PRINCIPLES OF NAVAL ENGINEERING 



Figure 11-16.— External view of main 

 boiler stop valve. 



38.52 

 steam 



In use, the main steam boiler stop is always 

 either fully open or fully closed. The valve can 

 be opened and closed manually at the valve 

 itself. In some installations, it can also be 

 closed pneumatically at the valve. The main 

 steam boiler stop can also be operated manually, 

 by remote control cables, from a remote oper- 

 ating station; as a rule, the valve can only be 

 closed (not opened) from the remote station. 

 For manual operation, the toggle operating gear 

 shown in figures 11-16 and 11-17 provides the 

 mechanical advantage required for closing the 

 valve against boiler pressure. 



Two-valve protection for each boiler is re- 

 quired on all ships built to U. S. Navy specifi- 

 cations. A second steam stop valve is there- 

 fore provided in the main steam line just beyond 

 the main steam boiler stop. 



Auxiliary steam stop valves are smaller than 

 main steam stop valves but are otherwise simi- 

 lar. Special turbogenerator steam stop valves 

 control the admission of steam to the turbo- 

 generator line. 



Safety Valves 



Each boiler is fitted with safety valves which 

 allow steam to escape from the boiler when the 

 pressure rises above specified limits. The 

 capacity of the safety valves installed on a 

 boiler must be great enough to reduce the steam 

 drum pressure to a specified safe point when 

 the boiler is being operated at maximum firing 

 rate with all steam stop valves completely 

 closed. Safety valves are installed on the steam 

 drum and at the superheater outlet. 



Several different kinds of safety valves are 

 used on naval boilers, but all are designed to 

 open completely (pop) when a specified pressure 

 is reached and to remain open until a specified 

 pressure drop ( blowdown ) has occurred. Safety 

 valves must close tightly, without chattering, and 

 must remain tightly closed after seating. 



There is an important difference between 

 boiler safety valves and ordinary relief valves. 

 The amount of pressure required to lift a relief 

 valve increases as the valve lifts, since the 

 resistance of the spring increases in proportion 

 to the amount of compression. Therefore a 

 relief valve opens slightly at a specified pres- 

 sure, discharges a small amount of fluid, and 

 closes at a pressure which is very close to the 

 pressure that causes it to open. Such an ar- 

 rangement will not do for boiler safety valves. 

 If the valves were set to lift for anything close 

 to boiler pressure, the valves would be con- 

 stantly opening and closing, pounding the seats 

 and disks and causing earlyfailureof the valves. 

 Furthermore, relief valves would not discharge 

 the large amount of steam that must be dis- 

 charged to bring the boiler pressure down to a 

 safe point, since the relief valves would reseat 

 very soon after they opened. 



To overcome this difficulty, boiler safety 

 valves are designed to open completely at the 

 specified pressure. In all types of boiler safety 

 valves, the initial lift of the disk is caused by 

 static pressure of the steam, just as it would 

 be in a relief valve. But just as soon as the 

 safety valve begins to open, a projecting lip or 

 ring of larger area is exposed for the steam 

 pressure to act upon. The increase in force 

 that results from the steam pressure acting 



290 



