Managing Motorized Valves - gefran 650 Installation And Instruction Manual

5.14. Managing motorized valves

In a control procedure, a motorized valve varies the flow rate
of a fluid based on the signal from the controller.
In an industrial process, the fluid may be a fuel, often corre-
sponding to the thermal energy introduced into the process.
To change the flow rate, the valve has an actuator that mo-
difies the valve's opening value, overcoming the resistance
produced by the fluid flowing in it.
Control valves vary the flow rate in a modulated manner, pro-
ducing finite variations in the fluid flow section correspon-
ding to finite changes in the input signal from the actuator.
A typical actuator consists of an electric motor connected
to the valve gate by means of a gearbox and a mechanical
drive system.
The actuator ca be integrated with various auxiliary com-
ponents, such as mechanical and electrical safety limit swi-
tches, manual drive systems, and position readers.
Setpoint
Actuator
Control
valve
Figure 21 - Valve connection diagram
Based on process dynamics, the controller determines the
output value that drives the valve actuator so that valve ope-
ning maintains the required process variable value.
5.14.1. Valve control parameters
The controller controls the valves with the following para-
meters of the VALVE submenu:
• TRAVL Actuator travel time: the time the valve takes
to go from completely open to completely closed (or
vice versa). Settable with resolution of one second, this
is a mechanical characteristic of the valve + actuator
group.
NOTE: if the actuator stroke is mechanically limited, re-
duce the TRAVL value proportionally.
80208G_MHW_650-1250-1350_01-2017_ENG
Process
variable
Process
5. Examples and applicative notes
• TIM.LO Minimum impulse: expressed as a percenta-
ge (with resolution of 0.1%) of actuator time, represents
the minimum change in valve position corresponding to
the minimum change in power supplied by the control-
ler (power below which the actuator physically does not
respond to the command).
Raising TIM.LO lowers wear on the actuator to the detri-
ment of precise positioning. Minimum impulse duration
is settable in TIM.ON as a percentage of actuator time.
• TIM.HI Impulse setpoint: expressed as a percentage
(with resolution of 0.1%) of actuator time, represents the
deviation in position (requested position – real position)
below which the maneuver request becomes impulsive.
TIM.HI is only active with TIM.OF=0
Impulse approach allows fine tuning of the position val-
ve, which is especially useful in case of high mechanical
inertia. Setting TIM.HI = 0 excludes positioning modu-
lation.
• TIM.ON: it is the shortest time accepted for the valve
command pulse, expressed as percentage of the
"actuator time"
• TIM.OF: it is the shortest time between two Valve
ON pulse command, expressed as percentage of the
"actuator time".
Setting TIM.OF=0 this function is excluded.
Setting TIM.OF <>0 the Valve movement becomes
pulsing; ON pulse time= TIM.ON and OFF pulse time=
TIM.OF
If the value TIM.OF<TIM.ON the value is forced to TIM.
ON.
• DEAD.B Deadband: this is a deviation band between
the control 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 full-scale and is symmetrical to the
setpoint. Once the process is defined, the deadband
is used to prevent stressing the actuator with repeated
commands that would be irrelevant to the control.
By setting DEAD.B = 0 the deadband is excluded.
5.14.2. Valve control modes
In valve control, every request for a maneuver greater than
minimum impulse is sent to the actuator via the relays with
function V.OPEN / V.CLOS.
Each action updates the assumed position of the virtual
potentiometer calculated on the basis of declared actuator
travel time. This mode always provides an assumed valve
position, which is compared with the controller's position re-
quest.
After reaching an assumed end position (fully open or fully
closed determined by the virtual potentiometer), the control-
ler supplies an additional command in the same direction,
thereby ensuring that the real end position is reached.
The actuators are normally protected against an OPEN com-
mand in fully open position or a CLOSE command in fully
closed position.
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