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Lubricant Requirements, Properties and Maintenance for Natural Gas CompressorsyG.E. Totten, G.E. Totten & Associates LLC Roland J. Bishop, Dow Chemical Company Tags: compressor lubrication, synthetic lubricants

Compressors are engineered in a variety of types and configurations. The final design selectiondepends on numerous factors. Gas type and required pressure are significant factors on both compressor and compressor lubricant selection. As temperatures and pressures increase, the stress on the lubricant increases as well. The first part of this two-part series appeared in the May-June issue of ML, and may be accessed at www.machinerylubrication.com. It addressed compressor types, common operating conditions and lubricant volume guidelines. In this second part, the authors address issues that pertain more specifically to the lubricant, with some consideration of natural gas transmission compressor and lubricant issues.

Compressor LubricantsThe 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 lubrication. Rotary compressors with final pressures below 1 mega Pascal (MPa), approximately 145 psi, are less difficult to lubricate. Rotary vane compressors require the use of an antiwear oil1 because an R&O oil is often insufficient for the crankcase splash lubrication of a reciprocating compressor. The selection of the proper compressor and application-dependent lubricant with the appropriate physical-chemical properties is vital to a successful process.2 ISO 6743 - Part 3A provides a classification procedure for compressor lubricants based on the type of equipment and operating conditions. Some of the most commonly reported oil-related service problems with compressors include:3 y y y y y y y y y y y y y y Increase in oil viscosity and total acidity Copper corrosion (oil turns green) Sludge deposits Substantial oil entrainment in discharge gas (air) due to decreased efficiency of the demister element Oil strainer plugging Bearing failure. For successful operation, compressor oil must exhibit the following properties:4,5,6 Oxidation resistance A wide operating temperature range (high flash point, low pour point, high viscosity index) Low volatility Superior antiwear performance Good demulsibility Adequate corrosion resistance Thermal/oxidative stability Rust and corrosion inhibition

y y y y y y

Hydrolytically stable Material compatibility Nonsludging performance Minimal oil loss to the system Nonfoaming behavior Nontoxic.

The successful development of a compressor lubricant will depend on how well the oil meets these technical requirements.

Gas Solubility in the LubricantThe solubility of natural gas and other hydrocarbons is much higher in petroleum oils and polyalphaolefin (PAO) synthetics compared to other commonly used synthetic base stocks such as diesters and polyalkylene glycols (PAG). That is expected because both hydrocarbon gas and petroleum-based oils are similar molecules consisting primarily of C-H bonds unlike diesters and PAGs, which are relatively polar. In fact, in a typical PAG molecule, every third atom in the polymer backbone is an oxygen atom, which makes it quite polar. Therefore, hydrocarbons are less soluble in PAGs. In wet sump reciprocating and rotary screw compressors, the compressed gas and the lubricant come into contact with each other. Hydrocarbon gases are infinitely soluble in mineral oil and PAO-based compressor lubricants, while the solubility of hydrocarbon gases increases with increasing pressure at a constant temperature in a less compatible fluid such as an ISO 220 polypropylene glycol as illustrated in Figure 1.7

Conversely, increasing the temperature at a constant pressure will result in lower gas solubility.7 Because increasing gas solubility decreases viscosity, at some point the viscosity reduction of the compressor lubricant may be too much, and lubrication failure may result because of loss of hydrodynamic lubrication, Figure 2.2

The solubility of various gases in lubricants has been measured and reported.5,7,8 The solubility was measured in a fixed volume apparatus. A known amount of gas and lubricant was allowed to reach equilibrium at a given temperature. Gas solubility was calculated using the gas laws. The lubricant was stirred to facilitate equilibrium. Solubility of methane at pressures up to 5000 psig is compared at 50C for three lubricants: PAG, PAO and petroleum oil in Figure 3.

The methane gas solubility in PAG is roughly half that for a PAO and petroleum oil, and that solubility was nearly as high in the PAO as in the petroleum oil. Gas solubility exhibits a significant effect on lubricant viscosity. The greater the solubility of the gas in the oil, the greater the viscosity loss (viscosity dilution). A lubricant viscosity dilution chart is shown in Figure 4 for methane at 50C.

Similar gas solubility comparisons for nitrogen and ethylene are provided in Figures 5 and 6.8

Lubricant Solubility in the GasThe lubricant solubility in the compressed gas should also be minimized to reduce carryover by absorption of the lubricant in the gas. Matthews, using a constant pressure flow through a load (gravimetric) cell 5 evaluated the absorption of the lubricant in natural gas. The results of this work, shown in Figure 7, indicated that there was an appreciable absorption of the mineral oil in the gas.

Compared to the PAG, the lubricant showed no appreciable loss.

Fluid Analysis ProceduresTable 1 shows a list of testing procedures that are commonly used to evaluate compressor lubricants. Click here to see Table 1.

In this article, the basic principles of natural gas composition and its compression have been discussed. An overview of various compressor designs used for natural gas, their lubrication and potential lubricant-related problems have been provided. Finally, various tests that have been reported for use with compressor lubrication have been summarized. This information offers a comprehensive overview of natural gas compressor lubrication and fluid maintenance.

References1. 2. 3.

4. 5. 6. 7. 8. 9. 10.

