Natural gas is widely used to heat homes, generate electricity and as a basic material used in the manufacture of many types of chemicals. Natural gas, like petroleum oil, is found in large reservoirs underground and must be extracted from these underground cells and transported to processing plants and then to distribution centers for final delivery to the end user. The gas is moved with the use of many types and sizes of compressors that collect, pressurize and push the gas though the distribution pipes to the various processing centers and points of use. The compressors that move the gas are located in ships and drilling fields, in chemical and process plants, and in the huge maze of pipes that makeup the distribution network, which brings gas to the market in a pure, useable form. This article explains various aspects of gas, gas compressor and compressor lubrication, including compressor lubricants, fluid maintenance and some basic compressor failure analysis guidelines.

Natural gas and petroleum oil formed as a result of the decay of plants and animals that lived on earth millions of years ago. The decaying matter was subsequently trapped in huge pockets called gas reservoirs in rock layers underground. These pockets may contain predominantly gas or they may exist together. It is estimated that the amount of recoverable natural gas within the United States alone is 900 to 1300 trillion cubic feet (Tcf).

The composition of natural gas at the well head is variable and often contains different compositions of volatile hydrocarbons in addition to contaminants including carbon dioxide, hydrogen sulfide and nitrogen. Commercial pipeline natural gas contains predominantly methane and lesser amounts of ethane, propane and sometimes fractional quantities of butane as shown in

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For transportation and storage, natural gas must be compressed to save space. Gas pressures in pipelines used to transport natural gas are typically maintained at 1000 to 1500 psig. To assure that these pressures are maintained, compressing stations are placed approximately 100 miles apart along the pipeline. This application requires compressors and lubricants specifically designed for this use.

Gas Compressors

Compressors can be classified into two basic categories, reciprocating and rotary. Reciprocating compressors are used for compressing natural gases and other process gases when desired pressures are high and gas flow rates are relatively low. They are also used for compressing air.

Reciprocating Compressors

Reciprocating compressors compress gas by physically reducing the volume of gas contained in a cylinder using a piston. As the gas volume is decreased, there is a corresponding increase in pressure. This type of compressor is referred to as a positive displacement type. Reciprocating compressors are typically a once-through process. That is, gas compression and lubricant separation occur in a single pass.

Reciprocating compressors may be further classified as single-acting or double-acting. Single-acting compressors, also classified as automotive compressors or trunk piston units, compress gas on one side of the piston, in one direction. Double-acting compressors compress gas on both sides of the piston.

To consider the lubrication process, it is convenient to divide the parts that need to be lubricated into two categories, cylinder parts and running parts. Cylinder parts include pistons, piston rings, cylinder liners, cylinder packing and valves. All parts associated with the driving end (the crankcase end), crosshead guides, main bearing and wristpin, crankpin and crosshead pin bearings are running parts.

The lubricant is then fed directly to the cylinders and packings using a mechanical pump and lubricator arrangement. Single-acting machines, which are usually open to the crankcase, utilize splash lubrication for cylinder lubrication. Compressor valves are lubricated from the atomized gas-lubricant in the system.

Compared with cylinder part lubrication, the lubrication of running parts is typically much simpler because there is no contact with the gas. The equipment manufacturer specifies the required viscosity grade.

Because gas temperature increases with increasing pressure, if heat is not removed, the lubricant will be exposed to high temperatures and undergo severe decomposition. Therefore, compressor cylinders are equipped with cooling jackets. One of the most important roles of the compressor cylinder lubricant is as a coolant. The coolant is usually water or a water-glycol refrigerant. Although the same lubricant can be used to cool both the cylinder and the running parts, there are many cases where different lubricants are used because the cylinder lubricant is exposed to compressed gas at high temperatures. Therefore, the lubricant should also exhibit thermal and oxidative stability. Table 2 compares compressor operating temperatures.

Rotary Compressors

Rotary compressors are classified as positive displacement or dynamic compressors. A positive displacement compressor utilizes gas volume reduction to increase gas pressure. Examples of this type of compressor include rotary screw, lobe and vane compressors.

