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Compressors are engineered in a variety of types and configurations. The final design selection depends on numerous factors. Gas type and required pressure are significant factors on both compressor and compressor lubricant selection. As temperatures and pressures increase, the stress on the lubricant increases as well. The first part of this two-part series appeared in the May-June issue of ML, and may be accessed at www.machinerylubrication.com. It addressed compressor types, common operating conditions and lubricant volume guidelines. In this second part, the authors address issues that pertain more specifically to the lubricant, with some consideration of natural gas transmission compressor and lubricant issues.
The choice of a compressor lubricant depends on the type and construction of the compressor, the gas being compressed, the degree of compression and the final outlet temperature. Piston compressors provide the highest gas pressures and are among the most difficult from the standpoint of cylinder lubrication. Rotary compressors with final pressures below 1 mega Pascal (MPa), approximately 145 psi, are less difficult to lubricate. Rotary vane compressors require the use of an antiwear oil1 because an R&O oil is often insufficient for the crankcase splash lubrication of a reciprocating compressor. The selection of the proper compressor and application-dependent lubricant with the appropriate physical-chemical properties is vital to a successful process.2 ISO 6743 - Part 3A provides a classification procedure for compressor lubricants based on the type of equipment and operating conditions.
Some of the most commonly reported oil-related service problems with compressors include:3
The successful development of a compressor lubricant will depend on how well the oil meets these technical requirements.
Gas Solubility in the Lubricant
The solubility of natural gas and other hydrocarbons is much higher in petroleum oils and polyalphaolefin (PAO) synthetics compared to other commonly used synthetic base stocks such as diesters and polyalkylene glycols (PAG). That is expected because both hydrocarbon gas and petroleum-based oils are similar molecules consisting primarily of C-H bonds unlike diesters and PAGs, which are relatively polar. In fact, in a typical PAG molecule, every third atom in the polymer backbone is an oxygen atom, which makes it quite polar. Therefore, hydrocarbons are less soluble in PAGs. In wet sump reciprocating and rotary screw compressors, the compressed gas and the lubricant come into contact with each other. Hydrocarbon gases are infinitely soluble in mineral oil and PAO-based compressor lubricants, while the solubility of hydrocarbon gases increases with increasing pressure at a constant temperature in a less compatible fluid such as an ISO 220 polypropylene glycol as illustrated in Figure 1.7
Conversely, increasing the temperature at a constant pressure will result in lower gas solubility.7 Because increasing gas solubility decreases viscosity, at some point the viscosity reduction of the compressor lubricant may be too much, and lubrication failure may result because of loss of hydrodynamic lubrication, Figure 2.2
The solubility of various gases in lubricants has been measured and reported.5,7,8 The solubility was measured in a fixed volume apparatus. A known amount of gas and lubricant was allowed to reach equilibrium at a given temperature. Gas solubility was calculated using the gas laws. The lubricant was stirred to facilitate equilibrium. Solubility of methane at pressures up to 5000 psig is compared at 50ºC for three lubricants: PAG, PAO and petroleum oil in Figure 3.
The methane gas solubility in PAG is roughly half that for a PAO and petroleum oil, and that solubility was nearly as high in the PAO as in the petroleum oil. Gas solubility exhibits a significant effect on lubricant viscosity. The greater the solubility of the gas in the oil, the greater the viscosity loss (viscosity dilution). A lubricant viscosity dilution chart is shown in Figure 4 for methane at 50ºC.
Similar gas solubility comparisons for nitrogen and ethylene are provided in Figures 5 and 6.8
Lubricant Solubility in the Gas
The lubricant solubility in the compressed gas should also be minimized to reduce carryover by absorption of the lubricant in the gas. Matthews, using a constant pressure flow through a load (gravimetric) cell5 evaluated the absorption of the lubricant in natural gas. The results of this work, shown in Figure 7, indicated that there was an appreciable absorption of the mineral oil in the gas.
Compared to the PAG, the lubricant showed no appreciable loss.
Fluid Analysis Procedures
Table 1 shows a list of testing procedures that are commonly used to evaluate compressor lubricants.
In this article, the basic principles of natural gas composition and its compression have been discussed. An overview of various compressor designs used for natural gas, their lubrication and potential lubricant-related problems have been provided. Finally, various tests that have been reported for use with compressor lubrication have been summarized. This information offers a comprehensive overview of natural gas compressor lubrication and fluid maintenance.