Compressor Lubrication Best Practices Heinz P. Bloch Tags: compressor lubrication mac hine that elevates the pressure of a compressible process fluid, typically A compressor is a type of machine air, or a host of ot her gases. Dynamic compressors are based on the principle of imparting velocity to a gas stream and then converting this velocity e nergy into pressure energy. In c ontrast, positive displacement compressors confine a certain inlet volume of gas in a given space and subsequently elevate this trapped amount of gas to some higher pressure level. The overwhelming majority of compressors in either the dynamic (axial/centrifugal) or positive displacement (reciprocating and screw type) category incorporate moving components. Nearly all compressors require a form of lubricant to either cool, seal or lubricate internal components. Only static jet compressors (ejectors) (ejec tors) and late 20th- and early 21st-century oil-free m achines with rotors suspended in magnetic or air bearings are exempt from the need for some type of lubrication. This article deals with the lubrication of dynamic compressors (Figure 1 ). Click here to see figure 1.
Key Components Dynamic compressors have a few key components that require a coolant/lubricant: gears, bearings and seals. To date, the majority of dynamic compressors continue to utilize oil film-lubricated seals, as illustrated in Figures 2d, 3a and 3b. Only labyrinth seals (Figures 2 a and 2b) or gas-lubricated seals (Figure 3c) operate without a liquid film separating the faces. On the more conventional liquidlubricated seals, the bearing and sealing lubricant are ofte n the same.
Figure 2a
Figure 2b
Figure 2c
Figure 2d
Figure 2. Traditional Compressor C ompressor Seal Designs (Dresser-Roots Co., Connersville, IN)
Figure 3a
Figure 3b
Figure 3c Figure 3. Modern Compressor Seal Configurations (Demag-DeLaval, Trenton, NJ) Lubricating Oil System Operation The lube oil system (Figure 4) supplies oil to the compressor and driver bearings and to the ge ars and couplings. The lube oil is drawn from the reservoir by the pumps and is fed under pressure through coolers and filters to the bearings. Upon leaving the bearings, the oil drains back to the reservoir. Click here to see Figure 4
The reservoir is designed to permit c irculation of its entire fluid volume between eight to 12 times per hour. Oil reservoirs often have thermal sensors for monitoring tem perature levels during start-up and constant operations. Reservoirs also often have oil temperature co ntrols that provide for preheating during cold start-up conditions and cooling to prevent overheating during peak operating cycles. The rese rvoir may be pressurized or vented. When in operation, the compressor lubricant oil is normally circ ulated by the main oil pump. An auxiliary pump serves as a standby. These two pumps generally have different types of drive or power sources.
When both are driven electrically, they are connected to separate supply feeders. On compressors with step-up gearboxes, the main oil pump may be driven mechanically from the gearbox, and the auxiliary pump operates during the start-up and run-down phases of t he compressor train. Relief valves protect both pumps from the effects of excessively high pressures. Check-valves prevent reverse flow o f oil through the stationary pump. Heat generated by friction in the bearings is transferred to the cooling medium in the oil coolers. Aircooled oil coolers may be employed as an alternative to water-cooled oil coolers. The former have long been used in regions where water is in short supply. A pressure-regulating valve is controlled by t he pressure downstream of the filters and maintains constant oil pressure by regulating the quantity of bypassed oil. A pressure switch activates the auxiliary oil pump. If t he oil pressure falls below a preset limit, a second pressure switch shuts down the compressor train. Filters c lean the lube oil before it reaches t he lubrication points and a differential pressure gauge monitors the deg ree of fouling (flow restriction) of the filters. The flow of oil to each bearing is regulated individually by orifices, particularly important for lubrication points requiring different pressures. Lube oil for the driver and other mechanical components is taken from branch lines. For instance, when a hydraulic shaft position indicator is used, it is supplied with oil from the lube oil system. Temperatures and pressures are measured at all important locations in the system, including temperatures from oil sumps, return lines from bearings, gears and other mechanical components. Temperatures and pressures are often recorded on the suction and discharge sides of each c ompression stage to offer the operator a sense of the health of the system. The readings can be taken locally or transmitted to a monitoring station.
