Table of Contents
Advertisement
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
corresponding to the thermal energy introduced into the
process.
To change the flow rate, the valve has an actuator that
modifies 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,
producing finite variations in the fluid flow section
corresponding 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
components, such as mechanical and electrical safety limit
switches, manual drive systems, and position readers.
If available, valve position is normally measured with a
potentiometer (feedback valve) to obtain more accurate
control.
The connection diagram with controller includes open/close
relay commands.
Figure 24 - Connection diagram for floating valve
If available, the auxiliary input of the controller can be confi-
gured for the valve position function.
Figure 25 - Connection diagram for feedback valve
80209C_MHW_850-1650-1850_01-2020_ENG_pag. 219
Based on process dynamics, the controller determines the
output value that drives the valve actuator so that valve
opening maintains the required process variable value.
5.14.1. Valve control parameters
The controller controls the valves with the following
parameters 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 sec-
ond, this is a mechanical characteristic of the valve +
actuator group.
NOTE: if the actuator stroke is mechanically limited,
reduce the TRAVL value proportionally.
•
TIM.LO Minimum impulse: expressed as a forcentage
(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 controller
(power below which the actuator physically does not
respond to the command).
Raising TIM.LO lowers wear on the actuator to the
detriment of precise positioning. Minimum impulse
duration is settable in TIM.ON as a forcentage of
actuator time.
•
TIM.HI Impulse setpoint: expressed as a forcentage
(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
valve, which is especially useful in case of high
mechanical inertia.
Setting TIM.HI = 0 excludes positioning modulation.
•
TIM.ON: it is the shortest time accepted for the valve
command pulse, expressed as forcentage of the
"actuator time"
•
TIM.OF: it is the shortest time between two Valve
ON pulse command, expressed as forcentage of the
"actuator time".
•
Setting TIM.OF=0 this function is excluded.
•
Setting TIM.OFF≠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 forcentage 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.
Advertisement
Table of Contents