Arbocus, G. (1977). Synthetic Compressor Lubricants - State of the Art. Lubrication Engineering, Vol. 34, No. 7, p. 372-374. Patzau, S. and Szchawnicka, E. (1989). Oils for Air and Technical Gas Compressors. Trybologia, Vol. 20, No. 4, p. 18-21. Sugiura, K., Miyagawa, T. and Nakano, H. (1982). Laboratory Evaluation and Field Performance of Oil-Flooded Rotary Compressor Oils. Lubrication Engineering, Vol. 38, No. 8, p. 510-518. Short, G. (1983). Development of Synthetic Lubricants for Extended Life in Rotary-Screw Compressors. Lubrication Engineering, Vol. 40, No. 8, p. 463-470. Matthews, P. (1990). Lubrication of Reciprocating Compressors. J. Synth. Lub., Vol. 6, No. 4, p. 292-317. Van Ormer, H. (February 1987). Trim Compressed Air Cost with Synthetic Lubricants. Power, p. 43-45. Tolfa, J. (1990). Synthetic Lubricants Suitable for Use in Process and Hydrocarbon Gas Compressors. Lubrication Engineering, Vol. 47, No. 4, p. 289-295. Garg, D. (1991). Polyalkylene Glycol-Based Compressor Lubricants. Paper presented at the Sixth Annual Reciprocating Compressor Conference, Salt Lake City, UT, September. Mang, T. and Jnemann, H. (1972). Erdl kohle- Erdgas-Petrochem. Verneigt BrennstoffChemie. Vol. 25, No. 8, p. 459-464. 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.

Natural Gas Compressors and Their LubricationyG.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 materialused 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

Click Here To See Tables 1 and 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.

Gas CompressorsCompressors 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 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 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 dryscrew 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 80C to 110C (180F to 230F), 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 oncethrough 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. 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 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. References1. 2. 3. 4.

5. 6.

7. 8. 9.

10.

Estimate obtained from the Natural Gas Week Web site: www.naturalgas.org/TERMDEF.HTM. Unit Course 2: For Natural Gas Compressors. Worthington Compression. Corpus Christi, TX. Wills, J. (1980). Chapter 14 - Compressors. Lubrication Fundamentals. Marcel Dekker Inc., New York, NY, p. 365-394. 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. Scales, W. (1997). Chapter 19 - Air Compressor Lubrication. Tribology Data Handbook, Ed. E.R. Booser. CRC Press, Boca Raton, FL, p. 242-247. Cohen, S. (1987). Development of a Synthetic Compressor Oil Based on TwoStage Hydrotreated Petroleum Basestocks. Lubrication Engineering, Vol. 44, No. 3, p. 230-238. Short, G. (1983). Development of Synthetic Lubricants for Extended Life in Rotary-Screw Compressors. Lubrication Engineering, Vol. 40, No. 8, p. 463-470. Miller, J. (1989). Synthetic and HVI Compressor Lubricants. J. Synth. Lubrication Engineering, Vol. 6, No. 2, p. 107-122. Tolfa, J. (1990). Synthetic Lubricants Suitable for Use in Process and Hydrocarbon Gas Compressors. Lubrication Engineering, Vol. 47, No. 4, p. 289295. Kist, K., and Doperalski, E. (1979). Brief Introduction to the Screw Compressor. AIChE 86th National Meeting, Paper 68E.

Managing Lubricant Viscosity to Maintain Compressor Healthy

Robert Kasameyer Tags: compressor lubrication, viscosity

If youre running one of the approximately 140 working refineries 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 dont 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 theres a bearing failure and production stops. Its 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. Theres 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 hydrocarbon gases, the lubricants 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 lubricants viscosity can only be gauged when measured in its natural position with gas vapors dissolved in the lubricant. In addition, monitoring lubricant temperature isnt sufficient to protect compressor bearings, especially in applications where process starts and stops can occur. Whats 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 CompressorsRotary compressors are used throughout oil refineries in applications ranging from vapor recovery to gasprocessing 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 found in intermediate and end-product applications, scroll compressors are valued for their reliability and smooth operation.

The Importance of Lubricant ViscosityIn 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 compressors bearing life and overall reliability. 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 humidity in the refinerys external environment can greatly reduce lube oils 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 RisksCompressor lube oils are formulated to work well 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, which 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 between 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 certain 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 companys 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.

Oil Analysis Boosts Compressor ReliabilityyDaryl Beatty, Dow Chemical Company Tags: oil analysis, compressor lubrication

There are several key elements that must be addressed to ensure an effective compressor lubricant analysis program. Errors or omissions related to these elements can lead to unnecessary expenditures and lower reliability. Knowing what actions should be taken based on compressor lubricant analysis is important for success. Understanding some common mistakes and misconceptions about water in compressor lubricants and the effect the environment has on the lubricants and the compressor is also key to a successful lubricant analysis program.

Inlet Air QualityFor centrifugal compressors, the significant air quality issues are particulate removal through filtration and the effect of acid gases on intercooler corrosion. In a centrifugal compressor, there is only minimal contact between the air and the lubricant and the sump sizes are typically quite large, resulting in dilution of any contaminants. For these reasons, it is rare for ambient air conditions to significantly affect fluid life, or have a detrimental effect on the running gear of the compressor. Double-acting reciprocating compressors are not totally immune to air contaminants, but are less subject to inlet air quality issues because of a continuous infusion of fresh cylinder lubricant. The fresh lubricant serves to flush contaminants through the system with a protective effect, even though these compressors have a high lubricant consumption.