Compressor Seals In general, the mechanical contact or oil face seal (Figure 3a) employs a spring-loaded stationary carbon ring in sliding contact with a rotating ring manufactured from high-quality material with a special finish. This type of seal is also effective w hen the compressor is at standstill and the oil pumps have been shut down. The main components of oil bushing seals (Figure 3b) are two stationary, but radially free-to-move (floating ring) breakdown bushings with small diametral clearances opposite a shaft sleeve (Figure 3 b). The floating ring clearance controls the flow of the seal liquid cooling the seal. Floating carbon ring seals (not shown) successfully combine some of the best features of all of the above. They, too, require seal face lubrication.
Seal Oil System Operations The seal oil, or seal liquid system (Figure 5) supplies the mechanical contact and floating ring seals with an adequate flow of seal liquid at all times, correctly ensuring proper function. An effective seal is provided at the settle-out pressure when the compressor is not running. The seal oil system m ay be combined with the lube oil system if the gas does not adversely affect the lubricating qualities of the oil, or provided the oil made unserviceable by the gas does not return into the oil system.
Click here to see figure 5
There are two methods of combining lube oil and seal oil systems: booster or combined systems. In the booster system, the oil pressure is raised to the pressure required for lubrication purposes and then part of it is raised further to the pressure needed for sealing. Alternatively, in the combined system, all the oil is initially raised to the required pressure and flow, then reduced to system component requirements.
The hardware and operation of each of these types of oil systems are identical or nearly identical. Mechanical face seals and floating ring seals are supplied with seal o il at a defined differential pressure above the reference gas pressure (pressure within the inner seal drain). The flow of seal oil is regulated by a differential pressure-regulating valve, which changes the pressure of the seal oil relative to changes in system gas pressure or, as shown in Figure 5, by a level-control valve that maintains a constant level in the overhead tank. The oil in the overhead tank is in contact with the reference gas pressure via a separate line, with a static head providing the required pressure differential. In addition, the oil in the overhead tank compensates for pressure fluctuations and serves as a r undown supply if pressure is lost. If the level in the tank falls excessively, a level sw itch shuts down the compressor. A moderate oil temperature is maintained by a constant flow of oil through the overhead tank. For the mechanical contact seal system, a regulating valve maintains the reference gas and the seal oil at a constant differential pressure. As the name indicates, the mechanical contact seal serves as a mechanical standstill seal when the compressor plant is shut down. The seal oil is split into two streams in the compressor seals. Most of the flow returns under gravity to the reservoir. A small quantity passes through the inner seal ring to the inner drain, where it is exposed to the gas pressure. This oil, mixed with the buffer gas, flows to the separator system, which consists of a separator and a condensate trap on each side. The separated gas flows to either the flare stack or to the suction side of the compressor while the oil flows into a tank for further degassing. If oil is used as sealing liquid and can be used again, degassing is accelerated by heating or by air or nitrogen sparging. Sparging units perform on-stream purification of oil which can kee p lubricants serviceable for long time periods. Only if the oil becomes unusable is it led away for separate treatment or disposal. The quantity of oil passing through the inner drain in a modern centrifugal compressors is small and ranges from 5 to 50 liters per day on new machines.
Compressor Lubricants The overwhelming majority of compressors are best serve d by premium-grade turbine oils with ISO viscosity grades of 32 or 46. However, there are many different types of compressors and each manufacturer is likely to recommend lubricants that have been used on a test stand and at contro lled user facilities. Premium-grade ISO VG 32 turbine oils are used more often than the heavier viscosity grades. The typical viscosity index is 97, with a pour point around -37ºC (-35ºF). Oxidation stability (per ASTM D943) should exceed 5,000 hours and the flash point (per ASTM D92, COC) should be 206ºC, or 403ºF. These lubricants must provide the following:
Long life without need for changeout
Prevention of acidity, sludge, deposit formation
Excellent protection against rust and corrosion, even during shutdown
Good demulsibility to shed water that enters the lubrication system
Easy filterability without additive depletion
Good foam control
It is not uncommon to operate these systems for many years on the initial fill of lubricant, in some cases beyond 30 years. These long-term lifecycles are associated with premium-grade product selection, large sumps, reasonably good contamination control and the occasional top- off “sweetening” effect on the oil in use. Extended lifecycles on turbine, turbo-compressor and other R &O type oils used in these applications are also facilitated by the relatively simple additive structure of the product, which minimizes kinds of complications associated with complex additive systems like those found in EP gear lubricants. Editor’s Note
Condensed, by permission, from ISBN 0-88173-296-6, Bloch, Heinz P. Practical Lubrication for Industrial Facilities. Lilburn, Ga: The Fairmont Press, 2000.
