Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGAuxiliary steam stop valves are smaller thanmain steam stop valves but are otherwise similar.Special turbogenerator steam stop valvescontrol the admission of steam to the turbogeneratorline.Safety ValvesFigure 11-16.— External view of mainboiler stop valve.38.52steamIn use, the main steam boiler stop is alwayseither fully open or fully closed. The valve canbe opened and closed manually at the valveitself. In some installations, it can also beclosed pneumatically at the valve. The mainsteam boiler stop can also be operated manually,by remote control cables, from a remote operatingstation; as a rule, the valve can only beclosed (not opened) from the remote station.For manual operation, the toggle operating gearshown in figures 11-16 and 11-17 provides themechanical advantage required for closing thevalve against boiler pressure.Two-valve protection for each boiler is requiredon all ships built to U. S. Navy specifications.A second steam stop valve is thereforeprovided in the main steam line just beyondthe main steam boiler stop.Each boiler is fitted with safety valves whichallow steam to escape from the boiler when thepressure rises above specified limits. Thecapacity of the safety valves installed on aboiler must be great enough to reduce the steamdrum pressure to a specified safe point whenthe boiler is being operated at maximum firingrate with all steam stop valves completelyclosed. Safety valves are installed on the steamdrum and at the superheater outlet.Several different kinds of safety valves areused on naval boilers, but all are designed toopen completely (pop) when a specified pressureis reached and to remain open until a specifiedpressure drop ( blowdown ) has occurred. Safetyvalves must close tightly, without chattering, andmust remain tightly closed after seating.There is an important difference betweenboiler safety valves and ordinary relief valves.The amount of pressure required to lift a reliefvalve increases as the valve lifts, since theresistance of the spring increases in proportionto the amount of compression. Therefore arelief valve opens slightly at a specified pressure,discharges a small amount of fluid, andcloses at a pressure which is very close to thepressure that causes it to open. Such an arrangementwill not do for boiler safety valves.If the valves were set to lift for anything closeto boiler pressure, the valves would be constantlyopening and closing, pounding the seatsand disks and causing earlyfailureof the valves.Furthermore, relief valves would not dischargethe large amount of steam that must be dischargedto bring the boiler pressure down to asafe point, since the relief valves would reseatvery soon after they opened.To overcome this difficulty, boiler safetyvalves are designed to open completely at thespecified pressure. In all types of boiler safetyvalves, the initial lift of the disk is caused bystatic pressure of the steam, just as it wouldbe in a relief valve. But just as soon as thesafety valve begins to open, a projecting lip orring of larger area is exposed for the steampressure to act upon. The increase in forcethat results from the steam pressure acting290