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August 8, 2012
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Compressor Lubrication, Part IV-A: Positive-Displacement Types

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Representing some of the most complicated and costly equipment in a plant, compressors, along with the gases they process, call for specialized lube strategies.

compressor is a machine that raises the pressure of a gas and then delivers it for use in a variety of applications, including those associated with combustion, pneumatic, refrigeration and gas-transmission processes. The main purpose of a compressor is to increase gas pressure to the point where it can be used in an industrial facility. Compressors are rated by discharge pressure in psi and capacity in cubic feet/minute (cfm). 

As shown in Fig. 1, compressor types fall into two major groups: positive displacement and dynamic. The various types of units in these two groups, along with the specific gases they compress (see Table I), call for different lubrication strategies. That’s why Part IV is divided into Sections A and B. This issue’s focus on positive-displacement types will be followed by discussions of dynamic designs and troubleshooting techniques in the September/October issue. 

NOTE: Although air is the most commonly compressed gas in today’s manufacturing facilities, air compressors per se will not be discussed in this series. Lubrication of air compressors was the focus of an article in the May/June 2009 issue of LMT (www.mt-online.com/thibault). 

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In positive-displacement units, pressure is attained by trapping a specified amount of gas and converting it to smaller volume. Reciprocating (recip) units, followed by helical screw units, are the most common positive-displacement designs. 

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Dynamic compressors operate on the principle of accelerating a gas by impellers or blades to increase its velocity. The gas is then slowed down, converting kinetic energy into potential energy, resulting in an increase in pressure. 

Typically, positive-displacement compressors—reciprocating designs in particular—produce higher pressures while dynamic compressors (such as centrifugal and axial designs) are able to move greater gas volumes.

The major factors involved in the selection of compressor lubricants include:

  • Type, size and speed of compressor
  • Gas being compressed
  • Number of stages
  • Pressure and temperature at each stage
  • Environment
  • Type of lubrication system

Reciprocating compressors
Operation… 
Reciprocating compressors incorporate a cylinder and piston, piston rings, inlet valve, discharge valve and a drive assembly consisting of a crankshaft, connecting rod and drive, such as an electric motor. The compression process begins with piston movement in the cylinder creating a greater volume that decreases the pressure. This allows the inlet valve to open, resulting in gas flow in the cylinder. Once the piston reaches the end of its stroke, the inlet valve closes and the piston moves in the opposite direction, reducing the volume in the cylinder and causing gas pressure to increase. When the pressure is high enough to overcome the gas pressure in the discharge line, the discharge valve opens, allowing the gas to escape in the discharge line. The cycle is repeated as the piston moves back and forth. For each cycle, gas is drawn into the cylinder, compressed and delivered to the discharge piping. The piston rings maintain a seal between the piston and the cylinder, which lets the gas be compressed without leaking past the piston.

Reciprocating compressors have many types of arrangements, from number of cylinders to stages of compression. In order to increase gas discharge volume, such compressors may have multiple cylinders that have separate inlet lines but common discharge lines. Single-cylinder compressors are classified as either horizontal or vertical. The most common arrangements for a two-cylinder compressor are the V- and L-shaped layouts. 

Another way to increase volume in a reciprocating compressor is to compress the gas on both faces of the piston. This is called a double-acting compressor. Through adiabatic compression, as the pressure on a gas increases, the temperature goes up proportionally and at high pressures can reach very high gas discharge temperatures.  The pressure-limiting factor in a reciprocating unit is the temperature achieved during the compression process. To achieve higher pressure, two or more stages are used. The gas is cooled with an intercooler between stages and then recompressed to higher pressure. The first stage has the largest piston and cylinder—with each successive stage being smaller. Figure 2 illustrates a double-acting, two-stage, L-shaped reciprocating design. (Most high-pressure reciprocating compressors are double-acting, multistage and water-cooled.)

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Lubrication… 
Lubrication of reciprocating compressors in terms of oil type and viscosity can vary widely based on the type of gas comp-ressed and the desired discharge pressure. The major lubrication areas and components on a compressor are as follows:

  • Compressor Frame
    • Main bearings
    • Connecting rod bearings
    • Cross head bearings
    • Motor bearings
  • Cylinder Region
    • Rider bands
    • Compression rings
    • Packing
    • Valves

If the frame bearings and cylinder are using the same oil for lubrication, the oil is pumped from a reservoir where the oil is filtered and cooled. The oil is distributed to the bearings in the frame by an oil pump. The same oil is also used, by way of injectors, to lubricate and seal piston rings. In many cases, two different lubricants are used. The frame bearings don’t require synthetics because of the moderate conditions. High-pressure, high-temperature conditions may require synthetics or compounded oils in the cylinder. Also, higher pressures require higher viscosities. A separate oil system is used to supply oil to the injectors for the cylinder. Too much oil in the cylinder can create problems, such as carbonizing the valves. It is better to under-lubricate than over-lubricate. One quart of oil will lubricate the sweep of the piston over 10,000,000 ft2. The following is the formula to calculate the oil required in quarts/day. This is an average amount—it can change based on operating conditions and gas being compressed:

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The injectors are set in drops/minute and have to be adjusted to the size of the drop to achieve quarts-per-day calculation. During commissioning and run-in of new compressors, this amount is usually doubled. Check with the OEM for its requirements.

