Application Note

Application Note
Valves, the actuators that move
them, and the electronic circuits
that control them, are all subject
to the effects of aging soon after
they are installed. The valve seat
wears not only from the repeated
seating of the valve, but from the
liquid or gas that passes through
it. Depending on the application,
a valve can be stroked from hun-
dreds to tens of thousands of
times over a one-year period.
This amount of mechanical
motion inevitably causes screws
to reposition, springs to weaken
and mechanical linkage to
loosen. In addition, electronic
components change value over
time. The results are valves that
don’t fully open or close, close
prematurely, or operate erratically
and cause improper regulation of
General steps in checking
valve positioning.
The first order of business is to
set up the ProcessMeter in the
sourcing mode using the appro-
priate range of current for the
positioner. The Fluke 789 uses a
separate pair of jacks to source
current. First connect the test
leads into the 24 V loop power
mA output jacks. Next, select the
4-20 mA range by moving the
function switch from Off to the
first mA output position. Now
you’re ready to connect the 789
to the input terminals of the valve
positioner.
With the test equipment prop-
erly set up and connected to the
positioner, it’s time to determine
if the positioner fully closes the
valve at the 4 mA input current
level. Using the push buttons on
the ProcessMeter, adjust the
source current to 4.0 mA. Now,
while watching the valve for any
movement, press the Coarse
Down button once to decrease
the current to 3.9 mA. There
should be no movement of the
valve. In setting the point at
which the valve starts to open,
you want to be sure there is no
counter pressure by the actuator
against the force holding the
valve closed when there is
4.0 mA on the controller’s input.
In a spring-to-close valve, there
should be no pressure on the
diaphram. With a double acting
piston actuator, there should be
no pressure on one side of the
piston. You may want to set the
start of opening between 4.1 and
4.2 mA to get that insurance at
the closed setting. To check the
opening of the valve, press the
Coarse button up from 4.0 mA.
The ProcessMeter will increase
0.1 mA for each press of the
Coarse button. You should adjust
the zero adjustment on the posi-
In-field valve positioner
checks using the
Fluke 789 ProcessMeter
™
From the Fluke Digital Library @ www.fluke.com/library
the gas or liquid under its control.
This is more commonly referred
to as “calibration drift.”
To keep a system operating
properly, a good preventative
maintenance program that man-
dates periodic checks of valve
positoners is required. These
checks need to be conducted
quickly to minimize down time.
When these checks reveal cali-
bration drift, recalibration of the
electronic valve positioner must
be performed quickly. With the
varied locations in which a valve
can be installed and the difficulty
in removing it, the equipment
used to perform the checks must
be brought to the valve positioner
itself. Therefore, this “in-field”
tester must be portable, easy to
use and rugged. Fluke’s 789
ProcessMeter™ is a perfect
solution.
With its signal sourcing capa-
bility, the Fluke 789 can simulate
a controller connected to a valve
positioner’s input. Through the
controls on the ProcessMeter, you
can set the positioner’s input
current to a specified level and
visually inspect the reaction of
the valve’s position using the
mechanical position indicator, the
valve stem position or flow indi-
cators. In addition, the Fluke 789
can continuously adjust the
source current in a ramping or
stepping fashion, allowing you to
check the valve’s linearity and
response time.
An example will help explain
how simple it is to use a Fluke
789 ProcessMeter for these
checks. This example only
demonstrates the basic principles
in making position checks on a
valve positioner. Manufacturer’s
specific instructions should
always be consulted for proper
and appropriate valve positioner
testing and calibration.