Product Overview

VP 656 Powermite Three-Way Mixing Valves (Flared Type) Technical Instructions
Document No. 155-086P25
March 7, 2005
Siemens Industry, Inc. Page 3
Operation
The control pressure from the controller is connected to the upper housing, above the
valve diaphragm. With no air pressure in the upper housing, the internal compression
spring exerts force against the piston plate which holds the stem assembly tight against
the upper seat permitting the flow from NO port to Common (C) port. An increase in
control pressure will overcome this spring force and the differential pressure of the
controlled medium to gradually move the stem assembly downward.
Full stem travel will bring the throttling plug in contact with the bottom seat for a
metal-to-metal close-off permitting the flow from the NC port to the C port.
The nominal spring span is identified as the change in air pressure required to operate the
valve from the opening of the upper port to the closing of the bottom port.
Figure 1. Operation.
Sizing
Correct sizing of the control valve for actual expected condition is essential for good control.
An undersized valve will not have sufficient capacity at maximum load. An oversized valve
may initiate cycling and the seat and disc may be damaged because of the restricted
opening.
Some variables that must be determined are:
1. The medium to be controlled; water, etc.
2. The maximum inlet temperature and pressure of medium at the valve.
3. The pressure differential that will exist across the valve under maximum load demand.
4. The maximum capacity the valve must deliver.
5. The maximum medium pressure differential the valve actuator must close against.
6. As the medium pressure differential across the valve increases, the spring span also
increases. This must be considered when sequencing valves.
7. Typical valve capacities are shown for water in Table 2. See AB-1 "Valve Selection and
Sizing" for further recommendations.