Opinions Wanted

A co-worker of mine had a suggestion regarding a 4-20 mA output which drives an electrically actuated modulating valve. He said it is a good idea to run the loop through a relay. When it is desired to shut the valve, apply 4 mA but also open up the relay. The loop is broken and zero mA is at the valve actuator. The valve is configured to close on loss of signal. This setup will guarantee a shut valve in case the 4 mA setting has drifted over time.
I don't agree. Are there any merits to his idea?

Well, it depends. Most positioners can be set to "fail" on loss of signal to the open, closed, or current position. However, for most applications the 4-20 mA signal is sufficient. You can use it in combination with a limit swich and time delay, so that if the valve doesn't incicate closed within the set time you trip an alarm, or for a very critical application open the relay.

One of the great advantages to 4-20mA as oppossed to 0-20mA is that you can verify circuit integrity with 0% output. If you have 0ma (0V) you know you are looking at a wiring problem. The actuator will go to zero just as fast and reliably with 4mA as 0.

And the drifting...well thats what regular calibration is for.

Remember us guys who have to maintain the equipment.



Brian.

Yes there are definitely merits to running it through a relay. I use this method quite frequently on burner controls. I usually use a set of auxilliary contacts on the main gas valve. That way when the main gas valve is off the modulating motor is also closed. The main benefit of this is that on a relight the main gas valve doesn't blow full pressure gas into a burner thats operating only on pilot. Instead it ramps up as the modulating motor moves to its output state.
Typically all systems I design to operate on high pressure gas (10psi or higher, yes PSI not "WC) I do this way. I have an incinerator flare at work that rumbles enough just ramping up to 100% output. I would hate to be standing there if it lit at 100%.

This is an especially useful trick if you are using a single point controller to run the system. Then you don't have to concern yourself with switching the controller to manual and zeroing the output.

Dave

I have automated controls on a 10MW gas burner, and I agree 100% with using contacts. This is the correct direction to be thinking in.

During ignition we used relays to drive the burner motor to fully open and then, and fully closed, feedback contacts to confirm these positions and then another contact to confirm arrival at ignition position.

Once lit the burner then goes to 4-20mA modulating control.

There are a number of critical safety considerations concerning the design of burner managment systems. This post does not constitute a formal recomendation, nor does it in anyway represent all of the information needed to implement such a system.

godfrey,

You asked for opinions instead of facts, so here is mine: For gas valves on furnaces, fire water pumps, NASA shuttle, and so on., maybe it makes sense to use a relay as a backup to the 4-20 ma signal. (But then what backs up the relay?) For everything else, I say it is going too far.

My motto: You can make it fail-proof, but you can't make it damm fail-proof.