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From the previous page, recall this diagram showing an example of a simplified pneumatic-assisted control valve and signal converter. The actual valve is slightly more complex. Ill. 1 shows a cutaway picture of an actual pneumatic-assisted control valve. Each of the basic parts of the valve are identified in Table 1. The legend for the numbers is listed in Table 1. The top of the valve contains the pneumatic operator. When air fills this part of the valve, it will change the position of the diaphragm. The position of the diaphragm controls the position of the valve actuator stem, which pulls the valve out of its seat.
The springs provide a counterbalance to the diaphragm and help to return the valve to its normal position when the air pressure returns to minimum (3 psi). This valve is a normally closed (NC) valve, which means it will be in the closed position when air pressure is at minimum. These types of valves are available as normally open (NO) valves also. The fail-safe condition of the system will determine whether NO or NC valves are used. For example, if the system is supplying cooling water for a process, one would want the valve to be an NO valve so that if the air pressure or valve failed, the springs in the valve would always cause the valve stem to cause the valve to be in the open position. If the valve controlled the supply of liquid to a tank, one would want the valve to be an NC type so that if the control part of the system failed, the valve would close and not allow liquid to enter the tank. If an NO valve were used in this type of application, the valve would return to its full open position when a failure to control air pressure or a valve failure occurred, which could allow the tank to overfill.
|Simplified Pneumatic-Assisted Valve