Users of oil-flooded rotary screw air compressors can unlock significant benefits by implementing the recommendations in their manufacturer-required oil sampling.
Oil sampling insights
- Oil is the lifeblood of an oil-flooded rotary screw compressor.
- As oil ages, additives are consumed and additive levels drop below design parameters, allowing oxidation and acid formation.
- Viscosity levels also increase with aging, potentially allowing contaminants and varnish to drop out of the oil.
- The primary purpose of oil sampling is to extend the life of the lubricant and the compressor.
Oil sampling is a crucial part of owning and operating an air compressor, alerting users to key maintenance and performance issues that could damage key components and shorten its lifespan.
Original equipment manufacturers (OEMs) consider oil sampling the most essential maintenance procedure, especially critical if the compressors are exposed to excessive heat or contaminants or do not run often enough.
Analyzing an oil sample can identify key performance issues, allowing compressor users to take preventive action before they become problems. These concerns include:
- Component wear
- Damage to key components
- Oil contamination
- Reduced lubricant life
- Acid formation
- Varnishing
- Loss of efficiency
- Oxidation
- Corrosion
They are severe enough that most, if not all, compressor OEMs require customers to draw an oil sample and have it analyzed by an accredited, third-party lab.
For example, Kaishan requires customers to collect and analyze an oil sample every 2,000 hours of operation with 1,000 hours recommended for food-grade applications to maintain extended warranty protection (see Figure 1). Failing to collect an oil sample is one of the most frequent causes of warranty denials.
How to read an oil sampling analysis
An oil sampling report may look intimidating, but it makes more sense to understand what’s being measured and why.
There are six key points to focus on:
- Wear metals
- Additive element levels
- Other contaminants
- Viscosity
- Acid number
- Water
1. Wear metals
As oil ages, key components begin to wear and form tiny particles, indicated by any wear metals detected in the analysis. If the analysis shows those particles are metal fragments, that’s an indication those components are experiencing mechanical wear:
- Iron particles point to steel parts, such as bearings, gears or rotors.
- Copper or aluminum might mean the problem is in seals or bushings.
2. Additives
Compressor oil contains antioxidant additives that improve thermal and oxidative stability and give the oil longer life. As a compressor is used, those additives are gradually depleted. The additives portion of the sampling analysis establishes how many of the elements, such as calcium and magnesium, from the additives are still in solution, helping to neutralize the acid and prevent oxidation.
Compressor manufacturers work closely with their lubricant suppliers to select the right combination of additives for their specific designs and optimize compressor performance. As a result, it is recommended that owners follow their manufacturer’s recommendations when choosing oil for their units.
There’s a common misconception that this number shows the concentration of the additives remaining in the oil. But because these elements remain in the oil even after the additives break down, it only shows that the correct lubricant was used.
3. Other contaminants
Dust and sand contamination in the report’s contaminants section indicates that there are impurities in the oil, most likely originating in its intake air. If the contamination is silicon only, that indicates dust is the contaminant. Dirt shows up in a 3-to-1 or 4-to-1 ratio of silicon to aluminum. Check the inlet air filter and replace it if necessary.
4. Viscosity
Viscosity is one of the primary properties of compressor oil, mainly its thickness and ability to resist flow. Thus, the report’s viscosity section measures the oil’s ability to lubricate the compressor’s key components and absorb harmful contaminants.
Viscosity will increase with age as the oil starts to break down.
5. Acid number
The report’s section on acid number, tracks the amount of oxidation and identifies the amount of acid in a sample. A high number indicates an elevated oxidation level and that the additives have been depleted, leading to acid buildup. One possible cause is running the compressor at a higher temperature than it should be.
Those oxidative byproducts can precipitate out of the oil solution when the concentration becomes higher than the fluid can keep soluble. The varnish that drops out can adhere to components, reducing efficiency. Solids will also drop out and form sludge.
As a result, a high acid level will damage a compressor if left uncorrected. At the very least, the oil should be changed, but maintenance personnel should also look for root causes, especially when a sample is drawn at less than the expected service interval.
6. Water
Water increases oxidation, reducing the oil’s effectiveness. This leads to increased rust and corrosion, even damaging rubber or polymers, such as seals and gaskets. Ultimately, it will lead to premature machine failure.
Water in compressor oil may also affect its viscosity and acidity. If the report shows water is detected in the sample, it may indicate the system is experiencing rapid cycling. It may not have time to exceed the pressure dewpoint temperature and remove moisture.
Using data from oil sampling analysis
An oil sampling analysis provides hard data on how long the oil will remain effective in an application and how often it should be changed.
In addition to the conditions of the rotary compressor oil, the analysis gives critical insight into the operational conditions of the compressor, indicating high levels of wear or corrosion.
It’s an all-encompassing review of the lubricant and its integrity and it can provide critical insight into a compressor’s operation and environment.
We summarize the key measures, their causes and the recommended actions below.
Making the most of an oil sampling report
Owners of oil-flooded rotary screw air compressors can optimize the operation of their compressed air systems significantly by carefully reviewing the oil sampling analysis from their lab.
Regularly sampling oil will indicate whether it is being exposed to excessive heat or taking in contaminants. It can also detect excessive bearing wear, allowing for proactive maintenance, even avoiding an unplanned shutdown. It can help extend the life of the lubricant and equipment and measure the level of contamination, lubricant oxidation and additive depletion.
It’s beneficial for compressors exposed to excessive heat or contaminants or units that don’t run often enough. Most compressor OEMs require oil sampling to maintain their warranties.
Shawn Wood is a project manager with Kaishan USA.
