The right lubricating oil for gas compressors

The question which lubricating oil to use in a rotary screw gas compressor is decisive not only for reliable machine operation but also for the profitability of the processing plant as a whole

The question which lubricating oil to use in an oil-injected rotary screw gas compressor is decisive not only for reliable machine operation but also for the profitability of the processing plant as a whole.

Frequent problems in such compressors are high wear, deposits, sludge formation and corrosion, which may all lead to extra downtime and high production loss.

This article explains the criteria to be taken into account when choosing a compressor lubricant.

Which lubricating oil is best suited? What happens when the process gas and the lubricant meet in the compressor? These and other important questions tend to arise when process gas compressors are commissioned and lubricated for the first time or after downtime of a compressor already in service.

The following real-life examples show that a sound understanding of chemical processes and some practical experience are required to satisfactorily answer all of these questions.

1) Oil-injected rotary screw gas compressors.

Depending on the requirements made on volume flow and pressure, a variety of process gas compressor types are used in many different areas of industry.

Turbo-compressors, reciprocating compressors and rotary screw compressors are the most commonly found types in industry.

Their application areas are mainly in the oil and gas industry as well as the chemical and petrochemical industry - wherever gases are compressed and processed.

Lubrication varies greatly for the different compressor types.

In turbo-compressors, the lubricating oil is required only for the lubrication of the bearings and has little or no contact with the process gas.

The lubrication of oil-injected rotary screw and reciprocating compressors is more challenging.

Here, the lubricating oil is injected directly into the compression chamber, sometimes under extreme operating conditions, to lubricate, seal and, above all, cool down the pistons or rotors.

In rotary screw compressors, in particular, the lubricating oil is in intense contact with the process gas.

Long oil service life is a major requirement for this type of oil circulation system, as opposed to the total loss lubrication in reciprocating compressors.

2) Every gas stream has a different flavour.

Apart from varying impurities in the air, air compressors simply compress a well-known gas - ambient air.

Gas compressors, however, are used to compress a wide variety of different gases and gas mixtures, from inert gases, such as nitrogen or helium, via reactive gases, such as ammonia or methyl chloride through to hydrocarbon gases, such as methane or heptane.

Some gas mixtures may include moisture or acid components, such as hydrogen chloride or hydrogen sulphide.

These very specific and varying gas flows make lubricant selection a difficult task.

Whilst oxidation of the lubricating oil is a major problem in air compressors, reactions in gas compressors may be considerably more complex and far less predictable.

Apart from chemical reactions between the gas and the lubricating oil, acidification of the oil, corrosion and sludge formation, the solubility of the process gases in the oil needs to be observed, as this may considerably affect oil viscosity during operation.

The possible problems resulting from these reactions include damaged bearings and rotors, corrosion, solid or sludge-type deposits, shortened oil change intervals, high oil consumption, foam formation or even damage to downstream process catalysts, resulting in lengthy and costly compressor downtime and subsequent standstill of the entire process plant.

Those are the problems - but what needs to be observed in selecting the correct lubricating oil for a gas compressor with a defined gas flow? 3) Typical problems and solutions.

3.1 Component wear and viscosity reduction.

Certain gases may dissolve in lubricating oils under pressure and thus reduce the oil's viscosity.

This can lead to undesirable wear of compressor bearings and rotors.

This effect is explained quite clearly in the following example: Carbon dioxide gas is dissolved in beer under pressure and escapes from the beer upon decompression - ie when the bottle or can has been opened - through foaming.

In the same way, when pressure is applied, process gases are dissolved into the lubricating oil in a compressor and separated from the oil once again as pressure drops, which usually manifests itself in the formation of foam.

In this context, the following rule applies: The better the solubility of the gas in the lubricant, the more significant the viscosity reduction of the oil.

The solubility of gases into lubricating oils depends largely on the following factors.

* Pressure: The higher the discharge pressure, the higher the tendency of the gas to be dissolved in the oil.

* Temperature: The higher the discharge temperature, the lower the tendency of the gas to be dissolved in the oil.

* Polarity of oil and gas: "Like dissolves like".

As a rule, polar gases have a higher tendency to be dissolved in polar lubricating oils than non-polar gases, and vice versa, non-polar gases are more easily dissolved in non-polar oils than in polar oils.

* Molecular weight of the gas: The higher the molecular weight of the gas, the higher its tendency to be dissolved in the oil and the higher the drop in oil viscosity.