Managing Lubricant Viscosity to Maintain Compressor Health
Robert Kasameyer Tags: compressor lubrication, viscosity If you’re running one of the approximately 140 working re fineries in the United States, the last thing you need is an unplanned shutdown. But a production standstill is exactly what is at risk if you don’t keep an
eye on the viscosity of the lubricating oil used in any of the rotary compressors in the plant, with the highest risk of these being the gas compressors. One minute all processes are up and running, and the next there’s a bearing failure and production stops. It’s not just the cost of lost production either.
A compressor failure in a single part of the refinery can cost tens of thousands of dollars a day in lost revenue, with similar amounts to rebuild a compressor, and hundreds of thousands of dollars for a replacement. There’s also the cost of maintaining spares. Clearly managing lubricant viscosity is critical to maintaining compressor health, but it is a common practice to monitor lubricant viscosity in each major compressor once a month by sending a sample to a lab for testing. For compressors where lubricant
comes in contact with methane and other light hyd rocarbon gases, the lubricant’s viscosity can break down much more quickly, increasing the risk of failure. Through hard luck, refiners also have found that real-time temperature monitoring is inadequate to monitor lubricant viscosity. A major Gulf Coast refinery claims it has solved the problem by moving to real -time monitoring of lube oil viscosity in critical compressors. “We recognized that in-line viscometers are the best way to know what is happening to the lube oil in our large screw compressors,” says the plant manager. “Further, we have found in-line lubrication
viscosity monitoring offers a cost- effective way to keep track of compressor health.” The true measure of the health of a lubricant’s viscosity can only be gauged when measured in its
natural position with gas vapors dissolved in the lubricant. In addition, monitoring lubricant temperature isn’t sufficient to protect compressor bearings, e specially in applications where process starts and stops can occur. What’s needed is in-line viscosity monitoring to help provide plant operators with real-time data on lubricant viscosity. There is a solution for refinery managers working to keep plants online and
producing. New, inexpensive and rugged in-line viscometers are able to monitor real-time changes in lubricant viscosity, offering a cost-effective way to keep track of compressor health in real time.
Refineries and Compressors Rotary compressors are used throughout oil re fineries in applications ranging from vapor recovery to gas-processing operations. Screw and scroll compressors make up a significant portion of this equipment. Screw compressors use two reciprocal screws to compress gases. Gas is fed into the compressor by suction and moved through the threads by the rotating screws. Compression takes place as the clearance between the threads decreases, forcing the compressed gas to exit at the end of the screws. Scroll compressors, often known as spiral compressors, use two interleaved spiral vanes to move and compress fluids and gases. Typically f ound in intermediate and end-product applications, scroll compressors are valued for their re liability and smooth operation.
The Importance of Lubricant Viscosity In both types of compressors, lube oil is used to seal the compressor from gas leaks, lubricate moving parts and manage temperature during operation. The condition of lubricant oil is a critical factor in extending a compressor’s bearing life and overall re liability. Monitoring and managing lubricant viscosity can prevent costly breakdowns due to bearing failure. Viscosity also plays a role in energy efficiency, as demand for more efficient compressors is driving the use of lower-viscosity lubricants. A range of lube oils, typically synthetic in composition, is available for use in compressors. Water resistance, thermal stability, long life, resistance to oxidation and resistance to absorption of process gases are all important characteristics. While the goal is a lubricant with a long and useful life, harsh environments, contaminants and even humid ity in the refinery’s external environment can greatly reduce lube oil’s useable lifespan.
Monitoring lube oil viscosity is the best way to prevent bearing wear and compressor failure. While some plants may monitor as infrequently as once a month, rapid changes in viscosity occur, and the results can be severe.
