Answer / indranil mukherjee

ON/OFF CONTROL VALVE WORKING IN TWO MODE EITHER ON OR OFF.
BUT PID CONTROL VALVE IS TOTALLY CONTROLABLE.MEAN ITS
OPENING OR CLOSING SCALE IS BETWEEN 0 TO 100% OR VICE-VERSA.

Answer / ytd

ON/OFF CONTROL VALVE WORKING IN TWO MODE EITHER ON OR OFF.
In ON off control process has shoot up and down. This
process is not suitable for critical processes.
E.G.controlled heating / cooling for Annealing cycle.


PID control generally used in close loop process where
feedback of output is used to correct the input depend upon
the desired process.
S(o/p)= S(i/p)+/-(S(correction).
From this simple equation o/p signal is corrected but
depending upon the responce of the process `Diffential
gain' or integral gain is introduced and this simple
equation is modified.
Depending upon the process performance responce time ang
gain values are to be programmed in the PID controller.
Then PID controller follows the equation to make process
more controlled.

Answer / m.v.ramesh

Most ofthe answers are right, but the basic question is a
bit diverting in to other direction.

Question has to be like this.
What is the difference between On-Off control valve and
regulatory control valve.

Dont call as PID control valve. offcource regulatory
control valve is controlled by p or I or PI or Pd or PID
controlling modes.

Regulatory control valves can be operated by hand wheel
also with out PID controllers, some applications with
manual pneumatic regulators.

rest all answers are ok. simply on-off valves have 2
positions and regulatory valves can have n number of
positions in its range. selection of these are process /
design dependent.

Answer / kavi

Both are used to control a process. The on/off valves are
actuated by simple digital signal ( may contain voltage of
24 vdc/ac to 220vac) from an on-off controller or from a
controller whose output is of "on-off PID". In the first
one when the process exceeds a set value ( increase or
decrease) the digital output goes to valve to operate a
solenoid whose air supply opens or closes the valve ( a
full close or open condition). If the controller is having
an on-off PID the digital signal going to the valve is the
output of PID block of the controller which is in the form
of pulse width modulated one. In this also the valve will
have only two states like fullopen or full-close. In the
so called PID valve, the valve can have any postion of
opening basing on the analog signal received from the
controller. We can say that the valve opens or closes
proportional to the analog signal from the controller. In
this the controller tuning ( setting the values of P, I D )
has to be done by the process operator or the concern
instrument engineer/technician.

Answer / surendra gupta

on/off controller are simple type controller, they simply
on or off the o/p. where as pid controller, two additional
functions are added that are integral and derivatives.

2) on/off controller are not used for the closed ckt where
as pid controller are used for closed ckt.

Answer / sabitha reddy

THE BASIC CONTROL VALVE SET UP WIL BE DESIGNED IN ORDER TO
UNDERSTAND THE CONTROL VALVE OPERATION AND IT'S FLOW
CHARACTERSISTICS..INHERENT FLOW CHARACTERISTICS CONSISTS OF
PNEUMATIC CONTROL VALVES OF LINEAR,EQUAL PERCENTAGE,QUICK
OPENING(ON/OFF)..
COMING TO THE DIFFERENCES BETWEEN CONTROLLERS IS--
ON/OFF IS DISCONTINUOUS WHERE AS PID IS CONTINUOUS ONE
(WHICH ATTEMPTS TO CORRECT THE ERROR BETWEEN MEASURED
PROCESS VARIABLE AND DESIRED SET POINT )AND IT'S OUTPUT
CHANGES SMOOTHLY...

Answer / jobby c johnson

on /off controller is a discrete state controller
whereas PID is an continous state controller

Answer / amit mukherjee

In on-off controlling the o/p of controller gets on as it goes
below setpoint and vice versa when it goes up.But in PID control the controller tries to open final control element and
analyse error(e=sp-pv) and act according to their to their action.

Answer / bell

on/off will make full open or full closed position of the
valve,but pid will give percentage of valve open/close
according to the controller output.

Answer / sudipta,iter

on/off control is not closed loop where pid is closed loop
system.

why do we use 4-20mA as o/p and why don't use 0-20mA or 5- 21mA?

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