The ambient air is a more serious concern for rotary screw compressors where the entire flow of air through the compressor contacts the fluid, and the fluid is effectively acting as a scrubber to absorb the acids and contaminants. Even a low concentration of acid is significant, when the sheer volume of air being handled is considered. Some of this acid will be absorbed by the fluid, which will show up analytically as a lower pH and higher acid number (AN). Lubricants for rotary screw compressors are formulated with good corrosion protection, but eventually even that is overwhelmed. Once this occurs, filters may plug more frequently due to corrosion particulate. This effect results in significantly shortened fluid life. It is not unusual in a contaminated environment to see the life of a nominal 8,000hour fluid reduced to 2,000 hours. The life of downstream components, such as aftercoolers and dryers, is also often compromised by corrosion caused by acid gases which pass through the compressor from the environment. These gases then condense with water in the coolers and dryers and drastically increase corrosion rates. What can be done to extend fluid life and solve these problems? y Remote air inlets may be installed to obtain inlet air from a source away from the contamination. This is typically outside the building. Ironically, inside air is rarely of better quality than the makeup air being taken into the building. Air can be tested by suspending corrosivity coupons of copper and silver in the air near the compressor. After a specified period, laboratory analysis of the resulting compounds on the surface of the coupons will reveal the type of contaminant in the air and the degree of contamination. Inlet air scrubbers may then be prescribed based on the degree and type of contamination to remove contaminants from the inlet air. The result is longer fluid life and decreased corrosive attack of compressor bearings, coolers, dryers and downstream equipment.

y

y

Lubricant Analysis ParametersThe key analysis parameters vary with the type of lubricant being used. Most new rotary compressors are equipped with polyglycol-based lubricants. With a polyglycol or polyglycol/polyolester-based compressor fluids, the following parameters are of great interest: y pH - A rapid or excessive decrease in pH indicates ingestion of acid gases or other contaminants from the environment. This will require a fluid change, but also indicates that the source of contamination needs to be eliminated. AN - The acid number is an indication of remaining useful fluid life. AN may increase with either oxidative degradation of the lubricant or accumulation of contaminants from the environment. Either way, this accumulated acid reflects the depletion of the corrosion inhibition package. Suggested change points vary, typically from 1.0 to 2.0. The fluid life from the time the AN reaches 1.0 until the time it will reach 2.0 is only 10 percent to 20 percent of the overall life span. Due to the difficulty of removing the last 20 percent of the fluid from the compressor, it is probable that stretching the change point from 1.0 until 2.0 actually costs more in terms of shortening the life of the next charge of fluid, than is gained on the first charge. Stretching the change point here is false economy and results only in a greater exposure of the compressor to fluid containing high levels of acid. Viscosity - The viscosity of some original equipment manufacturer (OEM) compressor fluids are specifically designed for the needs of that compressor application and do not fit in either the ISO 32 or 46 viscosity ranges. With polyglycol fluids, viscosity will normally increase about 10 percent with use, then stabilize. If lab personnel are not aware of the initial viscosity of a fluid, they often assume it originally fit into an ISO range and then mistakenly condemn it for high or low viscosity. It is important to always compare viscosity to the specification for that fluid, not an ISO range. With polyglycols, it is unusual for fluid to fail due to viscosity change, because the fluids are resistant to varnish and sludge formation and dont have a tendency to gain viscosity. Contaminants - Hydrocarbon contamination is typically monitored to assure that operators are not mixing fluid types. If fluids are mixed, the life of the fluid may be compromised.

y

y

y

y

y

Oxidation - Polyalphaolefin (PAO) or mineral oil change points can be determined by the degree of oxidation of the base fluid. This is not necessary with polyglycols, because AN is a reliable indicator of fluid condition. High Particulate - If the corrosion particles are mostly small particulate, the filters should be changed and measures should be taken to determine what acidic condition is causing the corrosion.

For PAO-based compressor lubricants, the pH, AN and viscosity must also be monitored. In addition, it is useful to monitor oxidation, typically by infrared spectroscopy. Monitoring the oxidation level is useful in preventing varnish and deposit formation. In cases where the fluid quantity justifies it, rotating pressure vessel oxidation test (RPVOT) can reveal the remaining useful life of a PAO-based fluid. With polyglycols these steps are not necessary, because AN is a reliable indicator of the remaining fluid life.

Table 1. Typical Values for New and Suggested Condemning Limits for Polyglycol Compressor Fluids

Water ContentTable 2 compares typical water content of samples from air compressors with other equipment in a plant. This specific plant is located in a humid environment in a southern U.S. state.

Table 2. Water Analysis of Lubricant Samples from Compressors and Other Typical Equipment in the Same Plant The water content of the air compressors ranges from 0.4 percent to 0.6 percent, while other types of equipment in the same plant have about 1/100 the water content. Labs often flag a sample like this with an alarm, when in fact these levels of water are normal for rotary compressors, and the compressor fluids that are specially made for them are formulated to function in this environment.