The rotary screw compressor illustrated in Figure 1 consists of two intermeshing screws or rotors which trap gas between the rotors and the compressor case. The motor drives the male rotor which in turn drives the female rotor. Both rotors are encased in a housing provided with gas inlet and outlet ports. Gas is drawn through the inlet port into the voids between the rotors. As the rotors move, the volume of trapped gas is successively reduced and compressed by the rotors coming into mesh.

These compressors are available as dry or wet (oil-flooded) screw types. In the dry-screw type, the rotors run inside of a stator without a lubricant (or coolant). The heat of compression is removed outside of the compressor, limiting it to a single-stage operation. In the oil-flooded screw type compressor, the lubricant is injected into the gas, which is trapped inside of the stator. In this case, the lubricant is used for cooling, sealing and lubrication. The gas is removed from the compressed gas-lubricant mixture in a separator. Rotary compressors, such as the screw compressor, continuously recirculate (1 to 8 times per minute) the lubricant-gas mixture to facilitate gas cooling and separation as opposed to reciprocating compressors, which are once-through processes.

A rotary vane compressor is schematically illustrated in Figure 3. Rotary vane compressors consist of a rotor with multiple sliding vanes that are mounted eccentrically in a casing. As the rotor rotates, gas is drawn into areas of increasing volume (A) and discharged as compressed gas from areas of small volume (B). In a rotary screw compressor, the lubricant is injected into the compressor housing. The rotors are exposed to a mixture of the gas and lubricant. In addition to providing a thin film on the rotors to prevent metal-to-metal contact, the lubricant also provides a sealing function to prevent gas recompression, which occurs when high-pressure, hot gas escapes across the seal between the rotors or other meshing surfaces and is compressed again. Recompression causes gas discharge temperatures to exceed the designed range for the unit. This often leads to loss of throughput and poor reliability.

The lubricant also serves as a coolant by removing heat generated during gas compression. For example, for rotary screw air compressors, the air discharge temperature may be 80ºC to 110ºC (180ºF to 230ºF), accelerating oxidation due to turbulent mixing of the hot air and lubricant.

In addition to these functions, the bearings at the inlet and outlet of the compressor must be lubricated. With rotary screw compressors, the lubricant is in contact with the gas being compressed at high temperatures and it experiences high shearing force between the intermeshing rotors. These are demanding use-conditions for the lubricant.

A simplified diagram for lubricant flow in a typical rotary screw compressor is shown in Figure 4. The lubricant and gas mixture from the compressor discharge line goes into a gas/lubricant separator where the compressed gas is separated from the lubricant. After separation, the lubricant is cooled and filtered, then pumped back into the compressor housing and bearings.

A schematic diagram for a rotary lobe compressor is provided in Figure 2. The principle of operation is analogous to the rotary screw compressor, except that with the lobe compressor the mating lobes are not typically lubricated for air service. As the lobe impellers rotate, gas is trapped between the lobe impellers and the compressor case where the gas is pressurized through the rotation of lobes and then discharged. The bearings and timing gears are lubricated using a pressurized lubricating system or sump.

As with reciprocating compressors, lubrication of rotary vane compressors is also a once-through operation. The lubricant is injected into the compressor casing and it exits with the compressed gas and is usually not recirculated. The lubricant provides a thin film between the compressor casing and the sliding vanes, while providing lubrication within the slots in the rotor for the vanes. The sliding motion of the vanes along the surface of the compressor housing requires a lubricant that can withstand the high pressures in the compressor system.

A dynamic compressor, such as the centrifugal compressor shown in Figure 5, operates on a different principle.

Energy from a set of blades rotating at high speed is transferred to a gas, which is then discharged to a diffuser where the gas velocity is reduced, and its kinetic energy is converted to static pressure. One of the advantages of this type of compressor is the potential to handle large volumes of gases.

In a centrifugal compressor, the lubricant and gas do not come into contact with each other, which is a major distinction from reciprocating, rotary screw and rotary vane compressors. The lubricant requirements are simpler and usually a good rust and oxidation-inhibited oil will provide satisfactory lubrication of the bearings, gears and seals.

The choice of a compressor lubricant depends on the type and construction of the compressor, the gas being compressed, the degree of compression and the final outlet temperature. Piston compressors provide the highest gas pressures and are among the most difficult from the standpoint of cylinder and valve lubrication and equipment reliability. However, R&O (rust and oxidation inhibited) oil is often sufficient for the crankcase splash lubrication of a reciprocating compressor.

