These valves are used extensively on naval ships. The valves and their control pilots are available in several designs to meet different requirements. They may be used as unloading valves to reduce pressure or to provide continuous regulation of pressure and temperature. They may also be used for the control of liquid levels. The air-operated control pilot may be either direct acting or reverse acting. A direct-acting pilot is shown in figure 13-30. In this type of pilot, the controlled pressure-that is, the pres-sure from the discharge side of the diaphragm control valve-acts on top of a diaphragm in the control pilot. This pressure is balanced by the pressure exerted by the pilot adjust-ing spring. When the controlled pressure in-creases and overcomes the pressure exerted by the pilot adjusting spring, the pilot valve stem is forced downward. This action opens the pilot valve to increase the amount of operating air pressure going from the pilot

to the diaphragm control valve. A reverse-acting pilot has a lever that reverses the pilot action. In a reverse-acting pilot, therefore, an increase in controlled pressure produces a decrease in operating air pressure. In the diaphragm control valve, operating air from the pilot acts on a diaphragm contained in the superstructure of the valve operator or posi-tioner. (See fig. 13-31.) It is direct-acting in some valves and reverse-acting in others. If the valve operator is direct-acting, the operating air pressure from the control pilot is applied to the TOP of the valve diaphragm. When the valve operator is reverse-acting, the operating air pressure from the pilot is applied to the UNDERSIDE of the valve diaphragm.

View A in figure 13-31 shows a very simple type of direct-acting diaphragm control valve. The operating air pressure from the control pilot is applied to the top of the valve diaphragm. The valve in the figure is a downward-seating valve. Therefore, any increase in operating air pressure

Figure 13-30.-Air-operated control pilot.

Figure 13-31.-Diaphragm control valves.

pushes the valve stem downward. This tends to close the valve.

Now look at view B. This is also a direct-acting valve. The operating air pressure from the con-trol pilot is applied to the top of the valve diaphragm. But the valve shown in view B is more complicated than the one shown in view A. The valve shown in view B is an upward-seating valve rather than a downward-seating valve. Therefore, any increase in operating air pressure from the control pilot tends to OPEN this valve rather than to close it.

As we have seen, the air-operated control pilot and the positioner of the diaphragm control valve may be either direct-acting or reverse-acting. In addition, the diaphragm control valve may be either upward-seating or downward-seating. These factors, as well as the purpose of the installation, determine how the diaphragm control valve and its air-operated control pilot are installed in rela-tion to each other.

To see how these factors are related, lets con-sider an installation; a diaphragm control valve and its air-operated control pilot are used to supply reduced steam pressure. Figure 13-32 shows one arrangement that we might use. We will assume that the service requirements indicate the need for a direct-acting, upward-seating, diaphragm control valve. Can you figure out which kind of a control pilot-direct-acting or reverse-acting- should be used in this installation?

Figure 13-32 .-Arrangement of control pilot and diaphragm control valve for supplying reduced steam pressure.

Lets try it first with a direct-acting control pilot. The controlled pressure (discharge pressure from the diaphragm control valve) increases. When that happens, increased pressure is applied to the diaphragm of the direct-acting control pilot. The valve stem is pushed downward and the valve in the control pilot is opened. This sends an in-creased 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 pushes the stem downward. Since this is an upward-seating valve, this action OPENS the diaphragm control valve still wider. Obviously, this wont work-for this application, an INCREASE in controlled pressure must result in a DECREASE in operating air pressure. Therefore, we made a mistake in choos-ing the direct-acting control pilot. For this particular pressure-reducing application, we should choose a REVERSE-ACTING control pilot.

You will probably not need to decide which type of control pilot and diaphragm control valve are needed in any particular installation. But you must know how and why they are selected so that you will not make mistakes in repairing or replac-ing these units.