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Comment PDF Operations & Maintenance

Lubricating Rotating Machinery

By Amin Almasi WorleyParsons Services Pty Ltd. |

 

 

 

Lubricants in rotating machines reduce friction and wear, dissipate heat, protect surfaces, keep out foreign contaminants and remove wear particles. Commonly used liquid lubricants fall into two main categories: mineral (petroleum-based) oils and synthetic oils.

Mineral oils are produced by refining crude petroleum. They usually contain trace amounts of some unwanted substances. By contrast, synthetic oils are engineered, so their properties can be more tightly controlled. Synthetic oil lubricants include synthesized hydrocarbons (such as polyalphaolefin, or PAO), organic esters (such as diesters and polyol-esters), polyglycols, polyalkylene glycol (PAG), phosphate esters and silicone lubricants.

Mineral oils are more frequently used in chemical process industries (CPI) applications, but the importance and use of synthetic oils has been steadily increasing in recent years. In general, they offer superior performance in terms of higher oxidation stability, improved corrosion resistance, and the ability to withstand both higher and lower temperatures.

 

Compressors and pumps

Some compressors operate with gas discharge temperatures exceeding 160°C. This calls for a lubricating oil that has good oxidation properties and thermal stability. Meanwhile, in some compressors or pumps, the lubricant is in contact with moisture (from the handled fluid), so the lubricant must have good demulsibility (that is, resistance to emulsification, or good resistance to mixing with water).

Today, the overwhelming majority of compressors and pumps are best served by premium-grade oils with ISO VG 32 or 46 (sometimes ISO VG 68 or 100 grades). However, there are many different types of compressors and pumps, and each manufacturer is likely to recommend only those lubricants that have been used successfully before. Occasionally, compressor lubricants have to be formulated for exceptional severe-service performance.

Reciprocating and screw compressors.During operation, lubricants used in screw compressors and in the cylinders of reciprocating compressors are in direct contact with compressed gas. With conventional mineral oils, such gas can become dissolved in the oil. Additionally, any oil that becomes dissolved in the gas can be carried away, depleting the lubricating film. This can result in machine component scoring and higher wear rates. Specially formulated synthetic lubricants generally perform better in these instances.

When selecting a lubricant for reciprocating or screw compressors, low solubility in compressed gas should be a key selection criteria. The selected lubricants should also have relatively high viscosity indexes, minimal viscosity loss, good thermal stability over an extended temperature range, excellent wear protection, excellent lubricity. They should be non-poisoning to catalysts, since some oil will be transferred downstream by the gas.

For reciprocating or screw compressors, the lubrication oils that meet all of these criteria are mainly synthetic lubricants formulated with PAG base stock. Overall, these provide excellent oxidative and thermal stability, which are particularly important for high-temperature applications. Relatively high viscosity indexes facilitate low-temperature startup and help to maintain acceptable viscosity over a wide temperature range.

PAG lubricants are highly stable, even at sustained high temperatures, and thus have very low deposit-forming tendencies. And importantly, any decomposition products that may form tend to be soluble in the lubricant and thus do not tend to separate as sludge or contribute to the formation of varnish or lacquer. Similarly, because of their close contact, it is important to select lubricants that are compatible with the elastomer and coatings used in the compressor’s wetted parts.

Minimizing lubricant carryover to downstream discharge streams is important for any compressor, particularly for screw compressors and reciprocating compressors. In general, gas solubility increases roughly linearly with increasing pressure. As a rough indication, PAG lubricants typically have a gas solubility that is less than half of that of mineral oils or PAO synthetic oils. High-pressure reciprocating compressors (that is, those above 300 bar, up to several thousand bar) need special attention in this regard.

Another important consideration is water solubility. PAG lubricants show less than 20% of the water solubility of typical mineral oils. Generally, mineral oils (or petroleum oils) are complex mixtures of naturally occurring hydrocarbons, but synthetic base fluids have a controlled molecular structure with predictable and advanced properties.

When it comes to reciprocating compressors, the traditional approach has been to ensure that the lube oil that is supplied to the crankcase (bearing) is the same as the lube oil supplied to the cylinder and packing. The goal has been to keep lubricant types to a small number, to bring down costs and avoid lubricant misapplication. However, in a majority of cases, it is more appropriate to provide a different lubricant to the cylinder and packing working space to improve overall performance.

For instance, lubricants for the crankcase and crankshaft mechanism should be suitable for the particular bearing application. In many cases, this will involve using a suitable mineral oil (or sometimes a synthetic lubricant that is suitable for bearing applications) in relatively large volumes in a closed lubrication loop.

On the other hand, cylinder and packing lubricants must be suitable for use in contact with gases. In many cases, separate lubricant-supply systems are used for working space (cylinder packings) and running components (crankshaft bearings), to enable the most optimal lubrication for each.