Rotary compressors with final pressures below 1 Mpa (approximately 145 psi) are less difficult to lubricate. Because of the potential for vane to cylinder or lobe-to-lobe contact, rotary screw and vane compressors require the use of an antiwear (AW) oil. The selection of the proper compressor and application-dependent lubricant with the appropriate physical-chemical properties is vital to a successful process, and will be addressed fully in the second part of this two-part series of gas compressor and compressor lubrication issues.

Natural Gas engines

Unlike gasoline or diesel engines, natural gas engines require somewhat different oil formulations. These engines can reach up to 16,000 horsepower with up to 20 power cylinders and oil reservoir capacities of 1,585 gallons. Their speed can range from 300 to 2,000 rpm.

The quality of the natural gas fuel used can vary widely. It can range from sweet methane to raw sour natural gas containing hydrogen sulfide, carbon dioxide and nitrogen, or even poorer quality digester gas from landfills.

Most of these engines are turbocharged and may be of either two- or four-stroke design. Natural gas engines are most commonly used to operate natural gas compressors, electric generators or cogeneration power plants.

The engine and driven compressor units can be separately joined by a crankshaft or be of the integral type, where the engine and compressor have a common drive shaft.

Natural gas engines also have special requirements that differ from diesel and gasoline engines. This is because the natural gas engine combustion process is affected by the type of fuel.

Most of these engines operate indoors, particularly in warm climates such as the southern United States where higher ambient temperatures affect operation.

The combustion process burns much more cleanly, therefore soot levels are very low. Consequently, high detergent additive levels are not generally necessary.

The process burns methane type fuel, so fuel dilution is not a problem. However, because of higher combustion temperatures, oil viscosity increases can be a serious concern. These higher combustion temperatures cause chemical oxidation and nitration to occur within the oil.

Operating speed is generally constant; therefore, these engines are prone to deposit formation.

Exhaust valve recession and burning are other concerns when operating natural gas engines. Valve recession is the gradual wearing of the valve into the head. It is caused by insufficient lubrication or insufficient ash deposit cushioning the valve seat area.

Frequently, a valve may also suffer damage called guttering, which is a deep-channel cutting across or into the valve seat area. The causes of these conditions are many and varied and can be quite different, depending upon the engine design.

Lubricant formulation (as it relates to ash deposits) and viscosity, operating temperatures, exhaust gas temperatures, natural gas fuel quality, engine design characteristics and air/fuel ratios should all be considered when investigating the root cause of an exhaust valve failure.

The issue of ash deposits in particular is much discussed among operators of natural gas engines. Ash deposits are the residue remaining after the oil is burned during operation. The ash residue is made up of metallic detergent additives, such as calcium, barium and magnesium compounds.

These ash deposits, if adequate, can prevent valve recession. However, if the ash content is too high, the result will be unwanted and harmful deposits. Consequently, lubricant manufacturers and blenders must take great care that the proper selection and quantities of anti-wear and detergent additives are applied when formulating natural gas engine oils. The selection and application also should always take the engine manufacturer’s recommendations into account.

Lubricants for Compressed Natural Gas Engines

Decreasing natural gas prices, incentive programs and technological changes have made fleet operators consider moving more truck and bus fleets from diesel fuel to compressed natural gas (CNG) and liquefied natural gas (LNG) engine technologies. However, there are misconceptions about the lubrication of these engines in the marketplace. Differences in fuel and combustion processes between diesel fuel and CNG/LNG result in different stresses on engine oil.

Diesel Fuel

Emissions control technologies have forced engine oil formulators to change their approach to heavy-duty diesel engine oils. Technologies such as diesel oxidation catalysts (DOCs) and diesel particulate filters (DPFs) have come with limitations on additives available for acid neutralization and wear reduction. Exhaust gas recirculation (EGR) has increased soot-loading in the oil, putting higher demands on engine oil dispersants to effectively prevent the agglomeration of abrasive soot particles and thickening of the oil caused by soot. Increased use of biodiesel and biodiesel blends, and new injection timing technologies have added a new challenge for heavy-duty engine oils. Diesel engine oils are uniquely designed for these new and changing conditions.