Changes in Viscosity and Consequent Risks Compressor lube oils are formulated to work w ell and remain stable at high temperatures and pressures. Hydro-treated mineral oils are used for their low gas solubility (1 to 5 percent). Synthetic compressor lubricants are used depending on the process and how much gas dilution is present. PAO (Polyalphaolefin) oils, for example, have excellent water and oxidation resistance. PAG (Polyalkaline Glycol) oils, which do not readily absorb gases, are used in applications where process gases are compressed. Many factors can affect lube oil viscosity. These include oxidation, dilution, contamination, bubbles and temperature changes. Oxidation occurs when churning lube oil foams, exposing more oil to surface air and causing oxidation that lowers viscosity and threatens useful lubricant life. Dilution is the result when lubricant oil is diluted with gas such as methane, dropping viscosity. Bubbles form as foaming oil churns against the screws or vanes of the compressor, instantly dropping the viscosity of the oil. In contamination, vapors from hydrocarbons being processed can mix with lube oil. This light hydrocarbon and methane contamination – sometimes called “a witches’ brew” – makes measuring viscosity challenging. Significant changes in temperature can occur – typically at start-up – that affect the viscosity of the underlying lube oil as well as any contaminants, further aggravating the situation. A range of compressor failures can result. Bearings, both rotary and thrust, can fail, w hich in turn cause wear on the rotor assembly. Replacing bearings is less costly than a total rebuild or replacement. Either way, the plant faces downtime. The unpredictability of viscosity changes means monthly checks are not enough to prevent bearing failure and subsequent plant downtime. Some compressor customers are designing in-line viscometers into compressors to monitor real-time viscosity changes that happen betwe en standard oil lab analyses, viewing this “preventative” approach as an ideal way to ensure bearing life and minimize the costs associated with unscheduled downtime.
Process Viscometer Approaches Not all process viscometers are created equal. Several instruments employ an innovative sensor technology that uses an oscillating piston and electromagnetic sensors. Other process viscometer technology approaches include falling piston, falling sphere, glass-capillary, U-tube and vibration designs. In all cases, plant managers should look for c ertain characteristics for in-line lubricant viscosity measurement, such as menu-driven electronic controls, self-cleaning sensors, built-in temperature
detection, multiple output signals, automatic viscosity control, data logging, quick-change memory settings, security and alerts. Menu-driven electronic controls can be powerful and easy to use , while a self-cleaning sensor uses the in-line fluid to clean the sensor as it is taking measurements to reduce unscheduled maintenance. With built-in temperature detection, the sensor should show temperature as an analog reading. For automatic viscosity control, look for a sensor that is pre-set but reconfigurable. The sensor should be able to “learn” how much control is needed for each fluid setting. Security and alerts are designed to prevent unauthorized changes and sound an alarm when set points are reached so operators can take action quickly. With multiple output signals, the sensors should display temperature and temperature-compensated viscosity readings. For process lines that run more than one fluid, quick-change memory settings simplify the process of changing settings. In data logging, the date and time code should be automatically logged, creating an audit trail and simplifying performance and quality-trend measurement.
About the Author Robert Kasameyer is the president and CEO of Cambridge Viscosity Inc., a global leader in fluid viscosity measurement. The company’s major applications include life sciences and pharmaceuticals as well as oil and gas exploration, oil analysis, chemical processing and coating. Kasameyer holds a BSME from Tufts University and an MBA from Harvard University. Tips for Sampling Oil from Compressors
Noria Corporation Tags: compressor lubrication
"Can you describe the best sampling point for a wettype screw compressor and the accepted oil cleanliness for this type of machine?"
There are three main objectives for good oil analysis: maximizing data density, minimizing data disturbance and sampling at the proper frequency. Regardless of t he machine type, these three objectives must be met to receive representative information from your equipment and the oil. Compressors can be challenging machines for lubricants, as there are often high temperatures, high pressures and many contaminants intermingling with the oil. One of the biggest factor s with compressor lubrication is the gas that is being c ompressed. In a wet screw compressor, the oil is flooding the compression chamber, so any gas being compressed will mix with the oil. Therefore, you need to ensure that this oil can handle the gas and m aintain its lubricating properties. The majority of these systems are connected to a circulating system. This means that the lubricant flows through the machine to filters, separators, coolers and perhaps other condition-control devices installed in the circuit. This provides a variety of locations from which to pull a representative sample. If you can find an elbow on the main return line prior to the oil draining into the reservoir, this could serve as a great primary sampling location, as it would provide a snapshot of the entire system. Many of these circulating systems lubricate more t han just the compressor, including the motor, an associated gearbox and sometimes even different sections o f the compressor (bearings, timing gears, screws, etc.). In these cases, a single sampling point isn’t adequate to pinpoint any alarming issues. With this type of configuration, it is a good practice to install secondary sampling ports after each lubricated component to help identify any problems being seen at the primary port. Of course, different systems can handle different amounts of co ntaminants. Your fluid cleanliness targets are great key performance indicators to track in order to ensure that the compressor will have a long service life. Set targets based on the criticality of the e quipment. While many compressors will run well at an ISO code of 17/14/11, if the compressor is highly critical, the target may need to be reduced to 16/12/10. Remember, it's always best to balance the manufacturer's recommendations with your own reliability initiatives when setting targets. In a perfect world, the oil would be as clean as possible to make sure you aren't inducing wear due to fluid contamination.