Synthetic lubricants with ISO VG 100 to 220 ratings are extensively used for reciprocating compressor cylinder-packing for process services. In the lubrication of double-acting compressor cylinders, one of the most important factors is the rate of oil feed. In general, the risk of over-lubrication is greater than that of supplying too little oil. Because many problems associated with reciprocating compressor operation can be overcome by preventing excessive lubrication, proper control of the supply of oil to the cylinder is the key.

Nearly 45% of reciprocating compressor shutdowns are due to cylinder valve and unloader problems. About 20% of reciprocating compressor shutdowns result from packing or piston ring problems. Proper selection and use of cylinder and packing lubricant is essential to improve the performance of cylinder valves, unloaders, packings, piston rings and bearings.

Operators should examine how closely the applied lubricant’s feedrate meets the actual cylinder packing’s lubricant requirements. This can be done by examining internal surfaces, such as cylinder walls. When properly lubricated, these surfaces should be covered with a thin film of oil. There should be no evidence of oil accumulation. If the cylinder surfaces are wiped with a piece of paper, oil should stain paper evenly, but should not soak in. If the paper is dry or unevenly spotted, the feedrate is too low. If the paper is saturated, the feedrate is too high. Lubricant feedrate should be adjusted to provide no more than the minimum lubricant requirement.

There is violent mixing of lubricant and gas inside rotary screw compressors (oil-flooded screw compressors). In general, screw compressors need lubricants with extra oxidation resistance to ensure long service life in closed circulatory systems. They also require lubricants with lower viscosity compared to cylinder-packing lubricants. In rotary screw compressors, the same lubricant is often used for bearings as well as internal compressor lubrication.

Inside screw compressors, the lubricant is used to lubricate mating metal surfaces and seal compression chambers (which form as a result of screw mashing) and to cool the compressed gas. Lubricant films seal and lubricate all contact lines between male and female screws.

Oil viscosity plays an important role in ensuring machine performance and minimizing power losses. The selected lubricants should maintain a dependably strong lubricant film to provide dependable wear protection, thermal stability, and appropriate viscosity across the temperatures they are likely to encounter during operation (Figure 1). At the same time, the viscosity should be appropriate to limit power losses.

 Figure 1. In an oil-injected, twin-screw compressor, the lubricating oil is injected into the gas stream to absorb the heat of compression and act as lubricant and sealant. This enables a much higher pressure ratio in a single stage and provides significant protection against corrosive gases. In multi-stage machines, inter-cooling is usually not required

Dynamic compressor and pump lubrication.Dynamic compressors and pumps are machines that achieve a pressure rise by adding kinetic energy and velocity to a continuous flow of fluid. These machines deliver relatively large volumes of fluid at relatively low or medium pressure increases. Common types of dynamic machines include centrifugal and axial compressors and pumps.

Optimal lubricants for dynamic compressors and pumps are usually premium extreme-pressure (EP), multi-purpose oils designed for dependable performance over a wide range of temperatures and operating conditions (extreme-pressure lubricants are oils that can work under shock or sudden high pressure rises). The most appropriate lubricants for dynamic machines tend to be synthetic lubricants formulated from PAG base stock, and that have dedicated anti-wear, severe-service and long-life properties.

Specifically, when specifying a lubricant for dynamic machinery, users should opt for ones that have the following characteristics:

1. Superb oxidative and thermal stability

2. Relatively high viscosity indexes (values depend on the application and can vary considerably)

3. Relatively low pour points for easy cold temperature startup (again, optimal values depend on application and can vary considerably)

4. Excellent lubricity for enhanced resistance to friction and wear

5. Extreme pressure lubrication

6. Good resistance to mechanical breakdown

7. Good resistance to sludge and varnish formation

8. Non-corrosive and stain resistant

9. Suitable compatibility with elastomers and coatings (particularly seal system components, gear unit internal paints and so on)

Lubricants with ISO VG 32, 46 and 68 grades are commonly used in dynamic compressors and dynamic pumps.

 

Turbine lubrication

Turbine lubricants must have excellent thermal and oxidation resistance at bearing oil temperatures, which can approach 100°C in typical steam turbines or heavy-duty industrial gas turbines, and can exceed 200°C in aero-derivative gas turbines. Turbine lubricants must control the rust and corrosion that could destroy precision surfaces, and resist foaming and air entrainment, which could impair lubrication and lead to equipment breakdown.

Such lubricants must also have suitably high viscosity indexes that allow more uniform lubricating performance over a wide range of ambient and operating temperatures. They should also be easily filterable without additive depletion (additive separation or sludge formation).

