SAFEGUARDING RISK OF FIRE & EXPLOSION IN CA SYSTEM

“SAFEGUARDING RISK OF FIRE & EXPLOSION IN CA SYSTEMS-”

Safety is one of the main assets of pneumatic tools & equipment. Unfortunately, we can’t claim the same advantage for compressed air plants since fires and/or explosions in compressed air systems occur all over the world. Fires and and/or explosions endanger the health and lives of the personnel being in the proximity of the delivery systems of compressed air plants such as piping, air receivers, pulsation dampers, aftercoolers etc. There are sometimes heavy financial losses when compressed air plants are damaged but there are occasions when more serious losses occur to stoppage of production which is dependent upon the delivery of the compressed air.

?Through this article, we had had the opportunity to study the causes of several explosions in a railway truck factory in a compressed air system operating at a pressure of 7 kg/cm2. In one case the auto ignition of degraded oil accumulated in the bend of the discharge piping caused such a heating of the tube that the steel became plastic. The wall of the tube had blown up followed by the bursting of the tube. The escaping compressed air threw two steel work benches from the proximity of the discharge system several tens of metre away and the accumulated burning oil was thrown onto the wall of a building at about 200 m. The effect of it was like that of a napalm bomb. It is interesting to note, that the accidents always arise either in the period preceded by a substantially reduced loading of the plant or during the reduced consumption of compressed air. The inspections of the piping after the accidents have shown considerable amount of carbon deposits and of the thickened oil.

?The compressed air plant in the railway truck factory was equipped with three two-stage reciprocating compressors each having the capacity 3650 m3/h and by a turbo compressor 6300 m3/h. The compressor oil used for lubrication of the reciprocating compressors had been manufactured from un appropriate paraffinic crude oil. But tests of the fresh oil, regenerated oil and of samples taken from the discharge system showed satisfactory results. The flash point of the oil was much higher than the air discharge temperature of about l50°C - at least 226°C and there was no explosion at the Kretz test at the pressure 7kg/cm2 using glowing coil for initiation. At the time of accidents there were no faults with the reciprocating compressors and according to records of operators, the compressors have continually worked with normal discharge temperatures. All the accidents happened in the cold seasons/ in December, January, March, and September.

?The rate of oil was at the critical time 500 g/h per compressor instead of 400 g/h as recommended by the manufacturer of the compressors due to a slight seizure of the differential pistons.

?After the mentioned disaster the search how to avoid dangerous situations was done and the solution was found in the change of oil from one with a higher flash point. But as we shall learn later it is not the final solution, since it means that even greater difficulties may occur later due to the increased oil deposits in the piping.

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The compressed air in this case was used primarily to feed pneumatic forge hammers about 2/3 of the air consumption and to reduce the compressed air consumption the discharge piping had been thermally insulated. This was the only deviation from the current outlay of compressed air plants. Apart from this only one point the case is a typical example of situations where fires and explosions in compressed air plants arise. It will be further analysed, to make conclusions from the findings of fire and explosions causes and to avoid these dangerous situations.

?CAUSES OF FIRE AND EXPLOSION IN DISCHARGE PIPING

There are many records of fires and explosions in discharge piping of air compressors in many countries like U.S.A. Great Britain, Germany, and Czechoslovakia etc.

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Oil or its deposits could be very dangerous when incomplete burning in cylinders or discharge piping of compressors occurs. This gives rise to carbon monoxide, which can be also present in the air polluted by exhaust gas from internal combustion engines operating in the proximity of the compressor suction line. The carbon monoxide escaping with the air from pneumatic tools is very poisonous to plant workers in closed conditions. The careful analysis of circumstances favourable to the rise of fires and explosions has proved these causes to be in discrepancy with the up-to-date suppositions and seem at first to be paradoxical.

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The first presumption how to prevent fires and explosions in the discharge piping, is careful oil section. Oil should have the lowest permissible viscosity, to make the evaporation of the oil in contact with hot piping walls easy, for the purpose of being carried by the compressed air into cooler parts of the piping and into air receiver. The oil breakdown & the carbon deposits, which are very hazardous from the point of view of fire prevention, will be avoided. The heavy Carbon deposits due to their insulating effect make possible local overheating and inflammation. Besides the high temperature the high air pressure contributes radically to the rapid oxidation of oil. The influence of the air pressure on the chemical activity of oil for oxygen can be deduced from the fact that however the air temperatures behind the first compressing stage are identical or even higher than that of the discharged air there are no fires or explosions in the interstage piping.

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Operation tests of the Atlas Copco Company with their compressors along with the laboratory tests of the Shell international have proved that paraffinic oils tend to deposit more coke on the piping wall than more volatile naphthenic oil and that the preference should be given to a straight distillate before a blend of' two oils of different viscosity to get the desired viscosity. There are records of explosions not only in the discharge piping but even in the aftercooler which was considered before as the best protection against explosions and fires.

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The above-mentioned tests on the applied oil have apparently shown that the oil was not the cause of explosions, but comparing the test conditions with those in the discharge piping such differences were found that the conclusive evidence of the tests was refuted. The flash point of the oil had been determined at atmospheric pressure while the pressure in the piping was 7 kg/cm2 which lowers considerably the flash point. But even more important is the fact that the oil was in the form of a mist, which can substantially reduce the flash point of oil. Stoichiometric ratio in such a mist is quite different from the ratio in the Kretz apparatus even if the used pressure was 7 kg/cm2.

