GB768214A - Improvements relating to the operation of supercritical pressure vapour generators - Google Patents

Abstract

768,214. Critical pressure boilers. BABCOCK & WILCOX, Ltd. April 7, 1955 [April 8, 1954], No. 10325/55. Class 123(2). A forced-flow, once-through steam generator producing steam at supercritical temperature and pressure comprises a preheater section 12a, a primary superheater 12b, a normally-dry separator drum 22 and a secondary superheater 12c arranged for serial flow of fluid therethrough. A feed pump 18 supplies water to section 12a at supercritical pressure, e.g. 5,000 lbs. per. sq. in., the water leaving the section at about 680‹F. and passing to superheater 12b through an unheated external transfer line 20. During normal operation, the water-steam conversion zone 12d is located in superheater 12b. Steam leaves the superheater and flows through drum 22, the secondary superheater wherein it is heated to about 1/100‹F. and the outlet valve 26 of the latter to the point of use. When it becomes necessary to remove scale from zone 12d this is carried out whilst maintaining a supply of steam from the generator by first reducing the generator pressure to below the critical, e.g. to 2;600 lbs. per sq. in., by opening valve 26 so that steam separates in section 12a, the steam-water mixture flowing through superheater 12b and removing some of the scale The water in the mixture is completely evaporated in superheater 12b and the scale particles are washed from the steam leaving the superheater by pure water admitted from line 28, the steam separating from the wash water in drum 22 and flowing through superheater 12c wherein it is raised to the correct temperature. When the lower pressure is reached, the firing rate is decreased to cause the conversion zone to move from section 12a through superheater 12b into drum 22. The water now flowing through superheater 12b flushes the remaining scale from zone 12d into drum 22. Scalecarrying water is drained from the drum through a line 32 and a reducing valve 34 into a receiving tank 40 provided with a blow-down valve 44 operated according to the level in the tank by a controller 42. The blow-dowri water passes through a cooler 48 and purifier 52 to the feed water storage tank 60. When the scale-forming zone 12d is clean, normal supercritical pressure operation is resumed. During flushing, a special scale-removing liquid such as caustic soda solution may be injected into conduit 20 at 62.