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Aviation History
1952
1952 - 1217.PDF
FLIGHT, 2 May 1952 531 MODERN FUEL SYSTEMS Gas-turbine Installation Requirements: Mr. J. E. Walker's R.Ae.S. Lecture IN a R.Ae.S. Main Lecture, given before the Luton Branch of the Society on April 24th, Mr. J. E. Walker, A.F.R.Ae.S. (chief power plant engineer, de Havilland Aircraft Co., Ltd.) dealt comprehensively with Fuel Systems for Turbine-En$ined Aircraft, basing his discussion primarily upon experience obtained with the Comet. Introducing his paper, Mr. Walker said that, although its tide suggested that turbine-engined aircraft required fuel systems fundamentally different from those of other aircraft, this was not entirely the case, and much of his material was applicable to piston-engined aircraft. Gas turbines did, however, make more exacting demands in certain aspects, while some relief was obtained by the use of less volatile fuels. In the first major section of the lecture, Mr. Walker examined the characteristics of the turbine fuels most used in this country, namely, kerosine and wide-cut gasoline. More volatile gasolines were capable of being used in turbine engines, but were not specifically catered for in turbine-engined aircraft design, and if they were used as an expedient it would be with limitations on fuel system performance. Those charac teristics of the two main turbine engine fuels aflecting fuel system designs are given in Table 1. After discussing vapour pressure, the release of air from the fuel, and fuel freezing, the lecturer passed to the subject of water in fuel. Free water could be pumped in with the fuel, water in suspension could be carried in by dirt, water could condense in the fuel tanks, or water could be present in solution in the fuel. Little trouble had arisen from the two last-named causes, but the presence of free water constituted a serious menace which would lead to filter blockage by ice. Free water and water in suspension should both be avoided by the use of adequate separation and filtering facilities. If the aircraft filters did become iced, this could be countered by Eroviding a by-pass, heating the filter element, or by injecting an anti-•eeze, such as methanol, upstream of the filter. A very suitable solution would be the provision of a solid anti-freeze insoluble in the fuel yet soluble in water. Chromium-trioxide had been suggested for this pur pose, but it suffered from a characteristic of such substances, namely, reaction with rubber tank sealant. The effect of fuel on fuel system components also had to be considered. The synthetic rubbers used in the construction of bag-tanks and seals were adversely affected by aromatic hydrocarbons, which were present in high proportion in the two turbine fuels considered. Similarly, unexpected corrosion effects had been met on metal parts from com binations of fuels and bag-tank materials, while the temptation to use magnesium had to be avoided wherever the metal might come into contact with water. Another factor brought into prominence by the use of capacitance-type fuel gauges was the permittivity of the fuel. Large variations in this factor made accurate fuel measurement difficult, and there appeared to be a good case for including values of permittivity in the fuel specification. • Mr. Walker then turned his attention to a consideration of fire risk and the explosive range of the fuels. The fire danger potential of the two fuels considered could be assessed from an examination of the pressure/temperature explosive range, the flash-point, and the spon taneous ignition temperature. Taking a fuel temperature range of +40 to —55 deg C, it would be seen from the envelope curves of Fig. 1 that kerosine would produce an explosive mixture in the tanks at the high temperatures, while wide-cut gasoline was dangerous at the lower temperatures. There was litde doubt that for civil aircraft in flight kerosine was the safer, wide-cut gasoline in this respect being worse than ordinary aviation gasoline. Under the heading of flash-point, Mr. Walker showed that the lower the fuel temperature dropped below 38 deg C (and it was seldom above this figure') the