Principles of naval engineering - Historic Naval Ships Association

Chapter 14. -PIPING,FITTINGS, AND VALVESVALVE SPRING VALVE GUIDEGLYCERINE SEALFILLING PLUGVALVE DISKCOOLINGFINSBLEEDER VALVE CHECK VALVEAIR GAUGE AND PUMP CONNECTIONHERE11.323Figure 14-22.— Pneumatic pressure controlledreducing valve for high temperature service.loading chamber is surrounded by a finned hoodwhich conducts heat away to atmosphere.A rubber diaphragm is installed in the middleof the dome. The bottom of the diaphragm isseparated from the bottom half of the dome by afixed steel plate. The area immediately above thediaphragm communicates with the upper part ofthe dome through holes in the shrouding. The upperhalf of the dome carries a level of water forsealing; the lower half of the dome carries alevel of glycerine for sealing. The area abovethe glycerine is charged with air, which exertsa downward pressure on the glycerine and forcessome of it to go up the tube toward the diaphragm.This pressure causes the diaphragm to move upward;and, since the stem of the valve is in contactwith the diaphragm, the upward movementof the diaphragm causes the valve to open. Whenthe valve is open, steam can pass through it.From the outlet connection, an actuating lineleads back to the upper part of the dome, asshown in the illustration. Steam at the reducedpressure is thus allowed to exert a force on thetop of the water seal; this force is transmittedthrough the water and tends to move the diaphragmdownward. When the pressure of thesteam from the actuating line exceeds the loadingair pressure in the lower half of the dome, thediaphragm moves downward sufficiently to closethe valve. The closing of the valve reduces thepressure of the steam on the discharge side ofthe valve. When the pressure on the outlet sideof the valve is equal to the air pressure in thelower half of the dome, the valve takes a balancedposition which allows the passage of sufficientsteam to maintain that pressure.If the load increases, tending to take moresteam away from the valve, the outlet pressurewill be momentarily reduced. Thus, the pressureof steam on top of the diaphragm becomes lessthan the pressureof air below the diaphragm, andthe valve then opens wider to restore the pressuretonormal.If the load is reduced, this causesa momentary increase in outlet pressure; andthis in turn increases the pressure on top of thediaphragm, making it greater than the air pressurebelow the diaphragm. The diaphragm istherefore displaced downward, and the outletpressure is again restored to normal.DIAPHRAGM CONTROL VALVES WITH AIR-OPERATED CONTROL PILOTS.-Diaphragmcontrol valves with air-operated control pilotsare being used increasingly on newer ships forvarious pressure- control applications. Thesevalves and pilots are available in several basicdesigns to meet different requirements. Theymay be used to reduce pressure, to augmentpressure, or to provide continuous regulation ofpressure, depending upon the requirements of thesystem in which they are installed. Valves andpilots of very similar design can also be used forother services such as liquid level control andtemperature control. However, the discussionhere is limited to the valves and pilots that areused for pressure- control applications.The air-operated control pilot may be eitherdirect acting or reverse acting. A direct-actingair-operated control pilot is shown in figure14-23. In this type of pilot, the controlled pressure—thatis, the pressure from the dischargeside of the diaphragm control valve— acts on topof a diaphragm in the control pilot. This pressureis balanced by the pressure exerted by thepilot adjusting spring. If the controlled pressureincreases and overcomes the pressure exertedby the pilot adjusting spring, the pilot valve stemis forced down. This action causes the pilot valveto open and so to increase the amount of operating381