Dear Experts,

We have a Metso Control valve with a positioner ND9206HE1. The positioner needs min 9.5V/20ma for operation and has a resistance of 475ohm.

The DCS open loop voltage is 21.9V. When the signal cables are connected to the positioner, it drops to 9.1V. Is this normal? Our Instrument team says that with 9.1V , this valve will not function properly.

The cables are OK because, I removed the positioner from field and connected it directly to our DCS FTA.

When I give 0% (4ma), voltage across positioner is 9V

When I give 100% (20ma), voltage across positioner is 9.35V

Open loop voltage of DCS signal wires is 21.9V

Or do you suspect if there is any problem with DCS power supply?

Another doubt which I have is , considering the positioner as a 475ohm resistor, at 4ma the voltage drop across it should be 4mA x 470 = 1.9V whereas in reality voltage is 9V. How is this possible.

regards,
tomsci
tomsci [at] gmail.com

 

tomsci,

I have run into some what of a similar problem in the past. We had a large butter fly valve with a positioner on it that we would control from our DCS. It failed one day and had to replace it. Well for the life of me i could not get the new positioner to operate correctly. I ended up using a Slimpak isolator just to get away from the GE field I/O power supply. It was not large enough to power this positioner. So i tied the Slimpak into the regular 24PS and just did a direct transfer of 4-20MA to 4-20MA

 

A few thoughts

Do you know what load the DCS is capable of accepting at 20mA ?

Is there anything else in the loop apart from the controller ? Try replacing it with a 250R fixed resistor and measuring the loop resistance at the DCS end with the field cables disconnected from the field end. Then check the operation of the loop by measuring the voltage across the terminals of the FTA for various values of loop current (4/12/20mA) with the fixed resistor still in place.

This should help identify where you are losing voltage

Steve

PS The positioner's resistance is only 475R at 20mA, it rises as the current drops.

 

You have not mentioned whether the valve is working or not.

 

What you are seeing seems normal to me. Your positioner is not a simple resistor, more like a zenner diode in series with a resistor. It uses the 9 Volts to drive it's electronics and provide the power to coil. Look at the specs of your DCS output module it will tell you if there is a potential problem or not, if it says 500 or higher you are OK

Roy

 

Thank you Mike / Steve / NG / Roy for your thoughts on this.

<b>DCS AO Card Specification :</b>

Maximum Resistive Load @ 20ma = 750 Ohms
Maximum Output Voltage when
sourcing 20ma in 750ohms
resistive load = 15V

<b>Intelligent Valve Controller (ND9200H) Specs :</b>

4-20ma HART Communication
Supply Voltage 30V DC.Load Voltage up to 9.5V DC at 20ma corresponding to 475ohm (maximum voltage drop)

From the above information, I think the voltage (9.36V @ 20ma) which I measure across + & - terminals of positioner is normal. I don't suspect DCS supply anymore. Moreover we have few other valves with exactly same positioner and they are working fine.When I measure the voltage across them, its also in the range of 9.1 ~ 9.35 depending on the current.

NG, yes. the valve is responding to stroke test. But sometimes I see it is getting disconnected and connected in our FDM (asset management software for all HART devices). This particular valve remains closed during normal operation of plant and will open during a shutdown. During a shutdown last week, it did not open even when operator was giving 100% frm DCS. That's why we started investigation about the possible reasons. Instrument team suspected DCS power supply but now it seems PS is ok. During a shutdown, there is too much vibration of this valve and positioner and I doubt if that is why it did not open. But still I could not find an answer why the HART comm indicates failure intermittently.

Kindly share if you are getting any ideas. I will update if I find any solution.

thanks
Tom

 

Do you think there might be a short circuit in the field wiring, that would stop HART but the DCS would still put out 20 mA (I assume it will alarm if it's open circuit)

Roy