-When process safety is top priority
Year after year, costly damages occur in manufacturing and processing plants. The main reason for this is the penetration of oil and germs into the compressed air system which is detected too late, and uncontrolled advancement to the points of consumption. There is however a highly efficient dual strategy to prevent this maximum credible compressed air accident.
The compression of air is a delicate issue. The main reason for this is the fact that unprocessed compressed air is anything but clean. It contains all components of normal ambient air in a compressed form: dusts, dirt particles, particulate matter, unburned hydrocarbons, moisture, germs, solvent vapours etc. The compression of normal ambient air leads to many times the concentration of these contamination”s.
To aggravate the situation, some contaminations even affect each other, thus forming further risk factors. For example, water forms an emulsion with oils, and dust can clot with oil or water and form larger dirt particles.
Today, high-quality, oil free compressed air is therefore required in nearly all production and processing sectors. Here, the problem focuses mainly on remaining oil aerosols and vapours which may impair sensitive tool parts at the point of consumption, wash out basic lubrication”s on components or even contaminate end products.
The oil fractions contained in the compressed air are expanded in tools and machines and escape into the ambient air, with negative consequences. They settle on surfaces, for example, and cause a disturbing oil film which, in turn, affects the adhesion of paints or complicates bonds. In many cases, oil mists also contain harmful nitrosamines. The fact that already a fraction of 0.3 mg oil per m³ is perceived in the breathing air is unpleasant at least.
The demands increase
To allow the classification and limitation of the risks, recommended quality classes exist for the quality of compressed air. These are defined in DIN ISO 8573-1.
Table: Quality classes according to DIN ISO 8573.1
In numerous fields of application, however, compressed air, which, according to this standard, is technically oil-free and classified as class 1, is in no way clean enough. According to this guide-value classification, the oil content (including the oil vapour) must not exceed 0.01 mg/m³ compressed air. For comparison: even relatively unpolluted country air shows an oil content of approximately 0.1 to 0.3 mg/m3. This amount is already so minuscule that it can barely be detected.
However, this amount is still fully unacceptable for many compressed air applications, for example for medical engineering, pharmaceutics, measurement technology, food processing, for the packaging of foodstuffs and medicaments or the provision of certain breathing gases, e.g. for diving with nitrox.
In these fields, absolutely oil free compressed air with a residual oil content of, in some cases, less than 0.003 mg/m³ is required.
Up to now, three technical solutions have mainly been used to try and achieve this aim: compressors for the generation of oil free compressed air, refrigeration drying with simultaneous oil separation of up to approximately 80 per cent, and finally oil separation filters. In many cases, a combination of these three technologies is applied.
Solutions with shortcomings
However, specific weaknesses of the individual components continue to exist in combined operation. Even oil free compressors, for example, do not guarantee reliability under most application conditions. They in fact theoretically produce compressed air quality which is identical to the quality of the inlet air – but usually the inlet air is already affected. This is particularly the case when the air is taken from the “normal” environment, meaning not from an especially-shielded compressor room.
In this manner, a multitude of hydrocarbons enters the compression process, for example solvents such as ketones (e.g. acetone), aromatic hydrocarbons such as benzene, toluene or xylene, and polycyclic aromatic hydrocarbons such as naphthalene. In addition, there are fuel residues in the ambient air, namely gas oils, petrol or kerosene. A highly aggressive mixture for com-pressed-air processing.
Even placing an oil free compressor in an area that has been shielded to a large extent against environmental effects is no guarantee for clean inlet air. Oil vapours can also reach the outside through the compressor-internal gear-box-casing ventilation. In large compressors, this is in fact an almost continuously occurring effect.
All this makes it clear: even downstream of dry-running screw and piston compressors where the true compression process takes place without oil as a lubricant, sealant and coolant, one cannot expect clean compressed air in the sense of absolutely oil-free compressed air. This, of course, applies in particular to the employment of oil-injected screw compressors or oil-lubricated piston compressors.
In the case of these compressors, the employment of downstream filter systems is therefore virtually an obligation and, with “oil free” compressors in sensitive applications an excellent recommendation.
