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In an adjustment process the adjustment valve has the function of varying fuel delivery (frequently corresponding to the thermal energy
introduced into the process) in relation to the signal coming from the controller.
For this purpose it is provided with an actuator able to modify its opening value, overcoming the resistances produced by the fluid passing
inside it.
The adjustment valves vary the delivery in a modulated manner, producing finite variations in the fluid passage inner area corresponding to
finite variations of the actuator input signal, coming from the controller. The servomechanism, for example, comprises an electric motor, a
reducer and a mechanical transmission system which actions the valve.
Various auxiliary components can be present such as the mechanical and electrical safety end travels, manual actioning systems.
The controller determines, on the basis of the dynamics of the process, the control output for the valve corresponding to the opening of
the same in such a way so as to maintain the desired value of the process variable.
- Actuator time (Ac.t) is the time employed by the valve to pass from entirely open to entirely closed (or vice-versa), and can be set with a resolution of one
second. It is a mechanical feature of the valve+actuator unit.
NOTE: if the actuator's travel is mechanically limited it is necessary to proportionally reduce the Ac.t value.
- Minimum impulse (t.Lo) expressed as a % of the actuator time (resolution 0.1%).
Represents the minimum change in position corresponding to a minimum change in power supplied by the instrument below which the actuator will not
physically respond to the command.
This represents the minimum variation in position due to which the actuator does not physically respond to the command.
The minimum duration of the movement can be set in t.Lo, expressed as a % of actuator time.
- Impulsive intervention threshold (t.Hi) expressed as a % of the actuator time (resolution 0.1%) represents the position displacement (requested position –
real position) due to which the manoeuvre request becomes impulsive.
You can choose between 2 types of control:
1) ON time of movement = t.on and OFF time proportional to shift and greater than or equal to t.Lo (we recommend setting t.on = t.Lo) (set t.oF = 0).
2) ON time of movement = t.on and OFF time = t.oF. A value set for t.oF < t.on is forced to t.on. To activate this type, set t.oF < > 0.
The type of movement approach allows fine control of the reverse drive valve (from potentiometer or not), especially useful in cases of high mechanical inertia.
Set t.Hi = 0 to exclude modulation in positioning.
This type of modulated approach allows precise control of the feedback actioned valve, by a potentiometer or not, and is especially useful in cases of high
mechanical inertia. Setting t.Hi = 0 excludes modulation in positioning.
- Dead zone(dE.b) is a displacement band between the adjustment setpoint and the process variable within which the controller does not supply any
command to the valve (Open = OFF; Close = OFF). It is expressed as a percentage of the bottom scale and is positioned below the setpoint.
The dead zone is useful in an operative process to avoid straining the actuator with repeated commands and an insignificant effect on the adjustment.
Setting dE.b = 0 the dead zone is excluded.
t.Hi
10
9 • ADJUSTMENT WITH MOTORIZED VALVE
SetPoint
Actuator
Adjustment valve
CONTROL EXAMPLE FOR V0 VALVE
Characteristic parameters for valves control
t.Lo
t.Lo
t0
t1
t2
Process
variable
Process
t.Lo
Graph of behavior inside the band with integral time ≠ 0.
With integral time = 0, movement ON time is always equal
to OFF time.
t0 = t.Lo
80379_MHW_600V-T73_06-2012_ENG
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