more difficult it became to start a kerosine fire, while wide-cut gasoline remained highly dangerous down to —23 deg C. The significance of this was that, in the event of a crash—unless this hap pened immediately after take-off in a hot climate—gasoline was ready to be ignited immediately, while kerosine required considerable heating to enable the flame to travel. This interval gave the crew and passengers in the crashed aircraft a chance to escape, and also gave fire-fighting crews a chance to extinguish the fire before it took control. This con siderable safety advantage now offered to civil aircraft should not be lost by the use of higher volatility fuels. TABLE I: TURBINE FUEL DATA Test-fuel specification Bulk-fuel specification Current American equivalent Specific gravity (60 deg F) Calorific value per lb Calorific value per Imp gal Volume coefft. of expansion Reid vapour pressure Dissolved air (S.L. equilibrium)* ... Dissolved water at 20 deg C Dissolved water at 50 deg C Freezing point (initial) Flash point Spontaneous ignition temp, (open steel plate)t Rate of flame propagation) Aromatics (by volume) Sulphur content (by weight) Kinematic viscosity at 0 deg F Aviation Kerosine - DERD/2482 MIL-F-5616 J.P.I 0.79-0.83 18,500 BTh.U. 147,000 B.Th.U. 0.00094/deg C 0.12 Ib/sq in 14 per cent by vol 0.006 per cent by wt 0.026 per cent by wt —40 deg C (max) +38 deg C (min) 650 deg C App. 4ft/sec 20 per cent (max) 0.2 per cent (max) 6 Cenctstokes (max) Wide-cut gasoline RDE/F/KER/210 DERD/2486 MIL-F-S624A J.P.4 0.739-0.825 18,700 B.Th.U. 143,500 B.Th.U. 0.00094-0.00120/degC 2-3 Ib/sq in 14-24 per cent bv vol 0.007 per cent by wt 0.028 per cent by wt — 60 deg C (max) -14 to-23 deg C 650-720 deg C App. 4ft/sec 25 per cent (max) 0.4 per cent (max) 2 Cencistokes (max) * The air dissolved in fuels is slightly oxygen rich. f The values of spontaneous ignition temperature are comparative only, varying with the test-method. The flim: doss not propagate at all below the lower limit of inflammability, i.e. at sea level, the flash point. In his second major section, Mr. Walker dealt with detail design of a fuel system. It was generally realized, he said, that fuel pumps were not only required to supply fue to the engine pump at sufficient pressure to prevent any vapour release in the feed system or engine pump where cavitation could occur, but also to provide the necessary agitation in a fuel tank to cause air to come out of solution in the fuel and escape from the tank vents before it could enter the fuel-pipe system. It was obvious from this consideration that pumps should not be installed in feed lines where there was no means of escape for such air and vapour. The particular characteristics of the pumps which were now generally used, whereby the delivery pressure fell to negligible proportions when the pump became uncovered, could be used to good advantage to reduce the unusable fuel in a tank to a minimum. In the Comet, fully sub merged pumps could be switched on in one tank while the previously used tank was nearly empty, the pumps being left running to empty the tank as completely as possible without fear of forcing air into the feed system. Although full duplication of pumps was provided in the Comet, no pump failures had occurred, reflecting the ready co-operation of the makers in undertaking extensive tests under conditions simulating flight cycles. The use of gas turbine power-plants made the capacitance-type gauge mandatory, especially since modern aircraft tended to use large WAYMOUTH UNITSS REMOVABLE PLATE -80 "60 "40 -20 O ZO 40 60 BO IOO TEMPERATURE (deaC) Fig. 1 (above). Exp/osive//m/ts of two turbine fuels. The left-hand loop refers to wide-cut gasoline and the other to kerosine with an R.V.P. of 0.126. The shaded area represents the fuel temperature-range measured during development flights on the Comet. Fig. 2 (right). Comet port inner tank. REFUELLING CUT-OFF SWITCH 'TWO STAGE) CLACK VALVE LOOPED VENT PI DRIPSTICK REFUELUNC VALVE WAYMOUTH TERMINAL BLOCK CONNECTION TO^VENT OUTLET VALVE REMOVABLE PLATE MANHOLE COVER FLOAT VALVE WAYMOUTH UNITS VENT CONNECTION TO OUTER TANK
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