Patent GB-875849-A (GB)
Title: Improvements in and relating to safety devices for boilers
IPC: F22B37/47
Filed: 1959-05-12
Abstract: 875,849. Low water safety and indicating devices. NORDLUND, K. F. May 12, 1959 [May 13, 1958], No. 16321/59. Class 123(1) In a safety device for the protection of a steam generator against low water operation and which can be used to give a warning signal, e.g., light a lamp at a central control station, a pipe 3, communicating with the water in a vessel 1 at the normal level 2, is closed at its outer end by an end wall of a hard steel socket 4 into which is screwed a copper plug 5 having engaged therein a fuse plug 6 which melts at a temperature slightly above the normal temperature of the water and steam. An uninsulated pipe 8 extends between the vessel 1 and the outer end of the pipe 3 to prevent the formation of sediment in the latter. At normal water level the heat from an insulated electric heating coil 7 wound around the socket and plug 4, 5 is carried off by the pipe 3 and the water in it the fuse 6 being maintained below melting point. Should the water level sink below the pipe 3 the heat is conducted away from the socket and plug 4, 5 by the pipe 3 only.so that the fuse 6 reaches a temperature to melt and break an electric circuit supplying energy to the coil 7, in parallel to one, or more motors 12, and also to other electric apparatus for the control of the firing and the feed-water supply of the steam generator. The circuit comprises a line 9 which supplies direct current to one terminal of a relay coil 10 of an automatic circuit-breaker 11 having its other terminal connected to the motor 12 &c. The current passing through the coil 10 closes the circuit-breaker contacts 11 so that the motors 12, firing, and feed-water apparatus can be started and current is supplied, through maximum and minimum current relays 18, 19 which may control a circuitbreaker, to the heating coil 7 and through a conductor 20 and the plug and socket 5, 4 to earth. From the coil 10 the current passes along a line 13, which may incorporate a limiting resistance, to a switch 14 which may have a novolt release and then, through a looped conductor 15 cast into the plug 6, to the plug and socket 4, 5 and to earth. An analogous circuit for an alternating current electricity supply is also described. The plug and socket 5, 4 may be formed in one piece having the fuse plug 6 screwed into it. In a modification, Fig. 3, a pipe 34 closed at one end by a wall 38 and at the other by a removable threaded plug 40 is, at these ends, connected to the vessel 1 by smaller upper and lower pipes 35, 36 repectively, the upper pipe 35 entering the vessel 1 at a water level 37 which is slightly higher than the lowest permissable level 2. A rod-shaped heating element 43 is accommodated in a copper cylinder 42 which preferably has a nickel or chrome-plated outer surface and is sweated within a short tube 41 which passes transversely through and is welded to the pipe 34 midway between the pipes 35, 36. The fuse plug, 45, with a lead 50, is cast around a threaded pin 49 projecting from a metal fuse holder 44 which is pressed in good heatconducting contact, against the cylinder 42 by means of a spring-loaded rod 46 supported in bearings 47, 47. The holder 44 and the fuse plug 45 unit is accessible for replacement. The fuse plug 45 may be cast in a threaded bore in the holder 44 instead of around the pin 49.