Overstrained systems
However, the modules downstream of the compressor are not always in a position to limit the residual oil content in the system to the range required for demanding applications – not to mention an effective, continuous residual oil monitoring.
In practice, despite all efforts, a considerable uncertainty therefore remains regarding constant, absolutely reliable quality of the processed, “oil free” com-pressed air. The factors exerting an influence on filtration and adsorption in a compressed-air plant are too multifarious.
This starts already when choosing the compressor type, because the compression principle already influences the shearing and heat input of the hydro-carbon molecules. The hydrocarbon chains downstream of an oil-free compressor, for example, are shorter than the chains downstream of an oil-lubricated compressor. The aerosol diameter, in turn, is smaller downstream of a screw compressor than downstream of a piston compressor. Strong nozzle atomisation of the oil in the airend causes a surface enlargement which is ideal for cooling; the intensive contact with the atmospheric oxygen, however, also effects early ageing of the oil.
Further factors include the operating mode of the plant, frequency regulation, cooling and design of the oil separators. In pressure-switch-controlled compressors, for example, a frequent change between load and no-load operation leads to an enormously high stress on the oil separator, while frequency-controlled compressors cause a fluctuating oil introduction which is significantly higher in the lower speed range than during full-load operation.
High oil introduction of frequency-controlled, oil-injection-cooled compressors in the lower speed range.
High-quality activated-carbon filters and adsorbers serve perfectly in com-pressed-air processing but they also require the plant operator”s respective attention. For these filters, regular maintenance and replacement in time are vital requirements for safe and reliable operation. This is not always easy be-cause the influences impacting the filters and adsorbers are often beyond the plant operator”s control.
Natural limits
The flow rate, the temperature and the moisture have considerable effects on the adsorption capacity of the activated-carbon filters and adsorbers. In practice, velocities of five to 15 metres per second exist in compressed-air plants. In contrast, the technically optimum flow rate for highest-possible adsorption is only 0.6 metres per second – poles apart from the real everyday working con-ditions. Primarily during the start-up of the machine with mostly particularly high velocities, there is always a risk of entrainment and atomisation of oils even in filter systems which are otherwise extremely stable.
Contact time in the activated-carbon bedComparison activated-carbon cartridge activated-carbon filter element with identical outside dimensioning
According to the technical data sheets, most manufacturers define the service life of an activated-carbon filter on the basis of an operating temperature of 20°C. In contrast, in most cases 30 to 40°C can be found when it comes to practical operating temperatures. As a result, the concentration when entering an activated-carbon filter or adsorber is increased by almost 10 times the value (see illustration). The service life of the filter or adsorber is reduced correspondingly drastically, contrary to the theoretical indications. In addition, from a temperature of more than 40°C onwards, short-chain hydrocarbons start redesorbing from the activated carbon. In such a manner, the operational capability even of filters which are virtually still functional is eliminated.
Compressed-air temperature Oil content
| Compressed-air temperature | Oil content |
|---|---|
| 10 | 0.01 |
| 15 | 0.025 |
| 20 | 0.0425 |
| 25 | 0.078 |
| 30 | 0.13 |
| 35 | 0.24 |
| 40 | 0.4 |
| 45 | 0.68 |
| 50 | 1.16 |
| 55 | 1.9 |
| 60 | 3.2 |
| 65 | 5.6 |
| 70 | 9.2 |
Oil content downstream of an ultrafilter or oil inlet concentrations into an activated-carbon adsorber
Air that is too humid is also a threatening enemy of activated-carbon filters and adsorbers, as the relatively large water molecules can easily clog their fine pores. Therefore, the efficiency of the filter also rapidly decreases when the relative humidity increases. In the event of a real water or oil surge, the absorption capacity of an activated-carbon filter can be exhausted within a few minutes.
The general rule is: subsequent to exceeding the saturation limit, activated carbon will release the concentrated oil content like a dripping sponge.
The worst case scenario for process safety
In the event that – no matter which of the mentioned factors is involved – oil penetrates the filter or adsorption stage, the entire system will be saturated with oil. This is the worst case scenario, the maximum credible accident for a system depending on absolute process safety in the sense of really oil free compressed air.