In most conditions, frames are lubricated with rust- and oxidation-inhibited (R&O) oil (usually an ISO VG of 150). In some cases, an OEM may recommend an ISO 100.

Cylinder lubrication is related to the type and pressure of the gas being compressed. Units that compress inert gases are the easiest to lubricate—with ISO 150 R&O oil under moderate pressures <1000 psi. As pressures increase to 5000 psi, there’s a corresponding increase in viscosity from ISO 150 to 680.

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Fig 3. A rotary screw compressor design.

Hydrocarbon or wet gases can have a large dilution effect on an oil; as pressures are increased oil viscosity should increase. Polyalkylene glycols (PAG) are very resistant to dilution by hydrocarbons (and also don’t form deposits). Under higher pressure conditions they are an excellent choice. Typically, ISO 150-220 is used. When discharge temperatures approach 300 F, synthetics are usually recommended—such as PAGs, which don’t cause deposits when oxidized, and diesters, which have high thermal stability and excellent solvency that prevents exhaust-valve deposits (a major problem at high temperatures). 

Reactive gases can pose many problems with regard to lubrication. In applications like oxygen compression, a hydrocarbon lubricant should not be used if there’s a chance of it coming in contact with the gas. Fluorocarbons have been used in this service. Compounded oils with synthetic animal fat have been used to provide protection from acidic components. In some cases, engine oils have been used for the same reason (their detergent packages neutralize acidic components). This approach isn’t recommended if moisture is present. If you are working with reactive gases, consult your OEM and lube supplier for the correct solution.

Screw compressors
Operation…     
Rotary screws are positive-displacement designs that use screw-shaped rotors for gas compression. The main components are the inlet and outlet ports and the main and secondary rotor (which mesh as they rotate). There is a groove on the secondary rotor, and when it passes the gas inlet port, the gas enters the groove. The gas becomes trapped after passing the inlet, forming a gas pocket along the entire length of the groove. The lobe then meshes with the lobe on the main rotor. As the lobes mesh, the volume of the gas pocket is reduced, compressing the trapped gas. The gas is released as the groove passes by the discharge. Figure 3 illustrates a rotary screw compressor.

Rotary screw compressors—in both one- and two-stage designs—can generate pressures up to 350 psi. The two major sub-set categories are wet and dry screws. The most common of these is the flooded, or wet screw, where the oil and gas come together in the compressor. The oil provides a seal to rotate the screws, whereas the dry screw has a timing gear to regulate the screw movement. (NOTE: Flooded screw compressors are the main compressor type used for air compression.)

Lubrication… 
The dry screw compressor is the easiest to lubricate because there is no contact between the oil and gas. Typically, an R&O ISO 32-100 is used, depending on application and temperature.

The most common is the flooded screw, where the lubricant is in direct contact with the compressed gas. The major functions of the lubricant are to lubricate bearings and speed gear if present, cool, seal the rotors and prevent rust. Most flooded screws for air compression use synthetics, of which polyalphaolefin (PAO) and a polyalkylene glycol/ester blend  are the major types. blend. (NOTE: Please refer to my article “Proper Selection and Monitoring of Compressor Lubricants,” published in May/June 2009 issue of Lubrication Management & Technology for a comprehensive discussion on the lubrication of rotary screw air compressors.)

Flooded screws have the same problems as reciprocating compressors in processing hydrocarbon and reactive gases that react with the lubricant. The typical viscosities used are 32-68. When a hydrocarbon gas is in contact with most lubricants, the viscosity will be lowered because of dilution. Therefore, to achieve the target viscosity, a higher-viscosity fluid will be needed. The best lubricant for this application is a PAG, which is diluted less than any other type of product. For normal applications, mineral oils, PAOs and diesters have been used. Remember that reactive gases pose special problems with regard to lubricants. Always consult the compressor OEM and lube supplier.

Other positive-displacement compressor types
Rotary lobe… 
Rotary lobe compressors are also called blowers. They can compress large volumes of gas at low pressures. They feature two figure-8 impellers in a casing that rotate in opposite directions. Small clearances are maintained between the lobes with the use of timing gears. The gas is trapped between the impeller and the casing. As the impeller rotates, successive volumes of gas are packed in the confined space resulting in an increase in pressure.

Lubrication is performed on the unit’s bearings and timing gear. Either an R&O 150-220 or an AW 150-220 should be used. (NOTE: The November/December 2012
issue of this magazine will cover blowers in detail.)

