Producing oil-free compressed air with continuous monitoring
Process safety without a bad aftertaste
In the daily use, there is usually considerable insecurity about the constant, reliable quality of process “oil-free” compressed air. For good reasons too, even oil-free compressors are not a 100% guarantee. In addition, the treatment downstream of the compressor is not always able to reduce the residual oil content in the system to the required degree. Finally, continuous monitoring is extremely difficult. Most companies using “oil free” compressed air have no idea how contaminant free it really is. In terms of process safety they “fly blind”
This applies, above all, to highly-sensitive applications where compressed air with a far lower residual oil content than specified for ISO 8573 – class 1 for technically oil-free compressed air is required, for example in medical technology, pharmaceutics, surface technology or in food processing. A general difficulty can develop into a real problem: the continuous monitoring and measurement of the – hopefully – extremely low residual oil content in the compressed air. After all, we are talking about values in the order of 0.003 mg oil introduction or less in one cubic metre of compressed air.
However, let”s first look at the “normal case”: the production of technically oil-free compressed air in accordance with ISO 8573-1 in class 1. This standard defines the oil content, including the oil vapour, of max. 0.01 mg/m³ com-pressed air. This corresponds to approximately four hundredths of what is contained in atmospheric air. This amount is already so small that it can only be guaranteed with multi stage downstream treatment. What happens in the event that this cannot reliably be achieved?
In German production and processing plants alone, damage running into mil-lions of euros occurs annually as a result of contaminated compressed air. Subsequent to expansion of the compressed air during the application, the oil fractions in it settle for example on surfaces and cause an oil film which, in turn, affects the adhesion of paints or complicates bonds. In most cases, the problem focuses on residual aerosols and oil vapours. These may, amongst others, impair sensitive tool parts at the point of consumption, wash out basic lubrications on components or contaminate end products.
In many cases, oil mists also contain harmful nitrosamines. The fact that al-ready a fraction of 0.3 mg oil per m³ in breathing air is perceived as an odour is at least unpleasant.
Oil-free does not necessarily mean oil-free
Even downstream of dry-running screw and piston compressors, in which the real compression process takes place without oil as a lubricant, sealant and coolant, clean compressed air in the sense of absolutely oil-free compressed air cannot be expected.
The danger lies in the “raw material” – the induced air.
With the intake air, a multitude of hydrocarbons enter the compression proc-ess, in particular when the air is taken directly from the environment and not from an especially protected compressor room. Solvents such as acetone, aromatic hydrocarbons such as benzene, toluene or xylene, and polycyclic aromatic hydrocarbons such as naphthalene, contaminate the compressed air. In addition, there are fuel residues in the ambient air, namely hydrocarbons such as gas oils, petrol or kerosene. It also contains all components of normal ambient air in a compressed form: dusts, dirt particles, particulate matter, moisture, germs etc.
To aggravate the situation, some contaminants even affect each other, thus forming further risk factors. For example, water forms an emulsion when com-bined with oil, and dust can aggregate with oil or water and form larger dirt particles.
In this respect, one has to keep in mind that compression to an operating pressure of 8 bar (g), for example, means nine times the concentration of the ambient air components.
Even when placing one”s oil-free compressor in an area that has been carefully shielded against environmental effects and provided with filtered ambient air, one is not at all on the safe side. Oil vapours can also reach the outside through the compressor-internal gearbox-casing ventilation. In large compressors, this is in fact an almost continuously occurring effect.
This means that even the best “oil-free” compressor in the ideal location needs support. In most plants, this is implemented by means of drying equipment with simultaneous oil separation and by oil separation filters – often employed as a combination.
Numerous hardly calculable influence factors
However, such components downstream of a compressor are not always in a position to limit the residual oil content in the system to the range required for demanding applications because the factors exerting an influence on filtration and adsorption in a compressed-air plant are too extensive.
This starts with the hydrocarbon molecules, which are modified by shearing and heat input in the compressor. The aerosol state downstream of a screw compressor, for example, is smaller than downstream of a piston compressor.
Other influential factors are the operation mode of the plant, the frequency control, cooling and the design of the oil separators. A continuous up and down during the compressor operation leads to an enormous stress on the oil separator while frequency-controlled compressors cause a varying oil intro-duction that is significantly higher in the lower speed range than during full-load operation.