Consider this scenario: A plant performs periodic routine analysis on each of these fluids. Each time analysis is done, the water level is reported as shown in Table 2, along with the recommendation that the fluid in the compressors be changed. What is not considered is that the water level in the new fluid will again reach these levels quickly. Fluid has been wasted with no benefit. Water levels will also vary with fluid basestock type, because some basestocks are capable of tolerating more water before free water is released into the fluid. The keys are: y Know the maximum amount of water that a type of fluid will tolerate before free water is released into the system. For example, polyglycol compressor lubricants, which are used by several compressor OEMs, will tolerate about 0.8 percent water before free water becomes a problem. With hydrocarbons and synthetic hydrocarbons, free water will typically become an issue at lower levels. Recognize that the water level in a sample is also a characteristic of the equipment application in which the fluid is used. Rotary air compressors are higher than nearly any other application because of the contact of the lubricant with large amounts of humid incoming air. When the air is compressed, water vapor is condensed. That water must either be absorbed by the fluid or allowed to circulate as free water. Other types of equipment, as demonstrated in Table 2, tend to be much lower. Analysis alerts should be set accordingly. Changing the fluid does not solve a water problem for long. The water level varies with humidity, ambient temperature, duty cycle and machine operating temperature. Water is being continuously ingested. Make sure the lab is aware of fluid type and application, so that water level limits can be set accordingly. Dont obsess about water levels in compressor fluids. The water level is high in these units compared to other types of equipment. Historically, compressor air ends (also called compressor units) from major OEMs typically last about 10 years, with a few reaching 20 years before having to be rebuilt. All of them have had high water levels and served long lives. Only recently have the high levels been noted. The compressor fluid should be specifically designed for use in rotary compressors, and contain corrosion protection adequate for this demanding application.

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Compressor CondensateThe analysis of compressor condensate is a useful tool in detecting some corrosive or acid gases in the air that may not be effectively trapped by the lubricant. A low pH or high AN in the condensate may reveal a corrosive condition, which if left unchecked, will lead to short aftercooler and refrigerated dryer life. The typical source of these problems is contaminated inlet air and remedies which were previously discussed. Metals analysis of the condensate can also reveal the rate of corrosion, which may already be occurring in aftercoolers and dryers. In addition, the total organic carbon (TOC) or total oil and grease (TOG) analysis indicate the carryover rate of lubricant from the compressor, which is an indicator of the efficiency and condition of the air/oil separator.

Figure 1. Two-stage Tandem Compressor

Fluid AnalysisCertain actions should be taken based on elemental analysis or particulate in the fluid. The sources and the importance of particulate and trace elements in a rotary compressor should be considered first. A rotary compressor is unique in that the metals and particulate in the fluid can originate from several sources. Primary sources include: y y y Ingestion with the inlet air, either through or bypassing the inlet filter Corrosion particles, primarily from the upper portion of the receiver tank Wear debris from rotors, housing, gears and bearings

The key to determining the type and source is analysis. When particulate is the concern, analytical ferrography is one simple and useful technique for differentiating between these three sources. Once determined, any of these problems can be readily resolved. Figure 2 demonstrates the various types of ferrous particulate that can readily be distinguished by analytical ferrography. In this case, wear debris and corrosion particles are present.

Figure 2. Analytical Ferrograph from a Rotary Compressor Showing a Variety of Particulate Types Figure 3 shows a variety of particulate types identified by this technique. Of particular interest in rotary compressors is the ability to distinguish ferrous particles originating as wear debris, which is a serious concern, from corrosion particles that typically originate from the surface of the receiver tank, and should be trapped by the bearing filter before reaching the bearings.

Figure 3. A Variety of Particulate Types Distinguished by Analytical Ferrography In addition, a variety of particulate is ingested with the air from the environment. While it is not usually necessary to specifically identify the items that are ingested, it does indicate an inlet air filtration problem, which can then be diagnosed and remedied. Identification of particulate into these three categories - wear, rust/corrosion and ingested material - will determine the action needed to alleviate these problems, thereby enhancing compressor life and reliability. Trace element analysis is also useful for the early detection of potential problems in compressors. Table 3 offers some possibilities for explaining the presence of various elements in compressor fluids and the significance of those elements. Click Here to See Table 3. It is important to note that many of the elements are useful for resolving specific issues, but are not tracked on a routine basis. Of the elements listed in Table 3, iron is the most useful wear metal for helping determine the compressors specific condition. In addition to spectroscopy, one of the particle size analysis methods will provide more useful data. An increasing trend in particulate levels should be investigated.

