Zero Adjustment; Span Adjustment - GE Masoneilan 4700P Maintenance Manual

Zero Adjustment

See Figure 19 on page 68 and Figure 20 on page 71.
The zero adjustment is made with the zero nut (4B). When the positioner is installed and the
pneumatic circuits are connected apply low instrument signal (i.e., 3 psi for 4700P/4800P, 4 mA
for 4700E/4800E), release locknut (4C) and turn zero nut (4B) to adjust valve plug to proper posi-
tion (open or closed). Then tighten zero lock nut (4C).
NOTE

Span Adjustment

See Figure 19 on page 68 and Figure 20 on page 71.
If the valve does not fully stroke for a full signal span, the feedback spring rate is too high. Loosen
screw (8) and turn spring (5) on the spring end (4) to increase the number of active coils and de-
crease the spring rate. Conversely, if full stroke is reached prior to full signal span, turn the spring
to decrease the number of active coils and increase the spring rate.
Adjustment of +1/4 turn of the spring changes its rate by approximately +10%.
After making a spring adjustment, it is necessary to reset the zero. This is done by turning the
zero adjustment (4B), and tightening zero locknut (4C).
Example: If the number of active coils is increased making the spring longer, it is necessary to re-
adjust the zero nut to compensate for the increased spring length.
When span adjustment has been completed, re-tighten the spring locking socket head screw (8).
CAUTION
48 | =GE Oil & Gas
Do not attempt to adjust zero nut with signal above low end of
signal range. If valve is seated at low end of signal range, after
making zero adjustment, decrease signal below low end of signal
range and slowly increase signal to ensure valve plug lifts off seat
at desired signal. If valve is open at low end of signal range, adjust
zero so valve indicates Open on travel indicator scale.
When installing positioner with cam in place, check the position of
the spring (5) on diaphragm S/A (9) to ensure that it is centered on
the diaphragm.
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