What about filtration? High-quality activated-carbon filters serve perfectly in compressed-air processing but they also require the plant operator”s attention. For these filters, regular maintenance and replacement in time are vital re-quirements for safe and reliable operation. This is not always easy because the influences impacting the filters often escape the plant operator”s control. The flow rate, the temperature and the moisture, for example, have considerable effects on the adsorption capacity of the filters.
Not enough time for an emergency shut-down
In the event that – no matter which of the problem factors mentioned is in-volved – an increased oil introduction into the compressed air takes place, the operator rarely has the time to react to the oil penetration with the disconnec-tion of the plant.
Even after a pre-alarm, he has only a few minutes to prevent the worst case: the emergence of oil-containing compressed air at the points of consumption. According to example calculations of the German compressed-air specialist BEKO Technologies GmbH, this occurs in less than five minutes in a com-pressed-air plant which is operated with a flow rate of seven metres per sec-ond and which has a pipework length of 2,000 metres. The extremely cost-intensive consequences are contaminated products, machine and process failures, production stoppages or even health problems for the employees.
Until recently, no really reliable technical solution was available for the proc-essing of constantly oil-free compressed air, not to mention the possibility of continuous residual oil content monitoring via measurement and control.
Conditions which have radically changed, with the introduction of an innovative solution for compressed-air processing: the catalysis method.
Catalysis breaks with the tradition of compressed-air processing
BEKOKAT, the catalytic converter developed by BEKO Technologies, over-comes all weak points of the previous technologies and offers compressed-air purity which, with a residual oil content of a barely measurable 0.003 milli-grams per cubic metre of compressed air, exceeds the requirements of ISO 8573-1 for technically oil-free compressed air of class 1 by far.
Combined with the METPOINT OCV measuring system for the detection of hydrocarbon vapours, which is also offered by BEKO, BEKOKAT represents a dual strategy for highest process safety during the processing of oil-free com-pressed air.
The BEKOKAT method opens the way to absolutely oil-free compressed air via a completely different technological approach than previous solutions: via catalysis.
This method realises the total oxidation of hydrocarbons – meaning in a con-centrated, comprehensive process step subsequent to compression. The complete removal of oil from compressed air thus takes place in only one sin-gle plant component. This component functions independently of the ambient conditions, the oil input concentration and the relative humidity of the com-pressed air.
What remains are clean air and clear water
BEKOKAT tackles the lubricants and oils in the compressed air supplied by the compressor. Subsequent to the compression stage, these exist in the form of gas, vapour or aerosol. Through the employment of the BEKOKAT, such air components are fully converted into carbon dioxide and water. Even the con-densate accumulating during the cooling-down of the compressed air is abso-lutely oil-free thanks to the catalysis method, and can also flow directly into the sewerage system without being processed.
Currently, no other system is capable of implementing this total oxidisation and the absolutely residue-free mode of operation.
In the BEKOKAT, granular material serves as the catalytic converter, which is heated up to an operating temperature of approx. 150 °C by means of heating elements. In the catalytic converter, the oil molecules are broken down until only one carbon atom remains. In the final catalysis phase, the oil molecules are oxidised down to H2O and CO2.
It is essential that the hydrocarbon chains, i.e. the oil molecules, can be “cracked” at any point. They are continuously broken down until only carbon dioxide and hydrogen remain. Therefore, the hydrocarbon chains from oil-free and oil-lubricated compressors varying in length pose no problem at all to BE-KOKAT.
It is not always well known that activated-carbon filters are not able to adsorb polar compounds such as alkenes, alcohols, glycols or ketones. In contrast, the BEKOKAT system also does a good job as far as these substances are concerned and completely removes them from the compressed air. This is generally independently of the inlet conditions.
Further on in the process, the purified compressed air is cooled down in a heat exchanger to approx. 10 to 15 °C above the inlet temperature and is available for the respective application.
The long service life of the special granular material of the BEKOKAT is also extremely advantageous as far as profitability is concerned: it only needs to be replaced after a good 20,000 operating hours. In contrast, the life of an acti-vated carbon filter is already depleted after approximately 500 operating hours, even under perfect operating conditions.
