BASICS
Back to
Back to
BASICS
ACCESSORIES FOR
CONTROL VALVES
BY JOHN DE VEAU
Control valves are a ubiquitous essential in today’s industrial
facilities. Whether they are managing
flow or controlling process conditions
such as temperature or pressure, they
are key to maximizing production,
maintaining product quality and
ensuring safety.
However, they cannot accomplish
these tasks on their own. Equally
important to the processes are the
accessories that work with a control
valve—helping ensure accuracy,
boosting output, making various control system components compatible
with one another and providing added
layers of safety.
In this article, we will review some
of these devices, the roles they play in
a control system and factors that
should be considered when specifying
these devices.
VALVE POSITIONER
Positioners are required in almost all
control valve applications. Many positioner types and communication protocols are available, but a positioner’s
primary function is always the same:
to accurately position the throttling
element within a control valve as
directed by a signal from the process
control system.
The positioner receives a sign from
an external process controller, compares the valve’s actual position to the
desired position, and sends a command change to the actuator to correct the valve position in a controlled
feedback loop. The process controller
measures the change in the process
and sends further changes to the position signal as necessary.
Without a positioner, a control
valve position is an open loop; the valve moves in
response to the signal but nothing is providing
feedback to ensure the valve is in the desired
position.
Four factors make positioners vital components in today’s process control systems:
Higher-pressure actuators: Positioners make it possible to use today’s higher-pressure actuators in traditional pneumatic control systems, which can reduce costs
and make the valve more responsive.
Most pneumatic control systems use an output signal range of 3 to 15 psig or 6
to 30 psig. Traditional spring-diaphragm actuators are widely available in these
ranges, but the output force from a given actuator size is significantly limited. And,
piston actuators almost always require higher air pressures to perform optimally.
As a result, most actuators are designed and sized to operate at air pressure
ranges well above the 3 to 15 or 6 to 30 psig delivered by the control system. Therefore, a positioner or other amplifying device is required to make them compatible
with the control system’s lower output pressure. The higher air pressure allows
selection of smaller actuators reducing overall equipment cost.
High-pressure, angle-style control
valves with high-pressure piston
actuators are equipped with digital
valve positioners, airsets and
volume boosters, along with other
accessories.
