Chemical Engineering August 2012 - 54

FIGURE 3. Drilled-hole shape and spacing
in the cage wall along with fl ow-down
design reduces and isolates cavitation to
prevent damage
FIGURE 2. This 1960s globe-valve design
has cage-style trim
and operating noise extremes, involved
use of expensive and often lessthan-successful
solutions. With simple
trim changes, these problems could
now be solved with cost-effective and
reliable solutions.
Staging the pressure drop so that
the pressure remains above the liquid's
vapor pressure can prove effective
in avoiding cavitation. With the pre1960s
valve, staging often involved the
use of an orifice plate downstream of
the main valve or the use of two valves
in series. However, these techniques
typically displaced cavitation from the
main valve to the downstream restriction
and did not effectively control the
cavitation in the system. They also
increased valve size, as less pressure
drop was available to process the same
amount of flow. Cavitation control was
often less than successful, and the continual
need to replace damaged piping
components proved costly.
The cage-style valve delivered an
answer to this cavitation dilemma
with its specialized cages. For example,
one cage design contains a multitude
of highly engineered, shaped
holes that improves the flow performance.
The holes are radially aligned
to flow cavitating jets of liquid to collide
in the center of the flow stream,
thereby avoiding damage to valve and
pipe surfaces (Figure 3).
A more-extreme pressure-drop
cage trim is designed specifically for
liquid applications where pressure
drops are above 207 bars (3,000 psi)
and cavitation is a serious problem. It
stages the pressure drop across successively
larger flow areas, such that
more than 90% of the overall pressure
drop is taken in the first stages
where there is little danger of bubble
formation. Cavitation is completely
avoided, thus protecting the valve,
and providing an answer to the high
cost of valve trim replacement.
The evolution of the specialty, anticavitation
trims has extended to services
where the fluid may have entrained
particulate matter that could
plug trim passages or cause erosion
damage to conventional anti-cavitation
trims. Used frequently in high pressure-drop
applications up to 4,200 psid,
this trim design employs a combined
axial and radial flow path with large
openings that allow particulate matter
up to ¾ in. in diameter to pass through
the valve. Due to the need for tight
shutoff in many applications, its multistage
design features a protected seat
where the shutoff function of the valve
is separate from the throttling areas of
the trim. All significant pressure drops
are taken downstream of the seating
surface, and the seating surfaces are
not worn away by throttling control
action. The result, once again, is that
replacement trim costs are avoided.
Continued product evolution of that
mid-60s valve innovation created answers
to other long-standing control
valve problems.
Protecting your investment with
noise attenuation trim. Throughout
the CPI, steam applications
with high pressures and large
pressure drops can be noise
generators that cause equipment
damage and control
issues. Because it can affect
plant availability and profitability,
control valve noise
is a concern shared by plant
operators and maintenance
personnel. Equally significant
is the fact that high noise levels
can cause health concerns for personnel
whose workday keeps them on
the plant floor.
The prevalent noise source of aerodynamic
flow is fluid turbulence within
the control valve body. Before cagestyle
trim, noise control techniques
did not consider the reduction of flow
turbulence as an answer. Instead,
they centered on the use of acoustic
wrap on the valve and adjacent piping,
which served only to mask the
noise. While this approach may have
been adequate in protecting personnel
who were working close to the valve,
the noise would reappear downstream
relatively unabated.
In the late 1960s and early 1970s,
studies focusing on the mechanisms
and abatement of control valve noise
led to the development of different
strategies to control fluid-generated
noise. Today, putting these strategies
to work are cage-style trims that utilize
unique flow-passage shapes and
multi-stage pressure reduction - capabilities
not possible with previous
generation valve designs.
A principle noise-reduction strategy
is to break the aerodynamic flow
stream into many small, parallel
flow passages that ensure exit jet
independence as flow exits the cage.
This technique reduces the power of
the noise source as it shifts the frequencies
to a higher, more easily dissipated
range. Up to 18 dB(A) of attenuation
is typical.
Yet another cage architecture utilizes
drilled hole technology to deliver
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Chemical Engineering August 2012

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