Oil Analysis Boosts Compressor Reliability

Daryl Beatty, Dow Chemical Company

There are several key elements that must be addressed to ensure an effective compressor lubricant analysis program. Errors or omissions related to these elements can lead to unnecessary expenditures and lower reliability.

Knowing what actions should be taken based on compressor lubricant analysis is important for success. Understanding some common mistakes and misconceptions about water in compressor lubricants and the effect the environment has on the lubricants and the compressor is also key to a successful lubricant analysis program.

Inlet Air Quality

For centrifugal compressors, the significant air quality issues are particulate removal through filtration and the effect of acid gases on intercooler corrosion. In a centrifugal compressor, there is only minimal contact between the air and the lubricant and the sump sizes are typically quite large, resulting in dilution of any contaminants.

For these reasons, it is rare for ambient air conditions to significantly affect fluid life, or have a detrimental effect on the running gear of the compressor. Double-acting reciprocating compressors are not totally immune to air contaminants, but are less subject to inlet air quality issues because of a continuous infusion of fresh cylinder lubricant.

The fresh lubricant serves to flush contaminants through the system with a protective effect, even though these compressors have a high lubricant consumption.

The ambient air is a more serious concern for rotary screw compressors where the entire flow of air through the compressor contacts the fluid, and the fluid is effectively acting as a scrubber to absorb the acids and contaminants.

Even a low concentration of acid is significant, when the sheer volume of air being handled is considered. Some of this acid will be absorbed by the fluid, which will show up analytically as a lower pH and higher acid number (AN).

Lubricants for rotary screw compressors are formulated with good corrosion protection, but eventually even that is overwhelmed. Once this occurs, filters may plug more frequently due to corrosion particulate. This effect results in significantly shortened fluid life. It is not unusual in a contaminated environment to see the life of a nominal 8,000-hour fluid reduced to 2,000 hours.

The life of downstream components, such as aftercoolers and dryers, is also often compromised by corrosion caused by acid gases which pass through the compressor from the environment. These gases then condense with water in the coolers and dryers and drastically increase corrosion rates.

What can be done to extend fluid life and solve these problems?

  • Remote air inlets may be installed to obtain inlet air from a source away from the contamination. This is typically outside the building. Ironically, inside air is rarely of better quality than the makeup air being taken into the building.

  • Air can be tested by suspending corrosivity coupons of copper and silver in the air near the compressor. After a specified period, laboratory analysis of the resulting compounds on the surface of the coupons will reveal the type of contaminant in the air and the degree of contamination.

  • Inlet air scrubbers may then be prescribed based on the degree and type of contamination to remove contaminants from the inlet air. The result is longer fluid life and decreased corrosive attack of compressor bearings, coolers, dryers and downstream equipment.

Lubricant Analysis Parameters

The key analysis parameters vary with the type of lubricant being used. Most new rotary compressors are equipped with polyglycol-based lubricants. With a polyglycol or polyglycol/polyolester-based compressor fluids, the following parameters are of great interest:

  • pH - A rapid or excessive decrease in pH indicates ingestion of acid gases or other contaminants from the environment. This will require a fluid change, but also indicates that the source of contamination needs to be eliminated.

  • AN - The acid number is an indication of remaining useful fluid life. AN may increase with either oxidative degradation of the lubricant or accumulation of contaminants from the environment. Either way, this accumulated acid reflects the depletion of the corrosion inhibition package.

    Suggested change points vary, typically from 1.0 to 2.0. The fluid life from the time the AN reaches 1.0 until the time it will reach 2.0 is only 10 percent to 20 percent of the overall life span. Due to the difficulty of removing the last 20 percent of the fluid from the compressor, it is probable that stretching the change point from 1.0 until 2.0 actually costs more in terms of shortening the life of the next charge of fluid, than is gained on the first charge. Stretching the change point here is false economy and results only in a greater exposure of the compressor to fluid containing high levels of acid.

