Viewpoint: the impacts of compressed air contamination
13 Jun 2016
Aerosols and vapours both present a contamination risk to compressed air, says Red Dragon Valves' Colin Wait.
The European Standard ISO 8573, which consists of nine parts, describes all types of contamination in compressed air. But while it is true that oil-lubricated compressors are a major contributor to oil content in compressed air, this isn’t the only source to consider.
Oil-based compounds such as aerosols and vapours are frequently present in the atmosphere within engineering plants. This means that industrial pollution can enter a system directly at the compressor inlet.
It also means that indoor emissions must be considered as a source of contamination.
Ask yourself: how many times have you walked around a plant and noticed the smell of oil, chemicals or hot plastic? If you can smell it, there is a chance it will be in the compressed air too.
While it is true that oil-lubricated compressors are a major contributor to oil content in compressed air, this isn’t the only source to consider
The siting and protection of the compressor inlet, whether indoors or outside, should therefore be an important consideration. The filtration plant after the compressor may remove some of this contamination using coalescent filters. However, the success of this is dependent on the efficiency of the filter, which will reduce over time, making maintenance an important factor.
Measure up
In general, if there is liquid or aerosol oil present, there will also be vapour. However, vapour can be present when there is no liquid or aerosol content. For this reason, vapour could be viewed as the most important factor to measure, although ISO 8573-1 dictates that all states of oil should be monitored and used to calculate the overall concentration between the different phases.
The methods of measuring oil aerosol contamination in compressed air for ISO 8573-2 compliance require either long-term testing or the introduction of a filter disc membrane. Samples taken using a filter disc are typically sent away for analysis. However, the delay in results could mean that any evidence of contamination would result in significant loss or recall of product.
Meanwhile, oil vapour according to ISO 8573-5 requires testing a sample extracted from a charcoal pack by solvents, via gas chromatography and mass spectroscopy. These methods are very accurate but are not convenient or cost-effective for regular use unless there is a suitable laboratory on your site.
Content monitoring
The importance of oil content is decided by site standards as well as any relevant external standard or legislation. This would have to take into account the quality and commercial impacts of oil contamination to the product, exposure levels for operator safety and many other risk assessment factors.
When working to standards such as ISO 8573-1 with defined contamination classes, regular auditing and continuous monitoring may be required to ensure compliance.
Isokinetic sampling
Isokinetic sampling refers to a method of ensuring the measured part is a completely accurate sample of the whole flowing media with respect to temperature, pressure and velocity.
Standards require the use of these sampling points in order for the measurements to be representative, comparable and repeatable — but this is only true where full flow monitoring is not possible or desirable.
Any measurements being stated as compliant with ISO 8573, which are not based on full flow techniques and not through using fixed isokinetic sampling points for reduced flow sampling, must be regarded as non-compliant with the standards of ISO 8573.
Continuous monitoring
Meanwhile, having a continuous monitoring system in place is a great way to monitor system trends and will quickly highlight transients that need investigation. Fast notification of problems and their rectification can reduce production losses and support the site quality regime. Currently however, the ISO 8573-approved methods do not support continuous measurement processes and any system installed would require additional periodic auditing using an approved test method.
- Colin Wait is marketing & product development manager at Red Dragon Valves.
