Powers 593 Series Flowrite II Heavy Duty Bronze Globe Control Valves - Manual (Page 8)

8
PRELOAD
The Normally Closed (NC, RC, ATO) valve assembly uses the
springs to hold the valve shut, against the upstream pressure
attempting to push the plug open.
1. All Normally Closed valve assemblies are factory set to a nominal
preload depending on size of 7-10 psi.
2. If your process requires a high air pressure signal (>7 psi) to start
the valve moving, it is necessary to decrease the actuator preload.
Adjust the preload so that the valve is fully closed with the
minimum air signal and fully open with the maximum air signal.
3. It is a characteristic of an unbalanced valve that as the pressure
drop across the valve increases, the effective spring range
increases. Consult one of our application engineers if you have
any questions about your particular application.
The Normally Open (NO, DA, ATC) valve assembly uses the
actuator springs to hold the valve open, and the upstream
pressure helps to hold the valve open.
1.If the maximum air signal applied to the valve does not allow the
valve to shut off tightly, decrease the preload.
2.If the valve starts closing at too low of an air signal, increase the
preload. The preload should be adjusted so that control air
pressure closes the valve tightly, and stem movement starts at an
acceptable point.
3. In some applications where a wide range of flow conditions are
encountered, it is desirable to install two valves in parallel and
sequence them.
Preload adjustment (also called spring tension adjustment) is
required under the following conditions:
1. The valve and actuator stem have been disengaged. (for packing
changes, stem changes, maintenance of valve assembly, etc.)
2. The action of the valve assembly has been reversed.
3. Valve sequencing is required. (Use of a positioner is
recommended if using a 3–15 psi control signal.)
4. Line pressure force exceeds the spring holding force on a
normally closed (NC) valve assembly, which causes the valve to
open with less than 7 psi signal preload.
5. Line pressure force adds to the spring holding force on a
normally open (NO) valve assembly, which causes the valve to
start closing at a preload greater than 3 psi signal
Deviation from this recommended preload setting will affect your
process. A higher preload setting requires a higher air signal at the
diaphragm to start stroking the valve. If the preload is too high,
with a given air signal a Normally Open valve will not close
completely, and a Normally Closed valve will not open completely
to overcome the initial preload setting.
To decrease preload on RA actuators or increase preload on DA
actuators:
1. To prevent the plug and seat from scoring, lift the valve plug off
the seat before adjusting the preload by applying air pressure to
the actuator, approximately 9 psi for a Normally Closed valve
assembly, and 5 psi for a Normally Open Assembly.
2. Hold the actuator stem in place (DO NOT turn actuator stem
during this process) and loosen the valve stem locknut by turning
the locknuts counterclockwise a number of times.Press the two
locknuts together.
3. Hold the actuator stem in place (DO NOT turn the actuator
stem), turn the valve stem clockwise and retighten the locknut.
(Fig. 1).
To increase preload on R.A. actuators or decrease preload on
D.A. actuators:
1. To prevent the plug and seat from scoring, lift the valve plug off
the seat before adjusting the preload by applying air pressure to
the actuator, approximately 10 psi for a Normally Closed valve
assembly, and 5 psi for a Normally Open Assembly.
2. Hold the actuator stem in place (DO NOT turn actuator stem
during this process) and loosen the valve stem locknuts by
turning the locknuts counterclockwise a number of times. Press
the two locknuts together.
3. Hold the actuator stem in place (DO NOT turn the actuator
stem), turn the valve stem locknuts counter-clockwise and
retighten the locknut (Fig. 1).
Fig 1 - Rotate Lock Nuts to
Adust Preload