Keeping your compressors safe from lubricant breakdown.
When it comes to lubricating rotary screw air compressors, a unique set of circumstances can make the lubrication process difficult. In a rotary screw air compressor, the lubricant is constantly injected into the compressor air end along with the intake air, and both are moved through the compressor chamber. In most lubricant applications, oxygen, heat and moisture are not continually combined. Two of those three factors are often combined, but not all three. As the air is compressed in the chamber, heat from this compression is generated. It is this combination, along with the moisture, that is present in the atmosphere. This makes the application a difficult one for lubrication.
Today's synthetic compressor lubricants represent some of the highest lubrication technology anywhere. These lubricants have a very complex set of additives designed to stave off the effects of the harsh conditions of a rotary screw compressor. As the lubricant is used, it is normal for the additive packages to slowly become depleted. As the oil life is consumed, and the additive packages are depleted, acids will begin to form in the lubricant. The acids are derivatives of sulfuric acid, which attacks and destroys the metal components inside a compressor unit.
When a new synthetic lubricant is installed, the Total Acid Number (TAN) will be 0.0 and the barium level will often be 350 to 400 PPM. Barium is a leading additive that retards the formation of acid. As the additive package begins to break down, the barium will deplete and the TAN will begin to rise. When the TAN hits 2, it's time to change your lubricant.
Over the years, we've seen TANs of 13 to 20 and sometimes even to 30 in many compressors. Customers often wonder how the TAN can be so high when they change the lubricant every year.
Here's how it happens:
First, when oil in a compressor is drained normally, about 10 percent of the lubricant remains in the oil lines, cooler, or in the bottom of the sump. If someone changes the oil and the TAN is 2.0, it may have taken 12 months for the TAN to reach that level. The oil that has remained in the compressor will contaminate the new oil. During the next change cycle, the TAN this time can reach 2.0 in 10 months and it may hit 4.0 by the time the annual oil change takes place.
The 10 percent is left behind again and, after the next oil change the TAN can reach 2.0 in eight months. By the time the next annual change takes place, the TAN could easily be 15. And the next year the TAN could hit 30 or even 40. When the TAN reaches those high levels, big trouble starts. Usually there is serious corrosion of the compressor's internal components leading to air end failure, in addition to premature air/oil separator blockage, oil filter blockage, and control devices working improperly due to the sludge and varnish.
In the case of polyglycol based lubricants, such as Sullair® Sullube 32® and Ingersoll Rand® Ultra Coolant®, the polyglycol has a unique characteristic which causes the lubricant to begin smelling extremely foul. The odor can be so offensive that it will force people out of the plant. This odor is simply a by-product of the acids reacting with the polyglycol base-stock of the lubricant. (There are other types of base stocks used in synthetic lubricants. The other most common base stock is Polyalphaolefin ( PAO ). PAO based lubricants generally do not generate this type of odor, but the acids and varnish develop the same way.)
By the time this foul smell begins to manifest itself, the TAN is very high. In order to prevent a compressor failure, the lubricant needs to be drained and special efforts need to be made to remove as much of the old lubricant as possible. All lubricant lines and the bottom of the compressor sump need to be cleaned thoroughly.
When this happens, we suggest installing a heat activated cleaner in the compressor, allowing it to circulate as the compressor comes to its operating temperature. The cleaner will actually dissolve the sludge and varnish deposits created by the high acid lubricant. Once the unit has operated for 30-40 hours at operating temperature, the entire load of lubricant and cleaner needs to be drained and discarded. It is important to drain this blended batch of lubricant while it is still warm. If this batch of lubricant is allowed to cool, the sludge and varnish will fall out of suspension, possibly causing more sediment problems than existed before hand. The compressor should then be refilled with the normal lubricant. (It should be noted that some compressor manufacturers now advise that the flushing process should be conducted using their normal compressor lubricant. They advise a customer to run a batch of regular lubricant through the system, as described above, then discarding the entire batch. This process may clean the compressor to a degree, but it is our experience that normal lubricant does not clean as well as the heat-activated cleaners. (What this approach does do, is allow the compressor manufacturer to sell more of their high-priced lubricant! )
In order to prevent the high acid levels from recurring, we recommend that compressor operators obtain a sample of the lubricant at least every 4,000 hours and have Air Engineering, perform a free laboratory analysis. The benefits of laboratory analysis by compressor experts are:
(1) Analysis will indicate the TAN as well as 27 other compounds in the lubricant.
(2) Wear metal and moisture content of the lubricant will be indicated.
When an unfavorable condition is discovered through lubricant analysis, there are many things an operator can do to correct the operating conditions in order to improve the compressors performance. The compressor experts at Air Engineering will help you determine the most effective solution.
There's one other thing to keep in mind as you manage your compressors and lubricant. While today's synthetic compressor lubricant is truly remarkable--it's designed to last 8,000 hours--you should know that for every 10 degrees above 190 degrees F that a compressor operates, the oil life can be cut in half. As the oil life is depleted, the additive packages are also depleted and the ability to stave off acid formation is decreased.
Even if a unit operates on one very hot day and the discharge temperature reaches 210 degrees (which happens often) that batch of oil is probably only good for 2,000 hours. ( 210º minus 190º = 20º difference. 20º is two increments of 10º, 8000 hours cut in half two times, is 2000 hours ) If it's allowed to operate for 6,000 more hours, chances are good that the acid level will get very high and then contribute to the spiraling effect described earlier. The best way to monitor this is through regular, consistent oil analysis.
(1) Air Engineering performs lubricant analysis for its current lubricant customers, free of charge. If a customer is not currently an active user of Air Engineering's performance compressor lubricants, we still perform the analysis free because we are so sure that smart companies will see the extreme value the Air Engineering's performance lubricants represent.
(2) It is important to keep in mind that the true value of a lubricant analysis program comes after several samples are analyzed. After several samples, trends in the sample results will become evident. Those trends are a better indicator of current conditions than a single sample result. Remember - this is a long-term management tool.
++Sullair and Sullube 32 are registered trademarks of Sullair Corporation. Ingersoll Rand and Ultra Coolant are registered trademarks of Ingersoll Rand Corporation.
Information contained in this document reflects the opinion of Air Engineering, Inc. These opinions are based on our experiences over the past 40 years and are provided to you as a value added service in order to keep your company as competitive as possible.
Other Products:
Rotary Screw Compressor Parts
Lubricants
Air End Rebuilding
Reciprocating Compressor Parts
Heat Exchangers
Coalescing Filters Point of Use Filters
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