Answer / sam

The feedback positioner on any control valve is just a "nice
to have" and is not necessary for any control loop to work.
Don't get confused between a feedback control system and the
feedback positioner. The controller receives it's feedback
on how to make valve adjustments from the transmitter in the
field, not this feedback positioner. You can therefore
disconnect any feedback positioner in the field permanently,
and everything in the control loop will still work.

Some plants use it only as confirmation that the valve is
working and reacting as the controller instructed it to. In
other words if the controller send a signal out to the valve
to open 30%, the feedback positior will should send a signal
back that will indicate 30% when the valve has reached the
instructed 30% position in the field.
As you can imagine control room operators like feedback
positioner very much, and they need them to be pin point
accurate all the time.

Anyway, so in the case with split range valves, nothing
changes since you still want to see that the valve has
opened from zero to 100% regardless what signal (4 to 12mA)
it uses to do so.
In the writeup below you will see I have used two separate
controllers working in a split range configuration from a
common transmitter. So this is your solution as well if you
need to get these feedback's working. Each valve has it's
own controller output and feedback. Read the previous
writeup below and you will see what I mean.

Also note there is another way, but I am a bit reluctant to
tell you about it since I don't think this is a very good
way of doing it but anyway decide for yourself:
Some plants do not use feedback positioners at all.
Where FB positioners are not used, the position indication
on the DCS is the same as the output signal to the valve.
Yes I know it is wrong, and should not be done like this but
I think the decision was made based on the amount of call
out we had just to repair these continues discrepancies
between the output and the feedback's.
So you don't need it to do control, but if you have to have
a indication on your DCS faceplate use the same signal as
the output signal to the valve.



Question

Explain Split Range Control In Control Valve Briefly.

Question Submitted By :: Jaydeep_6494u
I also faced this Question!! Rank Answer Posted By

Re: Explain Split Range Control In Control Valve Briefly.
Answer
# 1

split range control is which the output of a controller is
split to two or more control valves. For eg,
For 0% controller output,valve A fully open & B fully
closed.
For 25%, Valve A 75% open,B 25% open
For 50%, Valve A& B- 50% open,
For 75%, Valve A 25%open, B-75%open..
For 100%, Valve A fully closed & valve B Fully Open..

Like this, d controller output action can be set depending
upon the application...


Is This Answer Correct ? 5 Yes 1 No

0
Arunnatraj

Re: Explain Split Range Control In Control Valve Briefly.
Answer
# 2

In addition wt ANS.1-
in split range control the control signal is split in two
ranges according to the application .for eg,4-20MA.signal
can be used as 4-12 MA. FOR VALVE A OPEN i.e.50%.& 12-20MA
for vlv b open.vice-versa.


Is This Answer Correct ? 4 Yes 0 No

0
Nilesh Karpe



Re: Explain Split Range Control In Control Valve Briefly.
Answer
# 3

Split range is as per answer 2 and not answer 1. Answer 1 is
using two valves, one is a fail open and one is a fail
close. The 4 to 20mA control signal is then controlling the
two valves but in opposite directions. This can also be done
by calibrating the one positioner as a direct acting and the
other as a reverse acting.
Yes this is 100% correct and can be done and the two valves
will work perfectly and in exactly opposite directions to
each other. In a case like this the two valves will have to
be identical as well otherwise the balancing act you are
trying to perform with this setup might not work.
Never seen a application where you would need this but if it
needs to work like that, it can be done.

Split range means you are splitting a 4 to 20mA control
signal in two.

This is very useful in applications where the product flow
increases very rapidly and then falls away again. a Good
example will be before a slug catcher where the incoming
product from the well heads is coming in suddenly very fast
and then falls away again after a couple of minutes.
Another application is to control the steam from a boiler.
At times you need a lot of steam and other times you only
need a small amount.
How do you size the valve needed to control these very big
changes. a Small valve is needed to control the normal flow
but if the flow increases the small valve will open fully
and it will still be to small to control the now very high
flow or pressure. If you install a big valve to control the
big pressure or flow the valve will only open 2 to 5% most
of the time and it will be impossible to do stable control
with such a valve opening. Say nothing about the plug and
seat that will only last a couple of days.
To control these very high variance in flow and pressures of
the process we install two valves in parallel in the same
line. Normally the one is a small valve and the other is
about twice the size of the small valve.
The design and process engineers will decide what minimum,
normal and maximum conditions are and do the valve sizing
accordingly.
We calibrate the small valve to open from fully close to
fully open with a signal of 4 to 12 mA. We then calibrate
the second bigger valve to open from fully close to full
open with a signal of 12 to 20 mA.

In a situation where you use identical valves you can take
the signal from one controller and send it to both valves
but in the above application it is better to use two
controller but with the same input from one pressure or flow
transmitter. Each valve will then be controlled individually
from its own controller and on it's own PID tuning set. I am
sure you can see that it will be quite impossible to find a
PID tuning set that are appropriate for both the big and the
small valve and it is therefore better to use two
controllers but with the same input.

During normal operations the small valve will control the
process without any problems based on the input from the
pressure or flow transmitter, and the bigger valve stays
close all the time. If the process changes to something
bigger than what the small valve can handle the second
bigger valve needs to help, and will start opening up, once
the small valve is fully open. Once the demand falls away
the big valve will start to close and then the smaller valve
until the process is back to normal operating conditions..
Both valves receive a full 4 to 20mA signal from it's own
controller, but will only react based on the 4 to 12 or 12
to 20mA calibration that was done on each valve's positioner.

In other simpler applications you can use two identical
vales and set their positioners as above and send the one
controller output signal to both. One PID tuning set will
also work for both valves and conditions.

The valve sizes will depend on what the conditions are so
they can be the same or they might need to be different sizes.
Good luck

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