Turbine lubricants should be versatile, able to serve as both lubricating oil and hydraulic fluid for various turbine systems, generators, driven equipment, gear units and other auxiliary components. The goal is to simplify lubricant inventories to a relatively small number of multi-purpose products, thereby minimizing the chance of potentially costly lubricant misapplications. Products of interest for turbine operators are ISO grades 32, 46 and 68.

Steam and gas turbine oils are expected to provide years of trouble-free operation (Figures 2 and 3). In-service monitoring of turbine oils is a valuable means of assuring optimum oil performance and extended turbine life. The following recommendations are intended as a general guideline. If the limit is passed, the lubricant should be replaced, and the problem root-cause should be studied so that required corrective action may be taken at the first opportunity:

FIGURE 2. A typical turbine lubrication oil system requires a combined oil system that supplies lubrication oil for bearings and hydraulic oil for the turbines

FIGURE 3. Tilting-pad bearings are commonly used for high-speed rotating machines. Each bearing consists of a series of pads. Because oil is basically incompressible, pressure builds within the oil film, which provides a means for the oil film to transfer the load

• Total acid number increase — The warning limit is 0.3 mg KOH/g)

• Water content — The warning limit is 0.2%)

• Cleanliness — It is necessary to find the source of particulates (for example, makeup oils, dust or ash entering system, wear and so on), so that steps can be taken to address the problem

• The Rotary Bomb Oxidation Test (RBOT) warning limit is less than half of the test result value of the original oil (RBOT is a method of comparing the oxidation life of lubricants. For more information refer to ASTM 4378 and ASTM D-2272)

Steam turbines. A steam-turbine oil system is usually required to provide oil for bearings, trip-and-throttle valves, governor systems, power cylinders and similar accessories. Trip-and-throttle valves have two major functions — as an emergency shut-off valve to trip the steam turbine (that is, to cut steam flow immediately at the inlet) and to admit and throttle steam to the steam turbine, particularly during startup. The use of a combined oil unit — one that provides both lubrication oil for bearings and hydraulic control oil for trip-and-throttle valves, governors and similar applications — is very common.

Steam turbine lubricants must readily shed any water that becomes entrained during operation. Water in the steam-turbine train’s oil reservoir typically comes from one of the three following sources (A water analysis can usually determine the source):

1. Simple condensation from air within the reservoir can be minimized by maintaining the manufacturer’s specified oil level within the reservoir and maintaining good ventilation around the turbine train.

2. A leak in the shell-and-tube oil cooler(s) may allow cooling water into the oil loop. If the oil pressure is greater than the water pressure, oil will be forced into the cooling water in the case of leakage. Operators should adjust or select oil operating pressures greater than the cooling-water’s operating pressure.

3. The main contributor to entrained water in the oil system is steam bypassing the steam seals, and subsequently mixing with the lubricant oil. This is particularly prevalent in steam turbines with high back pressure or high first-stage pressure, once the seals are worn. It is good practice to provide air purge connections on the bearing seals of steam turbines. Dry instrument air will provide positive pressure in the lube oil area.

Gas turbine lubrication. In general there are two classes of gas turbines:

1. Heavy-duty gas turbines. Lubricant selection for these types of gas turbines is similar to the selection for steam turbines. Standard components of these turbines are fairly massive and the bearings are typically located at some distance from the heat sources. In most cases, petroleum-based lubricants perform suitably for these gas turbines.

2. Lightweight aero-derivative gas turbines. These turbines tend to be compact, as they are mainly based on aircraft gas turbine engines. As a result, size and weight are extremely important, and the bearings are typically located relatively close to sources of heat. Aero-derivative gas turbines tend to require that the oil not only lubricates under more-severe thermal and oxidative conditions, but that the oil also serves as a heat transfer fluid, to carry heat away from the bearings and shafts. Additionally, aero-derivative gas turbines are subjected to repeated and rapid starts, as well as hot peaks. The extreme operating conditions of aero-derivative gas turbines generally require a high-quality synthetic-base-oil (often one with an ester base).

 

Gear unit lubrication

Lubricants in gear units are often subjected to shock loads and associated overloading. This creates extreme pressure (EP) requirements for gear-unit lurication oils. Gears should be continuously lubricated, and at the same time, the oil must be kept clean.

Viscosity is probably the single most important factor in lubricant selection for a gear unit. A lubrication oil must be selected with a viscosity that can withstand the anticipated load, speed and temperature. Other important factors are: EP additives (relates to load and speed), viscosity index (relates to temperature), and oxidation stability (relates to temperature and contamination).

Lubrication-oil film thickness is mainly a function of operating speed. Based on experience, high-speed gear units (above 5,000 rpm) often require heavier oil (for example sometimes, heavier than ISO-grade 100).

Mineral oils.Mineral oils are still widely used for gear units. EP additives of the lead-napthenate, sulphur-phosphate or similar types are recommended for gear drives when a lubricant with higher load capacity is required. As a general rule, mineral oils should be used in relatively low speed, highly loaded gear drives, with a low or medium operating temperature (below or around 75°C). EP oils are more expensive compared to straight mineral oils. Some EP oils have a relatively short life at operating temperatures above 75°C.