The operator rarely has the time to react to the oil penetration with the disconnection of the plant. Even after a pre-alarm, he has only a few minutes to prevent the worst case scenario: the emergence of oil-containing compressed air at the points of consumption. With a flow rate of 7 m/s, for example, the penetrating oil will reach a compressed air consumer at a distance of 2,000 metres in less than five minutes.
The dramatic and extremely cost-intensive consequences are contaminated products, machine and process failures, production stoppages or even health problems. Consequences which no economically and responsibly acting company can afford, irrespective of the industrial sector.
Simulation of a cracked oil separation cartridgeCatalysis introduces the long-awaited turning point
Thanks to a newly-developed catalysis method, the situation has changed. This method overcomes all weak points of previous solutions and guarantees compressed-air purity which, with a residual oil content of a barely measurable 0.003 milligrams per cubic metre of compressed air, exceeds the requirements of ISO 8573-1 for oil-free compressed air of class 1 with 0.01 milligrams per cubic metre by far.
Measurement of the oil content downstream of a BEKOKAT catalytic con-verter
Combined with a METPOINT OCV residual oil monitoring system, the catalyti-cally acting BEKOKAT represents an extremely effective dual strategy during the processing of oil-free compressed air.
The new method opens the way to absolutely oil-free compressed air via a completely different technological approach than previous solutions: via ca-talysis.
Oxidation of hydrocarbonsWhat is this method about exactly?”
The technology realises the total oxidation of hydrocarbons – meaning in a concentrated, 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, even at oil inlet concentrations of more than 20 milligrams per cubic metre and a relative humidity of the compressed air of up to 100%. To make clear: the oil absorption capacity of ordinary activated carbon is already significantly restricted from a relative humidity of 50%! Beyond 80%, activated carbon loses its oil-removing properties and will only serve as a desiccant. Therefore, one should not be surprised that, in a two-stage filter combination (1st stage micro- or coalescence filter, 2nd stage activated-carbon filter), the activated-carbon stage needs to be replaced after a short period of time al-ready because, if no dryer is available, the compressed air leaves the first stage with a relative humidity of 100%.
1. Lubricated compressor
2. Cyclon separator with BEKOMAT
3. General purpose filter (grade G) with BEKOMAT (OPTION for highly contaminated compressed air)
4. Vessel
5. BEKOMAT for vessel drainage
6. BEKOKAT
7. Dustfilter (grade F) with BEKOMAT Oil- and grease-free section
8. Min. requirement: refrigerant air dryer
9. Superfine filter (grade S) with BEKOMAT
10. Sensor unit METPOINT® OCV
11. Display METPOINT® OCV
H1/H2 Ball valve for measuring section
H3 Ball valve for by-pass
H4 Aeration valve for measuring section
Only clean air and pure water
The new method tackles all contaminations in the compressed air supplied by the compressor, meaning lubricants, oils, sulphur dioxide, carbon monoxide, nitrogenous gases etc. Subsequent to the compression stage, these exist in the form of gas, vapour or aerosol. Hydrocarbon compounds are fully converted into carbon dioxide and water. This water can be separated through the employment of a refrigeration dryer and discharged. The condensate is now pure to such a degree that it stays clearly below the limit value for hydrocar-bons and may therefore be introduced directly into the public sewer system.
Condensate that accumulated through coolingLeft (upstream of the catalytic converter):
turbid condensate at the aftercooler of an oil-injection-cooled compressor
Right (downstream of the catalytic converter): The turbidity, determined by means of a nephelometer, is far below 10 FNU (formazine nephelometric units). With this, the condensate lies below the legally stipulated limit value for the introduction into the local sewer system.
No other mechanism of action currently realises this total oxidation and the absolutely residue-free method of operation.
Granular material serves as the catalytic converter, which is heated up to its reaction temperature by means of heating elements. In the catalytic converter, the oil molecules are broken down until only one carbon molecule remains. With this, the BEKOKAT exceeds the requirement of ISO 8573 according to which oil is defined from a carbon molecule number of six onwards. In the final catalysis phase, the oil molecules are oxidised down to H2O and CO2.
