GB1560551A

GB1560551A - Fluid flow control system

Info

Publication number: GB1560551A
Application number: GB92576A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1976-01-10
Filing date: 1976-01-10
Publication date: 1980-02-06
Also published as: DE2700473A1; IT1085851B; FR2337903A1; JPS5285613A; FR2337903B1

Description

(54) FLUID FLOW CONTROL SYSTEM (71) We, LUCAS INDUSTRIES LIMITED, a British Company of Great King Street, Birmingham B19 2XF, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to flow control systems for gases, and in particular to such systems when intended for control of a fuel gas supply to a gas turbine engine.

According to the invention a gas flow control system comprises a variable flow metering device, means for generating a first signal which is a function of a desired gas flow through said device, means, responsive to the gas pressure upstream of said device and to the effective flow area of said device, for generating a second signal which is a function of a choked gas flow through the device, means, responsive to said upstream pressure and said flow area and also to a pressure differential across said device, for generating a third signal which is a function of an unchoked gas flow through the device, actuator means responsive to a difference between said first signal and either said second or third signals, for varying said effective flow area, and selector means, responsive to a predetermined level of the ratio of said pressure differential to said upstream pressure, for rendering said actuator means responsive either to said second or said third signal, whereby said actuator means can control said device in accordance with a difference between desired and actual gas flows either in an unchoked or a choked flow condition.

Embodiments of the invention will now be described, by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a gas flow control system adapted to deal with both choked and unchoked gas flows to a gas turbine engine, Figure 2 shows diagrammatically, a metering valve forming part of the system of Figure 1, and Figure 3 is a block diagram of the electrical control circuit of Figure 2.

It is an object of the system shown in Figures 1 to 3 to provide a flow control system for a fuel gas supply to an engine, wherein the gas supply pressure may vary so as to result in either choked or unchoked flow through a metering orifice, and in which a regulating device for the gas supply, separately from the metering valve, is unnecessary.

As shown in Figure 1, a gas turbine engine 50 is supplied with gaseous fuel whose flow is regulated by a metering arrangement 51. The metering arrangement 51 is located in a fuel supply passage 52.

The metering arrangement 51 is responsive to a control signal on a line 53 from an electronic control circuit 54, shown in more detail in Figure 3. Control circuit 54 is responsive to a digital first signal QD, corresponding to desired fuel flow, and supplied on a line 55 from a computer 56. The computer 56 is responsive to operating parameters of the engine 50, for example engine speed N and combustion temperature T, and also to a signal S dependent on the setting of a power demand control for the engine 50.

A temperature-responsive device 57 is responsive to the temperature T of the gas in the passage 52 upstream of the metering arrangement 51 to supply, on a line 58, a signal proportional to 1/ VT.

A transducer 59 is responsive to the fuel pressure in the passage 52 upstream of the metering arrangement 51 to supply, on a line 60 a signal P proportional to this pressure. A further transducer 61 is responsive to the pressure difference across the metering arrangement 51 to supply, on a line 62 a signal Dp proportional to this pressure difference. The electronic control circuit 54 is responsive to the aforesaid signals 1/ VT, P, Dp on lines 58, 60, 62 respectively.

The metering arrangement 51 is shown in detail in Figure 2 and includes a main hydraulically-operated flow control valve 70 having an axially-slidable control element 71. Control element 71 is movable by a double-acting piston 72. Piston 72 is urged in opposite directions by pressures applied to respective lines 73, 74, these pressures being controlled by an electro-hydraulic valve 75 which is responsive to the control signal on line 53 from the circuit 54. Hydraulic supply pressure and return lines 76, 77 communicate with the valve 75 and with a hydraulic pressure source 78, the lines 76, 77 being interconnected by a relief valve 79. A displacement transducer 80 is responsive to the position of the control element 71 to provide, on a line 81 a signal A proportional to the gas flow area of the main flow control valve 70.The signal A is also supplied to the electronic control circuit 54. Upstream of the main flow control valve 70 is a shut-off valve 82.