  • Viscosity - The viscosity of some original equipment manufacturer (OEM) compressor fluids are specifically designed for the needs of that compressor application and do not fit in either the ISO 32 or 46 viscosity ranges. With polyglycol fluids, viscosity will normally increase about 10 percent with use, then stabilize.

    If lab personnel are not aware of the initial viscosity of a fluid, they often assume it originally fit into an ISO range and then mistakenly condemn it for high or low viscosity. It is important to always compare viscosity to the specification for that fluid, not an ISO range. With polyglycols, it is unusual for fluid to fail due to viscosity change, because the fluids are resistant to varnish and sludge formation and don’t have a tendency to gain viscosity.

  • Contaminants - Hydrocarbon contamination is typically monitored to assure that operators are not mixing fluid types. If fluids are mixed, the life of the fluid may be compromised.

  • Oxidation - Polyalphaolefin (PAO) or mineral oil change points can be determined by the degree of oxidation of the base fluid. This is not necessary with polyglycols, because AN is a reliable indicator of fluid condition.

  • High Particulate - If the corrosion particles are mostly small particulate, the filters should be changed and measures should be taken to determine what acidic condition is causing the corrosion.

For PAO-based compressor lubricants, the pH, AN and viscosity must also be monitored. In addition, it is useful to monitor oxidation, typically by infrared spectroscopy. Monitoring the oxidation level is useful in preventing varnish and deposit formation.

In cases where the fluid quantity justifies it, rotating pressure vessel oxidation test (RPVOT) can reveal the remaining useful life of a PAO-based fluid. With polyglycols these steps are not necessary, because AN is a reliable indicator of the remaining fluid life.

Table 1. Typical Values for New and Suggested
Condemning Limits for Polyglycol Compressor Fluids

Water Content

Table 2 compares typical water content of samples from air compressors with other equipment in a plant. This specific plant is located in a humid environment in a southern U.S. state.

Table 2. Water Analysis of Lubricant Samples from Compressors
and Other Typical Equipment in the Same Plant

The water content of the air compressors ranges from 0.4 percent to 0.6 percent, while other types of equipment in the same plant have about 1/100 the water content. Labs often flag a sample like this with an alarm, when in fact these levels of water are normal for rotary compressors, and the compressor fluids that are specially made for them are formulated to function in this environment.

Consider this scenario: A plant performs periodic routine analysis on each of these fluids. Each time analysis is done, the water level is reported as shown in Table 2, along with the recommendation that the fluid in the compressors be changed. What is not considered is that the water level in the new fluid will again reach these levels quickly.

Fluid has been wasted with no benefit. Water levels will also vary with fluid basestock type, because some basestocks are capable of tolerating more water before free water is released into the fluid. The keys are:

  • Know the maximum amount of water that a type of fluid will tolerate before free water is released into the system. For example, polyglycol compressor lubricants, which are used by several compressor OEMs, will tolerate about 0.8 percent water before free water becomes a problem. With hydrocarbons and synthetic hydrocarbons, free water will typically become an issue at lower levels.

  • Recognize that the water level in a sample is also a characteristic of the equipment application in which the fluid is used. Rotary air compressors are higher than nearly any other application because of the contact of the lubricant with large amounts of humid incoming air. When the air is compressed, water vapor is condensed. That water must either be absorbed by the fluid or allowed to circulate as free water. Other types of equipment, as demonstrated in Table 2, tend to be much lower. Analysis alerts should be set accordingly.

  • Changing the fluid does not solve a water problem for long. The water level varies with humidity, ambient temperature, duty cycle and machine operating temperature. Water is being continuously ingested.

  • Make sure the lab is aware of fluid type and application, so that water level limits can be set accordingly.

  • Don’t obsess about water levels in compressor fluids. The water level is high in these units compared to other types of equipment. Historically, compressor air ends (also called compressor units) from major OEMs typically last about 10 years, with a few reaching 20 years before having to be rebuilt. All of them have had high water levels and served long lives. Only recently have the high levels been noted. The compressor fluid should be specifically designed for use in rotary compressors, and contain corrosion protection adequate for this demanding application.

