POWER February 2012 - 81

CONTROL VALVE DESIGN
hind a bluff body in a free stream.
The CFD pressure plots shown in Figure
2 and Figure 3 illustrate how these low-pressure
areas can be formed. Each shows natural
gas flowing through a valve. Both valves
have the same mass flow and a pressure drop
ratio of approximately 2:1. The valve in Figure
2, however, is undersized, while the valve
in Figure 3 is adequately sized. In the undersized
valve, flow velocity increases in the
valve inlet, creating a large low-pressure area
where detached vortices could form.
Identifying a potential source of eddies is
only half the battle, though; it says nothing
about the possibility of detachment or the
subsequent movement of the vortices. Fluent
CFD, using the detached eddy simulation
(DES) model illustrated in Figure 4, can
complete the analysis and help predict transient
eddy detachment.
2. Undersized valve. A CFD pressure plot illustrates flow through an undersized valve
oriented in the flow-to-open direction. Note the large low-pressure zone adjacent to the valve
seat ring. Under certain conditions, this low-pressure region will detach from the valve body
wall, enter the free stream, and form a free eddy. Source: GE Energy
DES uses a transient (nonsteady) analysis in
combination with viscous turbulence to predict
the time-varying nature of the flow. This analysis
requires selecting very small time steps in
order for the CFD software to capture changes
in the fluid flow over time and show the eddies
forming and detaching. In this example, the
images showing contours of constant pressure
were taken at discrete time intervals roughly 0.4
milliseconds apart and show the low-pressure
eddies forming and then breaking away.
It is also important to note the pressure
pulsations that form inside the hollow cavity
of the valve plug. Many cage-guided valves,
such as compressor anti-surge valves and
main steam valves, have hollow plugs to reduce
weight and balance the pressure across
the plug. The pressure pulsations create a
similar periodic forcing function inside the
hollow cavity of the valve plug.
Another unintended consequence of the
Low
Flow
pressure
area
low-pressure zone, shown in Figure 2, is that
it effectively narrows the seat ring orifice, increasing
the pressure drop across the seat and
reducing the pressure drop across the valve
cage, thereby diminishing controllability and
reducing flow capacity. While acceptable results
may be achieved through proper valve
sizing and noise calculation, these unforeseen
phenomena must be understood to avoid
problems after installation.
In sum, the above analysis shows that vortex
Contours of static pressure (psi)
3. Adequately sized valve. A CFD pressure plot showing flow through an adequately
sized valve oriented in the flow-to-open direction. Note the absence of low-pressure zones.
Source: GE Energy
shedding is generated by high-velocity flows
navigating sharp bends in valves, that vortex
shedding is periodic in nature, and that the
vortices interact with the valve trim. The next
question that must be answered is: Can vortex
shedding cause severe valve vibration?
Natural Frequency Resonance of
the Plug-Stem System
Figure 5 shows an idealized view of a soft
mass-spring system-a control valve plug
at the end of a relatively slender valve stem.
The lateral modes of this assembly are not
allowed to develop, as the valve cage effectively
restricts the side-to-side motion of the
plug before any amplitude can build up. The
torsional first natural mode, however, is unconstrained,
as the plug rotates freely around
about the stem axis like a tetherball oscillating
around the pole.
The top of the stem is held steady by the
Low
pressure
area
actuator and is defined as the node of the system,
where the amplitude is zero. The bottom
of the system, at the plug, is defined as the
anti-node, where the amplitude is greatest.
Real-world valves include damping in the
Contours of static pressure (psi)
February 2012 | POWER
www.powermag.com
form of valve stem packing friction, contact between
the plug and the cage, and contact between
the stem and a guide bushing. Natural frequency
in a simple rod-mass system is inversely proportional
to the polar moment of inertia (plug mass
81

http://www.powermag.com

POWER February 2012

Table of Contents for the Digital Edition of POWER February 2012