GB1288015A - - Google Patents

Abstract

1288015 Fluid-pressure servosystems TOKYO SHIBAURA ELECTRIC CO Ltd 26 May 1971 [4 June 1970] 17133/71 Heading G3P [Also in Division F1] A turbine plant comprising a boiler 1 supplying steam through a control valve 2 to a high pressure turbine stage 3 and thence through a water separator 4 and intercept valve 8 to low pressure turbine stages 6, 7 includes a braking turbine 12 to which steam can be supplied through valve 13 to prevent plant overspeed. Valves 2, 8 are controlled by speed responsive governors such that should turbine speed rise to 102À5% of the rated value valve 8 becomes fully closed and if the speed rises further to 105% then valve 2 becomes fully closed. Braking valve 13 remains closed during normal operation but in one arrangement, Fig. 3 (not shown) begins to open if the speed rises to 106% and is fully open at 108À5%, thus generating a braking torque having a maximum value of 10% of the rated output of the main turbines. In the arrangement shown, however, braking valve 13 opens when the rate at which intercept valve 8 closes exceeds a predetermined rate as sensed by a differentiating mechanism 72. The system includes a speed governor controlled pilot valve 67 regulating the flow of liquid to a servomotor 60 connected to intercept valve 8. Movement of the piston rod 64 of servomotor 60 is transmitted via links 66, 73, 74, 75 to the left end 80a of a lever 80 forming part of the differentiating mechanism 72. During normal operation movement of rod 64 is slow and hence lever 80 pivots about its right end 80c since the latter is supported by springs 81, 82 which resist movement and because piston 77 of a dashpot 76 can follow the slow movement of lever 80. Lever end 80c is connected through links 83, 84 to the pilot valve 85 of braking valve servomotor 87 and since end 80c does not move pilot valve 85 and braking valve 13 remain closed. However should intercept valve 8 close rapidly, e.g. resulting from loss of load, the left end 80a of lever 80 moves rapidly downwards. Since piston 77 of dashpot 76 cannot move quickly lever 80 rotates around point 80b causing the spool of pilot valve 85 to rise and admit liquid to servomotor 87 to open braking valve 13. The characteristics of dashpot 76 can be adjusted by a needle valve 79 in by-pass passage 78. In the arrangement shown braking valve 13 is in series with valves 2, 8 so that braking turbine 12 is supplied with residual steam remaining in line 9 and stages 6, 7 after closure of valve 8. However in a modification, Fig. 1 (not shown), valve 13 is in parallel with valves 2, 8 and receives steam directly from boiler 1.