Advances into new purity dimensions
With the BEKOKAT method, it is for the first time possible to constantly achieve oil-free compressed air with a barely measurable maximum residual-oil content of 0.003 milligrams per cubic metre of compressed air, meaning a quality which is required in the extremely demanding fields of medicine and pharmaceutics, of food processing and packaging and of surface technology.
In these fields however, there was until now not only a lack of absolutely safe processing but also, above all, a lack of an uninterrupted, continuous meas-urement and monitoring of the oil-free compressed air. This was a problem, because according to the golden rule in process assurance, it is a fact that “Only what can be measured can be managed”.
It is of course possible to determine the residual oil content in compressed air via multiple regular laboratory investigations, but in most cases the respective results are only available after several days or weeks. This is far too late to react quickly and in an effective manner to any quality problems caused by oil-contaminated compressed air – as is substantiated by the previous calculation example of BEKO: less than five minutes for an emergency shut-down subse-quent to an “oil accident” in the system.
Here, one should not follow the “ostrich method” for lack of monitoring possi-bilities: hiding one”s head in the sand according to the principle that everything is OK as long as no failures can be ascertained in the product or in the proc-ess.…
In particular in highly-sensitive applications, this is of course intolerable – and with the establishment of stricter international quality assurance methods, this also becomes more and more risky with regards to legal aspects.
The end of the loss of control
Therefore, a technology allowing stationary online measurement and monitor-ing of the vaporous residual oil content of the compressed air – without requir-ing any time-consuming laboratory investigations – was eagerly awaited by the market.
The compressed-air specialist BEKO Technologies was also successful in making a big step forward in matters of process safety: with a measuring sys-tem for the detection of hydrocarbon vapours.
METPOINT OCV – which is the name of the innovative product – monitors the residual oil amount in the compressed-air flow online down to the range of a thousandth milligram per cubic metre. Even extreme limit values of 0.001 mg/m³ residual oil content can be continuously monitored online during running operation.
This system has been certified by TÜV Nord in accordance with the require-ments of ISO 8573-1, classes 1 – 4. This makes it the first online system for the detection of the oil vapour content in compressed air worldwide that can boast a certificate from an independent accredited institution.
As METPOINT OCV is able to continuously monitor the residual oil content of the compressed air, complicated samplings and time-consuming laboratory evaluations are no longer required.
The new system functions far more efficiently: the partial volume flow taken from the flowing compressed air is supplied via a rising main to a separate sensor unit, in which the hydrocarbon vapour content is measured by means of a PID (photoionisation detector). Dimensioning of this sampling and the measuring section correspond – as mentioned above – to the stipulations of ISO 8573.
The electrical signal resulting from the PID measurement is amplified and electronically evaluated. Subsequently, the results are indicated on a touch-screen display and simultaneously stored in the internal memory.
With a capacity of two gigabytes, the memory is able to record up to ten years of detected values.
The acquired data can be used for both the documentation of the compressed-air quality and the identification of contamination sources.
In the event that the recorded oil-vapour values in the monitored compressed air exceed the tolerance limits, the system will automatically trigger an alarm. In this way, inadmissible concentrations of residual oil are detected and indi-cated in time to prevent a large number of possible problems, such as con-tamination of pipe-work and products, damage to machines and plants, or high costs for rejects or repair.
Able to network and easy to maintain
In addition, the new measurement technology is future-proof thanks to its dis-tinct ability to network. It allows an absolutely smooth and effective IT connec-tion of the devices and thus the integration into the existing IT infrastructure of the plant operator. The maintenance concept of METPOINT OCV also points the way to the future. If a bypass is available, it can be serviced and calibrated on location without requiring the pressure in the system to be reduced or even the plant to be completely closed down.
The documentation in the event of maintenance or calibration measures is via an “electric signature” of the person in charge.
To sum up: the dual strategy ensures highest process safety
With the catalytic total oxidation of hydrocarbons according to the BEKOKAT method and with the continuous online oil vapour monitoring with the MET-POINT OCV measuring system, the quality in compressed-air processing reaches a level which, until now, was thought to be unachievable. As individu-ally acting systems, these two innovations already perform in an exemplary manner. When combined, they become a dual strategy implementing a maxi-mum of process safety and quality assurance.
The times of the “flying blind” in production and in the uninterrupted monitoring of oil-free compressed air can therefore be consigned to history.
