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"Can you describe the best sampling point for a wet-type screw compressor and the accepted oil cleanliness for this type of machine?"
There are three main objectives for good oil analysis: maximizing data density, minimizing data disturbance and sampling at the proper frequency. Regardless of the machine type, these three objectives must be met to receive representative information from your equipment and the oil.
Compressors can be challenging machines for lubricants, as there are often high temperatures, high pressures and many contaminants intermingling with the oil. One of the biggest factors with compressor lubrication is the gas that is being compressed. In a wet screw compressor, the oil is flooding the compression chamber, so any gas being compressed will mix with the oil. Therefore, you need to ensure that this oil can handle the gas and maintain its lubricating properties.
The majority of these systems are connected to a circulating system. This means that the lubricant flows through the machine to filters, separators, coolers and perhaps other condition-control devices installed in the circuit. This provides a variety of locations from which to pull a representative sample. If you can find an elbow on the main return line prior to the oil draining into the reservoir, this could serve as a great primary sampling location, as it would provide a snapshot of the entire system.
Many of these circulating systems lubricate more than just the compressor, including the motor, an associated gearbox and sometimes even different sections of the compressor (bearings, timing gears, screws, etc.). In these cases, a single sampling point isn’t adequate to pinpoint any alarming issues. With this type of configuration, it is a good practice to install secondary sampling ports after each lubricated component to help identify any problems being seen at the primary port.
Of course, different systems can handle different amounts of contaminants. Your fluid cleanliness targets are great key performance indicators to track in order to ensure that the compressor will have a long service life. Set targets based on the criticality of the equipment. While many compressors will run well at an ISO code of 17/14/11, if the compressor is highly critical, the target may need to be reduced to 16/12/10.
Remember, it's always best to balance the manufacturer's recommendations with your own reliability initiatives when setting targets. In a perfect world, the oil would be as clean as possible to make sure you aren't inducing wear due to fluid contamination.