In detail, heterogeneous catalysis requires seven sub-steps:
- Diffusion of the reactants – meaning the oil components of the “con-taminated” compressed air – on the catalytic surface of the granular material.
- Diffusion of the reactants into the pores.
- Adsorption.
- Surface reaction.
- Desorption of the reaction product.
- Diffusion from the pores.
- Diffusion into the homogeneous phase.
It is essential that the hydrocarbon chains, namely the oil molecules, can be “cracked” at any point. They are continuously broken down until only carbon dioxide and hydrogen remain. The hydrocarbon chains from oil-free and oil-lubricated compressors varying in length pose no problem at all to the process.
It is not always well known that activated carbon is not able to adsorb charged microparticles, so-called polar compounds such as olefins, alcohols, glycols or ketones. These substances do not raise any problems for the catalytic method as they are also hydrocarbon compounds. They are also completely removed.
In the further course of 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.
Representative for the whole series, a system, based upon BEKOKAT CC-060, Refrigerant air dryer DRYPOINT RA 18 and CLEARPOINT filter passed a long-ter, test. The downstream total oil content with significant less than 0.01 mg/m3 was measured and certified (certificate TÜV Nord Umweltschutz GmbH & Co. KG, test report 108 GMT 019 / 8000623757, 2009-04-08). A quality which is required in the extremely demanding fields of medicine and pharmaceutics, of food processing and packaging.
Fantastic side effect:
Catalytic converter generates germfree compressed air
In particular in the pharmaceutics or in the food industry, compressed air comes into direct contact with the product or the packaging – and it goes with-out saying that the production stages need to take place under aseptic condi-tions, meaning germfree, to ensure that, for example, the durability of the products is not impaired by introduced bacteria.
Microorganisms are first separated at the surface, grow through the filter medium when moisture is available and organic substances enter, and are finally entrained by the air flow. The air is now no longer sterile!
Already during the development of the catalytic converter the mode of action of the BEKOKAT® would not only remove oil but also kill any microbiological burden. This theory was confirmed after three reproducible long-term tests by an accredited, independent institute and a certificate issued (validation certifi-cate Gesellschaft für Produktionshygiene und Sterilitätssicherung GmbH dated 20 December 2010). To provide evidence, the inlet air was contaminated with bacterial spores of Bacillus atrophaeus ATCC 9732 with a concentration of 108 CFU (colony-forming units). This bacillary, flagellated bacterium was chosen because it represents, so to say, the “worst case scenario”. It is extremely active, very resistant to modified ambient conditions and very adaptive. Furthermore, oxygen serves for the cell respiration during the total oxidation. The flagellum allows active locomotion.
Subsequent to the infusion, an average bacterial load of 2265 CFU/m3 was reached at the inlet of the converter. Sampling was effected in a repro-ducible manner in accordance with ISO 8573-7 “Compressed air – Part 7: Test method for viable microbiological contaminant content”. It could be successfully demonstrated in several test runs that downstream of the converter, and also downstream of the super fine filter and the refrigeration dryer, the compressed air was free from germs or spores.
The germfree processing of compressed air is a typical task of sterile filters which, subsequent to internal sterilisation, separate germs at the filter surface. If the compressed air is not sufficiently dry and still contains small traces of oil, the germs encounter a growth-promoting culture medium. As a result of the cell growth, the germs grow into the filter medium with the flow and may reach the outlet side when the sterilisation intervals are too long. At that point the entire compressed-air system risks becoming contaminated when the germs are seized by the air flow.
Sterilisation with saturated steam
To avoid this, the filters need to be sterilised at regular intervals and their in-tegrity needs to be proven by means of suitable test methods. Sterilisation takes place with high-temperature saturated steam with a maximum number of cycles of <200, which is reached at the latest after seven months at a daily sterilisation.
In contrast, the catalytic material of the converter only needs to be replaced after 20,000 operating hours or, at a three-shift operation, after more than two years. This increases the productivity tremendously as, during the cycle time, operation must not be interrupted for sterilisation.
Sterile filter elements with a finite number of sterilisation cycles
Safety maximum through residual oil monitoring
But even in view of the very good procedural method and mode of operation of the converter, documented “watertight” process safety during compressed-air processing requires additional quality surveillance in most companies.
