Components of a Pneumatic-Assisted Control Valve

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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.

Cut-away diagram of a pneumatic-assisted control valve.
Above: Ill. 1: Cut-away diagram of a pneumatic-assisted control valve.

Table 1 (List to identify parts of the pneumatic-assisted control valve in Ill. 1)

No. Description

1 Plug/Stem Assembly

2 Packing Flange Stud

3 Packing Flange Nut

4 Packing Flange

5 Packing Follower

6 Drive Nut

7 Packing

8 Bonnet

13 Body

9 Hex Head Cap Screw

10 Body Gasket

11 Guide Bushing

12 Seat Ring

14 Indicator

15 Hex Jam Nuts

16 Yoke

17 Indicator Plate

18 Machine-Screw

19 Actuator Stem

20 Bushing

21 Hex Nut

22 Stem Spacer

23 Lower Case S/A

24 Hex Nut

25 Hex Head Cap Screw

26 Diaphragm

27 Upper Case

28 Diaphragm Plate

29 Spring

30 Spacer

31 Spacer

32 Hex Head Cap Screw

33 O-Ring

34 O-Ring

35 Actuator Vent Plug

36 Information Plate Ref.

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Butterfly Pneumatic-Assisted Control Valves