Answer / sam

Before we start, something in your question is a bit of a
concern. Split range control is done by installing two
valves in parallel in the SAME line, and NOT two different
lines. The easiest to do this modification is to built a
bypass line beside the original valve and install your
second valve in this bypass line right next to the original
valve. The bypass line only starts only about 1 meter before
and end about 1 meter after the original valve.

Ok back to your question.

If I read correctly between the lines, it seems that you are
doing a modification. The “one positioner” and “one
feedback”, I have to assume must be from the original valve
that was in there. You therefore only have two cables
available to work four signals. One cable from the CCR for
the control and another one for the feedback signal back to
the CCR.
I understand why you would prefer not to pull in any extra
cables. These kind of modifications are big and troublesome
and pulling in new cables will mean documentation updates
and DCS software modifications and all the rest.

Ok so, I will do my write up based on the assumptions that
you only have the two cables, one positioner and one
feedback positioner to work with and that you have already
installed two valves in parallel in the same line. I will
also assume the original valve is one of the valves you have
uses and we will call that the small valve. The second valve
which should be a bigger valve is installed in a bypass line
in parallel with the original valve in the same line, and we
will call that the big valve.

All stand alone FB positioners work with a variable
resistor(VR). Movement of the valve stem will turn this VR
and the resistance will change of this VR. This resistance
is then converted by the electronics of the FB positioner
into the 4 to 20 mA signal back to the CCR controller. You
need to measure this resistance and also see if it increases
or decreases during the upward movement of the valve stem.
Based on this you need to replace this VR with a smaller or
bigger VR in order to get only half or double the resistance
of the original full stroke value.
In other words if the resistance value in the original valve
changed from 0 to 1000 Ohm during a full, 0 to 100%, stroke
of the valve, the resistance will only be 500 Ohm at 50% of
the valve stroke.
Assuming the resistance value increases with upward movement
you need to change to VR with a 50% smaller VR (0 to 500
ohm) and install a identical VR in the big valve as well. If
you now connect them in series they will give you a total of
1000 Ohm which is the same as your original feedback
positioner resistance when you used only one valve. Assuming
the calibration of this FB positioner is still as it was in
the original valve, at 100% stroke the FB positioner will
only see 500 Ohm and the output to the CCR will only be
12Ma. As the big valve starts to open up the resistance of
the VR in the big valve will start to add itself to the now
fully open small valve’s resistance and the mA out to the
CCR will change accordingly until both valves are fully
open. In which case the total resistance will be as per your
original valve’s resistance, 1000 Ohm, and therefore your mA
out will be 20mA.

To do this:
1)
Small original valve:
Install the original FB positioner on the valve and connect
feedback signal cable to CCR as per normal for a one valve
installation. Do a calibration on it as normal. Stroke it up
and down and measure the VR’s resistance change and then
disconnect the variable resistor and remove it completely.
Replace it with the newly calculated VR.
2)
Valve Big:
Either install a new small JB on the valve, with the newly
calculated variable resistor in it or a complete new FB
positioner, if available. If you install a new FB
positioner, disconnect the original variable resistor inside
and remove it completely. Replace it with the newly
calculated VR.

To calculate the new variable resistors values:
Values used are examples only.
Exp.
Original VR value = 0 to 1000 Ohm increasing with valve opening.
R1 refers to the new VR you will install on the small valve.
R2 refers to the new VR you will install on the big valve.

RT = R1 + R2 = 500 + 500 = 1000 Ohm

Replace the original variable resistors in each FB
positioner with the newly calculated variable resistors (in
exp = 500 Ohm) and connect them in series and then to the
small valve FB positioner electronics.

Suggestion how to do this:
Use other gland entry on the small valve FB positioner and
connect a instrument cable to a small 2 way JB mounted close
by. From this JB connect another cable to the big valve FB
postioner. Connect the two variable resistors in series and
then back to the PCB inside the small valve’s FB positioner.

Another way to do this is to install the VR’s on each valve
and then connect them to a remote mounted SMART temperature
transmitter. The temperature transmitter is then configured
with the HART to take a resistance input instead of “RTD
input” and VOILA you have built your own FB positioner.



About the “only one” control positioner

To use only one positioner and try and do split range
control is NOT possible. I think you are more concerned
about pulling in new cables and it is not a matter of that
you do not have any more positioners available. I am sure
you can get another one from your stores.

Ok so to do the split range control you need to get another
positioner and install one positioner on each valve.
Make use of the second gland opening in the small valve’s
positioner and just daisy-chain( connect in parallel) the
wires to the big valve from the small valve's positioner.

Both valve positioners will now receive the full 4 to 20 mA
signal from the controller in the CCR simultaneously, and
that’s what you want. Calibrate the positioner on the small
valve for 4 to 12mA = 0 to 100% stroke and the big valve 12
to 20 mA = 0 to 100% stroke.

Ok to summarize, your two positioners will be controlled
from a single controller in the CCR. During normal
operations, the small valve will control the process and the
operator will see a feedback in the CCR of below 50%. This
is in relation to both valves and not just one valve, so you
need to explain this to the operator before you sign this
mod off as complete.
If the process changes and the small valve cannot handle the
process, the bigger valve will starts to open up and the
operator will see a feedback of more than 50% in the CCR.
Again explain and train the CCR operator on this control
system and the way it works.

You might find it difficult to find a PID tuning set that
will suit both valves, due to the size difference, but you
might be able to do it if you compromise a little on both.
In other words neither will give perfect control but you
will get them to control good enough with a average PID set.

Above is just one way to do this and it will depend on what
you use out there and what is available.

Some basic rules of thumb about installations and positioner
types:
On all the split range installations I have worked on we
used a pneumatic positioner with internal or a remote
mounted I/P converter. The reason for the pneumatic
positioner instead of a SMART positioner, as well as the
remote mounting of the I/P, was heat. The electronics don’t
like too much heat so in very hot areas, rather use a
pneumatic positioner with a I/P remotely mounted where it is
a bit cooler. You can do this for the feedback as well by
installing the VR’s in small JB’s on the valves and mount
the electronics where it is a bit cooler.
Good luck

Answer / ansarru

Not only one positioner.we should have positioner for each valve, that is there are 2 positioners.
Normally one valve work with (4-12) ma and another work with (12-20) and by this way,we are ecoordinating two manipulated variable for a single controlled variable, achieving "SPLIT RANGE CONTRL "

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