Recovery and processing of 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 its lubricant.
Different physical properties and performance levels are required for different engine oils depending on the engine. Marine diesel engine construction as Crosshead Piston or Trunk Piston will have separate oil performance requirements as well as condition monitoring considerations. The performance properties of marine engine oils consider the fuel type burned as well as the mechanical configuration of the engine.
Various factors affect the lubricants performance, which includes product formulation, equipment design and the makeup of the natural gas composition. Understanding of 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.
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 under lubrication of pistons and rings in reciprocating compressors.
Over lubrication, on the other hand, can cause oil carry-over into the gas stream, causing formation of hard deposits in the valves and gas passages. Over lubrication can also unseat packing rings, which will allow gas leakage and cause packing and rod overheating.
Natural Gas Composition:
Knowledge of the composition of the natural gas stream is important, since this will influence the performance of the compressor lubricant. Lubrication problems originate from a variety of sources, which can include the following:
- Deposits formation in valves and gas passages caused by over lubrication and carry over into the gas stream.
- Dilution of the oil by the process gas will lower viscosity, sometimes dramatically. Excessive dilution of the lubricant can be caused be either the compressor design, the gas stream composition or a combination of both. Solubility of the gas 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. Also, there are specially formulated lubricants that are 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 quality oil 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 effect proper lubrication of the cylinders and packings.
- Absorption of lubricant 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 that will monitor increases or decreases of different lubricant properties.
The particular challenges associated with lubrication of natural gas compressors will directly effect 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 down time. Regular sampling and testing is a valuable tool for optimizing performance and maintenance practices.