GB854446A - Fluid-pressure relay and control system incorporating the same - Google Patents

Abstract

854,446. Fluid-pressure servomotor control systems. BRISTOL CO. July 9, 1957 [July 19, 1956], No. 21699/57. Class 135. In a process control system which maintains a predetermined ratio between the flow of fluids in the pipes 10, 15 a pneumatically operated ratio control relay 33 is provided. A transmitter 12 sends a signal indicative of the flow in the pipe 10 to the ratio control relay 33 and to an indicating and recording unit 86, 89. A signal proportional to said flow is sent from the relay 33 to the force-balance controller 25 where it is compared with a signal from a transmitter 17 indicative of the flow in the pipe 15. The 'controller 25 normally controls the flow regulating valve 18. A capsule 87 which actuates a pen 93 and pointer 94 also receives the signal from the transmitter 17. The output signal from the relay 33 is also transmitted to a capsule 88 which operates a pointer 96. The transmitters 12, 17 receive an air supply from a source 20 via a pipe 19. Between the controller 25 and the regulating valve 18 is a direct-acting valve 30 supplied from the pipe 19 via a valve 71 which, in the position shown, is vented so that the controller 25 is controlling the valve 18. The pressure being transmitted to the valve 18 is indicated by a pointer 82 operated by a Bourdon tube 80. The pipe 19 is also connected to a reducing pressure regulator 65 of the supply- and-waste-type and to a manually settable reducing valve 66. The valve 65 is connected to the relay 33 and the valve 66 to a Bourdon tube 78 which operates a pointer 81 and to a shut-off valve 70 having the same actuating shaft 75 as the valve 71. The shaft 75 may be rotated to supply air to the valve 30 and to open the valve 70 and so put the valve 18 under manual control. " Bumpless " transfer from automatic to manual control and vice versa is obtained by prealigning pointers 81, 82 and 94, 96 respectively. The relay unit 33 comprises a body having four chambers 34, 42, 43, 44, 60. The chamber 34 has an inlet 35 connected to the transmitter 12 and an inlet-outlet 36 connected to the valve 65. A fixed restrictor 37 and a variable restrictor 38 are located adjacent the ports 35, 36 respectively. The chamber 34 communicates with the chamber 42. The chamber 43 which vents to atmosphere is defined by two diaphragms 40, 41 interconnected by a member 45 oppositely loaded by springs 51, 57. The chamber 60 is connected to the air supply and communicates with the chamber 44 via a port controlled by a valve 62 the extension 61 of which controls a passage 56 through the member 45 from the chamber 44 to the chamber 43. The arrangement is such that the pressure in the chambers 34, 42 exceeds the datum value of the input signal from the transmitter 12 by a fraction of the remainder of said signal depending on the adjustment of the restrictor 38. The pressure in the chamber 44 which is transmitted to the controller 25 is made up of a datum component and a component which is a multiple of the above fraction depending on the relative areas of the diaphragms 40, 41. Specifications 525,674, 785,009, 785,010 and 785,011 are referred to.