Chapter 14. -PIPING, FITTINGS, AND VALVES 



'.•i?^.* 



h 



HIGH 



PRESSURE-^ 

 STEAM 



^OPERATING 

 AIR PRESSURE 

 FROM CONTROL 

 PILOT 



REDUCED 



PRESSURE 



STEAM 



38.123X 

 Figure 14-24,— Direct-acting downward seating 

 diaphragm control valve. 



in operating air pressure from the control pilot 

 tends to open this valve rather than close it. 



As we have seen, the air-operated control 

 pilot may be either direct acting or reverse act- 

 ing, the superstructure of the diaphragm control 

 valve may be either direct acting or reverse act- 

 ing, and the diaphragm control valve may be 

 either upward seating or downward seating. 

 These three factors, as well as the purpose of the 

 installation, determine how the diaphragm con- 

 trol valve and its air-operated control pilot are 

 installed in relation to each other. 



To see how these factors are related, let us 

 consider an installation in which a diaphragm 

 control valve and its air-operated control pilot 

 are to be used to supply reduced pressure steam. 

 Figure 14-26 shows one arrangement that might 

 be used. We will assume that the service require- 

 ments indicate the need for a direct-acting up- 

 ward seating diaphragm control valve. What kind 

 of a control pilot— direct acting or reverse act- 

 ing would have to be used in this installation? 



Suppose that we try it first with a direct-act- 

 ing control pilot. As the controlled pressure (dis- 

 charge pressure from the diaphragm control 

 valve) increases, increased pressure would be 

 applied to the diaphragm of the direct-acting con- 

 trol pilot. The valve stem would be pushed down 

 and the valve in the control pilot would be opened, 

 thus sending an increased amount of operating 

 air pressure from the control pilot to the top of 

 the diaphragm control valve. The increased 

 operating air pressure acting on the diaphragm 

 of the valve would push the stem down and— since 

 this is anupwardseatingvalve— this action would 

 open the diaphragm control valve still wider. Ob- 

 viously, this will not work. For this application, 

 an increase in controlled pressure must result 

 in a decrease in operating air pressure. There- 

 fore, we should have chosen a reverse-acting 

 control pilot rather than a direct-acting one for 

 this particular pressure- reducing application. 



It is left as an exercise to the student to trace 

 the sequence of events as they would occur with a 

 reverse-acting control pilot installed in the ar- 

 rangement shown in figure 14-26. 



OPERATING 



AIR PRESSURE 



FROM CONTROL 



PILOT 



HIGH 



PRESSURE 



STEAM 



38.124X 

 Figure 14-25.— Direct-acting upward seating 

 diaphragm control valve. 



383 