"Heavy" hydrocarbons, such as toluene C7H8, are more easily dissolved than "light" ones, such as methane CH4 or propane C3H8.

The following example should provide a better understanding of this phenomenon.

The gas flow (indications in Mol %) to be compressed is: 20% Methane CH4; 18% Ethane C2H6; 32% Butane C4; 24% Propane C3H8; 4% C5; 1.5% C6; 0.5% C8+.

The compressor used is an oil-injected rotary screw gas compressor.

The gas mixture is compressed from 0.5 to 6.0 bar at a temperature of approx 95C.

To lubricate the bearings, the compressor manufacturer stipulates a minimum oil viscosity of 10 mm/s at an oil temperature of 70C, and for rotor lubrication a viscosity of at least 8.5 mm/s at 95C oil temperature.

Ambient temperatures are between 0C and 45C.

The questions are: Which lubricating oil is best suited for this type of gas mixture? What is the oil viscosity under its operating conditions once the gas has dissolved in the oil? Is the remaining viscosity sufficient to ensure reliable long-term operation of the compressor? An answer to all these questions can be obtained by using a special calculation programme for determining the solubility of each gas contained in the gas flow in the lubricating oil.

The benefit is a precise forecast of oil viscosity under operating conditions and, hence, a stable lubricating film as well as reassurance for the responsible operators when commissioning and operating the compressor.

This complex calculation can only be done by a very specialised lubricant manufacturer.

It is based on years of hands-on experience, extensive calculations and many online viscosity measurements on operational gas compressors.

The solution for the above example was to use a polyglycol oil with a viscosity of ISO VG 150.

During operation, the viscosity drops from 150 mm/s to approx 62 mm/s at a temperature of 40C.

The viscosity attained at operating temperature is sufficient to ensure reliable lubrication of the compressor.

The compressor should be operated with a final compression temperature of over 80C to prevent condensation of the heavy hydrocarbons (C6+).

If an ISO VG 150 mineral oil would be used for this application, problems would be imminent.

As mineral oils are non-polar and dissolve non-polar hydrocarbon gases quite easily, the viscosity of the mineral oil drops much faster and considerably lower than that of the polar polyglycol oil.

Moreover, there is no gas saturation point in the oil.

During operation, the viscosity keeps dropping and eventually falls below the stipulated viscosity requirements.

Wear protection is insufficient, and the compressor bearings or even the rotors may be susceptible to damage.

The consequences are downtime of the compressor, and often of the entire process chain, resulting in high productivity losses.

Calculation of viscosity under operating conditions is essential to ensure long-term reliable operation of a compressor.

Therefore, a lubricant specialist should be consulted to assist in the selection of the correct lubricating oil.

3.2 High oil consumption of the compressor.

Often, oil-injected rotary screw compressors consume a relatively large amount of oil, which manifests itself in high oil re-fill quantities, resulting in increased maintenance and operating costs.

Moreover, excessive oil consumption in a compressor may cause the formation of oil deposits in downstream components or damage the process catalysts.

For many processes, the specified maximum permissible oil content in the process gas is becoming stricter, a fact which should be considered when selecting the oil.

Quite often, the oil itself is the reason for relatively high oil consumption.

If the oil is injected into a rotary screw compressor, it is not only used for lubrication, but also for the cooling of the gas stream.

Some lubricating oils evaporate at the compressor operating temperatures, which may often be above 90C.

The oil vapour is carried along with the gas stream and - as opposed to the oil droplets in the gas stream - not collected by the oil separator.

Hence oil consumption depends, among others, on the evaporation stability of the lubricating oil.

As compared to synthetic oils, conventional mineral oils are characterised by a higher vapour pressure - ie they evaporate more easily - and thus lead to higher oil consumption.

Apart from the evaporation rate of the oil, its absorption in the gas flow also plays an important role.

Oil molecules may be absorbed by the compressed gas and carried along with the gas flow.

Here, the same rule applies as for solubility: Non-polar oils are absorbed far easier by non-polar gases than polar oils, and vice versa.

Once oil has been absorbed into the gas flow, it will not be collected by the oil separator.

The effects of absorption and evaporation show that oil consumption of a compressor can be influenced by selecting the correct lubricating oil.

Some process gas compressor operators are interested mainly in oil refill quantities, while others would rather know the oil content in the compressed gas stream.

Different interests - however, with the same reason, as will be explained in the following section.

