Unigas RN510 Installation, Use & Maintenance Manual Download

Page: 50 / 138

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.

9 • ADJUSTMENT WITH MOTORIZED VALVE

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.

Adjustment valve

Actuator

SetPoint

Process

CONTROL EXAMPLE FOR V0 VALVE

Process

variable

t.Hi

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

Characteristic parameters for valves control

- 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.

80379_MHW_600V-T73_06-2012_ENG