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FIRE PERILS FROM STATIC ELECTRICITY.

22nd September 1925
Page 19
Page 19, 22nd September 1925 — FIRE PERILS FROM STATIC ELECTRICITY.
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The Effect of Atmospheric Conditions. How Electric Charges may be Generated. Some Actual Instances. Petrol Punip Precautions.

(INF OF the most interesting contributions to the literature concerning the production of fire during the storage and use of inflammable liquid, such as petrol, is a paper read recently by Mr. A. M. Cameron, B.Sc., .A.I.FireE., at the annual tournament of the National Fire Brigades' Association, held at Torpiay.

Mr. Cameron pointed out that fire risks due to static electricity had received a great deal of attention in America, but had been somewhat neglected in this country, although it is true that the conditions over there are often much more favourable for its deyelopment than they are here, but it is undoubtedly a fact that there are many risks in this country, and that a number of accidents which has occurred here in the handling of motor spirit can only be accounted for by sparks occurring through the generation of static electricity.

Modern research work has tended to modify our views with regard to the productian of this form of electricity. Charges can be generated not only by friction, but also by the separation of two dissimilar surfaces; for instance, a leather belt running over a pulley will generate a static charge, even when no belt slip is present. The charges are usually extremely small 'and, if frequently earthed, are not dangerous ; it is only when they are allowed to accumulate that their effects will be noticed.

The author pointed out that it is evident that, in the case of spirit flowing through pipes, two cases can be described: (1) that in which the pipe is of a conducting material ; (2) that in -Which the pipe is of a non-conducting materiaL The first is probably the

most important, as it is through metal pipes that the flow of petrol, beazole, etc., mostly occurs, but the second affords more risk. As an example, we may consider the ease of petrol flowing through a rubber or rubber-canvas pipe.

Teatsmade by a German showed that, by passing benzole at varying speeds through a copper pipe into a receptacle which was earthed, a potential of 1,000 volts was generated at a speed of one metreper second; at two metres per second the potential was 2,000 volts, the charge in this case was negative. As a rule, 500 volts were required to produce a spark sufficient to ignite a mixture of vapour and air.

It is important to notice that both pipes and containers may be earthed, but liquids such as petrol and benzole are such bad conductors that they will hold their charges even after coming into contact with the earthed metal. It is this fact that makes the control of static currents so difficult.

Take, for instance, the case of a road tank wagon being filled from a storage tank by means of a pump and pipe line. As the spirit flows through the pipe into the wagon, static charges can be built up on the pipe, the wagon (as the rubber tyres insulate it front the ground), on the spirit and probably also on the storage tank, if this be not-well earthed.

In these circumstances a spark can be drawn from the wagon by any metal connection that happened to be earthed, or even by-the body of anyone approaching the wagon. Therefore, before running motor spirit into such a receptacle, the latter should be well earthed by taking a wire from it to, saY, a water pipe on a building. A similar wire should

be taken from the petrol pipe and, to ensure that no difference of potential can exist between it and the wagon, it should make good metallic contact.

There now remains the question of earthing the charge on the petrol, as if this be not done a spark may take place from the surface of the liquid to the sides of the tank and an explosion may result. To earth the petrol, the filling pipe should be taken almost to the bottom of the tank, so that the fluid is well spread over the earthed metal at a low velocity and so loses its charge.

As an example of this charging effect, the author mentioned that some comparatively heavy oil was filled into steel tanks 20 ft. high by 15 ft. in diameter at a temperature of about 140 degrees F., the oil falling in streams of 20 ft. or less into a strainer. On dry days charges of enormous voltage accumulated on the surface of the oil, the discharges sometimes being 4 ft. long, occurring in jagged lines across the tank and accompanied by sharp reports.

As rubber is a non-eanductor, it cannot be earthed by grounding at one point; in fact, it is possible, if it be charged, to draw sparks from it a few inches from the point of grounding In one instance a petrol tank wagon was backed close to an underground tank, a funnel was inserted into the filler of this tank, and two pieces of rubber tube were used to connect the tap on the tank wagon to the funnel. When the petrol was turned on, the first thing that happened was that the rubber tube began to flap to and fro, spilling a certain amount of petrol on the ground, and as the temperature of the air was fairly high, the petrol immediately vaporized, producing an explosive mixture.

To avoid any more spilling, the driver and boy attempted to hold the rubber tube in the funnel, and while doing so the boy noticed that the hair ou his arm stood out straight, being attracted towards the rubber tube ; the next moment there was a flash of flame and an explosion occurred.

This is a point of importance in connection with the flexible tubes nsed with petrol-pump installations. Every tube should have a metallic spiral running throughout its length, connected to the tap at one end and to the earthed body of the pump at the other__ The filling tap should be placed in contact with the orifice of the tank before the tap is turned on and during the whole time that the petrol is running. It is most important that the tubes should be examined occasionally to make certain that the metallic connections are intact, so avoiding the risk of potential being built up on isolated sections. If used for filling a number of tanks in quick succession, a faulty tube might carry a considerable potential. It often happens that, through a miscalculation, a tank is overfilled, resultinnin a cascade of fuel over the tank and, sometimes, the engine. This might easily be prevented by using a nozzle that would cease to pass spirit when the level of the liquid in the tank rose above the nozzle.


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