4.3 High oil vapour content in the gas stream.

Many industrial processes - in particular the compression of helium and nitrogen - will permit only very low oil content in the compressed gas stream.

In this context, besides the effectiveness of the oil separator, the tendency of the lubricating oil to evaporate plays an important role.

High oil consumption of the compressor usually goes hand in hand with high quantities of oil being carried along with the gas stream.

In extensive tests, the oil vapour content in a compressed air stream was measured and recorded over a period of 80 hours.

There are variations around the factor 20 between specially developed synthetic oils and conventional mineral oils.

These measurements prove that by using specially developed compressor oils, the oil absorption into the gas stream can be reduced and the purity of the gas stream optimised, thus increasing reliability and long-term efficiency and effectiveness of the entire process.

4.4 Damage to the process catalyst.

In many industrial processes, there are catalysts downstream from the compressor, which play an important role in the further processing of the gas stream.

Traces of lubricating oil contained in the compressed gas stream may reduce the efficiency of those catalysts or even destroy them.

This phenomenon is called "catalyst poisoning".

In order to avoid this, the amount of oil in the gas stream should be as low as possible and the type of oil used should not be detrimental to the catalysts.

Mineral oils, for example, contain unsaturated hydrocarbons and sulphur compounds, which may damage the catalyst.

Some additives may also cause negative reactions, which may, in the longer term, lead to further damage.

To be on the safe side, special lubricating oils should be used, which, ideally, have been approved by the catalyst manufacturers, such as UOP.

Lubricant manufacturers who specialise in this area can offer suitable oils.

The base oils and additives contained in their special lubricants have been carefully selected to ensure compatibility, in particular with the catalyst materials.

These oils ensure reliable process operation and long and efficient operation of the catalysts.

4.5 Deposits in the compressor.

As mentioned before, process gases consist of a wide variety of gas mixtures.

In the compressor, the individual components of these mixtures are in close contact with the lubricating oil injected into the compression chamber.

Just as there many different gas mixtures, lubricating oils may consist of different base oil types and additives.

The lubricating oil, the additives and the gas stream together make up a colourful mixture of all sorts of different chemical elements.

Hence, selection of the correct oil plays a decisive role when it comes to avoiding the formation of undesirable deposits.

This real-life example illustrates the extreme effects this may have: In this particular case, the oil used for injection lubrication of a rotary screw gas compressor was unsuitable for the gas stream.

During the first 500 operating hours, the oil changed to a greenish colour.

Control of the compressor was lost, as the slide valve for volume flow control had stopped functioning.

Shortly afterwards, the current consumption of the drive motor increased and the compressor shut down.

When the oil container was opened, the operators soon realised why the compressor had stopped working: The rotors and the entire oil system were covered in green sludge.

The compressor had to be dismantled and sent back to the manufacturer for manual cleaning and repair.

Obviously, the resulting downtime cost for the operator was immense.

What can be done to prevent the formation of deposits? This requires a lot of experience and in-depth and detailed chemical know-how.

Reactive gases may react with additives or unsaturated hydrocarbons contained in mineral oils, which may lead to the formation of many different reaction products.

Possible consequences are acidification of the oil, precipitation, sludge formation and clogging of valves and other components.

Therefore, when selecting a lubricating oil for a specific gas stream it is essential to consider the possible reactions.

Often the gas stream contains aggressive components and potential hazards need to be assessed prior to the selection of the oil.

What could happen, what could be the results and what can be done to ensure effective monitoring during compressor operation? Consideration of all these questions will provide us with the optimum solution, thus saving the operator any unwanted surprises, aggravation, downtime and repair costs.

Particularly in these cases, the practical experience of the lubricant supplier is vital in achieving the right conclusions.

In summary, selection of the correct lubricant for oil-injected rotary screw gas compressors has a great impact on the reliability and profitability of the entire process plant.

The lubricant supplier should, therefore, be able to provide more than just the right lubricant.

Thanks to extensive and detailed chemical know-how, a great deal of practical experience and specially designed lubricating oils, a lubricant specialist today is able to check and predict the effects of the gas stream on the lubricant.

The costs for a tailor-made service package including in-depth consulting, gas stream analyses, on-site support and the lubricating oil will pay off, even if only one short, unplanned downtime period can be avoided.

In many cases, such a partnership between OEM, operator and lubricant specialist, over a period of time, offers significant potential to increase productivity of the entire process plant.

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