Compressor Condensate

The analysis of compressor condensate is a useful tool in detecting some corrosive or acid gases in the air that may not be effectively trapped by the lubricant. A low pH or high AN in the condensate may reveal a corrosive condition, which if left unchecked, will lead to short aftercooler and refrigerated dryer life.

The typical source of these problems is contaminated inlet air and remedies which were previously discussed. Metals analysis of the condensate can also reveal the rate of corrosion, which may already be occurring in aftercoolers and dryers. In addition, the total organic carbon (TOC) or total oil and grease (TOG) analysis indicate the carryover rate of lubricant from the compressor, which is an indicator of the efficiency and condition of the air/oil separator.

Figure 1. Two-stage Tandem Compressor

Fluid Analysis

Certain actions should be taken based on elemental analysis or particulate in the fluid. The sources and the importance of particulate and trace elements in a rotary compressor should be considered first. A rotary compressor is unique in that the metals and particulate in the fluid can originate from several sources. Primary sources include:

  • Ingestion with the inlet air, either through or bypassing the inlet filter

  • Corrosion particles, primarily from the upper portion of the receiver tank

  • Wear debris from rotors, housing, gears and bearings

The key to determining the type and source is analysis. When particulate is the concern, analytical ferrography is one simple and useful technique for differentiating between these three sources.

Once determined, any of these problems can be readily resolved. Figure 2 demonstrates the various types of ferrous particulate that can readily be distinguished by analytical ferrography. In this case, wear debris and corrosion particles are present.

Figure 2. Analytical Ferrograph from a Rotary
Compressor Showing a Variety of
Particulate Types

Figure 3 shows a variety of particulate types identified by this technique. Of particular interest in rotary compressors is the ability to distinguish ferrous particles originating as wear debris, which is a serious concern, from corrosion particles that typically originate from the surface of the receiver tank, and should be trapped by the bearing filter before reaching the bearings.

Figure 3. A Variety of Particulate Types
Distinguished by Analytical Ferrography

In addition, a variety of particulate is ingested with the air from the environment. While it is not usually necessary to specifically identify the items that are ingested, it does indicate an inlet air filtration problem, which can then be diagnosed and remedied. Identification of particulate into these three categories - wear, rust/corrosion and ingested material - will determine the action needed to alleviate these problems, thereby enhancing compressor life and reliability.

Trace element analysis is also useful for the early detection of potential problems in compressors. Table 3 offers some possibilities for explaining the presence of various elements in compressor fluids and the significance of those elements.

Table 3.

It is important to note that many of the elements are useful for resolving specific issues, but are not tracked on a routine basis. Of the elements listed in Table 3, iron is the most useful wear metal for helping determine the compressor’s specific condition. In addition to spectroscopy, one of the particle size analysis methods will provide more useful data. An increasing trend in particulate levels should be investigated.

Figure 4. Lubricated Compressors

Figure 5. Two-stage Tandem Compressor

Good Observation Skills

Analytical techniques are only part of the story. There are many simple operational issues related to the fluid and fluid system that the lab has no way of detecting or resolving. There is no substitute for effective observation of operating conditions, trends and the environment in which the compressor is operating. For centrifugal compressors, trending of vibration readings on each stage will reveal the formation of bearing deposits or other subtle changes that can then be remedied before significant damage occurs.

The following items should be considered for rotary screw air compressors:

Inlet air temperature, humidity and contamination. Contaminated air contaminates the compressor fluid, resulting in equipment failures and downtime. Avoid ingestion of air from sources containing acid gases. These would include boiler exhaust, diesel exhaust, any operations discharging acid and others.

Draeger tubes used for personnel air monitoring are usually not sensitive enough to detect the concentrations of contaminants that will cause a problem. Air quality test kits, utilizing silver and copper coupons are useful in evaluating the quality of the air that is in contact with the lubricant.

