ES341536A1 - Safety system for steam generator - Google Patents

Abstract

A control system for a boiler is provided with a safety system which over-rides the normal control of fuel, air and feedwater when the ratio of fuel to air flow exceeds a predetermined limit above a normal ratio or when the ratio of fuel to feedwater flow moves outside a predetermined range about a normal value, the safety system operating according to the nature of the ratio change to increase the air supply, decrease the fuel supply, increase or decrease the feedwater flow, or hold the fuel supply constant and so return the ratio to its predetermined range or limit. As shown in Fig. 1 the invention is applied to a control system for a boiler having an economizer 4, evaporater 6 and superheater 8 and supplying a steam turbine 20 driving an electric generator 22, the system using independent variable signals representative of fuel, air and feedwater flow and dependant variable signals representative of steam pressure and temperature from the superheater 8, oxygen content in the flue gases, and power output from the generator 22. The dependant variable signals are compared with corresponding set point values to produce further signals which are combined in summation and differential amplifiers inter-connected as shown, of which the amplifiers 66, 68, 70 respectively control the feedwater supply elements 26, 28, the fuel supply elements 34, 36 and the air supply elements 38, 40 by establishing set point values at the comparators 116, 128 and 140 respectively supplied with signals representative of measured feedwater, fuel and air flow. The layout and operation of the control system is stated to be conventional and is briefly discussed. The safety control system shown in Fig. 2 uses duplicate circuits to ensure reliability and comprises ratio computers 200, 204, 300, 304 supplied via connections A, B, C, D, E, F with signals representing fuel, air and feedwater flow, the ratios so determined being compared in proportional amplifiers 208, 308 which operate alarms if these ratios differ due to a fault in the safety system. The outputs of computers 200, 204 are also fed to proportional amplifiers 216, 226, 242, 250 of which amplifiers 216 and 242 operate alarms when the fuel to air ratio exceeds 1À1 and amplifiers 226 and 250 pass signals to an AND gate 238 when the ratio exceeds 1À2 to take over control of the members 38, 40 and 34, 36, the air and fuel supply being respectively increased and decreased. The computers 300, 304 feed control circuits similar to those described above and having additional proportional amplifiers and circuit components to operate when the fuel to feedwater ratio falls to 0À9 or 0À8, the water and fuel being respectively increased and decreased when the ratio exceeds 1À2 and respectively decreased and held constant when the ratio falls below 0À8. The ratio correction may be made by regulating only one of the two variables involved in each case, the guiding principles of operation being that fuel is never increased, air is never decreased and feedwater may be varied in either direction.