Principles of naval engineering - Historic Naval Ships Association

Chapter 11-BOILER FITTINGS AND CONTROLSHAND LIFTING LEVER*SPINDLE1SPINDLE LOCK CLIPADJUSTING RINGSET SCR EWADJUSTING RINGDISK INSERTFigure 11-19.— Steam drum safety valve (nozzle reaction type).29.219connected by piping to the space above the diskin the superheater actuated (or unloading) valve.The unloading valve has no spring and reliessolely on pressure differential for its operation.Normally there is a static pressure above thedisk of the unloading valve, since a small orifice(or in some designs a small clearance aroundthe disk) allows some steam at superheater outletpressure to enter the space above the disk.When the drum valve is opened by steamdrum pressure, the mechanical connection betweenthe drum valve and the actuating valvecauses the actuating valve to open also. Withthe actuating valve open, steam flows from thespace above the disk of the superheater unloadingvalve, through the actuating line, throughthe actuating valve, to atmosphere. The suddenrelief of pressure above the disk of the unloading293