infinite number of possible system configurations exist. When specifying a
solenoid valve, it is important to
describe the desired action under loss of
electrical power, as well as the required
voltage and current ratings.
cisely machined and fitted components,
so even a small debris particle or
droplet of condensate can affect performance. An airset ensures a clean air
supply to keep everything operating as
designed. Periodic maintenance is
required to ensure the filter is clean and
condensate is drained from the dripwell.
actuator, allowing small signal changes
to flow through the bypass without
opening the main plug within the booster. Controlling the amount of bleed
adjusts the sensitivity of the booster, and
prevents overshoot that can be caused
by excess capacity, ensuring smooth,
controlled responses to small changes.
AIRSET
Airsets, also known as filter regulators,
are small pressure-reducing regulators
that manage the air supply to pneumatic
instruments and valves. They perform
two critical functions: providing a constant air supply pressure to the instrument or valve, and filtering the instrument air.
The pressure-reducing
function is essential to a
plant’s performance and
safety. Most plant instrument air systems operate at
pressures of 100 psi ( 6. 9
bar) or higher, while
most control valves and
other instruments are
designed to run at much
lower air supply pressures—as low as 20 psi
( 1. 4 bar) in some cases.
Exceeding the rated supply
pressure can lead to early failure,
mechanical damage, system shutdowns and potentially unsafe conditions.
Air supply pressure requirements can
vary significantly from device to device,
and having separate instrument air systems to meet the demands of each control system component would be impractical. Therefore, the standard practice is
to have a single air system and install
airsets on individual devices to reduce
the instrument air pressure to the
appropriate level.
Control valve actuators are designed
to be sealed devices with no air leakage.
As a result, any internal leakage
through the control devices could create
pressure buildup. Airsets, therefore, typically have an internal relief that will
vent any undesired pressure buildup
within the system.
Finally, clean instrument air is critical for consistent instrument performance. Valve positioners are made of pre-
VOLUME BOOSTER
A volume booster is a mechanical relay
that amplifies the flow of air.
Valve positioners, controllers and
transmitters have limited flow capacity.
To ensure accuracy, the internal nozzles,
relays and other mechanical devices
must be kept small to maximize responsiveness. Without external amplification, the output capacity of these instruments can be quite low.
Valve actuators, on the other hand,
often become quite large as capaci-
ties and/or pressures increase. These
large actuators contain much greater
volumes of air that must be loaded or
exhausted as the valve moves through
its travel. To handle the higher capaci-
ties, valve positioners often include an
output relay or high-capacity spool
valve to amplify flow capaci-
ty. Even with this
built-in amplifica-
tion, however, the
response of large valves
can be slower than
required.
The solution is an external
volume booster. It provides a
1: 1 amplification of flow
while keeping the output pressure as close as possible to signal
pressure. Since the booster has a
separate supply connection, it is not
limited by the capacity of the
devices ahead of it in the loop.
Volume boosters used on control
valves are normally applied with a
bypass or gain adjustment to provide
stability. The volume booster normally
has a built-in deadband where a certain
amount of signal change is necessary to
activate the volume booster. The bypass
or adjustment works by mixing the positioner output with the pressure in the
MANUAL OVERRIDE
(HANDWHEEL/HANDJACK)
A manual override is an emergency
backup device that allows manual operation of a control valve in the event
power and/or air supply is lost. This
should not be confused with a manual
operator. Unlike a manual operator, an
override device is not intended for continual use. Instead, the override is a lim-ited-use device, often designed to counter the built-in spring return within the
actuator to push the valve to its fully
open or closed position, depending on
fail action.
Several designs are available to
accommodate various valve and actua-
tor types and force requirements. They
range from a simple wheel and drive
screw attached to the top of a spring
diaphragm actuator to a mechanical
wheel and lever system connected to the
valve stem or hydraulic handjacks
(pumps) that attach to or are
integral within a piston
operator.
Control valves are
indeed essential to the
operation of any process
facility. Equally impor-
tant, however, are the
accessories that allow a con-
trol valve to perform optimally.
This article has only scratched the
surface, touching on the most
commonly used accessories.
Numerous other options are available,
giving control system designers and
plant operators a virtually bottomless
toolbox to draw from in meeting the
needs of any application. VM
An airset
Volume boosters
JOHN DE VEAU is global product manager for
rotary products for Dresser Masoneilan
( www.dressermasoneilan.com). Reach him at
john.deveau@dresser.com.