Compounded oils, that incorporate several different additives, are also available for gear units. The most commonly available additive is a molybdenum disulfide compound, which has been successfully used in some gear applications. However, it is difficult for a gear manufacturer or operator to recommend these oils since some of these additives have a tendency to separate from the base-stock. As a result, such compounded oils are not generally recommended by vendors.

Similarly, viscosity improvers in gear drives should be used with great care. In some cases, these polymer additives can nominally improve the viscosity index and extend the operating temperature range of oil. However, what must be remembered is that polymers are non-Newtonian fluids (so shearing reduces viscosity). A gear unit is a very high-shear environment, and as a result, the viscosity of the oil will be reduced rapidly if too much polymer is added.

Synthesized hydrocarbon lubricants. Synthesized hydrocarbon lubricants are gaining more wide-spread acceptance in gear unit applications. If properly formulated, synthesized hydrocarbon lubricants (typically based on diesters and PAO) can significantly improve gear unit (gears and bearings) reliability.

Synthesized hydrocarbons (diesters and PAOs) are highly recommended for gear units. Other synthetic lubricants, such as polyglycols (high-temperature lubricants), phosphate-esters (fire-resistant lubricants) or silicone lubricants (high-temperature and heat-resistant lubricants) are not recommended for gear unit applications. This is because of their very high cost (because of high volume of oil required in a typical gear unit), possible reliability issues and lack of referenced experience in gear unit applications.

In extreme applications (those involving higher or lower temperatures or with a need for fire protection), true synthetic lubricants (such as polyglycols, phosphate-esters, or similar) may be used for gear units. The user must be careful when selecting these lubricants since some of them remove paint and attack rubber seals.

The more recent synthesized hydrocarbons (again, based on diesters and PAOs) have many desirable features such as compatibility with mineral oils and excellent high- and low-temperature properties.

 

Engine lubrication

Engine manufacturers and lubricant manufacturers offer lists that recommend lubricants that are suitable for each engine type and model (in general, the engine makeer’s preferred lubricants must take precedence). If experience indicates abnormally severe conditions, it may be necessary to reduce the oil drain interval or recommend an oil that provides higher detergency.

Detergent additives are often added to help keep the engine clean by minimizing sludge buildup. For instance, superior engine lubricants are usually formulated from specially selected, solvent-extracted naphthenic base stocks, which have inherent resistance to carbon formation in the engine’s combustion chamber, port and valves.

Generally, engine oils (whether petroleum-based or synthetic, which are more common) are available in a wide range of viscosities and are suitable for both crankcase and cylinder lubrication. They should be highly effective to reduce ring-zone suppressing, port deposits and the formation of crankcase sludge.

The following list indicates desirable properties for engine lubricants:

1. Full synthetic engine oil designed for superior performance under severe conditions, suitable for critical service engines 1 MW or above.

2. Premium ash-less lubricant for two cycle engines.

3. Detergent dispersant lubricant (with around 0.4% ash), recommended for most four-cycle engines.

4. Premium medium-ash lubricant for lean-burn and cogeneration applications.

For most operators, an engine overhaul at two- to five-year intervals is common (five-year interval is reported for low-BMEP gas engines; BMEP means brake mean effective pressure). Piston-ring and valve problems are often reported as the main reasons for the unscheduled shutdown of engines, and this is closely related to proper lubricant selection and use. When using ordinary petroleum-based lubricants, it is necessary to punch carbon (remove carbon deposits) from engines ports more frequently (let say every 12–18 months). When using superior oils (mainly synthetic lubricants), there is no need to punch carbon between major overhauls (for instance, once during three- to five-year intervals). n

Edited by Suzanne Shelley

 

Suggested reading

Bloch, H.P.,”Practical Lubrication for Industrial Facilities,” 2nd Ed., Fairmont Press, Lilburn, Ga., 2009.

Author

Amin Almasi is a lead rotating equipment engineer at WorleyParsons Services Pty Ltd. in Brisbane, Australia (amin.almasi@ worleyparsons.com). He previously worked in Technicas Reunidas (Madrid) and Fluor (various offices). He holds a chartered professional engineer’s license from Engineers Australia (MIEAust CPEng-Mechanical), and a chartered engineer certificate from IMechE (CEng MIMechE), RPEQ (Registered Professional Engineer in Queensland). He also holds M.S. and B.S. degrees in mechanical engineering. He specializes in rotating machines including centrifugal, screw and reciprocating compressors, gas and steam turbines, pumps, condition monitoring and reliability. He has authored more than 45 papers and articles dealing with rotating machines.

 

 

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