Sliding vane…
A sliding vane compressor consists of a cylinder, a slotted rotor and vanes that fit in the rotor slots. The vanes are free to slide in and out of the slots as far as the distance between the rotor and cylinder walls. The rotor is maintained off center so a crescent-shaped space is left. Centrifugal force holds the vanes against the cylinder wall during operation. Each pair of vanes forms gas pockets of varying size. Where the distance between the rotor and cylinder is greatest, the gas pocket is the largest. As the rotor turns, the volume of the gas pockets gets smaller, compressing the gas. As the vanes pass the discharge port, the compressed gas flows out of the cylinder. These compressors are small, run quiet and require low maintenance at low to moderate pressures. They are sensitive to particles. The vanes contact the cylinder walls in a boundary lubrication condition; therefore antiwear oils are used ranging in viscosity from 32-150, depending on the application.

Liquid piston… 
This compressor has only one moving part: a rotor with blades. There are openings between each pair of blades. The compressor is partially filled with water. During operation, the water is spun outward by the rotor and forms a liquid ring around the casing. Separate gas pockets are formed between each pair of blades. Since the rotor is mounted off center in the casing, the gas pockets are large near the gas inlet port and small near the discharge port. Gas enters the gas pockets as they pass the inlet port. The gas becomes trapped and compressed by the liquid, reducing the size of the gas pocket and producing the increase in pressure. At full compression, the gas pocket’s opening 

passes the discharge port, and the compressed gas flows through the discharge line. Liquid piston units are used in low-pressure applications. The rolling element bearings are the only lubricated components (with ISO 32-68 R&O oil or, in some cases, an NLGI 1 or 2 lithium grease.)

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Basic troubleshooting techniques 
Compressor troubleshooting calls for a strong knowledge of machine component design, operating parameters, lubrication requirements and OEM specifications. In-depth troubleshooting usually becomes a one-on-one proposition: With a problem compressor, the troubleshooter must take a deep look into each piece of the puzzle. The following points comprise a basic troubleshooting approach for all types of compressors. 

Temperature. Changes in temperature from an established norm is a reliable indicator of changes in machine condition. Daily temperature inspections should, at least, include: suction and discharge of gas, gas interstage coolers, afterstage coolers, lube-oil coolers, cooling water, mechanical seals, crankcase and bearing oils. Periodic checks of bearings, valves and cylinder-head temperatures are important.

Levels. Liquid levels in compressor components must be monitored diligently. Correct crankcase, bearing housing, reservoir oil levels, feed rates on cylinder injectors and circulating oil systems must be kept constant. Compressed-gas receivers, intercoolers, aftercoolers and process piping must be drained and kept liquid-free. Free water should be drained from oil reservoirs and oil-filter housings daily.

Pressures. All compressors are designed to operate in specific pressure ranges; this is one governing factor determining what type of compressor is used in what service. Pressure differentials between suction, interstage and discharge gases must be tracked and variances out of the norm investigated. Bearing, mechanical seal and oil-filter pressures should be checked, at least daily. Air compressor inlet filter differential pressure should be checked daily. 

Changes in vibration or sound. Knocks, pings, rattles or ticks should be investigated as soon as possible after detection. 

Oil analysis should be conducted on no less than a quarterly basis—and on a monthly basis in severe service. Tests should include: viscosity, particle counts, wear metals, water content and FTIR or Ruler for remaining useful oil life. Modifications to this basic test slate will be required, depending on compressor type and service.

Troubleshooting specifics for positive-displacement types. . . 

Reciprocating compressors. Note pressure changes outside the norm (i.e., high intercooler pressure makes second-stage valves or unloaders suspect; low intercooler pressure points to first-stage valves and unloaders).

Monitor valve temperatures (hot ones are trouble). Check cooling-water inlet and outlet temps for efficiency.

Track crankcase oil viscosity on reciprocating gas compressors; oil dilution is common. When a recip compressor develops a “knock” and it’s not caused by insufficient lubrication, it’s likely due to mechanical looseness of bushings on the wrist pins, crossheads or cranks, or loose fasteners on foundations, cylinders, head bolts, etc.

Flooded rotary screw compressors. Proper functioning of the oil separator is one of the keys to long machine life. The lube oil is exposed to gross particle ingression and/or viscosity dilution from being mixed with the compressed air or process gas. The oil separator must remove these particles or the oil will soon be contaminated.

Track particle counts and wear metal in the lube oil and oil-filter differential pressure.

Rotary lobe and liquid piston compressors. These units only require lubrication for their shaft support bearings. Troubleshooting and oil analysis can be accomplished per the basic approach notes above.

Sliding vane compressors. These compressors can also be monitored via the basic approach above, with the addition of wear-debris analysis, particle-count trending and tracking of oil-filter differential pressures.

Coming up
Part IV-B in the September/October issue will focus on lube strategies and troubleshooting tips for dynamic (i.e., centrifugal and axial) compressor types. LMT

Ray Thibault is based in Cypress (Houston), TX. An STLE-Certified Lubrication Specialist and Oil Monitoring Analyst, he conducts extensive training for operations around the world. Telephone: (281) 250-0279. Email: rlthibault@msn.com.

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