Figure 4. Lubricated Compressors

Figure 5. Two-stage Tandem Compressor

Good Observation SkillsAnalytical techniques are only part of the story. There are many simple operational issues related to the fluid and fluid system that the lab has no way of detecting or resolving. There is no substitute for effective observation of operating conditions, trends and the environment in which the compressor is operating. For centrifugal compressors, trending of vibration readings on each stage will reveal the formation of bearing deposits or other subtle changes that can then be remedied before significant damage occurs. The following items should be considered for rotary screw air compressors: Inlet air temperature, humidity and contamination. Contaminated air contaminates the compressor fluid, resulting in equipment failures and downtime. Avoid ingestion of air from sources containing acid gases. These would include boiler exhaust, diesel exhaust, any operations discharging acid and others. Draeger tubes used for personnel air monitoring are usually not sensitive enough to detect the concentrations of contaminants that will cause a problem. Air quality test kits, utilizing silver and copper coupons are useful in evaluating the quality of the air that is in contact with the lubricant. Armed with this knowledge, a remote air inlet may be installed to obtain air from a cleaner area, or an air scrubber may be installed to protect both the lubricant life and the compressor. Low operating temperature. Proper discharge air temperature is important to maximizing fluid and compressor life. As a general rule of thumb for a humid environment, the discharge temperature should be 100F (55C) higher than the ambient temperature of the inlet air to prevent accumulation of water in the lubricant. Air-cooled compressors typically have coolers sized to prevent the compressor from running below this temperature and thus automatically avoid this problem. A watercooled compressor may cool too efficiently and condense water in the fluid. If the water separates from the fluid and collects in the receiver tank, the free water may not be detected by lab personnel, because the sample will not be representative of what is truly occurring in the compressor. One solution is to observe the temperatures and make adjustments as necessary. Depending on fluid type, temperature and humidity, water levels of up to 0.7 percent in a rotary screw compressor are normal. Levels above that amount indicate free water in the system and require intervention. High operating temperature. High operating temperature presents a different set of problems. The fluid will easily remain below 0.5 percent water content with no free water, but high temperature is detrimental to fluid life. Every 18F (10C) increase in temperature will reduce fluid life by approximately half. The temperature at which a fluid is rated for its nominal life, typically 8,000 hours for most polyglycol, ester or PAO compressor fluids, will vary. A fluid rated for 8,000 hours at 200F would be expected to significantly outlast a fluid rated for 8,000 hours at 180F. Load and unload performance. Load and unload performance affects carryover and energy savings. A compressor, which is allowed to run unloaded with a minimal air demand, will typically experience high lubricant consumption and more internal condensation and corrosion. In addition, it will use much more electricity, resulting in massive energy waste and increased expense. This cycle and loading should be observed and the compressors usage should be adjusted to match the air demand. Compressor OEMs have developed computerized control systems to continually monitor and adjust to maximize savings.

Leaks and condition of all couplings. Minor fluid leaks may be a warning of an impending failure of a coupling, gasket or seal. They should not be ignored. The amount of makeup fluid. The volume of makeup fluid added to each compressor should be logged for frequency and amount. Attempt to do a material balance, or accounting of the fluid; how much fluid is lost to leaks versus what is carried over into the condensate traps or plant air. A sudden increase in fluid makeup rate might correspond with the change to an inefficient, defective or improperly installed air/coolant separator. Finding this quickly can result in fluid savings and avoid the contamination of downstream components. OEM separators are typically closely matched to the air flows and other characteristics of specific compressors. Also, the materials of construction are compatible with the OEM fluids, to prevent compatibility failures. Savings in lubricant and energy can greatly exceed the cost of separator elements. Separator and bearing filter monitoring. It is important to track separator and bearing filter differential pressures and frequency of changes. These are useful trends for several reasons. First, the depletion of corrosion protection packages in the lubricant may result in shortened filter element life, due to blinding with corrosion particles. Progressively shorter intervals between filter changes, while on the same charge of lubricant, are strong indicators of this condition. The ingestion of particulates in the air may also result in short separator life. Particulate levels and element life may relate to the quality of the inlet filter element, an air leak or improper installation. Also, changing separator elements at the OEM-prescribed differential pressure (usually about 10 psi) results in energy savings and avoids the potential collapse of a separator, which results in massive amounts of fluid being discharged downstream into the air system. These are a few examples of lubricant-related observations which are easily made and may be consistently monitored to improve compressor reliability.

A Strategy for Compressor and Compressor Fluid HealthA strategy should focus on leading rather than trailing indicators. In trying to anticipate compressor reliability issues, the focus should be on the indicators discussed above, which can anticipate and prevent problems. Fluid condition, in terms of pH, AN and contamination should be monitored before corrosion results in high metallic content. The delivery of clean inlet air to the compressor to prevent contamination of the lubricant and corrosion of the system should be a main focus. Analysis is also critical, but only if the proper parameters are assessed and action is taken to prevent and resolve future problems. http://www.machinerylubrication.com/View/28594/compressor-oil-loss

PAGs are Rising to the Top of the Synthetic MarketyDaryl Beatty, Dow Chemical Company Martin Greaves, Dow Chemical Company Tags: synthetic lubricants

For centuries, lubricants have been utilized as a way to reduce friction and wear onmoving parts. In 2005, 40 million tons of lubricants were produced. While natural mineral oil-based fluids represent the majority of the market demand, many technological advances in equipment and machinery would not be possible without the benefits offered by improvements in synthetic lubricants, which currently make up only two percent of the market. While polyalphaolefins (PAOs) fill some of these needs, a growing number of applications are demanding higher performance requirements, or require unique specifications that are not met by traditional lubricants.

One of the most versatile types of synthetics is polyalkylene glycol (PAG) lubricants. PAGs are generally known as compressor lubricants, and their use in industry has increased since the 1980s. Increasing performance standards in the automotive and industrial markets peg these sectors as areas that show promise for growth. This article offers an overview of the main synthetic base stock chemistries and an indepth analysis of the benefits and uses of PAGs.

Synthetic Lubricant Base StocksThere are six major base stock types used in the development of synthetic lubricants, with each offering its own set of unique properties and applications. Silicones are valued for their low volatility, inertness to most chemical contaminants and thermal stability in severe applications, as well as their performance in lowtemperature environments. These qualities make them an excellent candidate for use as heat transfer fluids, specialty grease applications and DOT Type 5 automotive brake fluids. However, there are two limitations of silicones that must be considered for lubricating applications. First, they cannot be used in the cylinder lubrication of internal combustion engines because the combustion by-product is silicon dioxide. Second, extreme pressure performance is limited, and common extreme pressure additives are incompatible with them. In their proper applications, the fluid life and hydrolytic stability of silicones is unsurpassed.

