Different States, Different Oils, Different Impacts
Water is the most abundant molecule on the planet. It covers about 71% of the Earth’s surface as a liquid and occupies up to 4% of the air we breathe as a gas. So, it should come as no surprise that water is also found in all rotary screw air compressor oils. Unfortunately for compressor users, and according to one of the world’s leading air compressor manufacturers, water is “the most common and most damaging contaminants in rotary screw compressors.”
A typical 100 HP rotary screw air compressor can ingest up to 200 gallons of water per day in hot humid months, and all that water gets pumped directly into the oil. Once inside the compressor, the water can change states, act as catalysts and reactants in destructive chemical reactions to transform and degrade the oil both physically and chemically and do harm to every component it contacts. Some of the most vulnerable components include the compressor’s bearings, shaft seals, separators, oil filters, oil coolers, thermal valves, and many more.
Due to its abundance, various states, and chemical and physical transformations that occur in service, water is one of the most complex, challenging, and often studied contaminants in the field of tribology. In this 3-part series, we will provide an overview of water, its destructive impact on rotary screw air compressor oils and internals, and offer suggestions for ways to monitor, prevent and correct water-related problems.
Different States, Different Oils, Different Impacts
Water coexists in compressor oils in three different states – dissolved, emulsified, and free water. These various states have different effects on the oil and compressor and depend on numerous factors. These include ambient conditions (moisture loading), compressor operating temperature and pressure, the specific oil base stocks and additives used, and the type and level of contaminants present.
Most rotary screw air compressor oils in use today are blends of synthetic base stocks. The most common of these include SHCs (synthetic hydrocarbons), PAOs (polyalphaolefins), Diesters, PAGs (polyglycols), and POEs (polyol esters). Each of these base stocks has its own unique physical and chemical properties and depending on the precise blend can differ dramatically in their water holding capacity (saturation point or solubility limit), reactivity, and overall tolerance of water.
As mentioned, water typically coexists in compressor oils in three states. Knowing the difference in these states, how to identify them, and their causes and effects on the different oil types, enables compressor users to take preventative and corrective actions to help avoid premature and costly oil replacement, nagging reliability problems, and unnecessary maintenance, repairs and downtime.
- Dissolved Water. Water normally enters air compressors in vapor form as humidity in the air. Once inside the compressor, the air and water are mixed with the oil and compressed raising its pressure and temperature. Some of the water vapor will condense into a liquid where it is mixed into solution with the oil until its oil-water-pressure-temperature equilibrium is met. In this “dissolved” state, water is molecularly bound to the oil-additive matrix and evenly dispersed as a homogeneous solution. As long as the water remains in a dissolved state, it will not separate from the oil, affect its performance, or cause serious problems. Water in a dissolved state does not change the oil’s appearance, leaving it translucent (like coffee with no cream), with no discoloration or visible layers. If you are going to have water in your oil, and there’s no escaping it, you want it to be in the dissolved state.
- Emulsified Water. Once an oil has reached its water solubility limit, known as its saturation or dew point, the water has no place to go and will no longer molecularly bond to the oil and go into solution. Instead, the excess water will coexist separately within the oil as suspended microscopic droplets known as an emulsion. With the extreme mechanical shearing and agitation provided by the air-end, oil-water emulsions tend to remain dispersed throughout the entire volume of oil and do not easily separate. Emulsions will give the oil a hazy appearance (like coffee with cream) known as turbidity. Oil-water emulsions have great potential to inflict both chemical and physical harm to the oil and compressor. With emulsions, the microscopic water droplets provide an abundance of water surface area to facilitate harmful chemical reactions and trap air bubbles that circulate freely to bearing load zones to interfere with lubrication and accelerate wear.
- Free Water. With increasing concentrations of water in the oil, the water moves from a dissolved state to an emulsion, to “free” water. Free water is characterized by the water’s ability to separate from the oil by the force of gravity. When this occurs a visible clear water layer can easily be seen below the oil. For many compressor oils, like mineral oils, SHCs, and PAOs, this means that when the compressor is shut down and the oil is allowed to cool, a water layer will form below the oil in the bottom of the sump. This can be especially problematic for intermittent duty compressors, which upon restart can send a slug of water and rust to the compressor’s bearings. Like emulsified water, free water will physically and chemically attack compressor oils and internals resulting in reduced component life accompanied by excessive maintenance and repairs.
Water Solubility vs. Oil Type. It’s extremely important for air compressor users and their oil analysis labs to know the solubility limit and “normal” water levels for the specific type of oil in use. As mentioned above, most compressor oils today are synthetic blends, and their water solubility limits will vary considerably depending on the specific base stocks, additives, and proportions used in their formulation. Generally speaking, water solubility in compressor oils can be lumped into two categories.
- Mineral Oils / SHCs / PAOs. These API Group II, III & IV oil base stocks are largely hydrophobic, non-polar, with poor solvency and little attraction to and tolerance of water. When new these oils can hold very little water in the range of 200 – 500 ppm before they reach their solubility limit, become saturated, and form either an emulsion or free water. Due to their intolerance of water, SHC and PAO compressor oils are best used in cooler drier environments and when compressors do not run intermittently with auto start/stop employed. Compressor manufacturers who use SHC and PAO oils as their factory fill often recommend their compressors have the water drained off the bottom of their sumps before each restart. This unfortunately is rarely done, often sending slugs of free water to compressor bearings during one of the most vulnerable periods when the compressor is ramping up in speed.
PAGs / POEs. At the other end of the water solubility spectrum are the API Group V synthetic base stocks. These oils are generally characterized as being hydrophilic, polar, with good solvency and relatively high attraction to and tolerance of water. Compared to SHC and PAO compressor oils, PAG/POE blends easily hold more than 10 times the amount of water in solution, with solubility limits that typically range from 6,000 – 8,000 ppm of water, or more. This greater water holding capacity makes PAG/POE oils a superior choice for hotter, more humid environments as they are much less susceptible to becoming saturated and forming harmful emulsions or free water. And because water in PAG/POE compressor fluids don’t easily separate, they perform well in both intermittent and continuous duties as they do readily form free water that needs to be drained off the bottom of the sump before restarts.
Don’t miss Part 2, where we will discuss the numerous problems created by water in compressor oils.