For choked flow through a metering orifice: Mass flow QF =K2 AP/ffT (1) For unchoked flow: Mass flow

Where K. and K4 are constants dependent on the characteristics of the gas and.

Where PB=P-DP.

The electronic control circuit 54 is operable to provide a control signal on line 53, this control signal being derived from second or third signals which are respective functions of mass flow equation (1) or (2) above, dependent on whether the flow through the main control valve 70 is choked or unchoked. The Dresence or otherwise of a choked flow condition is dependent on the function Dp/P, and the control circuit 54 is responsive, in a manner to be described, to the value of this function to select the manner in which the control signal on line 53 is calculated.

Control circuit 54 includes an analog to digital converter 90 which is responsive to the signal on line 58 to provide, on a line 91, a digital signal proportional to 1/ Analog to digital converters 92, 93 are respectively responsive to the signals on lines 60, 62 to provide, on lines 94, 95 digital signals proportional to P and Dp.

A dividing circuit 96 is responsive to the signals on lines 94, 95 to provide, on a line 97, a signal proportional to Dp/P. A function generator 98 is responsive to the signals on line 97 to perform the calculation indicated at equation (3) above and to provide, on a line 99, a function signal proportional to the value of +e.

The signal on line 97 is also supplied to a discriminator circuit 100 which supplies an output signal on a line 101 when the value of Dp/P is greater than or equal to .47, which corresponds to choked gas flow. A square root calculator 108 is also responsive to the signals on line 97 to provide, on a line 109, an output signal corresponding to /Dp/P.

A store 102 has two locations whose contents can be set to selected values of the constants K2 and K4 in equations (1) and (2) above. These values may be read out as respective fourth and fifth signals on lines 103, 104.

An analog to digital converter 105 is responsive to the signal on line 81 to provide, on a line 106 a digital signal A corresponding to the flow area of the main control valve 70.

A first multiplying circuit 110 is responsive to the signals on lines 91, 94, 103, 106 to supply, on a line 111 an output corresponding to QF =K2 AP/ v'T. A second 1 multiplying circuit 112 is responsive to the signals on lines 91, 94, 99, 104. 106 and 109 to provide, on a line 113, an output corresponding to QF =K4 A +2 s/P.DP/s/T.

A circuit 120 is responsive to the signals on lines 106, 111 to provide, on a line 121, a second signal proportional to QF -A, for the choked flow condition. A further circuit 122 is responsive to the signals on lines 106, 113 to provide, on a line123, a signal proportional to QF -A for the unchoked flow condition. The reason for the introduction of the A term will later be made apparent.

In the presence of a signal on line 101, corresponding to choked gas flow, a selector circuit 124 permits the signal on line 101 the unchoked control signals on converter 125. In the absence of a signal on line 101 the unchoked control signals on line 123 pass to the digital to analog to converter 125.

Output signals from the converter 125 are supplied to one input of a comparator 126. The digital signals on line 55, corresponding to desired fuel flow QD are supplied via a digital to analog converter 127 to a second input of the comparator 126.

Operation of the electro-hydraulic valve 75 may be expected to be vary rapid and to cause correspondingly rapid movement of the main flow control valve 70. Since as indicated, the greater part of the operations performed by the control circuit 54 are digital, these operations, together with analog to digital, or digital to analog convnrsion, may take up to 20 milliseconds to complete, during which time the control element 71 of the valve 70 may have moved appreciably. Accordingly therefore the valve position signal A on line 81 is supplied to the comparator circuit 126, to provide a negative feedback control. The selected QF-A signal from the digital to analog converter 125 is subtracted from the feedback signal A and the result compared with the Ql, signal from the converter 127.The comparator circuit 126 is such that there is no output signal on the line 53 when the result of this latter comparison is zero, that is when Q=Q,.

The system thus provides, in either choked flow or unchoked flow conditions, as appropriate, an actual fuel gas flow which corresponds to demanded gas flow, without the need for an additional gas pressure regulating valve.