Diesters, or dibasic acid esters, were developed during World War II and are the reaction product of long-chain alcohols and carboxylic acids. Historically, they have been effective as reciprocating compressor lubricants due to their low coking tendency at temperatures of 400F or higher. They also provide excellent solvency and detergency. The aggressiveness of diesters toward elastomers, seals and hoses has limited the usefulness of these fluids. Newer fluids, such as polyol esters, meet the needs of many applications formerly filled by diesters. Polyol esters, or Neopentyl poly esters, have largely replaced diesters in hightemperature applications where oxidative stability is critical. Common applications include their use as lubricants in aircraft engines, high-temperature gas turbines,

hydraulic fluids, and as heat exchange fluids. They can also be used as a co-blended basestock with PAOs to enhance additive solubility and reduce the tendency of PAOs to shrink and harden elastomers. PAOs are hydrocarbon polymers manufactured by the catalytic oligomerization of linear alpha olefins like alpha-decene. They are considered high-performance lubricants and provide a high viscosity index and hydrolytic stability. PAOs are the most commonly used, and are generally less expensive than other synthetic lubricants. They have been used in passenger car motor oils, as well as numerous industrial lubricant applications. Phosphate Esters are valued in applications where safety and fire resistance are critical considerations, which include fire-resistant hydraulic fluids and aircraft fluids. High flash points and fire points enhance their resistance to ignition, and their low heat of combustion makes them excellent self-extinguishing fluids. However, they do have several weaknesses including poor hydrolytic stability, which can lead to the formation of aggressive acidic by-products. Care must be taken when used because they can also react and degrade a variety of commonly used sealants and paints. PAGs offer quality lubricity, high natural viscosity index and good temperature stability. PAG base fluids are available in both water soluble and insoluble forms, and in a wide range of viscosity grades. They offer low volatility in high-temperature applications and can be used in high- and low-temperature environments. They are commonly used as quenchants, metalworking fluids, food-grade lubricants and as lubricants in hydraulic and compressor equipment. However, the water soluble PAGs are incompatible with petroleum oil, and care must be taken in transitioning equipment from hydrocarbon oils to PAGs.

The Development of PAGsPAGs were one of the first synthetic lubricants to be developed and commercialized. They were created under mandate from the U.S. Navy in response to hydraulic fluid fires on ships resulting from ordnance strikes during World War II. In 1942, and for the next 30 years, the Navy began to exclusively use PAG-based water glycol hydraulic fluids that were fire-resistant and could operate over a wide temperature range. Later, PAGs began to see extensive use as textile lubricants and as quenchants in metal heat treating. PAGs are classified by their weight percent composition of oxypropylene versus oxyethylene units in the polymer chain. PAGs with 100 weight percent oxypropylene groups are water insoluble; whereas those with 50 to 75 weight percent oxyethylene are water soluble at ambient temperatures. Although PAGs have long been used as industrial lubricants, recent work has led to the development of PAG lubricants for use in equipment in the food processing industry. These products are known as food-grade approved lubricants. In these applications, they offer excellent lubricity, increased oxidative stability, a high viscosity index (180 to 280) and low pour points. They are one of the few synthetic substances identified in the FDAs food additive regulation for food-grade lubricant base stocks, 21 CFR 178.3570, for use in industrial machinery when incidental food contact with a lubricant may occur.

PAG Applications and BenefitsBecause of the properties that make up PAG lubricants, they are uniquely suited for a number of industrial and manufacturing applications. Their water solubility allows for easy clean-up of equipment. PAG lubricants offer high viscosity indexes, and are shear stable. PAGs are also valued for their low volatility in high-temperature applications, and for resistance to formation of residue and deposits. Their biodegradability makes them ideal for environmentally sensitive applications.

PAGs are best known as compressor lubricants. PAGs are also the lubricant of choice in high-pressure natural gas and ethylene compression, where the viscosity stability of hydrocarbon-based lubricants is adversely affected due to solubility of the gas in the fluid. In refrigeration compression, PAG and polyol ester-type lubricants are used almost exclusively with the current generation of environmentally friendly HFC refrigerants such as R-134a and R-152a. The two largest U.S. air compressor OEMs have used PAG lubricants as the standard factory fill in rotary screw air compressors for almost 20 years. More recently, a third compressor OEM has begun to offer PAG as an optional fluid. From the laboratory perspective, the condition of PAG fluids is relatively easy to monitor. In most applications, as the end of the useful life approaches, the only significant change is the increase in acid number (AN) from fluid oxidation. Depending on the additive package, fresh PAGs will typically have an AN of 0.1 to 0.5 mg KOH/g. An increase of 1.0 from the new fluid specification is a good condemning limit. Viscosity remains fairly stable, even during the latter stages of fluid life. Water limits may be set higher for PAGs than hydrocarbon fluids because they are more water tolerant than other fluid types. Even a water insoluble PAG will tolerate as much as 0.7 percent water contamination before allowing free water to exist in the fluid. PAGs are also useful in industrial equipment operating year-round without seasonal changes. Their superior heat transfer characteristics and thermal and oxidation stability make them ideal for use as heat transfer fluids in large, open vented systems and for process fluids in the production of plastics, elastomers, threads or fabricated parts where compatibility of the fluid with the processed part is important. Textile fiber production is another industry that benefits from the use of PAGs. These lubricants do not stain or discolor fibers, and are easily removed during the scouring process. PAGs are also the lubricant of choice for many high-speed, high-temperature fiber processes where shear stability is a requirement. In addition, they are often used as lubricants in textile manufacturing equipment as extreme-pressure gear lubricants. A renewed emphasis on energy conservation has increased interest in energy-efficient gear lubricants. For example, the extreme demands of gear lubrication in wind turbines are being met by PAGs. The low velocities and high surface loadings on the gears in these units have resulted in micropitting problems with conventional hydrocarbon oils that have been overcome with PAG-based fluids. In other gearbox applications, especially worm gears, the naturally low coefficient of friction found in PAG fluids results in energy savings, lower temperatures and lower wear rates.

