POWER TRANSMISSION | SPECIFICATION
than the diaphragm designs.
Actuators are usually made from
stainless steel or polyphenolsulphide
(PPS), which is a chemically resistant
plastic. The actuator is topped off by
the control head or positioner.
Older, pneumatically operated
positioners had a apper/nozzle
arrangement and operated on 3-15
psi, so no matter if the valve is open,
closed or somewhere in between, the
system was always expelling some
compressed air into the atmosphere.
Compressed air requires
considerable energy to generate
and when a manufacturing line is
equipped with multiple process
control valves all venting into
the atmosphere, this can
equate to a large waste
of energy. It is important
to establish not only the
most appropriate valve
design, but also a costeffective
solution that
takes account of annual
running costs.
Modern, digital, electropneumatic
valves that use
micro solenoid valves to control
the air in and out of the actuator have
introduced signi cant improvements
for operators. This design means that
while the valve is fully open, fully
closed or in a steady state, it is not
consuming any air. This, and many
other engineering improvements,
have made substantial advances in
both economy and precision.
FLEXIBLE DESIGNS
Valve seats can be interchangeable
within a standard valve body, which
allows the valve to t existing pipework
and the valve seat to be sized to the
application more accurately. In some
cases, this can be achieved after
the valve has been installed, which
would enable a process change to be
accommodated without replacing the
complete valve assembly.
Selecting the most appropriate seal
materials is also an important step
to ensure reliable operation; Steam
processes would normally use metalto
metal seals, whereas a process
that included a sterilisation stage may
require chemically resistant seals.
After installing a valve, set-up is
now comparatively easy and less
time-consuming. In-built calibration
procedures, such as Bürkert’s X-tune,
perform the initial setup
procedures automatically,
measuring the air required
to open and close the valve,
the resistance of the piston seals
on the valve stem and the response
time of the valve itself. Once complete,
the valve is ready for normal operation.
IMPROVING SAFETY
Control valves should be speci ed so
they operate in the 40-85% range so
if the valve is commanded to a 10%
setting, it can detect that something
has potentially gone wrong with the
control system and the best course of
action is to close the valve completely.
If the valve is commanded to a
position of 10% or less this can cause
very high uid or gas velocities, which
have damaging effects on the system
and cause considerable noise and
damage to the valve itself.
Modern control functionality can
offer a solution that acts as a safety
device to prevent damage to the
process pipework and components.
By building in a fail-safe mechanism,
any valve position setting below a
pre-set threshold will result in the
valve closing completely, preventing
damage to the surrounding system.
Control inputs can also include
safety circuits to ensure safe operating
conditions within the equipment.
For example, if an access panel on a
vessel containing steam is opened,
an interlock switch will open and the
valve controlling the steam supply to
the vessel can be closed automatically
helping to mitigate any risks.
IMPROVING RELIABILITY
Many process control environments
offer less than ideal conditions for
long-term reliability. Moisture-laden
atmospheres, corrosive chemicals
and regular wash-downs can all
shorten the service life of a process
control valve. One of the potential
weaknesses is the actuator’s spring
chamber where atmospheric air is
drawn in each time the valve operates.
One solution is to use clean,
instrument air to replenish the spring
chamber. This offers a defence against
the ingress of airborne contaminants
by diverting a small amount of clean
control air into the control head,
maintaining a slight positive pressure,
thus achieving a simple, innovative
solution. This prevents corrosion of
the internal elements and can make a
signi cant improvement to reliability
and longevity in certain operating
conditions.
Ultimately, choosing the most
appropriate process control valve
can be a complex task that is often
best achieved with the assistance of
expert knowledge. Working directly
with manufacturers or knowledgeable
distributors enables process control
systems to be optimised for long-term
reliability as well as precision and
ef ciency. !
32 WWW.EUREKAMAGAZINE.CO.UK | APRIL 2020
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