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Aviation History
1954
1954 - 0321.PDF
RESSRVWRS RiMPUS 151 ETE1NTS Fig. 7 (above). Simple internal mechanism of a non-return valve. Fig. 8 (right). Face of "tanks full" indicator, the warning lamps of which are extinguished as the various filling valves close. Fig. 9 (below). The modern type of pipe connector, which, able to handle high pressures, is replacing the familiar link made of fuel-resistant hose. CIRCLIP LOCKING CIRCLIP INNER SLEEVE OUTER SLEEVE RUBBER SEAL SPLIT COLLAR Before leaving the transfer system, a word or two about the non-return valves will not be out of place. Although Fig. 1 shows two non-return valves only, these components have a very wide general use in fuel systems and there are many types avail able. While their mechanism is so simple as not to warrant any description, it is interesting that the type shown (Fig. 7) apart from being line-mounted, is extremely light, with a very low pressure-loss; e.g., a ljin valve which weighs 4| oz shows a pressure-loss of 4j Ib/sq in at 80 gal/min. A further important feature is that, unlike many other types of non-return valve, they are positively leakproof from 0.1 in water gauge to 100 lb/sq in. So far no mention has been made of the venting system, but it would be erroneous to think that it plays only a small part in fuel system design. The basic need for an adequate venting system is to provide a means of escape for the air displaced in each tank as the fuel level increases, and to provide an inlet or outlet for air as the aircraft altitude changes. This latter point is of tremendous importance today for some military aircraft can climb at rates exceeding 15,000ft/min. During ground fuelling it is essential that the vent in each tank can pass to atmosphere the full quantity of fuel entering that tank, so that damage is prevented in the event of a failure of the normal shut-off device. Should such failure occur, the inertia of the fuel leaving the tank after the tanker supply has been cut off may create an internal depression of such magnitude as to cause a tank collapse. There is more likelihood of this occurring where the flexible type tank is used. To eliminate the possibility of such collapse, anti-siphon valves are placed in all vent lines. Because of the danger of boiling of the fuel in aircraft which operate at high altitudes, it is necessary to pressurize the tanks, and for this purpose nitrogen is used in preference to air, since there is less risk of fire in the event of a bullet strike during combat or a forced landing. The vents obviously cannot be in use when the tank is pressurized and these lines are used for the introduction of the nitrogen, the change-over being effected by the positioning of suitable cocks in the vent outlets. The control of these cocks must be automatic so that there is no possibility of ground fuel ling being commenced with the vents closed, and their operation is usually carried out electrically, being controlled by a master fuelling switch positioned in the cockpit, or at the fuelling point. It will be seen from the fuel system diagram that a scavenge box is incorporated. As its name implies, it is used to scavenge the fuel lines after the fuelling operation is completed. In fighter systems there is little need for such scavenging, since the amount of fuel remaining in the lines does not warrant the addition of the extra equipment. In larger types, however, this residual fuel can represent a considerable number of gallons which would be so much dead weight, and as such are certainly worth recovering. The scavenge box is fitted at the lowest point of the system and is gravity fed. A small pump interconnected with a float switch is installed in the container and, as the fuel level in the container rises, the pump is switched on and the residual fuel is forced into the centre fuselage tank. This process continues until the fuel ling gallery is emptied and the float switch switches off the fuel pump. Although not shown in Fig. 1, it is now a definite require ment that a method of indicating "tanks full" is included in the system other than by the normal fuel-contents gauges in the cockpit. This is achieved where solenoid-operated valves are used in conjunction with float switches by fitting a small indicator (Fig 8) adjacent to the ground fuelling point; this indicator is inter-connected with the valve and float switch. The wiring is so arranged that, when a master switch energizes all shut-off valves for fuelling, the indicator lamps are illuminated. This serves as an electrical check of the complete installation prior to fuelling; and, as fuelling is completed and the filling valves close, the warning lamps will be extinguished. Any lamp not extinguished will indicate a failure, and the tank in which the failure has occurred. Where float-operated shut-off valves are used it is, of course, necessary to introduce a float switch or pressure switch in that particular tank. Interconnected with this electrical system and master switch are the vent and pressurization cocks to which previous reference has already been made. The preceding paragraphs have followed the system through from filling point to tanks and have shown the method of appli cation of the various fuel shut-off and safety devices employed, but so far no mention has been made of the method of connecting one pipe to another in the system. This may appear to be a very simple matter, but when the fuelling pressures are considered together with the need to keep the pressure-loss to a minimum, it must be realized that to find a suitable pipe connector that will comply with all the requirements was not easy. In the past it was usual to make pipe joints with a length of fuel-resistant hose secured by a number of hose clips. Today, with increased pres sures, this method is both useless and dangerous. The connector illustrated has no captive end fittings, is light in weight and offers a degree of flexibility both axially and radially. These features make it simple to install, particularly where access is very limited. From the weight viewpoint it compares very favourably with the old type of connection; for example, a l^in connector weighs 5 oz. Variants of this type of connector are made in the form of bulk head and termination connectors which have the same advantages as the standard type. Heat shields are also available to shroud these connectors and to improve their fire resistance. The emphasis so far has been on fuelling the aircraft, but it is just as essential to be able to defuel with the minimum delay. While it is a requirement that defuelling must be possible, the rate at which it should be carried out is left to the individual designer. The system illustrated is defuelled by the use of an engine pump which draws the fuel from the port wing tanks direct and from the starboard tanks and fuselage tanks via a collector box. No provision for defuelling the tip tanks is made since it is assumed that the contents of these tanks have already been trans ferred to the rear fuselage-tank. To avoid the addition of extra piping it is desirable that in future fuel systems the defuelling should be made through the fuelling gallery; but if this is done then the filling valves must also incorporate provision for defuel ling. As mentioned earlier the later types of in-line valves are fitted with the necessary mechanism. It is hoped from the facts given in this short article that the reader will have gained some idea of the modern method of fuelling aircraft and the type of equipment that is employed in such a system. Naturally, the story does not end here and with the new development of fuelling equipment which is going ahead all the time, it should not be long before filling rates of 1,000 gal/ min will be commonplace. It has taken a long time for pressure fuelling to be accepted; but, now that it is in practice, it is obvious to all that any weight penalty incurred by the addition of extra equipment is by far outweighed by the advantages gained, and that the time is coming when the problem of aircraft turnround time both from the military and civil viewpoints will cease to be the bogey of the aircraft operator so far as fuelling is concerned.
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