- Buyer's Guide
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).1
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 Table 1.2
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.
Compressors can be classified into two basic categories, reciprocating and rotary.5 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 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 units5, 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.
An equation recommended by Scales for estimating the amount of oil to inject into a cylinder for lubrication is:4
Q = BxSxNx62.8 / 10,000,000
Where: B is the bore size (inches), S is the stroke (inches), N is the rotational speed (rpm) and Q is the usage rate expressed as quarts of oil per 24-hour day.
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.6
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 (Figure 1,7,8,9 Figure 23 and Figure 33).
Figure 1. Screw Compressor
Figure 2. Lobe Compressor
Figure 3. Vane Compressor
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.10 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.10
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.6
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 4.8
Figure 4. Lubricant Flow in a Rotary Screw Compressor
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 2.3 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.
A rotary vane compressor is schematically illustrated in Figure 3.3 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).
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 53, 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.