Hydromechanical pressure compensation control of a variable displacement pump in a centrifugal pumping and metering system and associated method

1. A pump assembly comprising:

a pump unit including a centrifugal pump and a variable pump supplying pressurized flow;

a fuel control assembly receiving flow from the pump unit, the fuel control assembly including at least first and second metering valves and at least first and second throttling valves where the first metering valve and the first throttling valve form a first metering valve/throttling valve pair, and the second metering valve and the second throttling valve form a second metering valve/throttling valve pair;

a control for the variable pump receiving first and second signals indicative of a pressure differential across one of at least the first and second throttling valves, or across one of at least the first and second metering valve/throttling valve pairs, for altering operation of the variable pump in response to the pressure differential; and

at least one selector valve that compares the pressure differentials across at least the first and second throttling valves, or across at least the first and second metering valve/throttling valve pairs, and communicates the signals representative of the throttling valve or the metering valve/throttling valve pair with the lowest differential pressure to the control.

2. The pump assembly of claim 1 wherein the first signal is received from downstream of the metering valve.

3. The pump assembly of claim 1 wherein the first signal is received from downstream of the metering valve and upstream of the throttling valve.

4. The pump assembly of claim 1 wherein the centrifugal pump of the pump unit is a high speed centrifugal pump that is operatively associated with the variable pump to supply fluid pressure needs for the fuel control assembly.

5. The pump assembly of claim 4 wherein at a minimum differential pressure, only the centrifugal pump provides pressure to the fuel control assembly.

6. The pump assembly of claim 5 wherein at higher differential pressures, the variable pump is commanded by the control to a minimum displacement output.

7. The pump assembly of claim 5 wherein when the differential pressure falls below the minimum differential pressure, the control signals for the variable pump to increase the displacement and boost pressure to maintain the minimum pressure level across the throttling valve and cutting off once the minimum differential pressure is achieved.

8. The pump assembly of claim 1 wherein the variable pump is a variable displacement pump having a freely rotating cam ring.

9. The pump assembly of claim 1 wherein the variable pump and the centrifugal pump are driven at different relative rotational speeds.

10. The pump assembly of claim 9 wherein the variable pump is driven at approximately 50% of the rotational speed of the centrifugal pump.

11. The pump assembly of claim 9 wherein the variable pump and the centrifugal pump are coaxially driven.

12. The pump assembly of claim 1 further comprising an inducer stage driven at a reduced rotational speed to operatively improve inlet performance characteristics of the centrifugal pump.

13. A method of operating a pump assembly comprising:

providing a pump unit that includes a high speed centrifugal pump and a variable pump to supply pressurized flow;

providing a fuel control assembly that receives flow from the pump unit, the fuel control assembly including a first metering valve and at least a first throttling valve pair, and a second metering valve and a second throttling valve pair;

monitoring pressure differential across the first and second throttling valves, or across the first and second throttling valve and metering valve pairs;

providing a selector valve to compare the pressure differentials from the monitoring step and communicating a lowest pressure differential set of signals representing the throttling valve or metering valve/throttling valve pair with the lowest differential pressure to a control of the pump unit; and

controlling operation of the pump unit via the control in response to the lowest pressure differential signals from the selector valve.

14. The method of claim 13 wherein the monitoring step includes receiving a first signal from downstream of the metering valve.

15. The method of claim 13 wherein the monitoring step includes receiving a first signal from downstream of the metering valve and upstream of the throttling valve.

16. The method of claim 13 wherein at a minimum differential pressure, the method includes using only the centrifugal pump to provide pressure to the fuel control assembly.

17. The method of claim 16 wherein at higher differential pressures, the method includes commanding the variable pump to a minimum displacement output.

18. The method of claim 16 wherein when the differential pressure falls below the minimum differential pressure, the method includes increasing the displacement of the variable pump to boost pressure to maintain the minimum pressure level across the throttling valve and cutting off once the minimum differential pressure is achieved.

19. The method of claim 13 wherein the monitoring step includes using first and second pressure signals, and the controlling step includes using the signals to hydromechanically alter operation of the pump unit.

20. The method of claim 13 wherein the pump unit providing step includes using a variable displacement ring pump as the variable pump and allowing a cam ring thereof to freely rotate to reduce viscous drag.

21. The method of claim 13 further comprising driving the variable pump and the centrifugal pump at different rotational speeds.

22. The method of claim 21 wherein the driving step includes driving the variable pump at approximately 50% of the rotational speed of the centrifugal pump.

23. The method of claim 21 wherein the pump unit providing step includes using an inducer driven at a reduced rotational speed to operatively improve inlet performance characteristics of the centrifugal pump.