Until now, the quality check of such superbly processed compressed air within a short period of time was much more complicated and unsuccessful. Uninter-rupted, permanent residual oil monitoring, documented at short intervals, was and still is out of the question.
Instead, until now, the agent of choice is a time-demanding laboratory investi-gation. The results of this are only available some time after the sampling. In the meantime, the milk could have been long since spilt.
Even less unsatisfactory than a laboratory test as a method is the frequently-practised “principle of hope”: everything is OK as long as no disturbances oc-cur to the product or during the process. A highly risky procedure.
The dilemma of the HACCP systems
Such more or less incalculable makeshifts were and are still possible against the background of another rule of process assurance: “Things that cannot be measured, cannot be made a standard.”
This dilemma becomes very clear using the example of the HACCP systems increasingly employed in the food sector or in the pharmaceutical industry. HACCP stands for “Hazard Analysis and Critical Control Point”. The quality assurance concept was originally developed for the American space agency NASA for the production of astronaut food. Today, the term “HACCP” specifies the systematic procedure of process cycles in the food industry.
This also includes the recording and documentation of all critical data relevant to quality, as well as the close-meshed examination of potential hazard sources.
This, in turn, requires the installation of powerful, continuously-operating measurement and evaluation technology. In the case of oil-vapour monitoring in compressed-air plants, this was utterly impossible until now, as the corre-sponding technology was not available.
The result: the specification of the compressed-air quality was omitted and virtually avoided. At the cost and in the awareness of process safety which, all in all, is considerably restricted as a result.
The same goes for application fields with similarly extreme quality require-ments as in the food industry, even when no HACCP systems are employed here, for example in the pharmaceutical-, medical-, laboratory- or precision-technology sector.
What they all have in common is the desire for maximum controllability of the oil-vapour content in the compressed air.
The end of the general loss of control
A new technology called METPOINT OCV promises to put an end to the gen-eral loss of control. It serves for the stationary online measurement and moni-toring of the vaporous residual oil content of the compressed air in accordance with ISO 8573-1.
This measuring system serves to detect hydrocarbon vapours. It monitors the residual oil content 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.
For this purpose, samples are taken continuously from the flowing compressed air and are supplied via a rising main to the sensor unit of the device the dimensions of which are approximately 23 x 20 centimetres. In the sensor unit, the hydrocarbon content is measured via a PID (photoionisation detector).
The measuring principle of the PID is based on the ionisation of the gas mole-cules through UV radiation and the registration of the resulting ion current. This electrical signal is measured, amplified, electronically evaluated and displayed. Simultaneously with the indication, the data is recorded.
In the event that the recorded oil-vapour values in the monitored compressed air exceed the tolerance limits, the device will automatically trigger an alarm. Inadmissible concentrations of residual oil are thus reliably detected and sig-nalised, and costly consequences of oil penetration prevented.
With this, continuous round-the-clock online measurement of the oil vapour content in highly demanding compressed air systems is possible.
Validated properties
Like the catalytic converter, the online system for the detection of the oil va-pour in compressed air is validated (TÜV Nord Umweltschutz GmbH & Co. KG, test report 109GMT003 / 8000624793 dated 31 March 2010). This repre-sents safety for the users through an internationally accepted standard.
Uninterrupted documentation of the compressed air quality
Even the documentation abilities of the system open up completely new pos-sibilities regarding the quality monitoring and process safety in such com-pressed-air plants. The acquired data can be used both for the uninterrupted documentation of the compressed air quality and for the identification of con-tamination sources. A benefit value that exceeds by far the limits of random laboratory samples which were previously standard.
Conclusion
With the catalytic total oxidation of hydrocarbons and uninterrupted online residual oil monitoring, quality in compressed air processing achieves a level which, until now, seemed to be out of reach.
Contact: Ruth Goodison
BEKO Technologies Limited, 2 & 3 West Court, Buntsford Park Road, Bromsgrove, Worcestershire, England, B60 3DX.
Tel: 01527 575778. Fax: 01527 575779.
E-mail:info@beko-technologies.co.uk Website: www.beko-technologies.co.uk
