8 Instruction Manual
SECTION 3 - GENERAL PERFORMANCE
The pneumatic capacity control assembly will
provide accurate incremental adjustment in both
directions as a linear function of the applied
instrument air pressure. There is no tendency for
the capacity control to “drift” off setting over long
periods of time because
1. There is no signicant force acting on the pump
capacity control adjustment spool.
2. The friction from, the sealing rings act to hold
the spool in position until the pneumatic capacity
control applies sufcient force to overcome this
friction.
With 40 psi supply air pressure, the pneumatic
capacity control cylinder will develop up to 40
pounds force in the direction to increase the
capacity setting and up to 110 pounds force
in the opposite direction. Then with 100 psi air
pressure applied, these forces are increased to
100 pounds and 275 pounds respectively. There is
no noticeable change in control position when the
supply air pressure is varied within the specied
pressure range. However, the nominal force
required to overcome the seal ring friction on the
control spool is in the range of 17-30 pounds, so
the pneumatic capacity control should have no
difculty providing rapid, accurate adjustment of
the pump capacity when the minimum specied
control air pressure is applied.
It should be noted that the instrument air pressure
is applied to a closed chamber in the positioner
valve so that the regulator or instrument system
establishing the air signal must incorporate a
venting air bleed to insure accurate air signal
response.
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