Powers 597 Series Flowrite II Heavy Duty Balanced Control Valves - Manual (Page 2)

Page 2 TI597SI
APPLICATION INFORMATION
Flowrite II Single Seated SI balanced valves are generally recommended for
steam, hot water and chilled water applications. They are particularly
suited for installations requiring tight shutoff and quick response.
THEORY OF OPERATION
On normally closed valve assemblies, the valve stem will start to open
whenever the control air pressure applied against the actuator diaphragm
area and the lower housing exceeds the holding force of the springs. A
further increase in control air pressure will initiate a continued upward
travel of the valve steam until the valve has fully opened.
On normally open valve assemblies, the stem will start to close whenever
the control air pressure applied against the actuator diaphragm area and
upper housing exceeds the holding force of the springs. A further increase
in control air pressure will initiate a continued downward travel of the
valve stem until the valve has fully closed.
The air pressure change to initiate full stem travel is known as the spring
range or span. This spring span is factory set and will vary slightly as the
pressure drop across the valve changes.
When the valve is at its’ “full open” position there is maximum flow
potential through the valve. At this position, valves are compared based
on flow that is directly related to the valve flow coefficient (see Cv
equations in table 1). The 597 SI valves are designed so that equal changes
in valve stem position provide equal percentage changes in existing flow
through the valve. This is otherwise know as an equal percent valve which
has a typical flow curve (figure 2) that can be used to determine flow based
on stem position, pressure drop, and Cv. As you can see from the graph,
these valves are less sensitive at the low end, which gives both high
rangeability and high flows. These types of valves are used extensively to
compensate for fluctuating system requirements (pressure, flow, load, etc.).
Valve actuators equipped with positioners provide feedback for enhanced
control strategies and, as an example, are required for valve staging. Valves
with positioners can utilize full control air pressure at any point in stem
travel to initiate stem movement or to maintain stem position. However,
the actuator springs still provide the necessary force to move the stem in
the opposite direction. Use of a positioner will tend to provide faster
response and ensure repeatability of stem position regardless of the load on
the actuator. However, in a system where available pressure and flow
requirements are relatively consistent, control valves can usually perform
adequately without a positioner.
Figure 1 A- Push to Open Figure 1 B- Push to Close
Figure 2