The design of an Oil Injected Rotary Screw Compressor allows for contact between the lube oil and the natural gas. Acting as sealant and protectant against rust and corrosion, lube oil is a crucial requirement for all rotating parts and also assists the cooling process by removing heat generated by the compression of the gas.

That being said, contact between natural gas and lube oil should be a consideration as interaction can result in the dilution of the lube oil due to natural gas ingress, which decreases the viscosity of the oil.

Therefore, the in-service (diluted) viscosity of the lube oil needs to meet the minimum viscosity requirement of the compressor. Field experience has shown that the optimum viscosity range for most compressors is between 20 – 30 cSt at oil injection temperature.

To effectively calculate the dilution that will occur, it’s important that the molecular weight of the natural gas components are considered when selecting the lube oil. Typically, the higher the molecular weight of the gas, the more dilution will occur and vice versa. With this and many other factors, like gas discharge conditions and the presence of hydro-carbon liquids influencing this correlation, it‘s advised that industry expertise is sought to determine the correct lube oil.

To help operators with this process, the Petro-Canada Lubricants Technical Services team uses a calculation program that can determine the percentage of dilution based on all parameters, in combination with the suction conditions of the compressor, to show an exact dilution percentage. Based on the results, a suitable oil can then be selected to meet the viscosity requirement of your compressor dependant on its specific operating conditions.

The differences between mineral and polyalkylene glycols (PAG) based fluids can be critical to your oil selection decision because the dilution of the lubricant with natural gas will largely depend on polarity of the lubricant.

Mineral based lubricants, polyalphaolefins (PAO) and natural gases are all classified as non-polar, meaning they naturally mix with each other, leading to a high dilution rate. Conversely, PAG fluids are polar, causing a lower dilution when they come into contact with natural gas.

Additionally, keeping the lube oil “dry” is important in creating optimal operating conditions in terms of bearing life. This is easier to achieve with mineral and PAO based lube oils. A PAG based oil with its higher polarity has more affinity to water than a mineral or PAO based oil and will therefore hold more water dissolved under the same conditions. In cases with too high an oil dilution, PAG based fluids can however help to meet the viscosity requirements of the compressor. Maintaining sufficiently high gas discharge temperature to ensure water remains in the vapour phase helps to keep the lube oil “dry”.

If water can’t be avoided, a water soluble PAG fluid is recommended over a mineral oil or PAO. For example, a water soluble PAG, like the SPX 7000 series of Petro-Canada Lubricants can hold more than 2% of the water dissolved, whereas under the same conditions using a mineral or PAO based lube oil, free water would form. Keeping water dissolved can minimize corrosion issues compared to conditions where free water is present.

Systems that compress heavy natural gas with a high hydro-carbon liquid content can be difficult to protect. Hydro-carbon liquids need to be removed at the compressor intake or kept in the vapour phase by a high discharge temperature, or these liquids will then meet the lube oil, resulting in heavy dilution of a mineral or PAO based lube. The resulting viscosity would be too low to provide sufficient film thickness inside a bearing.

Although not ideal for these situations, PAG oils are more suitable. It must be noted that phase separation between the hydro-carbon liquid and PAG can occur in the separator vessel. That being said, if no pure hydro-carbon liquid enters the bearings, a PAG based fluid will be acceptable.

PAG based fluids are incompatible with all mineral-based lube oils and PAO’s. We recommend draining and flushing existing lube oil before switching to an alternative type to keep your equipment healthy and up and running. A thorough drain of the system, which will include replacing the filter, is required as part of the flushing process, before a new oil can be used.

Whichever lubricant you choose, you need to be confident of its quality. By working with Petro-Canada Lubricants, customers can be confident in the consistent quality and performance of all their lubricants. Our products are manufactured to the world’s most demanding standards so you can count on maximum uptime and minimum downtime and look forward to higher productivity and profits.

For more information and guidance on selecting a lubricant for natural gas screw compressors, please visit Gas Plants, Pipelines & Power Generation.