WHAT WE CLAIM IS: 1. A gas flow control system comprising a variable flow metering device, means for generating a first signal which is a function of a desired gas flow through said device, means, responsive to the gas pressure upstream of said device and to the effective flow area of said device, for generating a second signal which is a function of a choked gas flow through the device, means, responsive to said upstream pressure and said flow area and also to a pressure differential across said device, for generating a third signal which is a function of an unchoked gas flow through the device, actuator means responsive to a difference between said first signal and either said second or third signals, for varying said effective flow area, and selector means, responsive to a predetermined level of the ratio of said pressure differential to said upstream pressure, for rendering said actuator means responsive either to said second or said third signal, whereby said actuator means can control said device in accordance with a difference between desired and actual gas flows either in an unchoked or a choked flow condition.

2. A system as claimed in Claim 1 which includes means for generating a signal dependent on a temperature of said gas, said means for generating said second and third signals being dependent on said temperaturedependent signal.

3. A system as claimed in claim 1 or claim 2 which includes means for generating fourth and fifth signals which are dependent on predetermined characteristics of the gas, said means for generating the second and third signals being respectively responsive to said fourth and fifth signals.

4. A system as claimed in Claim 3 in which said means for generating the second and third signals comprise respective first and second multiplying circuits.

5. A system as claimed in any preceding claim which includes means responsive to said ratio of said differential and upstream pressures for generating a function signal characteristic of an unchoked flow through said device, said means for generating the third signal being responsive to said function signal.

6. A gas flow control system substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. circuit 122 is responsive to the signals on lines 106, 113 to provide, on a line123, a signal proportional to QF -A for the unchoked flow condition. The reason for the introduction of the A term will later be made apparent. In the presence of a signal on line 101, corresponding to choked gas flow, a selector circuit 124 permits the signal on line 101 the unchoked control signals on converter 125. In the absence of a signal on line 101 the unchoked control signals on line 123 pass to the digital to analog to converter 125. Output signals from the converter 125 are supplied to one input of a comparator 126. The digital signals on line 55, corresponding to desired fuel flow QD are supplied via a digital to analog converter 127 to a second input of the comparator 126. Operation of the electro-hydraulic valve 75 may be expected to be vary rapid and to cause correspondingly rapid movement of the main flow control valve 70. Since as indicated, the greater part of the operations performed by the control circuit 54 are digital, these operations, together with analog to digital, or digital to analog convnrsion, may take up to 20 milliseconds to complete, during which time the control element 71 of the valve 70 may have moved appreciably. Accordingly therefore the valve position signal A on line 81 is supplied to the comparator circuit 126, to provide a negative feedback control. The selected QF-A signal from the digital to analog converter 125 is subtracted from the feedback signal A and the result compared with the Ql, signal from the converter 127.The comparator circuit 126 is such that there is no output signal on the line 53 when the result of this latter comparison is zero, that is when Q=Q,. The system thus provides, in either choked flow or unchoked flow conditions, as appropriate, an actual fuel gas flow which corresponds to demanded gas flow, without the need for an additional gas pressure regulating valve. WHAT WE CLAIM IS:

1. A gas flow control system comprising a variable flow metering device, means for generating a first signal which is a function of a desired gas flow through said device, means, responsive to the gas pressure upstream of said device and to the effective flow area of said device, for generating a second signal which is a function of a choked gas flow through the device, means, responsive to said upstream pressure and said flow area and also to a pressure differential across said device, for generating a third signal which is a function of an unchoked gas flow through the device, actuator means responsive to a difference between said first signal and either said second or third signals, for varying said effective flow area, and selector means, responsive to a predetermined level of the ratio of said pressure differential to said upstream pressure, for rendering said actuator means responsive either to said second or said third signal, whereby said actuator means can control said device in accordance with a difference between desired and actual gas flows either in an unchoked or a choked flow condition.