Research shows that reciprocating compressors that operate within North America alone consume lubricating oil worth more than $250,000 per annum. Compressor lubrication is highly essential as this ensures that all internal components of the equipment function optimally for a long time. Some of these components include gears, seals, bearings, etc.
However, the lubricating oil used for reciprocating compressors is not the type you just pick out in any hardware store downtime or anywhere. Lubricants must meet specific criteria before they are used for lubricating compressors.
This rule applies to every reciprocating compressor lubrication system. Here’s what you need to know about this system’s best practices.
The Reciprocating Compressor Lubrication System
Most reciprocating compressors rely heavily on two lubrication systems. These two lubrication systems deliver lubricants or oil to vital components within the cylinders, drivetrain, and packings.
The first lubrication system the compressor relies on is actually a recirculating system. This recirculating system is designed to protect the following:
- Crossheads in the drivetrain
But the second system, which is referred to as a ‘total-loss’ system, delivers lubricating oil to the following components:
- The cylinder bore
- Packing rings
- Piston rod
- Piston rings
The primary difference between these two lubrication systems is how the lubricant is used.
The drivetrain re-uses its lubricant several times before it requires replacement. But the cylinder lube system makes use of its oil only once before the latter is consumed during the compression process.
This is why it is crucial for the cylinder lubrication system to use lubrication oil efficiently. It helps maximize equipment reliability and minimizes operating costs.
The cylinder lubrication system – otherwise called the ‘force-feed’ system – makes use of a positive displacement plunger pump to deliver oil to the divider valve. The divider valve apportions the lubricant to vital areas within the packing and cylinder. The oil’s flowrate in these critical areas is measured by the number of oil drops per minute.
The lubrication oil must be fed at a sufficient pressure to readily overcome the gas pressure in every vital area. The pressures within these areas range from nearly atmospheric pressure to 3,000 PSIG.
As soon as the lubricant is injected into the critical area, it either mixes with the gas flowing through the cylinder within the compressor or moves to lower-pressure regions.
Reciprocating Compressor Design Considerations
Reciprocating compressors are employed in a wide variety of applications. Therefore, the compressor manufacturer must carefully analyze the details of every application in order to fully determine the worst operating circumstance for a proper recommendation of the lube rate and oil type.
In most cases, the ‘worst operating case’ is the one with the highest or greatest final-stage discharge pressure. Therefore, the recommended oil type is based primarily on the expected viscosity loss as soon as the oil gets injected into every critical area of the cylinder lube system.
Oil viscosity is significantly impacted by the following:
- Gas composition
- Operating temperature, and
More often than not, heavier hydrocarbon gases and higher discharge pressures impact cylinder oil viscosity. As a result, the cylinder oil becomes diluted, resulting in decreased viscosity. This means the oil films protecting components become thinner.
Therefore, synthetic lubricants or heavier ISO-grade mineral oils – resistant to dilution – are usually recommended and utilized to prevent loss of too much viscosity.
Other Causes of Oil Viscosity Loss
Oil starvation, as well as liquid contamination – e.g., hydrocarbon, water, etc. – can interfere with the quality of the oil film. These decrease oil viscosity or complete removal of the oil film. This results in poor lubrication, which can generate excessive temperatures that considerably decrease the lifespan of components.
Extreme poor lubrication cases can even bring about a thermal runway condition, leading to rapid component failure.
Lubrication rates influence how often oil must be injected into critical areas in a cylinder lubrication system.
Reciprocating Compressor Lubricants
Most reciprocating compressors function optimally using high-quality turbine oils with ISO viscosity grades of approximately 32 or 46.
Nevertheless, as mentioned earlier, manufacturers must specify the most suitable lubrication oil to use in a particular reciprocating compressor. Bear in mind that this recommendation from the manufacturer may have been used when test-running the compressor under highly controlled user facilities.
However, premium-grade ISO VG 32 is the turbine oil usually utilized over oil grades with much heavier viscosity. Premium-grade ISO VG 32 usually has a viscosity index of 97 and comes with a pour point of approximately –37 degrees Celsius.
Here are the basic characteristics that an excellent compressor lubricant must possess:
- Excellent foam control
- Protection against dust and corrosion, more so during shutdowns
- Long lifespan without requiring changeouts
- Excellent demulsibility in order to get rid of water that finds its way into the lubrication system
- Easy and swift filterability with zero additive depletion
- It prevents the formation of deposits, acidity, and sludge
Reciprocating compressors are known for their durability and ruggedness. Many can operate for an incredibly long time, even up to 3 decades in many cases.
This is mainly attributed to excellent contamination control, premium-grade product selection, large sumps, etc.
Reciprocating Compressor Seals
Compressors have oil face seals that rely heavily on spring-loaded stationary carbon rings in sliding contact with rotating rings. Therefore, the ring in a compressor must be made from highly durable material with a remarkable and high-quality finish.
An oil face seal is perfect, especially when the equipment is not operating and the oil pumps have been shut down. Moreover, the major components of oil bushing seals are the 2 stationary breakdown bushings with small diametral clearances positioned opposite a shaft sleeve.
The floating ring clearance controls the flow of seal liquid that cools off the seal. Floating carbon seals also require seal face lubrication.
Reciprocating compressors are employed in a wide range of applications in numerous industries. Proper lubrication is one of the keys to ensuring a compressor’s long lifespan. This is why it is crucial to take note of the type of lubricating oil used for compressors, including its attendant properties.
Taking note of and sticking to these best compressor lubrication practices guarantees the long life span of your reciprocating compressor.
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