Patent GB-1463507-A (GB)
Title: Apparatus for controlling a flow of a fluid additive
IPC: G01F1/696
Filed: 1974-06-12
Abstract: 1463507 Measuring fluid-flow J AGAR INSTRUMENTATION Ltd 16 Dec 1974 [28 Dec 1973 12 June 1974] 59933/73 and 26062/74 Headings G1N and G1RApparatus for controlling a flow of a fluid additive into a flow of waste gases comprises a gas conduit 10, Fig. 10, an additive duct 146, a valve 145, a flow passage 140 arranged to receive part of the gas flow, a chamber 30, Fig. 2 communicating with the flow passage 140 so as to be adapted to receive a substantially stagnant or by-pass supply of the said flow therefrom, first and second electrical resistance devices which are respectively mounted in the passage 140 and chamber 30 and whose resistance varies with temperature in a predetermined manner, and output means connected by an electrical circuit to said first and second devices and which is adapted to produce an electrical output which is functionally related to relative electrical parameters of the first and second devices, the output being representative of the rate of gas flow and controlling opening and closing of the flow control valve. The gas pipe 10, Fig. 1, e.g. for waste or flare gases 11 from an oil refinery, contains a probe 12 so that gas may be by-passed through a port 13 and a valve 35 to a flow sensor 17, Fig. 2, which is provided with a thermistor 25 on the inflow and an identical thermistor 26 on the outflow. The by-pass flow continues on from the sensor 17 through flow shapers 21, 24 back to the pipe 10 via a port 14, and a steam supply is connected to the sensor to effect periodic cleaning of 17. The heat dissipated by the thermistor 25, and consequently its resistance in an electrical measuring circuit, will depend on the temperature, composition and also the cooling or heating effect of the gas flow, whereas the heat dissipated by the thermistor 26 in the shielded stagnant gas chamber shown in 17 will depend on all these parameters except the flow-rate. In Fig. 3 the thermistor 25 is incorporated in a bridge 44 which with an amplifier 52 forms a circuit which is balanced at a predetermined temperature, and the thermistor 26 is connected in a bridge 57 also in a circuit balanced at a predetermined temperature. The output voltages at the points 53, 55 and 66, 68 will be altered as the resistances of the thermistors vary according to the rate of gas flow and in such a way as to charge the input voltages to the bridges and maintain the temperatures of the thermistors substantially constant. The outputs from the bridges are connected to a difference amplifier 72 and to a timer 42 controlling a motor 81 or relay ganged to a meter 82 forming part of an output display 41, Fig. 1. Thus any variation in the gas flow-rate will produce a linear change in the display 41, and during steam cleaning the meter 82 will continue to show its last reading. The output from the amplifier 76 may be used to control the amount of steam or other gas mixed with the waste gas. In Fig. 4 (not shown) the probe itself within the gas pipe contains both thermistors so that the gas flow is not by-passed and the probe may be moved to determine flow variation across the pipe. In Figs. 5-7 (not shown) various probes are illustrated which deflect the gas and soot before sampling. In Fig. 8 a condensation and gas scrubbing trap 117 is provided and the sensor 130, which is similar to that of 17 in Fig. 1, connects with a density meter 126 and a mass flow, S.G. or gas density indicator or meter 128.  In Fig. 10 gas is by-passed through a density meter 136 and a flow sensor 137 similar to the sensor 17 and these detectors are connected by way of Zener diodes 141, 143, to a mass flow calculator 142 and a recorder/controller 144 receiving signals from a differential pressure cell across a restrictor 148 in a steam pipe 146 communicating with a ring atomizer 149 in a flare stack 150. Thus it is possible to control the amount of injected steam and crack the flue gas into hydrogen or methane which does not smoke.