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Aviation History
1954
1954 - 0320.PDF
150 FLIGHT, 5 February 1954 (1) Static Condition. Solenoid de-energized. Piston assembly held in closed position by return spring. (2) Flow. Solenoid energized. Plunger and ball pulled away from seat. Piston assembly moves downwards and fuel flows through the body outlet ports and through the piston assembly and out of the solenoid throat ports. (3) Shut-off. Solenoid de-energized. Solenoid spring forces plunger and ball back on to its seat and the lower chamber of the valve is sealed. Pressure built up on underside of piston assembly. Since pressure on piston crown is equal to pressure on underside of piston body, and because the underside piston area is greater than crown area, piston assembly moves upwards and closes the valve. (4) Relief. If inlet pressure rises above shut-off pressure, piston crown is depressed against relief-springs while piston body remains in fully closed position. PRESSURE REFUELLING . . . magnet. As the float arm rises or falls, an armature attached to the float arm attracts or repels, through a diaphragm, the magnet on the rocker arm. Movement of the rocker actuates a small switch which makes or breaks the electrical circuit. Fuel passes into the rear fuselage tank through a float-operated filling valve which can also function as a transfer valve. The principle on which this valve operates is identical wirn the solenoid-operated valves in the forward tanks, but in this instance the float and float arm directly operate the valve and therefore no float switch is required. At this stage the question may be raised as to why solenoid-operated valves should be used in the forward tanks, instead of the less complicated float valve. The answer is, of course, that with an electrical valve, provided a manual switch is placed in the circuit, selective fuelling can be achieved, thus varying the fuel load to meet different aircraft load/range cases. The use of the float-operated valve as a transfer valve is discussed later. The manner in which the wing tanks are fuelled is identical with that applying to the two forward fuselage tanks, i.e., through solenoid-operated valves and float switches, and the 'fuel gallery which carries fuel to the wing tanks is extended on each wing into the tip tanks. It would obviously not be practical from the viewpoint of economy to install a filling valve in these tanks, since they may be jettisoned, and a line-mounted valve is therefore positioned between the outer wing tank and tip tank. This again is a solenoid-operated pressure-differential type of valve controlled by a float switch installed in the tip tank. The in-line type of valve is of particular interest and is a comparatively recent development. A valve of this kind is extremely easy to install and offers enormous advantages from the maintenance viewpoint, since it is no longer necessary to remove a tank or to disconnect tank fittings when valve servicing becomes necessary. The photograph (Fig. 5) shows show simple the instal lation really can be. No relief mechanism is incorporated in the valve because of the low rate of fuel flow which it handles. Later types of this valve, which are designed for higher rates of flow, Fig. 5. Compact and simple installation of in-line type of valve. PISTON SEAL PISTON BO0Y PISTON RELIEF PISTO RETURN SPRING BODY Fig. 4. On the right, shown in sectioned form, is a solenoid valve of the type sup plied for the Comet. The series of dia grams on the left shows the sequence of operations in a typical solenoid valve. N/R VALVE ASSEMBLY SOLENOID ASSEMBLY CONDUIT FITTING incorporate such a mechanism as well as a provision for defuelling. It will be noticed that each tip tank is fitted with a separate relief valve to protect the tank in the event of a failure of the filling valve. This relief valve (Fig. 6) has interesting features made necessary through its particular application in the wing tip tanks which are normally designed to withstand a comparatively low pressure because of the desire to keep their weight to a minimum. If, for example, the maximum permissible pressure in such a tank is 12 Ib/sq in and it is desired to use a valve "cracking" pressure of 71 Ib/sq in, the permissible tank pressure would be exceeded before any reasonable flow could pass through the open valve and thus relieve the internal pressure. The characteristics of the valve illustrated overcome this problem by "cracking" at 7i lb/sq in and remaining in the fully open position until the internal pressure drops to 3 Ib/sq in. While open, the valve is capable of passing up to 65 gal/min, without exceeding the 1\ lb/sq in "cracking" pressure. From the fuel system diagram it will be seen that there is a by-pass fuel line from outboard of the in-line valve to inboard of the inner wing tanks, a non-return valve being inserted in this line. This line is used during fuel transfer from the tip tanks to the rear fuselage tank and, if a non-return valve were not inserted, the tip tanks would fill direct and not through the in-line valve; this would, of course, produce disastrous results. Fuel transfer is accomplished by applying an air pressure to the tip tanks from the engine compressor and forcing the fuel through the by-pass lines and non-return valves into the main gallery and into the rear fuselage tank via the float-operated filling valve. For this hypothetical case, and remembering the tip-tank relief valves are set at % lb/sq in, let us assume that this transfer pressure is 6 lb/sq in. At this pressure the valve will open sufficiently to allow transfer to take place. During transfer the solenoid-operated valves will remain closed, since no current is being fed to the solenoids. Fig. 6. Pressure relief valve as fitted to tip tanks. 0
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