US2997846A - Control system for turbine driven positive displacement pumps - Google Patents

Description

1961 D. R. TROWBRIDGE ET AL 2,997,846

CONTROL SYSTEM FOR TURBINE DRIVEN POSITIVE DISPLACEMENT PUMPS Filed July 9, 1957 2 Sheets-Sheet l 1961 D. R. TROWBRIDGE ET AL 2,997,846

CONTROL SYSTEM FOR TURBINE DRIVEN POSITIVE DISPLACEMENT PUMPS Filed July 9, 195'? 2 Sheets-Sheet 2 NIL mmnn MW United States Patent M 2,997,846 CONTROL SYSTEM FOR TURBINE DRIVEN POSITIVE DISPLACEMENT PUMPS David R. Trowbridge, Hutton Mount, near Brentwood, Norman Moss, London, and Michael J. Broad, Enfield, England, assignors to The Plessey Company Limited, Ilford, England, a British company Filed July 9, 1957, Ser. No. 670,832 2 Claims. (Cl. fill-39.28)

This invention relates to means for automatically controlling the speed of the turbine in a turbine-driven positive-displacement pump under varying load conditions of a hydraulic system fed by the pump. In British Patent specification No. 847,372 there is described a turbinedriven positive-displacement pump for operation under varying load conditions of a hydraulic system fed by the pump, wherein an auxiliary pump is driven by the turbine, and the pressure produced by this auxiliary pump acts upon a pressure-responsive member in opposition to the pressure produced by the positive displacement pump; an automatic control means acts to increase the power input to the turbine when the pressure produced by the positive-displacement pump increases and/or the speed of the turbine decreases, and to decrease said power in put in the converse case.

In one form of the apparatus described and claimed in the said British specification the turbine driving the positive displacement pump is a hot-gas turbine, and the auxiliary pump serves to supply fuel to a combustion chamber feeding the turbine, the pressure-responsive device being arranged to control a spill valve to deflect and return a variable portion of the flow from the fuel pump. The present invention has for an object so to modify the last-mentioned construction as to make it suitable for the use of a centrifugal pump as the fuel pump.

Another object is to provide an improved automatic control valve of high sensitivity for such automatic control system, which will eifect a rapid reduction in fuel supply when the delivery pressure of the positive displacement pump drops below a predetermined value, a rapid decrease in fuel supply when the delivery pressure of the positive-displacement pump drops below a predetermined value, a rapid decrease in fuel supply when the delivery pressure reaches a second higher value, and in either case a progressive decrease or increase in the fuel supply thus established when and according as the turbine speed rises above, or falls below, a predetermined value.

According to the present invention a metering orifice is interposed in the fuel-pump delivery line between the fuel pump and the spill valve, the valve device being preferably so constructed as to apply fuel-pump pressure direct to the appropriate control surface through a separate passage.

FIG. 1 of the accompanying drawing illustrates one simple embodiment of the invention, while FIGS. 2 and 3 of the drawings accompanying the specification are two modifications of the valve assembly shown in FIG. 1.

Referring now to the drawings, the complete unit illustrated in FIG. 1 comprises a combustion chamber 2, to which liquid fuel is fed, either before or after vapourisation, by a centrifugal fuel pump 30, and which supplies a How of combustion gases to a gas turbine 1, which drives both the centrifugal fuel pump 30 and a gear pump 4 which constitutes the turbine-driven positive-displacement pump and is interposed between an intake conduit 6 from a supply tank 32 and a delivery conduit which is connected through a nonreturn value 33 to a hydraulic accumulator 34 which is also connected by a pressure line 38 to a hydraulic load system (not shown). The accumula- Patented Aug. 29, 1961 ICC tor 34 is equipped with a pressure-responsive actuator 35 which, when the accumulator pressure exceeds a predetermined value, operates an unloading valve 36 to open a by-pass line 37 and thus establish a direct connection from the delivery outlet 5 of the gear pump 4 to the tank 32, thereby unloading the pump 4. Return flow from the accumulator 34 to tank 32 is prevented in these circumstances by the non-return valve 35. This combination of a hydraulic accumulator with a nonreturn valve and with automatic unloading means is well known in the art and is accordingly not claimed as novel. A metering orifice 31 is interposed between the parts 12 and 13 of a line leading from the centrifugal pump 30 to the combustion chamber 2, and this metering orifice 31 is followed in series therewith by a chamber 10a which is equipped with a spill valve 18, 18a and a spill passage 17 by which excess fuel is returned to the fuel reservoir (not shown). The valve element 18a is a slim cone, connected by means of a stern 18b to a bellows 9 which is situated within the chamber 10a; the interior of the bellows communicates with the delivery side of the gear pump 4 by a passage 7. 11 is a spring opposing the pressure in the line 7. This spring has been shown as a separate element but may in practice alternatively be constituted by the resilience of the bellows 9.

When the device is in operation and turbine speed rises, the pressure of centrifugal pump 30 will rise, tending to deliver more fuel through metering aperture 31 and passage 13 to the combustion chamber 2. As soon however as the pressure in chamber 10a begins to increase, this increased pressure will act upon the bellows 9 to open, or increase the opening of, valve 18, thereby allowing some of the fuel delivered through line 12 to be returned to the reservoir through spill passage 17. Since this spill flow, as well as the flow to the combustion chamber 2, is supplied by the pump 30 and thus constitutes an additional flow through the metering orifice, it causes the pressure drop across said orifice to increase. An increased pressure drop resulting from the increased flow through metering orifice 31, lowers the pressure in the chamber 10a, relative to the pressure produced by the centrifugal pump which at a given speed tends to be constant over a wide range of delivery, and thus counteracts the tendency of the flow to chamber 2 to increase. When the delivery pressure of gear pump 4 drops, as it will when a fiuid pressure reservoir in the consumer system is fully charged and pump 4- is therefore unloaded, this will cause the valve 18 to open wide, thus allowing maximum spillage, reducing the fuel supplied through line 13 to the minimum required for maintaining combustion and operation of the turbine under idling conditions.