Compressed and Liquefied Natural Gas

Unlike diesel fueled engines, engines run on gaseous fuel tend to run much hotter and produce much more complete combustion. While having virtually no soot as a by-product of combustion, high temperatures and high nitrogen oxides production rates create a new set of problems for these engines. These engines place much less demand on dispersants and detergents, but also require much higher anti-oxidant concentrations to maintain and even extend drain intervals over their diesel-fueled counterparts.

CNG and LNG engines also require a carefully balanced sulfated ash level, which is typically lower than that found in diesel engine oils. Because the gaseous fuels are ‘dry’ and provide absolutely no lubricant value, engine oils must provide a soft ash deposit on the exhaust valves as a lubricant to prevent valve recession. Excess ash content can cause piston deposits, especially on the piston crown area, which can harm engine efficiency and cause irregular combustion. Eventually, these piston deposits can result in extreme knock and engine failure. In CNG and LNG engines, both the quantity and the type of ash-producing chemicals must be balanced to prevent high-temperature deposits and provide adequate valve lubrication.

Conclusion

Diesel-fueled engines and CNG/LNG engines place very different demands on engine oils. Diesel-fueled engines require oils with total base number (TBN) or alkaline reserve, and require much more dispersant for increased soot-handling capabilities. CNG and LNG engines require carefully balanced sulfated ash levels for valve lubrication, and require much higher anti-oxidant concentrations to prevent harmful effects of oxidation and nitration. In order to get the best performance out of your engines, you must select the appropriate oil. Using the correct oil for your heavy-duty engines will result in longer drain intervals and better engine protection.

Comparing Gasoline and Diesel Engine Oils

This article is to explore the similarities and differences between gasoline and diesel engine oils to appease your curiosity.

In the broadest sense, gas and diesel engine oils have the same anatomy or makeup. They are formulated from the blending of base oils and additives to achieve a set of desired performance characteristics. From this simple definition, we start to diverge when examining the lubricant’s required performance for each engine type.

Emissions and the Catalytic Converter

A catalytic converter is a housing that contains porous metal filler located between the engine and muffler in the exhaust system. Its role is to convert toxic emissions coming from the engine to stable byproducts before they enter the atmosphere. Some of the byproducts of combustion (lead, zinc and phosphorus) can severely cripple the converter’s ability to perform this job. Therein lies the first major difference between the oils. Diesel engine oils have a higher anti-wear (AW) load in the form of zinc dialkyldithiophosphate (ZDDP). The catalytic converters in diesel systems are designed to be able to deal with this problem, while the gasoline systems are not. This is one of the main reasons you don’t want to use a diesel engine oil in your gasoline engine. If your automobile was built prior to 1975, there is a good chance it does not have a catalytic converter, and thus the above statements do not apply.

Viscosity

Viscosity is the single most important property of a lubricant. When I am working as a consultant and designing a lubrication program, one of the first steps I take is to calculate required viscosities. Getting the right viscosity is of the utmost importance. The selected viscosity needs to be pumpable at the lowest start-up temperature while still protecting the components at in-service temperatures.

Typically, diesel engine oil will have a higher viscosity. If we were to put this higher viscosity in a gasoline engine, several problems might arise. The first is heat generation from internal fluid friction. I’ve covered before how this heat affects the life of an oil. A good rule of thumb is that for every 10 degrees C you increase the temperature, you cut the life in half. The second problem is the low-temperature pumpability of this higher viscosity. During cold starts, the oil may be very thick and difficult for the oil pump to deliver to the vital engine components in the lifter valley. This most certainly will lead to premature wear, as the components will be interacting without the benefit of lubrication.

Additive Levels

Diesel engine oil has more additives per volume. The most prevalent are overbase detergent additives. This additive has several jobs, but the main ones are to neutralize acids and clean. Diesel engines create a great deal more soot and combustion byproducts. Through blow-by, these find their way into the crankcase, forcing the oil to deal with them. When you put this extra additive load in a gasoline engine, the effects can be devastating to performance. The detergent will work as it is designed and try to clean the cylinder walls. This can have an adverse effect on the seal between the rings and liner, resulting in lost compression and efficiency.

So how do you know if an oil has been designed for gasoline or diesel engines? When reading a label, look for the API (American Petroleum Institute) doughnut. In the top section of this doughnut will be a service designation. This designation will either start with an “S” (service or spark ignition) for gasoline engines or a “C” (commercial or compression ignition) for diesel engines.