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According to the statement of competent person the spontaneous inflammation of carbon deposits in the piping, sound dampers and aftercoolers was found to be result of the exothermic oxidation. The heat generation by oxidation was increased exponentially with the temperature while the heat at transfer was proportional only to the temperature. The carbon deposits in the piping can grow to a certain amount, when the heat removed from the surface is no more equivalent to the heat generated by the oxidation. The inflammation sets in at a certain critical air temperature whose value will vary considerably, namely from 70°C to 300°C.

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Besides the oil quality the operating mode has a great influence on conditions promoting fires. In UK it was found that compressors running at intervals with no load at present more severe conditions in comparison with those running continuously on full load. The fully loaded continuously running compressors remain without deposits even when using oil prone to deposit formation, while similar compressors running intermittently with no load using very good oils tend to form deposits.

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Interesting is the statement of one of the competent persons that big compressors are more prone to deposit formation although their efficiency is better and the discharge temperature lower than those of small compressors. The lower temperatures are inconvenient since the oil evaporation takes a long time and being degraded by oxidation the oil forms solid deposits. This observation was confirmed by explosions in the mentioned railway truck factory because the explosion occurred always during the cold season.

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In accordance with observation of the Shell Centre, that the fire risk was increased in cases of the substantially reduced air flow through the piping. Since the carbon deposits are not cooled by the flow of air their temperature may rise above the autoignition temperature. This observation is in full agreement with the facts found out in the railway truck factory since there also the explosions or fires came just at the time of the reduced loading of the compressed air plant. The lubrication of the compressors being independent of the compressor loading, the machines are overlubricated when on reduced load. At the same time oil is not carried into the discharge piping during the period of no load. The oil covering the carbon deposits flows off and so after restoration of the air delivery, the air necessary for the burning of the oil deposits has a free access and the autoignition then starts. This statement appeared to be confirmed by explosion in the piping of a high-pressure air compressor in a chemical plant in Czechoslovakia. These explosions always arose after the compressor had been shut down for several days.

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THE MEANS TO AVOID FIRE AND EXPLOSION RISK

Some means how to avoid fire and explosion hazards or to reduce drastically their occurrence follows from the analysis of their causes:

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1)?????Selection of an appropriate oil is primary importance.

2)?????Over lubrication of compressors must be avoided. It can result in worse consequences than under lubrication. Double acting reciprocating compressors are much better than the single-acting ones as far as the oil consumption is concerned.

3)?????The oil consumption can radically reduce by using the Teflon piston rings which is similar to the service life of cast iron rings operating with abundant lubrication and is several times longer than the service life of Teflon rings of oil-free compressors.

4)?????Switching from hydrocarbon mineral oil to a synthetic oil as a cylinder lubricant may very much improve the safety of the compressed air plant. Such synthetic oil is based mostly on tricresyl phosphate can drastically cut maintenance costs or compressor turndown. However, some of them may bring additional problems such as not being safe with all materials used in compressor manufacturing or having not the capability of corrosion prevention like the hydrocarbon oils and sometimes the problem of the high cost will arise.

5)?????Some experts recommend the removal of carbon deposits and oil from the discharge system at least every six months. This method aims to prevent the formation of such a thick layer that could promote the autoignition of the carbon deposits. We must agree that such a method is time consuming, laborious, and expensive.

6)?????Experts affirms that it is difficult to separate oil vapours from a dry gas, but the injection of steam makes the separation very easy. The usual air pressure of 7 kg/cm2 is corresponding to the saturated steam temperature of 170 C, which is not far from the compressed air temperature. This means that the steam injection into the discharge piping near the compressor and following oil separation will keep the other part of the discharge system free of oil and carbon deposits. The excessive humidity may be sometimes prejudicial to this mode of fire prevention.

7)?????The injection of water instead of steam was also recommended by one of the experts. There is a closed circuit of water circulation. The water is injected in the compressed air by a spray nozzle in the delivery branch of the compressor and passes with the air through the aftercooler to be separated in the following separator. The separated water is recycled by a pump whose head is set by the spray nozzle operating pressure.

8)?????Someone reported of a specially designed electrostatic filter to remove oil from the hot compressed air, operating with the efficiency of 88.4 to 91.9 percent.

9)?????Non-expensive mode of fire and explosion prevention both from the primary cost and operational cost is the cooling of air in the aftercooler. It is necessary to avoid the aftercoolers of conventional type since these may be themselves potential sources of fire or explosion hazard according to past experiences.

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Applying the aftercooler in the proximity of the delivery branch of a compressor is advantageous, despite the apparently increased compressed air consumption, due to the reduced air temperature and hence the increased weight of the air feeding the pneumatic tools and/or equipment.

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On the other hand, placing the oil and water separator immediately behind the aftercooler makes the use of other separators in the piping dispensable. No loops or expansion compensators in the piping are necessary and in

this way the pressure drop of the piping will be reduced. The cooling of the air and separation of the air humidity close to the compressor prevents or at least drastically reduces the condensation of the air humidity in the piping and the carrying over of the condensate into the pneumatic tools and machinery. By washing off the oil film it can cause corrosion and rapid wear by a substantially increased compressed air consumption.

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The expedient water removal cuts radically the maintenance costs of the pneumatic tools and prevents the pipes bursting due to ice formation during frost weather. From the point of view of power economy, it io important that the leakages may bring about considerable losses of compressed air. A great asset of the compressed air cooling is the increased accumulation capacity of the discharge system thereby reducing the number of interventions of the compressor capacity control.

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Thanks to V. Chlumsky for this insight.