Principles of naval engineering - Historic Naval Ships Association

Chapter 14. -PIPING,FITTINGS, AND VALVES'.•i?^.*h HIGHPRESSURE-^STEAM^OPERATINGAIR PRESSUREFROM CONTROLPILOTREDUCEDPRESSURESTEAMSuppose that we try it first with a direct-actingcontrol pilot. As the controlled pressure (dischargepressure from the diaphragm controlvalve) increases, increased pressure would beapplied to the diaphragm of the direct-acting controlpilot. The valve stem would be pushed downand the valve in the control pilot would be opened,thus sending an increased amount of operatingair pressure from the control pilot to the top ofthe diaphragm control valve. The increasedoperating air pressure acting on the diaphragmof the valve would push the stem down and— sincethis is anupwardseatingvalve— this action wouldopen the diaphragm control valve still wider. Obviously,this will not work. For this application,an increase in controlled pressure must resultin a decrease in operating air pressure. Therefore,we should have chosen a reverse-actingcontrol pilot rather than a direct-acting one forthis particular pressure- reducing application.It is left as an exercise to the student to tracethe sequence of events as they would occur with areverse-acting control pilot installed in the arrangementshown in figure 14-26.Figure38.123X14-24,— Direct-acting downward seatingdiaphragm control valve.OPERATINGAIR PRESSUREFROM CONTROLPILOTin operating air pressure from the control pilottends to open this valve rather than close it.As we have seen, the air-operated controlpilot may be either direct acting or reverse acting,the superstructure of the diaphragm controlvalve may be either direct acting or reverse acting,and the diaphragm control valve may beeither upward seating or downward seating.These three factors, as well as the purpose of theinstallation, determine how the diaphragm controlvalve and its air-operated control pilot areinstalled in relation to each other.To see how these factors are related, let usconsider an installation in which a diaphragmcontrol valve and its air-operated control pilotare to be used to supply reduced pressure steam.Figure 14-26 shows one arrangement that mightbe used. We will assume that the service requirementsindicate the need for a direct-acting upwardseating diaphragm control valve. What kindof a control pilot— direct acting or reverse actingwould have to be used in this installation?HIGHPRESSURESTEAMFigure38.124X14-25.— Direct-acting upward seatingdiaphragm control valve.383