Patent GB-828628-A (GB)
Title: Control system for supercritical pressure boiler
Inventor: SMOOT CHARLES HEAD
IPC: F22B35/12
Filed: 1956-11-09
Abstract: 828,628. Fluid-pressure servo motor-control systems. ELECTROFLO METERS CO. Ltd. Nov. 9, 1956, No. 34287/56. Class 135. [Also in Group XXXV] In a boiler operating at super-critical pressure, means are provided for automatically controlling the water supply and firing rate in accordance with changes in demand. The control systems are modified in accordance with steam temperature to ensure that the steam remains at the desired temperature. As shown, a boiler 10 is supplied with fuel through a conduit 11 via a valve 12 and air for combustion is supplied by a blower 13 under the control of a valve 14. Water is supplied via a pump 18 the rate of supply being controlled by a valve 22 operated by an actuator 23. The boiler supplies a load 19, e.g. a turbine. To control the firing rate a transmitter 24 is connected to opposite sides of a restriction 25 and measures the steam flow. The transmitter 24, Fig. 3, includes a balance beam 26 movable by a diaphragm 27 which is connected on its opposite sides to the opposite sides of the restriction 25. The beam 26 co-operates with a nozzle 28 supplied with air past a restriction 29 so that pressure back of the nozzle is varied in accordance with variations of steam flow and acts on a diaphragm 31 to rebalance the beam, and via a cylinder 32 operates a wiper 35 over a resistor 36 to produce a voltage proportional to the flow through the conduit 17. This voltage is combined in a transformer 37 with a further control voltage derived from a temperature-responsive member 42 and via a maximum limiting unit 44 controls regulators 45 and 46 controlling valves 12 and 14 respectively. A sample of combustion gas may be tapped off at 15 and via a gas analyser 48 utilized to modify the action of the regulator 46 to vary the fuel air ratio. In the maximum limiting unit 44, Fig. 4, the line 43 is normally connected via a switch 49 and line 43<SP>1</SP> to the regulators and is also connected to a primary winding 51 of a transformer, the other end of the winding being connected to a constant voltage source 53. The anode and cathode of a control valve 55 are connected to an alternating source 56 in series with a relay winding 57 and the secondary winding 52 of the transformer is connected to the control grid. The phasing is such that when the voltage on the line 43 is less than that of the source 53 the anode of the valve 55 will be negative when the grid is positive and the valve will not conduct. When the voltage on the line 43 exceeds the potential of the source 53, the phasing in the transformer 44 is reversed, the valve conducts, and the relay 57 is energized and the line 43<SP>1</SP> is connected to the source 53. Since the transformer primary has a relatively high impedance, the maximum signal which can be transmitted through the line 43<SP>1</SP> is determined by the source 53. To control the supply of water, a conventional pressureresponsive device 58 applies a control voltage from a potentiometer 83 to the regulator 23, and this control is modified so that when the demand exceeds the capacity of the boiler, the temperature of the steam will be above the critical value at all times. For this purpose a transformer 59 has its primary connected across the limiting device 44 and its secondary connected to a motor 61 which operates to vary the tension in a spring 77 controlling the balance beam 75 of the device 58. Under normal conditions there will be no voltage drop across the limiting device and maximum tension will be maintained on the spring 77. When the limiting device operates there will be a voltage across it which is proportional to both temperature and the flow of steam to adjust the motor 61 and the spring 77 and correspondingly adjust the operation of the device 58. In a modification, Fig. 2 (not shown), the load-responsive transmitter 24 is omitted and the movable wiper of a potentiometer connected to a voltage source is mechanically connected to the steam control valve. In this case the regulator 46 is connected directly to the wiper so that the air supply is responsive solely to the demand. The valve 22 is omitted and the water feed varied by variation of the pump speed. Specification 709,419 is referred to.