Apparatus for shut-down of a forced flow vapor generating unit

916,173. Forced-flow steam boilers. BABCOCK & WILCOX CO. July 15, 1959 [July 17, 1958], No. 24370/59. Class 123 (2). In a forced flow, once-through boiler operating at high, perhaps supercritical, pressure, provision is made in the event of feed pump stoppage for cooling the heating tubes which would be endangered by overheating due to the residual heat in the setting by producing a thermal circulation of steam and water through those tubes. The supercritical pressure boiler shown is of the kind described in Specification 899,359. It has a tube-lined, upright furnace chamber A fired by pulverized fuel burners 18 and surmounted by a convection gas cooling chamber B which is divided into three passes 22, 23, 24 by tubulous partitions 20, 21. The gas flow through the passes is proportioned by outlet dampers 33, 34, 35. The upper and lower portions of all the passes contain respectively an economizer 32 and a secondary superheater 29 whilst the central portions of the passes respectively contain a final primary superheater 28, a first stage reheater 30 and an initial primary superheater 27 and a second stage reheater 31. A feed pump 46 supplies water through a conduit 42 to the economizers 32 whence it flows through a downcomer 44 and then upwardly in steam generating tubes lining the walls of the chambers A and B to a header 47. From this header steam-water mixture descends in downcomers 48 to enter tubes forming slag screens 25, 26 and the partitions 20, 21 through which it rises to a header 49. From this header steam passes into the initial primary superheater 27, leaving through a conduit 50 for passage through the superheaters 28, 29. A conduit 51 containing a check valve 52 connects the outlet of the pump 46 with the outlet conduit 50 from the superheater 27. The check valve 52 comprises a piston 52A moving in a cylinder 53. In its uppermost position the piston contacts a seating 56 and closes the conduit 51, the piston occupying this position when the generator is working normally since the pump pressure holds it on to the seating. If the feed water supply to the unit fails, the fuel firing equipment is automatically shut off and the piston 52A, no longer supported by the pump pressure, falls to open the conduit 51, whereupon a thermo-syphon circulation is established in a flow path including the economizers 32, the downcomer 44, the wall lining, screen and partition tubes, the superheater 27 and the conduits 51, 42. The generating tubes and superheater