Reference Manual

www.Fisher.com
Chapter 1
Control Valve Selection
In the past, a customer simply requested a control
valve and the manufacturer offered the product
best-suited for the job. The choices among the
manufacturers were always dependent upon
obvious matters such as cost, delivery, vendor
relationships, and user preference. However,
accurate control valve selection can be
considerably more complex, especially for
engineers with limited experience or those who
have not kept up with changes in the control valve
industry.
An assortment of sliding-stem and rotary valve
styles are available for many applications. Some
are touted as “universal” valves for almost any
size and service, while others are claimed to be
optimum solutions for narrowly defined needs.
Even the most knowledgeable user may wonder
whether they are really getting the most for their
money in the control valves they have specified.
Like most decisions, selection of a control valve
involves a great number of variables; the everyday
selection process tends to overlook a number of
these important variables. The following
discussion includes categorization of available
valve types and a set of criteria to be considered in
the selection process.
What Is A Control Valve?
Process plants consist of hundreds, or even
thousands, of control loops all networked together
to produce a product to be offered for sale. Each
of these control loops is designed to control a
critical process variable such as pressure, flow,
level, temperature, etc., within a required operating
range to ensure the quality of the end-product.
These loops receive, and internally create,
disturbances that detrimentally affect the process
variable. Interaction from other loops in the
network provides disturbances that influence the
process variable. To reduce the effect of these
load disturbances, sensors and transmitters collect
information regarding the process variable and its
relationship to a desired set point. A controller then
processes this information and decides what must
occur in order to get the process variable back to
where it should be after a load disturbance occurs.
When all measuring, comparing, and calculating
are complete, the strategy selected by the
controller is implemented via some type of final
control element. The most common final control
element in the process control industries is the
control valve.
A control valve manipulates a flowing fluid such as
gas, steam, water, or chemical compounds to
compensate for the load disturbance and keep the
regulated process variable as close as possible to
the desired set point.
Many people who speak of “control valves” are
actually referring to “control valve assemblies.”
The control valve assembly typically consists of
the valve body, the internal trim parts, an actuator
to provide the motive power to operate the valve,
and a variety of additional valve accessories,
which may include positioners, transducers, supply
pressure regulators, manual operators, snubbers,
or limit switches.
It is best to think of a control loop as an
instrumentation chain. Like any other chain, the
entire chain is only as good as its weakest link. It
is important to ensure that the control valve is not
the weakest link.