Natural Gas Compressors and Their Lubrication
G.E. Totten, G.E. Totten & Associates LLC Roland J. Bishop,Dow Chemical Company Tags: compressor lubrication, oil oxidation
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 petro leum 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 m ay 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 we ll 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 Click Here To See Tables 1 and 2.
For transportation and storage, natural gas must be compressed to save space. Gas pre ssures 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, r eciprocating and rotary.5Reciprocating compressors are used for compressing natural g ases 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 decrease d, there is a corresponding increase in pressure. This type of compressor is referre d 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 5, compress gas on one side of the piston, in one direction. Double-acting compressors compress g as 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 r unning 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 wr istpin, crankpin and crosshead pin bearings are running parts. An equation recommended by Scales for estimating the amount of oil to inject into a c ylinder 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 t he cylinders and packings using a mechanical pump and lubricator arrangement. Single-acting machines, which are usually open to t he 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 r equired 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 ar e many cases where different lubricants are used because the cylinder lubricant is exposed to compressed gas at high t emperatures. Therefore, the lubricant should also exhibit thermal and oxidative stability. Table 2 compares compressor operating temperatures.6
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 (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 screw s 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 o utlet ports. Gas is drawn through the inlet port into the voids between the rotors. As t he 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 gaslubricant 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 t he 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 t hroughput 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 t he lobe impellers rotate, gas is tr apped 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 ecce ntrically 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 t he compressor housing requires a lubricant that can withstand the high pressures in the compressor system. A dynamic compressor, such as the c entrifugal compressor shown in Figure 53, operates on a different principle. Click Here to See Figure 5.
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 r educed, 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 w ith each other, which is a major distinction from reciprocating, rotary screw and ro tary 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 recipro cating compressor. Rotary compressors with final pressures below 1 Mpa (approximately 145 psi) are less difficult to lubricate. Because of the potential for v ane to cylinder or lobe-to-lobe contact, rotary sc rew and vane compressors require the use of an antiwear (AW) oil. The selection of the proper c ompressor and application-dependent lubricant with the appropriate physical-chemical properties is vital to a successful process, and will be addressed fully in the sec ond part of this two-part series of gas compressor and compressor lubrication issues.
References 1. Estimate obtained from the “Natural Gas Week” 2. “Unit Course 2: For Natural Gas Compressors.” Worthington Compression. Corpus Christi, TX. 3. Wills, J. (1980). “Chapter 14 - Compressors.” Lubrication Fundamentals. Marcel Dekker Inc., New York, NY, p. 365-394. 4. “Unit Course 1 - For Natural Gas Compressors - An Introduction to the Basic Function and Components of a Gas Compressor Package.” Weatherford Compression. Corpus Christi, TX. 5. Scales, W. (1997). “Chapter 19 - Air Compressor Lubrication.” Tribology Data Handbook, Ed. E.R. Booser. CRC Press, Boca Raton, FL, p. 242-247. 6. Cohen, S. (1987). “Development of a Synthetic Compressor Oil Based on Two-Stage Hydrotreated Petroleum Basestocks.” Lubrication Engineering, Vol. 44, No. 3, p. 230-238. 7. Short, G. (1983). “Development of Synthetic Lubricants for Exte nded Life in Rotary-Screw Compressors.” Lubrication Engineering, Vol. 40, No. 8, p. 463 -470. 8. Miller, J. (1989). “Synthetic and HVI Compressor Lubricants.” J . Synth. Lubrication Engineering, Vol. 6, No. 2, p. 107-122. 9. Tolfa, J. (1990). “Synthetic Lubricants Suitable for Use in Process and Hydrocarbon Gas Compressors.” Lubrication Engineering, Vol. 47, No. 4, p. 289 -295. 10. Kist, K., and Doperalski, E. (1979). “Brief Introduction to the Screw Compressor.” AIChE 86th National Meeting, Paper 68E.