Planning (2): hidden safety factor
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(IN Tuesday this week nine people died and 34 were injured when a French liquid oxygen road tanker exploded on a bridge at Martelange, Belgium. It has happened before. And in Britain, too. What are the planners doing to prevent it recurring? The answer: everything in their power. The hidden safety factor is ever in their minds. They are aided by the work of such splendid bodies as the Fire Protection Association, from whose FPA Journal (July issue) I reprint, without further comment, this FPA report:—
Two men, the crew of a liquid oxygen road tanker, were killed when the tanker exploded at an engineering works at Biddulph, Staffordshire. Seven men employed at the factory and three women from nearby houses were injured, and several people received minor cuts from flying glass.
The vehicle was fitted with a horizontal cylindrical tank unit, which consisted of a carrying tank with an outer shell, both made of aluminium alloy. The space between them was filled with pearlite insulating material and kept under vacuum conditions. A cabinet containing the control valves and electrical switches for emptying operations was fitted at the rear of the tanker.
Safety devices on the carrying tank were two spring-loaded relief valves, designed to lift at 281b p.s.i., and a lin diameter bursting disc, set to operate at 401b p.s.i. Two pressure-relief discs were also fitted as a safety measure in the event of a leakage of the load into the vacuum space. The working pressure of the tank was 4-251b p.s.i.
To make transfer of the liquid oxygen into the storage tanks more efficient, a pump and electric motor unit had been installed inside the tank, immersed in the liquid oxygen. This was the only one of the fleet of tankers to be fitted in this way. The electric motor was a 10 h.p., three-phase induction motor, 200 volts, 26.9 amps, 11,900 r.p.m. The bearings were fitted in a polytetrafluoroethylene cage suitable for use in liquid oxygen.
The wiring was a four-core, mineral-insulated, copper-sheathed able. The electricity supply for the motor was obtained from an A.C. three-phase generator, 200v, 43 amp, 2,740 r.p.m., driven from :he road engine by an hydraulic power take-off. Instruments ndicating generator and motor speed were sited both in the cab of he vehicle and the control cabinet.
Pump motor tests
Tests to study the effects of faults had been carried out for about .hree-and-a-half years by the company which owned the tanker. Vial-operation tests included arranging for the electric motor to mat up and ignite while immersed in liquid oxygen, but no evidence was found of an explosion occurring or of a rapid build-up of )ressure. Tests on an electric motor similar to the one involved in the ncident included 181 hours continuous running, whereas a delivery veration usually took about half an hour.
In April 1966 the unit was considered safe to be put into cornnercial operation. At first the loads carried were inert gasesiquid nitrogen and liquid argon—and 205 deliveries were made. In [illy 1966 the vehicle started carrying liquid oxygen, and up to the ime of the accident 139 deliveries had been made.
On Friday, September 2, the tanker arrived at the factory on its irst call at about 9.20 a.m. Its load consisted of 5+ tons of liquid )xygen (150,000 cu.ft) at a temperature of —183 deg C. It was 'riven straight to the fixed tank installation and unloading began.
A factory employee walking towards the tanker saw one of the :rew turn on a valve at the rear of the vehicle where the control :abinet was situated. Immediately, he heard a loud hiss and saw a niff of "light blue smoke" at the rear of the vehicle. This appeared o alarm the operator who turned away.
The next instant a violent explosion occurred: parts of the tanker vere blown in all directions; the body of the driver was found 30ft from the rear of the vehicle; the second man was found 8ft away, near a propane storage tank. It seems that the tanker crew had little warning that anything was wrong until a few seconds before they were killed.
Investigations showed fairly clearly that a fault occurred in the electric motor which caused it to catch fire. This fault could have been either electrical or frictional heating; there is no evidence that the motor seized up.
Oxygen in a gaseous form would be present inside the motor casing, as liquid would be thrown out by centrifugal force when the motor was rotating. Deposits found on the dished end of the tank indicated that flames had come out of the ventilation louvres at the top end of the electric motor.
Pressure build-up
It was estimated that about 10+lb of iron and steel, which would burn in a pure oxygen atmosphere, had been consumed by fire. The heat produced had vaporized the liquid oxygen, which would then have expanded to 850 times its liquid volume, the pressure build-up being so rapid and so enormous that the safety devices were unable to cope, and the tank ruptured.
Apparently, both the bursting disc and the relief valves operated. There was no evidence that the tank failed prematurely.
The force of the explosion moved the vehicle forward about five feet. The dished end of the inner carrying tank was found 63ft from the tanker. The pump, shaft and motor of the electric motor were found 30ft away; parts of them were severely damaged by fire.
The front portion of the carrying tank, although projected forward so that it damaged the tanker cab, was itself only slightly damaged. The rear of the tank—the area surrounding the electric motor and pump unit—suffered considerable damage from internal pressure and from fire.
A small timber lean-to building was completely destroyed by fire. Half the structure and part of the contents of another building, 250ft by 80ft in area, were severely damaged by blast and falling debris. Half the structure of a third building, measuring 400ft by 120ft, was damaged by blast and flying debris, and nearly half its contents were slightly damaged. A number of private dwellings were damaged by blast.