Versatility Meets PerformanceFor more than 60 years, synthetic lubricants have provided a viable alternative to traditional hydrocarbon lubricants. Each type serves unique roles, with PAGs performing in both high- and low-temperature environments, in areas of extreme pressure and where water solubility is desired. PAGs can be designed to form a wide variety of polymers. The design of the polymer can be tailored to the lubricant application to provide, for example, the desired viscosity, pour point, solubility and other attributes. This versatility and the applications in which they are used shows that PAGs account for about 24 percent of the entire synthetic lubricant market. Low pour points, a wide range of viscosities, resistance to varnish formation, increased solvency and a wide range of solubility all add to PAG lubricants reputation as a high-performance synthetic lubricant on the market. With continuing emphasis on environmentally acceptable lubricants in industry, these qualities will continue to push PAGs to the forefront of the synthetic market.

Bearing basics for gas-industry screw compressorsTags: Cylindrical roller thrust bearings | Machinery and equipment | Reliability 16 September, 2005 Technology

Optimum bearing selection can extend bearing life and improve compressor performance. Screw compressorusage in natural gas applications has risen steadily over the past decade and continues to grow. This is chiefly due to lower gas pressures in many older natural-gas fields, which make reciprocating compressors uneconomical to install and operate. Today screw compressors are found in both sweet-gas (not containing H2S) and sour-gas (containing H2S) applications. They are employed at the wellhead to draw natural gas from the ground and boost the pressure to feed gas into pipelines.

As the usageof screw compressors increases, so does the focus on a major screw compressor component rolling bearings. Bearings play a critical role in screw compressor performance. Bearing-related problems, however, can impair compressor reliability. In the worst case, they can lead to catastrophic failure, interrupting gas production. Wells and scrubbing stations are often located in remote, hard-to-reach areas, making repairs difficult and expensive. Although standby compressors are sometimes available, they must be maintained to avoid potential problems at start-up. Accordingly, reliability-conscious gas-industry professionals and well operators seek to avert bearing-related problems before they occur. A big first step is becoming familiar with the basics of bearing operation in screw compressors and learning how to specify the correct bearing arrangement, bearing types and materials for compressor applications.

In twin screw compressorstwo meshing rotors turn in opposite directions inside the compressor housing. On the suction side, gas is drawn into a cavity produced between the housing wall and the two rotors. As the rotors turn, the cavity decreases in size, compressing

the gas and then discharging it. Gas field compressors are usually driven by a gas-fuelled engine, either directly or though a gearbox. The function of rolling bearings in screw compressors is to provide accurate radial and axial positioning of the compressor rotors and to properly support rotor loads. Bearings are employed at the suction and discharge ends; their number and configuration vary depending on load and application requirements. Typically, there are two cylindrical roller bearings at the suction end, one supporting each compressor rotor. A second pair of cylindrical roller bearings is used at the discharge end. Some compressors use journal bearings for radial support. The discharge end is equipped with two or more thrust bearings, which are usually single-row angular contact ball bearings or four-point contact ball bearings (QJ bearings). These bearings support pure axial loads.

As the gasundergoes compression, the temperature increases. Oil injected into the compression cavity lubricates the screws, seals leakage paths and cools the gas. After being discharged with the compressed gas and separated out, the same oil lubricates and removes heat from the bearings. Operators should recognize the risks of injecting too much oil into screw compressors. This as natural gas contains a mix of gases and is usually saturated with water vapor and sometimes acids and hydrogen sulfide are present. Injecting too much oil can lead to excessive cooling, causing condensation of water and acids. The condensates will mix with the oil and impair bearing lubrication, cause corrosion, and can lead to bearing failure. The oil used should have the proper viscosity and should not contain additives that cause water emulsification. Engine crankcase oils are typically not suitable as compressor oils. Oil samples should be taken periodically and analyzed for degradation, viscosity and water content. Natural gas also contains solid particles that can damage bearings and other components. To prevent damage, compressors should have effective oil filters.

Sour natural gas, which contains hydrogen sulfide (H2S), poses a particularly difficult challenge. Hardened steel and materials containing residual stresses are susceptible to sulfide stress cracking. This is a brittle crack phenomenon that occurs at low stress levels. In addition, hydrogen sulfide can mix with water and form sulfuric acid (H2SO4) leading to corrosion. Bearing cages should be made of stress-free materials to avoid damage from sulfide stress cracking. Well operators should carefully consider cage materials when specifying compressor bearings.