Armed with this knowledge, a remote air inlet may be installed to obtain air from a cleaner area, or an air scrubber may be installed to protect both the lubricant life and the compressor.

Low operating temperature. Proper discharge air temperature is important to maximizing fluid and compressor life. As a general rule of thumb for a humid environment, the discharge temperature should be 100°F (55°C) higher than the ambient temperature of the inlet air to prevent accumulation of water in the lubricant.

Air-cooled compressors typically have coolers sized to prevent the compressor from running below this temperature and thus automatically avoid this problem. A water-cooled compressor may cool too efficiently and condense water in the fluid. If the water separates from the fluid and collects in the receiver tank, the free water may not be detected by lab personnel, because the sample will not be representative of what is truly occurring in the compressor.

One solution is to observe the temperatures and make adjustments as necessary. Depending on fluid type, temperature and humidity, water levels of up to 0.7 percent in a rotary screw compressor are normal. Levels above that amount indicate free water in the system and require intervention.

High operating temperature. High operating temperature presents a different set of problems. The fluid will easily remain below 0.5 percent water content with no free water, but high temperature is detrimental to fluid life. Every 18°F (10°C) increase in temperature will reduce fluid life by approximately half.

The temperature at which a fluid is rated for its nominal life, typically 8,000 hours for most polyglycol, ester or PAO compressor fluids, will vary. A fluid rated for 8,000 hours at 200°F would be expected to significantly outlast a fluid rated for 8,000 hours at 180°F.

Load and unload performance. Load and unload performance affects carryover and energy savings. A compressor, which is allowed to run unloaded with a minimal air demand, will typically experience high lubricant consumption and more internal condensation and corrosion. In addition, it will use much more electricity, resulting in massive energy waste and increased expense.

This cycle and loading should be observed and the compressors’ usage should be adjusted to match the air demand. Compressor OEMs have developed computerized control systems to continually monitor and adjust to maximize savings.

Leaks and condition of all couplings. Minor fluid leaks may be a warning of an impending failure of a coupling, gasket or seal. They should not be ignored.

The amount of makeup fluid. The volume of makeup fluid added to each compressor should be logged for frequency and amount. Attempt to do a “material balance,” or accounting of the fluid; how much fluid is lost to leaks versus what is carried over into the condensate traps or plant air.

A sudden increase in fluid makeup rate might correspond with the change to an inefficient, defective or improperly installed air/coolant separator. Finding this quickly can result in fluid savings and avoid the contamination of downstream components. OEM separators are typically closely matched to the air flows and other characteristics of specific compressors.

Also, the materials of construction are compatible with the OEM fluids, to prevent compatibility failures. Savings in lubricant and energy can greatly exceed the cost of separator elements.

Separator and bearing filter monitoring. It is important to track separator and bearing filter differential pressures and frequency of changes. These are useful trends for several reasons. First, the depletion of corrosion protection packages in the lubricant may result in shortened filter element life, due to blinding with corrosion particles.

Progressively shorter intervals between filter changes, while on the same charge of lubricant, are strong indicators of this condition. The ingestion of particulates in the air may also result in short separator life. Particulate levels and element life may relate to the quality of the inlet filter element, an air leak or improper installation.

Also, changing separator elements at the OEM-prescribed differential pressure (usually about 10 psi) results in energy savings and avoids the potential collapse of a separator, which results in massive amounts of fluid being discharged downstream into the air system.

These are a few examples of lubricant-related observations which are easily made and may be consistently monitored to improve compressor reliability.

A Strategy for Compressor and Compressor Fluid Health

A strategy should focus on leading rather than trailing indicators. In trying to anticipate compressor reliability issues, the focus should be on the indicators discussed above, which can anticipate and prevent problems. Fluid condition, in terms of pH, AN and contamination should be monitored before corrosion results in high metallic content.

The delivery of clean inlet air to the compressor to prevent contamination of the lubricant and corrosion of the system should be a main focus. Analysis is also critical, but only if the proper parameters are assessed and action is taken to prevent and resolve future problems.

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