Lubrication for Centrifugal Compressors
At Simple Air Performance, we specialize in air compressor lubricant for industrial air compressors of all types, but centrifugal compressors have some unique characteristics related to the lubricant application. Here are some interesting Q&A's about centrifugal compressors put together by our formulator, a Society of Tribologists and Lubrication Engineers member and Certified Lubrication Specialist (STLE CLS®).
Q: What are the key functions of a lubricant in centrifugal compressors?
A: The centrifugal compressor is very unique, in that the lubricant:
- Does not directly contact the air, in the compression process - Serves as a high speed, hydrodynamic gear lubricant - Functions to lubricate journal and thrust bearings, again in
hydrodynamic mode. - Serves virtually no extreme pressure or boundary functions - The operating temperatures are relatively mild
On the surface, it looks like a relatively easy lubricant application, which causes many users to select a standard hydrocarbon based turbine oil.
Q: What are some key problems or issues in lubricating centrifugals?
A: The one issue that has been seen most is varnish on the surfaces of both axial and thrust bearings. This is most devastating in a compressor which has fixed tilt
pad journal bearings. In these machines, there is a clearance of about 0.001”. If even 0.0002” (two-ten-thousandths) of an inch of varnish forms on this surface, it will change the rotor dynamics, resulting in an increase of vibrations. Ultimately, there is a thermal rise at the bearing, and babbit on the bearings begins to deform. On a pinion/impeller assembly turning as much as 50,000 RPM, high vibration shutdown, failure, and a costly rebuild follows.
Q: What is the best way to avoid varnish formation on the bearings?
A: OEM’s began to recognize this problem in about 1997, when Ingersoll-Rand introduced Techtrol-Gold, a Polyalkylene Glycol (PAG) based lubricant. Prior to
this, a variety of turbine oils and PAO based fluids had been used in the machines. The introduction of PAG based fluids for centrifugal compressors totally eliminated the problem of bearing failures due to varnish formation.
Q: Why is PAG best for centrifugal compressors?
A: PAG is unique among lubricant base stocks in that when it degrades, the molecules break into smaller fragments, rather than cross-linking to form
insoluble varnish plaques on bearing surfaces. 19 years of experience with these fluids has demonstrated that PAG’s are the only type that has demonstrated the ability to totally eliminate the varnish problem.
Q: Who already uses PAG in Centrifugals?
A: Ingersoll-Rand has long adopted the PAG based fluid, Techtrol Gold as the recommended fluid for their compressors. A number of compressor service
groups upgrade compressor lubricants to PAG’s, often only after they have discovered the need for them the expensive way, because of varnish leading to a costly rebuild.
Q: Do all centrifugals need it?
A: Actually no, it is not mandatory for all. Some centrifugals, specifically those with floating tilt pad bearings can tolerate some varnish on the surface, as the tilt pads will retract as the varnish builds up. But knowing that there is some varnish
building on these surfaces, and that it could eventually cause a problem, wouldn’t you rather not have that there?
Q: Are there any other considerations when choosing PAG’s for centrifugal compressors?
A: Fluid life is a bonus. PAG fluids with advanced additive formulations, such as the one produced by American Chemical Technologies for centrifugal
compressors, have a life expectancy as high as 100,000 hours in these compressors. (NOTE: NOT rotary compressors) You read that right, not 10,000, but 100,000, as in 12 years. The temperatures and operating conditions in these machines are relatively mild. When you combine that with the superior additive technology built into these fluids, the life expectancy is extreme. If you are a fluid distributor reading this, you may cringe at the idea of a fluid you will sell only one time in a decade. Even though the fluid will last almost indefinitely, it is interesting to note that many customers will take the initiative to change a fluid after about 5 years, just because it is difficult to accept the idea that this is close to a “fill-for-life” fluid. Also, distributors have found that the loss of repeat fills for a given machine is offset by the positive effect of customer recommendations resulting in conversion of more machines at the same or other companies. So, please don’t fear the lack of repeat fill business, due to long life.
Simple Air Performance is a company with 60+ years combined experience in the industrial compressor distribution sector. We know the compressor industry and we know the needs of distributors like yourself. We only sell to distributors; never end users.
We welcome your comments on our blog and hope this information is helpful!