Polyamide cages,which are made of an injection-moulded glass-fiber reinforced polymer, also offer a possible solution. These cages have proved successful in sour-gas applications with operating temperatures up to 70C (158F). Higher temperatures, however, can cause polyamide to age prematurely when exposed to aggressive gases and acids. A new solution is

now available with cages made of PEEK, a polymeric material with superior chemical, temperature and aging resistance. PEEK also has superior properties as a bearing material with low friction and wear rate, making it suitable as a cage material in bearing applications with marginal lubrication. Traditionally, the natural-gas industry has refrained from using brass and other yellow metals in compressor and pipeline applications because of potential sulfide stress cracking. Residual-stress-free brass, however, is not subject to stress cracking. Machined brass cages made from centrifugally cast tubing are free from residual stresses. A large number of single row angular contact ball bearings, four-point contact ball bearings and cylindrical roller bearings are available with this type of cage. Although somewhat contradictory to industry practice, several screw compressor manufacturers have successfully employed such brass cages in sour natural gas compressor bearings since the mid-l980s.

Pressed brass cages, made from brass sheet, are not suitable in sour-gas compressors, since the pressing oper-ation leaves residual stresses in the material. Machined steel cages are commonly asked for by compressor users. Although there is no stress cracking with machined steel, there is a greater risk of metal smearing between the cage pockets and the rolling elements, especially in marginal lubrication. With the availability of PEEK and stress-free machined brass cage materials, there is no longer any reason to use machined steel cages. Rolling element bearings must be made of hardened steel to function reliably. Hardened steel components, however, can suffer hydrogen sulfide damage. SKF has investigated the use of special coatings to protect bearing steel in sour-gas applications. At the SKF Engineering and Research Center in The Netherlands, SKF scientists have identified critical properties for coating materials. The coating hardness must bond securely to the bearing steel and resist flaking under load. It must also also be nonporous so there is no diffusion of hydrogen sulfide through the coating. Another even more promising solution is to use a new grade of a super tough stainless bearing grade steel that is resistant to sulfide stress cracking. SKF is developing and testing such steels with very promising field test results in sour gas screw compressor applications. In these new bearings the balls or rollers are made of ceramic materials. Alternative solutions include SKF NoWear coated balls and rollers made of high nitrogen stainless steel.

The SKF NoWearcoating provides very low friction, protection against smearing and reduced wear, but NoWear is not dense enough to prevent diffusion of hydrogen sulfide. NoWear coated bearings provide a solution in difficult applications working under poor lubrication or low loads. SKF Explorer performance class bearings feature improved steel quality and heat treatment. SKF Explorer single row angular contact ball bearings, as standard manufactured for universal matching, four point-contact ball bearings and cylindrical roller bearings yield longer service life than conventional bearings in demanding screw compressor applications.

Once compressor bearings have been selected and installed, they should be monitored. Handheld monitoring devices for periodically monitoring or permanently installed monitoring devices for online surveillance can provide trending data and detect eventual bearing damage in the early stages and enable well operators to schedule maintenance and repairs for periods of planned shutdown.

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MiscibilityMiscibility refers to the ability of a gas or liquid to dissolve uniformly in another gas or liquid.Gases mix with each other in all proportions, but this may or may not apply to liquids, where miscibility depends on chemical affinity. Ethanol, an alcohol, and water are miscible because they are chemically similar, but benzene and water are only slightly miscible because of the very large differences in their chemical properties. Some liquids are essentially insoluble in other liquids, e.g., gasoline in water (and water in gasoline);such liquids are said to be immiscible. Other liquids are only soluble up to a point. In the case of water and ethyl ether (CH3CH2OCH2CH 3), it is possible to dissolve up to about 4 g of ethyl ether in 100 g of water, but the addition of more ethyl ether results in the production of separate layers of the less dense diethyl ether above the denser water layer. And some liquids are completely soluble in other liquids, regardless of the amounts combined; such liquids are said to be miscible in each other in all proportions. (Traditionally, the term solubility has been used interchangeably for miscibility in reference to liquids, even though it should strictly only be applied to solids.) There are several fundamental rules governing the miscibility of liquids in other liquids. First, the solubility of liquids in liquids increases with increasing temperature. Second, the more similar two compounds are in terms of polarity, the more likely that one is soluble in the other, i.e., polar compounds dissolve in polar compounds, and non-polar compounds dissolve in non-polar compounds. (Polar molecules dissolve in polar molecules because the dipole of one attracts the dipole end of the other.) Thus, benzene and carbon tetrachloride, being both non-polar, dissolve in each other, but neither will appreciably dissolve in water, which is polar. Both alcohols and ethers with up to three or four carbons are miscible in water because the OH groups in these molecules form hydrogen bonds with the water molecules. Alcohols and ethers with higher molecular weights are not miscible in water, however, because the water molecules can not completely surround those molecules. The molecule 1-heptanol, for example, consists of an alkyl chain of seven carbons and an OH group. The OH group forms hydrogen bonds with water molecules, but the alkyl portion of the molecule exerts no attraction on the water molecules. This part of the molecule is called hydrophobic, meaning water-hating. Because this part of the molecule cannot be surrounded by water, 1-heptanol is immiscible in water. In aqueous solutions, globular proteins usually turn their polar groups outward toward the aqueous solvent, and their non-polar groups inward, away from the polar molecules. The nonpolar groups prefer to interact with each other, and exclude water from these regions, leading to immiscibility. This type of interaction is usually weaker than hydrogen bonding, and usually acts over large surface areas.

Many gases are miscible with liquids. The miscibility of gases in liquids almost always decreases with increasing temperature, and increases with increasing pressure. For example, more oxygen is dissolved in the blood at higher than normal pressures.

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