FIG. 2 is a sectional elevation of a valve assembly corresponding substantially to that of FIG. 1. The same reference numbers as in FIG. 1 are employed for the connections, except that the connections to lines 7 and 13 are respectively referenced at 7a and 13a, and that the fuel-pump connection 12 has been replaced by two parallel connections 12a and 12b. The connection 12a leads through a metering orifice 22 to a spill valve and via the pipe connection 13, to the combustion chamber, while the connection 12b is unrestricted and serves to apply the delivery pressure of the fuel pump to the pressureresponsive member controlling the spill valve. It will be appreciated that, due to the provision of the metering orifice 22, the variation of the pressure at the fuel pump in response to a variation in turbine speed is much greater than the variation of pressure of the spill valve, so that the provision of the unrestricted second connection 12b renders the control more sensitive.

The bellows 9 of FIG. 1 has been replaced by two oppositely acting sliding piston members of different areas, namely a piston 19 of relatively large area, on which the delivery pressure of the centrifugal-type fuel pump acts through pipe 12b and a second piston 20 of much smaller area to which the delivery pressure of the positive-displacement pump 4 of the hydraulic system is applied through pipe 7.

Fuel for feeding the combustion chamber 2 is derived (through pipe 13) from a control chamber b, to which it is supplied through pipe 12a, and metering orifice 22, while a spill port 21 leading to spill passage 17 is controlled by a shuttle-type slide valve 23. The latter is integral with the piston 19 at one end, while the piston 24 assisted by a spring 24, acts on its other end. The position illustrated is the off-load position, in which the hydraulic accumulator 34 supplied by pump 4 is fully charged and pump 4 is unloaded in a well-known manner.

The hydraulic pressure in line 7 has therefore fallen to a small fraction of the working pressure, whereupon the pressure produced by the fuel pump 30 and admitted to piston 19 through the unrestricted passage 12b, has rapidly moved the shuttle valve 23 to a position, determined by the spring 24, in which the spill port 21 is throttled only sufficiently to ensure the delivery of the fuel necessary for maintaining the normal turbine speed under idling conditions.

FIG. 3 shows a modification of the valve just described, in which the spring 24a that biases the shuttle valve against the action of the fuel pressure is interposed between the shuttle valve 23a and the piston 20a on which the delivery pressure of the positive-displacement pump 4 acts. The piston 20a is for this purpose provided with a thrust collar 25. The references used are the same as in FIGURE 2, except for the addition or modification of an index letter a or b where an element has been modified, and the addition of some references for additional elements. The operation of the valve assembly is similar to that of FIG. 2, except that the device is insensitive to any small change in the delivery pressure of the hydraulic pump 4. This is achieved due to the fact that the hydraulic piston 20a will take up either one or the other of two extreme positions determined respectively by stops 23c and 23d, according as pump 4 works under load or is unloaded. Accordingly, the valve 23 passes a large or small volume of fuel, mainly according to the loading of the hydraulic system, but any small changes in load requiring fuel variation in order to maintain the system in equilibrium are sensed through the fuel delivery pressure as the turbine speed rises and falls. The fuel delivery pressure acts on the valve similarly as described above, trimming the fuel requirements about one or the other of two mean values alternatively provided by the valve according as the hydraulic pump works under load or under idling conditions.

In each of the embodiments of FIGS. 2 and 3 stops 23b (FIG. 2) or 23c, 23d, (PEG. 3) may be provided if desired for limiting the movement of the shuttle valve 23 or 23a in one or both directions.

What is claimed is:

1. In or for an automatic fuel control system for a hotgas turbine driving a positive displacement pump working against variable pressure wherein the system includes a dynamic fuel pump delivering fuel to the hot-gas turbine through a delivery line that includes a metering orifice and an automatic control valve operable to divert a variable part of the fuel in said line to a spill line for varying the quantity of the fuel delivered to the turbine, the construction of the automatic control valve, comprising in combination a valve housing having two co-axial bores separated by an interstice communicating with atmosphere, a piston in each said bore, a thrust spring interposed between said pistons, abutment means in one of the pistons which cooperate with the other piston to limit the mutual approach of the piston, means for establishing communication of the outer ends of said bores respectively with the variable delivery pressure of the positive-displacement pump and with the fuel, delivery line between the fuel pump and the metering orifice, means positively limiting the inward and outward strokes of one of the pistons, and a valve element connected at least operatively to the other piston and operative to increase the fiow diverted to the spill line upon inward movement and decrease the flow diverted to said spill line upon outward movement of said other piston.

2. An automatic control valve as claimed in claim 1, wherein said valve element is a valve piston fixed coaxially to said other piston and sealingly movable in the same bore, the valve housing further having a passage provided with means for establishing communication between said passage and the fuel line from the fuel pump, said passage leading from the fuel line to said bore between the innermost position of said other piston and the outermost position of said valve piston, the metering orifice being arranged in said passage, and a spill passage from said bore and co-operating with said valve piston in said bore for control, by said valve piston, of the flow from said passage through said bore to the spill passage, and a branch passage communicating with said passage between said communication and establishing means and the metering orifice and having means for connection through the fuel line to the hot gas turbine.

References Cited in the file of this patent UNITED STATES PATENTS 2,581,275 Mock Jan. 1, 1952 2,670,599 Davies et a1. Mar. 2, 1954 2,808,702 Dotson Oct. 8, 1957

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