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A rotary vane compressor is schematically illustrated in Figure 3. Rotary vane compressors consist of a rotor with multiple sliding vanes that are mounted eccentrically in a casing. As the rotor rotates, gas is drawn into areas of increasing volume (A) and discharged as compressed gas from areas of small volume (B). In a rotary screw compressor, the lubricant is injected into the compressor housing. The rotors are exposed to a mixture of the gas and lubricant. In addition to providing a thin film on the rotors to prevent metal-to-metal contact, the lubricant also provides a sealing function to prevent gas recompression, which occurs when high-pressure, hot gas escapes across the seal between the rotors or other meshing surfaces and is compressed again. Recompression causes gas discharge temperatures to exceed the designed range for the unit. This often leads to loss of throughput and poor reliability.

The lubricant also serves as a coolant by removing heat generated during gas compression. For example, for rotary screw air compressors, the air discharge temperature may be 80ºC to 110ºC (180ºF to 230ºF), accelerating oxidation due to turbulent mixing of the hot air and lubricant.

In addition to these functions, the bearings at the inlet and outlet of the compressor must be lubricated. With rotary screw compressors, the lubricant is in contact with the gas being compressed at high temperatures and it experiences high shearing force between the intermeshing rotors. These are demanding use-conditions for the lubricant.

A simplified diagram for lubricant flow in a typical rotary screw compressor is shown in Figure 4. The lubricant and gas mixture from the compressor discharge line goes into a gas/lubricant separator where the compressed gas is separated from the lubricant. After separation, the lubricant is cooled and filtered, then pumped back into the compressor housing and bearings.

A schematic diagram for a rotary lobe compressor is provided in Figure 2. The principle of operation is analogous to the rotary screw compressor, except that with the lobe compressor the mating lobes are not typically lubricated for air service. As the lobe impellers rotate, gas is trapped between the lobe impellers and the compressor case where the gas is pressurized through the rotation of lobes and then discharged. The bearings and timing gears are lubricated using a pressurized lubricating system or sump.

As with reciprocating compressors, lubrication of rotary vane compressors is also a once-through operation. The lubricant is injected into the compressor casing and it exits with the compressed gas and is usually not recirculated. The lubricant provides a thin film between the compressor casing and the sliding vanes, while providing lubrication within the slots in the rotor for the vanes. The sliding motion of the vanes along the surface of the compressor housing requires a lubricant that can withstand the high pressures in the compressor system.

A dynamic compressor, such as the centrifugal compressor shown in Figure 5, operates on a different principle.

Energy from a set of blades rotating at high speed is transferred to a gas, which is then discharged to a diffuser where the gas velocity is reduced, and its kinetic energy is converted to static pressure. One of the advantages of this type of compressor is the potential to handle large volumes of gases.

In a centrifugal compressor, the lubricant and gas do not come into contact with each other, which is a major distinction from reciprocating, rotary screw and rotary vane compressors. The lubricant requirements are simpler and usually a good rust and oxidation-inhibited oil will provide satisfactory lubrication of the bearings, gears and seals.

The choice of a compressor lubricant depends on the type and construction of the compressor, the gas being compressed, the degree of compression and the final outlet temperature. Piston compressors provide the highest gas pressures and are among the most difficult from the standpoint of cylinder and valve lubrication and equipment reliability. However, R&O (rust and oxidation inhibited) oil is often sufficient for the crankcase splash lubrication of a reciprocating compressor.

Rotary compressors with final pressures below 1 Mpa (approximately 145 psi) are less difficult to lubricate. Because of the potential for vane to cylinder or lobe-to-lobe contact, rotary screw and vane compressors require the use of an antiwear (AW) oil. The selection of the proper compressor and application-dependent lubricant with the appropriate physical-chemical properties is vital to a successful process, and will be addressed fully in the second part of this two-part series of gas compressor and compressor lubrication issues.

Natural Gas engines

Unlike gasoline or diesel engines, natural gas engines require somewhat different oil formulations. These engines can reach up to 16,000 horsepower with up to 20 power cylinders and oil reservoir capacities of 1,585 gallons. Their speed can range from 300 to 2,000 rpm.