GB831175A - Apparatus and method for controlling a forced flow once-through steam generator - Google Patents

Abstract

831.175. Forced-flow steam boilers. SULZER FRERES S. A. Feb. 3, 1958 [Feb. 7, 1957], No. 3523/58. Class 123(2) In a forced-flow, once-through boiler which may operate at supercritical pressure, a pump 10 supplies feedwater through a regulator valve 22 to a header 20 whence it flows through and is evaporated in heating tubes 32 lining the walls of a furnace 24 and connected to an outlet header 34. From the outlet header steam flows through a superheater 40 and outlet pipe 44 to the point of use. To reduce the power requirements and pressure head of the feedwater pump and to ensure an adequate cooling flow of working fluid through the tubes 32 at low loads a pump 50 is provided to recycle working fluid from the header 34 to the header 20. In operation, the fuel supply to the furnace burners 26 and the setting of the feedwater regulator 22 are respectively controlled according to variations in steam pressure and steam flow by regulators 56, 60 receiving impulses from a pressure sensing device 55 and a steam flow indicating device 58. The amount of working fluid recirculated is controlled either by impulses from the flow indicator 58 or from a pressure difference indicator 66 which compares the pressures in the headers 20, 34. The minimum pressure drop in the tubes 32 required to maintain an adequate cooling flow therethrough is maintained throughout any selected steaming load range by either control method which superimposes the recirculating flow on the main flow as that minimum is reached and thereafter increases the amount recirculated as the load further decreases or decreases the amount if the load increases.. In modifications the recirculation rate is controlled according to variations in electrical loading of a generator driven by a turbine supplied by the boiler or the speed of the recirculating pump is varied inversely as the speed of the main feed pump.