February 15, 2010


NATURAL GAS COMPRESSOR LUBRICATION

by David Doyle, CLS, OMA 1 & 2
Vice President & Operations Manager

Recovery and processing natural gas streams in developing energy reserves places significant demands on the performance of natural gas compressors. One of the key players in compressor reliability is the performance of the compressor’s lubricant. Various factors affect the lubricant’s performance, including product formulation, equipment design, and the makeup of the natural gas composition. Understanding the general requirements and problems associated with the lubrication of natural gas compressors, as well as routine testing of the lubricant, can greatly benefit the performance life of a compressor in a natural gas stream.

Compressor design can influence lubrication performance. Reciprocating compressors are used where high pressures are required.  Because there is a higher tendency for oil and gas mixing in reciprocating compressor design, higher viscosity mineral oils or synthetic polyglycol-based lubricants are sometimes used due to their low hydrocarbon solubility. Centrifugal designs operate well in applications where the compressed hydrocarbon gas must be kept free from lubricating oil contamination. Other factors that affect lubrication performance include operating temperatures, operating pressure, gas stream composition, and the lubricating system design.

The lubrication system in a compressor has to be designed properly in order for the circulating system to properly perform its function. Poor design will cause continuous operational and maintenance problems. Inadequate lubricator pump pressure caused by leaking fittings can create poor lubrication flow in the circulating system. Filtration is an important factor in system design. Large natural gas compressors commonly use an independent oil supply system. Proper design is critical in order to prevent piston and ring under-lubrication in reciprocating compressors. Deposits are formed in valves and gas passages due to lubircant carryover into the gas stream, which is due to over-lubrication. Over-lubrication can also unseat packing rings, which will allow gas leakage and cause packing and rod overheating.  

Oil coolers should operate sufficiently to maintain a maximum operating temperature of 190°F (88°C) to 200°F (93°C). A minimum temperature of 150°F (65°C) should be maintained to drive off water vapor. Cooler performance is affected by proximity to the compressor and piping size. The oil cooler should be located as close to the compressor as possible. Piping size should minimize pressure drop. In addition, if the oil cooler operating temperature is too low, the oil will become too thick, not circulate well, and create a pressure drop. 

Oil heaters are recommended in extreme cold temperature environments during startups. This will allow quicker circulation of the lubricating oil and prevent bearing lubricant starvation. This can be particularly important for bearing life. The surface temperature of heater coils should not be so high that it causes the formation of coke or carbon deposits, which will insulate the heater element and prevent the efficient transfer of heat. Deposits can also break loose and introduce abrasive material into the lubricating system.

Understanding the composition of the natural gas stream is important, since this will influence the compressor lubricant’s performance. Lubrication problems originate from a variety of sources, including:

  • Deposits form in valves and gas passages due to over-lubrication and then carry over into the gas stream.
  • Dilution of the oil by the process gas will lower viscosity, sometimes dramatically. Excessive lubricant dilution can be caused by the compressor design, the gas stream composition, or a combination of both.  Gas solubility into the lubricant is more prone at lower temperatures. Some oils are more prone to dilution than other oils. The use of higher viscosity oil or synthetic-based lubricants can compensate for dilution. There are also specially formulated lubricants designed to be less prone to viscosity decrease from dilution by liquids in the gas stream. Higher discharge gas pressure will cause greater oil dilution; higher discharge gas temperature will cause less oil dilution.
  • Poor oil quality or high temperatures due to oil cooler problems can lead to accelerated oxidation. Oxidation will increase viscosity and acidity, leading to poor lubrication.
  • Carried-over moisture and condensed gases can collect and wash the lubricant from cylinder walls, causing lack of lubrication on metal surfaces, excessive wear, and accelerate oxidation.
  • Formation of hard deposits in the cylinder area can be due to the use of high ash oil.
  • Poor cold temperature pumpability during startups will lead to excessive wear.
  • Lubricating oils that have higher detergency and additive compounding will hold water, which can affect proper lubrication of the cylinders and packings.
  • Lubricant absorption into the gas stream will deplete the protective lubricant film in the cylinder area, leading to dry cylinder walls, or metal-to-metal contact. This usually occurs at higher operating pressures (>3500 PSI).

Routine analysis of the in-service compressor lubricant is an important maintenance and preventative action tool. Testing of the in-service oil will allow an operator to judge whether the correct lubricant is being used and if the lubricant and equipment are operating properly. Test results can be trended over a period of time to monitor increases or decreases of different lubricant properties.

The particular challenges associated with natural gas compressor lubrication will directly affect the performance of in-service equipment. A well-designed lubrication system, along with the proper lubricant that is compatible with the processed gas stream will minimize equipment downtime. Regular sampling and testing are valuable tools for optimizing performance and maintenance practices.

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