Starting Point And Reference Variable; Adjustment After Mounting The Positioner On The Valve; Setting The Air Delivery (Volume Restriction Q) And Proportional Band Xp - Samson 4765 Mounting And Operating Instructions

Operating the positioner
6.2 Starting point and refer-
ence variable
The attached lever and the installed range
spring of the positioner are assigned to the
values of rated valve travel and the reference
variable (input signal) as in Table 1.
In normal operation, the reference variable
span is 100 % = 0.8 bar. A smaller span of,
for example 50 % = 0.4 bar is only required
for split-range operation (Fig. 12).
The span can be changed by exchanging the
range spring (section 6.4).
On making adjustments to the positioner, the
travel must be adapted to the reference vari-
able and vice versa.
With a reference variable, for example 0.2
to 1 bar, the valve must move through its en-
tire travel range from 0 to 100 %.
The starting point then is 0.2 bar and the up-
per range value 1 bar.
In split-range operation, the controller output
signal is used to control two control valves,
dividing it such that the valves move through
their entire travel range at half the input sig-
nal range each (e.g. first valve set to 0.2 to
0.6 bar, second valve set to 0.6 to 1 bar). To
avoid overlapping, allow for a dead band of
0.05 bar as shown in Fig. 12.
The starting point (zero) is adjusted at the
screw (4); the reference variable span, i.e.
the upper range value, is adjusted at the pin
(2).
20
6.3 Adjustment after mounting
the positioner on the valve
Connect the control signal input to a com-
pressed air source of max. 1.5 bar using a
remote adjuster and a pressure gauge.
Connect the supply air to the supply input
(supply 9).
6.3.1 Setting the air delivery
(volume restriction Q) and
proportional band Xp
1. Close the volume restriction (14) as far
as the required positioning speed per-
mits.
Check the positioning speed by pushing
the range spring (6) as far it will go.
2. Adjust the reference variable at the input
to approx. 50 % of its range. Then, turn
the zero adjustment screw (4) until the
valve is at approximately 50 % valve
travel.
On setting the Xp restriction, observe the re-
lationship with the supply pressure as indi-
cated in Fig. 9. The preset value of Xp should
be approximately 3 %.
3. Check the plug stem's tendency to oscil-
late by pressing the range spring (6)
briefly as far as it will go.
The Xp value is to be adjusted to be as
small as possible, without considerable
overshooting occurring.
Additional points that apply concerning
adjustment:
Î Always adjust the Xp restriction before
setting the starting point.
EB 8359-1 EN