Overview of Primary Product

Technical Instructions Powermite Two-Way Flared Valves Normally Open/Normally Closed
Document No. 155-311P25
March 7, 2005
Page 4 Siemens Industry, Inc.
Normally Open Normally Closed
Figure 1. Nominal Close-off Ratings.
Operation
The actuator spring provides the necessary force to hold the stem in the raised or
normal position.
An increase in control pressure overcomes the spring pressure and moves the stem
downward. In a normally closed valve, this increases the flow of fluid through the valve.
In a normally open valve, an increase in pressure decreases the flow of fluid though the
valve.
With the loss of control pressure, the spring returns the valve to its normal position.
Sizing
The sizing of a valve is important for correct system operation. An undersized valve will
not have sufficient capacity at maximum load. An oversized valve can initiate cycling,
and the seat and throttling plug can be damaged because of the restricted opening.
Correct sizing of the control valve for actual expected conditions is essential for good
control.
Some variables that must be determined are:
• The medium to be controlled: water, etc.
• The maximum inlet temperature and pressure of the medium at the valve.
• The pressure differential that will exist across the valve under maximum load
demand.
• The maximum capacity the valve must deliver.
• The maximum line pressure differential the valve actuator must close against.
• See Application Bulletin (AB)-1 Control Valve Selection and Sizing (155-285) for
further recommendations.
See Tables 2 through 5 for valve capacities.