Split Range Controller

[heaterguy]

Can someone recommend a split range controller? We like to use recognized brand names in our panels.

 

[danw2]

What do you mean by split range?

To me split range means a 4-20mA output runs to two valves, each of which is ranged differently, for instance, one valve opens continuously over the 4-20 range, the other valve starts to open at 12mA and opens fully at 20mA.

Any controller with a 4-20mA output can do that, because the ranging is set up on the valves positioner.

If by split range, you mean two outputs that operate on either side of the setpoint,for example, one output for heating (operates below setpoint), one output for cooling (operates above setpoint), then your choice is far more limited.

For duplex operation like heat/cool, I tend to use Honeywell UDC 1/4 DIN controllers 2500, 3200), because the setup is straightforward and it doesn't use numerical codes, it uses descriptive terms. The new P.I.E. software is great help for keeping track of different setups on different controllers.

Dan

 

[JLSeagull]

What danw2 said - and more. Several techniques exist and in combination. A 4-20 mA signal can be applied to the pneumatic signal transucer and valve positioners split 3-9 and 9-15 psi (or other splits). One 4-20 mA signal can be in series with two valve transducers with a split calibration so that 4-12 mA sends 3-15 psig (OK 6-30 psig) and the other transducer is calibrated 12-20 mA, etc. You can do two positioner split and two I/P split for four valves on one 4-20 mA DC. Better still is to have one PID loop in the system with the output configuration split for seperate analog outputs for each valve. Again you can do some of all of these selections.

 

[heaterguy]

4-12ma controls the coolling valve with 4ma full open
12-20ma controls heater with 20ma full on

I would like to have one 4-20 ma controller setpoint input and two 4-20 ma outputs, with one of the 4-20 ma outputs inverted compared to the other.

 

[JLSeagull]

Different failure positions too? With one or two 4-20 mA outputs the zero mA or power failure output opens the cooling media, the safe choice. Zero output to the heater could also be the safe failed choice. This would not require inverting the signal. The PID function whether one or two outputs seems the same. You can get a digital controller with two outputs. Depending upon the configuration algorithms you may or may not be able to split the internal signal such that you sequence the 4-20 mA outputs - one 0-50% = 4-20 mA and the other 50-100% - 4-20 mA. That would be my preference - if available. This is an available DCS function but many configuration engineers are not aware of this. With the "individual" controllers that have two outputs I would want the sales person to demonstrate this algorithm to suit me. Usually each output can be selected with direct or reverse features to inverse if needed. I am not sure that it would be needed here.

 

[danw2]

cooling - 12ma closed, 4 ma full open
heating - 12ma closed, 20 mA full open

1) I don't understand the "dual output with one inverted from the other".

A single 4-20mA output from a duplex controller running to both valves will accomplish the task you cited, just as you've described it:

Heating full open at 100% output (20mA)
Heating valve closed at 49.9% output (11.99 mA)
Cooling valve closed at 50.1% output (12.01 mA)
cooling valve full open at 0% output (4mA)
with a small deadband between heating and cooling

A second 4-20mA output could be configured to duplicate the primary output, so that you could use one analog output for one valve and the other analog output for the other valve without having to run a single 4-20 in series through both valve (positioners).

But the action of any 4-20mA output would have to be "reverse" to get the action you cited.

Both 4-20s would span the full 4-20mA range, with each valve responding in its own "half range", either 4-12 or 12-20.

What do you want to accomplish with the 2nd inverted 4-20 output? Why inverted? An inversion of the 4-20 to either valve would not provide the correct action.

2) "one 4-20 ma controller setpoint input" means the controller's setpoint (not process variable or temperature) is supplied from some other device generating a 4-20 remote setpoint for the controller, right?

Dan

 

[controlnovice]

4-12ma controls the coolling valve with 4ma full open
12-20ma controls heater with 20ma full on

Where do you want the valve positions to be if you loose the 4-20mA signal or air supply? Do you want the heating valve to close and the cooling valve to open? (Usually the safe setting)

If this is the case you do NOT want to invert one of the signals. Also, you can do this with just one 4-20mA signal and set up the valve positioners to act accordingly as stated by the others.

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