In engine fuel systems for aircraft, it is known to utilize a two-stage pump for delivering fuel to a fuel control. A first boost stage is provided by a centrifugal pump which delivers fluid to a second stage fixed displacement pump, such as a gear pump, with the fuel delivered under pressure from the latter pump to the fuel control. The components are normally sized to deliver adequate fuel under maximum operating conditions. Any fuel not demanded by the fuel control is returned as bypass flow from the fuel control to a location between the centrifugal boost stage and the second stage fixed displacement pump. Some heat is generated within this fuel; however, this is minimal under maximum operating conditions.FORGED STEEL VALVES

When the aircraft engine is idling or set in a descending altitude condition, the total fuel required by the engine is considerably less and there is a resulting high bypass flow from the fuel control, with a rise in temperature of the fuel. The temperature of the fuel may reach 300° F. Under these same conditions, there is a substantial pressure rise within the centrifugal pump boost stage which promotes added recirculation flow within the centrifugal pump which directs hot fuel toward the inlet of the centrifugal pump.

In prior art centrifugal pumps, high temperature fluid could enter into areas of low pressure within the centrifugal pump, with resulting vaporization of the fluid which adversely affects the operation of the centrifugal pump.Water Power Control Valves A centrifugal pump using conventional centrifugal labyrinth seals to minimize bearing thrust loads requires the venting of a space behind the rear shroud of the impeller. This venting has been to a low pressure area, such as the inlet of the impeller, as by openings through the rear shroud of the impeller.Water Control valves Another way of accomplishing this without openings through the rear shroud of the impeller is shown in the Williams U.S. Pat. No. 3,677,659 wherein a leakage space behind the rear shroud is connected through passages to the inlet of the inducer associated with the impeller. However, this is still returning the fluid to a low pressure area with resultant possible vaporization of the fluid and degradation in the operation of the centrifugal pump.

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2012-01-17

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