar and from other suppliers. A list of regulators and sources is available from Caterpillar.
2. Keep the engine oil temperature high enough to prevent condensation of water vapor in the oil. Generally, if the coolant temperature is kept above 88°C (190°F), vapor will not condense in the oil.
3. A CD grade oil which has a sulfated ash content of less than 1 percent should be used in the engine. The CD oil has a higher Total Base Number (TBN), or alkalinity reserve, to neutralize the sulfurous and sulfuric acids better than the oils generally used in natural gas engines.
4. The oil should be regularly analyzed for its Total Acid Number (TAN or the acidity of the oil), nitration, oxidation, pentane insolubles and viscosity increase. The analysis will find early indications of engine problems. Also, it will make sure that the change intervals are not extended beyond the oil’s ability to provide protection for the engine. The amount of make-up oil added to the engine also can affect the oil’s condition. The oil should carefully be monitored.
5. When possible, the engine should be started and brought to operating temperature on sweet gas, then switched to sour gas. To shut off the engine, switch to sweet gas and run the engine for 10 minutes at full load. Then, remove the load and run the engine at half the rated speed for five minutes. Finally, slow the engine to low idle for 30 seconds and shut it off. This procedure will decrease condensation at lower engine temperatures during start-up and shut down.
Propane is a heavier-than-air gas that is moved in a liquid state to a job location and is transformed into a vapor at the location. Since some states will not permit the use of liquid propane inside a building, Caterpillar’s recommendation is to check local building codes before final plans are made for a propane system.
Propane which is 95 percent pure with the remainder no heavier than butane and meets HD-5 specifications can be used in all naturally-aspirated or turbocharged and aftercooled engines. However, all high compression configurations must be used only in no-lug applications.
Commercial bottled fuels such as liquid propane and liquid bottled gas may not meet HD-5 specifications.
Propane-butane is a commercial mixture in which the butane content is more than 5 percent by volume. This mixture should be used only in naturally-aspirated engines with low compression ratios.
Propane-air mixtures, which have the same Btu contents as natural gas, are generally used as a standby fuel or as peaking fuels for natural gas systems. The same pressure regulating systems can be used for both propane-air and natural gas. But, the timing must be adjusted for propane because the ignition qualities of propane are present.
Other mixtures some times used are field gas and wellhead gas which have 5 percent or less butane and have less than 1 percent heavier hydrocarbons.
Other sources of information on fuels and lubricating oils which are available from Caterpillar are:
Petroleum Engine Application And Installation Guide, LEBW2177
EMA Lubricating Oils Data Book, SEBU5939
Fuel Gases For Gas Engines, LEO21242-01
Gas Engine Application And Installation Guide, LEBH2363
Caterpillar’s recommendation is to use these sources when specifications are written for applications which used wellhead or sour gas. Additional information is available from Caterpillar.Categories: Service Magazine | Leave a comment