Home > ScienceResidual oil content, oil content in the compressed air, where does it come from?Residual oil content and monitoring of oil content
The presence of oil mist in compressed air is a significant problem. Inhalation of either mineral and synthetic oil mist can cause respiratory irritation, dizziness, nausea and even unconsciousness. The presence of oil mist also results in the pipes of a compressed air system being coated with oil. This increases the risk of fire, particularly if used with enriched oxygen, and can be difficult and costly to clean up afterwards.
With the advent of synthetic oils the detection of oil mist has become more difficult. This is ironically due to some of the benefits of using synthetic oils in the first place. Their greater stability means that much higher temperatures are required to cause evaporation, meaning that oil vapour detectors are less effective at detecting the presence of oil mist. Their higher operating temperatures also mean that combustion is less likely and hence the presence of combustion products such as carbon monoxide is less likely too.
Mineral oil is essentially a natural product derived from crude oil. Produced as part of the distillation of crude oil, it comprises a mixture of numerous light oil fractions (typically in the range C15 to C50, alkanes and cyclo-alkanes).
Synthetic oil is a synthesized product made from methane, carbon monoxide and carbon dioxide. By controlling the quality of the feedstocks and the synthesis process, the oil produced is much more uniform in composition (commonly poly-alpha-olefins or esters) and has fewer impurities than mineral oils. The key benefits are greater resistance to oxidisation and thermal breakdown, longer service life, and better chemical and shear stability.
All compressors require lubrication to minimise frictional heating and wear of their moving parts, the majority are oil lubricated. During normal operation, movement of a compressors internal parts leads to the deposition of a thin oil residue on all surfaces of the compression chamber. The high speed of motion and the great forces required to compress the air, result in some of the oil being atomised. As a result all compressed gas at the output of the
compression chamber will contain some oil mist. Under normal conditions this oil mist is then removed by the after cooler and filters on the output of the compressor, typically a mixture of coalescing and charcoal filters.
Other sources of oil mist can be as the result of filter carry over. Under certain fault conditions the oil collected by a filter is re-atomised back into the air flow.
In the respect of oil mist creation, both synthetic and mineral oils behave in much the same way.
A properly maintained compressor should never generate oil mist above statutory levels in its filtered output.
There are however a number of mechanisms that can cause oil mist to appear in the output of a compressor:
4.1. Poor maintenance practices on the compressor filters.
If filters are not replaced or maintained in a timely manner, they can fail to remove the oil mist from the gas flow or even become a source of oil mist in their own right. In the case of coalescing filters they can re-entrain oil mist if the gas flow is prone to sudden high flow pulses or if they become flooded because the drain mechanism fails.
Particulate or charcoal filters can fail by becoming exhausted and effectively saturated with oil or by being partially blocked, leading to higher than expected flows that can reentrain oil from the filter media. These situations will result in oil mist being carried over into the compressor output.
If the filter media is not fitted correctly such that gas can bypass the media or the gas path is much less torturous than intended, oil mist carry over can also occur.
4.2. High compressor output temperatures.
The output of a compressor’s compression chamber will always contain a quantity of oil mist. The quantity of this contamination is dictated by the temperature in the compression chamber and the volatility of the oil used. Under normal operating conditions the temperature of the gas leaving the compression chamber is well below the flash point of the oil so the quantity of contamination will be low. The filtration on the output will be designed to remove this level of contamination so the quantity of oil contamination is controlled.
If the discharge temperature of the compressor rises, more oil mist will be present at the compression chamber output. If the temperature rises significantly, the oil will break down and significant quantities of contaminants will be produced. Any increase of contamination will have the effect of consuming the filters at a higher rate than expected and result in their early failure. Oil mist can be carried over these exhausted filters.
There can be many causes of increased discharge temperature at the compression chamber:
In extreme cases the temperature of the gas leaving the compressor can be so high that it will melt the casings of filter elements or even worse cause the oil that has been collected by the filters to burn. This produces large volumes of contaminants.
4.3. Failure of seals.
The seals on many of the working parts of a compressor are critical in separating both bulk quantities of lubricating fluids from the compressed gas and unfiltered and filtered flows of compressed gas.
If a seal fails in the compressor and bulk oil is introduced into the gas flow, the filters will quickly be overwhelmed and oil mist will be introduced into the compressor output.
If seals fail on filter elements, such that unfiltered gas can bypass the filter it will then be possible for any oil mist in the gas to pass straight to the compressor output.
One key factor to bear in mind with all three of the failure mechanisms listed above, is that other contaminants will also be generated or present at the output as a result of these failures.
Yes, as many of the mechanisms that generate oil mist and Oil vapour. The online sensor for these contaminants can give an indication that problems may be present with the compressor, that are resulting in the generation of oil mist. These measurement can be used as a prompt to stop the Compressor line.
It should be noted that the use of the OilControl is substitute for regular preventative compressor and filter maintenance. The first line of defence against oil mist is ensuring that the compressor and filters are operating correctly!
According to ISO 8573-5: 2010 all hydrocarbons with 6 or more carbon atoms per molecule in compressed air- Oil vapore + Oil aerosol - need to be measured ! The TPS OilControl (Compressed Gas Oil monitor) is capable of measuring the compleat residual oil with a minimum detection limit of 0,001mg/Nm3.
The Oil monitor is easy to install and optionally it is possible to expand the measurement range to CO2, CO, O2 and other possibilities up to ISO 8573-1.
Because residual oil is an important measurement for the end quality of your produced product it is therefore the ideal monitor to install and monitor your compressed air quality.
The complete set is delivered including a sample take-off point, a measurement system and a monitor system, providing you with a 4..20mA output or other options.
The system only needs 230VAC, 50HZ power supply to start your measurement.