TWO WAY CONTROL VALVES NORMALLY OPEN VALVE NORMALLY CLOSED VALVE Compressed air drives the valve closed. Compressed air drives the valve open. The return spring forces the valve open The return spring forces the valve closed on a decrease in air pressure. on a decrease in air pressure. AIR CONNECTIONS DIAPHRAGMS DIAPHRAGM PLATES ACTUATOR RETURN SPRINGS PACKING SEALS SET SCREWS STEMS DISC ASSEMBLIES VALVE SEATS FLUID FLOW VALVE BODIES Pneumatic valves stroke through ranges dependent on the range of the return spring. Some valves have fixed start points while other valves may have the start point altered via spring tension adjustment on the actuator. The spring range on the valves specification sheet is a bench spring range. Typically, in a system, a normally open valve with a bench spring range of four PSIG to eight PSIG will start at four PSIG, but close above the eight PSIG bench closing point. The fluid system differential pressure, from the supply to the return, causes a force against the disc assembly, shifting the spring range closing point of eight PSIG, for example, to ten PSIG. Valves must be installed with the fluid flow as illustrated. If a valve is installed backwards, it will usually cause hammering in the pipes just as the valve is close to the normal closing point. Honeywell and some Johnson Control valves have a normally open body, with an actuator that drives the valve closed when no pressure is on the diaphragm. These valves have to be considered normally closed in the logic of a system. Some older Johnson Control valves have the potential to be very dangerous during servicing. Consider the illustration of the normally open valve. If the disc assembly is detached from the stem, the set screws are the only means of holding the spring in the actuator. Some valves have an 8R actuator and a valve range of nine to thirteen PSIG. In this situation, the actuator could be thrown, if the set screws are loosened, with a force around fifteen hundred pounds. It could cause severe injuries or death. 2.9
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