tioners, particularly contemporary digital positioners, are incredibly stable.
However, they can have an open-loop
gain exceeding 1000: 1. A 0.1% (0.0012
psi for 3 to 15 range or .0016 mA for
the corresponding 4 to 20 mA electrical
signal range) change in signal can command up to full supply pressure if necessary to move the valve to a new position.
If a small incremental signal change is
commanded, digital positioner algorithms make a corrective signal to adjust
the output pressure until the valve’s
response matches the signal where older-technology analog force-balance
positioners might not even sense the
Many off-the-shelf, unmodified rack and
pinion actuators have been tested beyond
one million cycles—cycling as frequently
as possible at rated torque—without
requiring maintenance. In fact, cases
where the actuator’s sensitivity and hys-
teresis actually improved with increased
use have been noted, and cases have been
documented of actuators lasting 15 mil-
lion cycles in frequently cycled, lightly
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To order a copy of the Valve Catalog, visit The VMA Store on www.VMA.org.
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lead to particulate contamination of the
actuator. Grit lodges in the seal rings and
the guide pads, scoring the cylinder wall.
Because the contamination is admitted
into the volume between the pistons, the
dirt may directly damage the racks. Once
the cylinder wall is scored, the actuator is
no longer serviceable or repairable. Rack
and pinion actuators are lubricated for
life, and clean air will maximize their
Heat: Heat causes accelerated aging
of the O-rings and can break down the
lubricating grease. The primary heat
path into a rack and pinion actuator is
from the valve stem, through the coupling, and into the bottom seal of the pinion spindle. An issue with a leak at the
bottom seal is characteristic of a heat-related problem. Viton O-rings and high-temperature lubricant generally solve
this problem, and a good practice is for
valves intended for high-temp services to
supply extended brackets and couplings
to lengthen the heat path.
Corrosive environment: Caustic
chemicals dripping on an aluminum actuator housing can destroy the actuator in
days, if not hours. However, several solutions exist, including repairing process
leaks, adding a corrosion-resistant coating such as a TFE-based deposition polyester powdercoat, or epoxy, and using
corrosion-resistant actuator material
such as stainless steel.
Corrosive atmospheres: Particularly
with spring-return actuators, when the
air is released and the valve strokes to
its failsafe position, the spring-chamber
inhales a volume of exterior air equal to
the piston displacement. If this air is
corrosive, salty or humid, the actuator
may corrode internally. Rust flakes from
the springs can cause abrasive damage
to the cylinder bore or the springs may
pit from corrosion, leading to fracture.
Generally, the most effective remedy is
to use a rebreather vent, which vents the
exhaust air over the vent port so it can
be inhaled (preferentially to the ambient
atmosphere). If the supply air is at 80
psi, the exhaust expands more than six
times as it vents to atmosphere, so the
available supply to the vent port is
sufficient to keep the corrosive atmosphere purged.