Will a digital HART controller rated at 9V at 4-20mA work on a 24 VDC control circuit? 2

[MaintEngnr]

I support a small test facility and we have purchased some Masoneilan control valves to replace some really old control valves. We decided to upgrade from pneumatic valve positioners to digital valve positioners. While looking through the manual another engineer found that the Masoneilan SVI II AP positioners we purchased use HART digital protocol and operate on ~9 VDC with a 4-20mA signal. The question is our control power is 24VDC so what would we need to do to make this work?

 

[davidbeach]

A 24 to 9 volt converter sounds like a good start.

 

[xnuke]

Seems to me 9 V is the minimum voltage the DVP needs to see at 20 mA current on the HART loop. Please consult the manual yourself and check the compliance voltage on your 4-20 mA controller output to ensure there will be at least 9 V at the DVP.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[itsmoked]

I agree with xnuke in that that is the 'compliance' voltage that the actuator will take out of your 24V, leaving 15V for everything else in the loop.

That actuator can 'see' 30V. You may need to play games if it is in a hazardous area and you want the lowest voltage for the barriers.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Add up the voltages of all the devices in the loop. The sum must be equal or lass than the power supply voltage.
We almost lost a 100 ft. incinerator stack due to insufficient power supply voltage.
The high, high shut-down setting was close to 100%. (Close to 20 ma.) Adding the individual device voltages indicated that we had enough voltage. However, one device in the loop was badly off spec and required several volts more than the rated voltage. As a result, the power supply was incapable of pushing 20 ma. through the loop. The maximum current possible was just below the level required to trip the shutdown circuit. We had a knock-out drum burp and give us a liquid carry-over into an incinerator intended to handle waste gas.
A stainless steel thermo-well inside the stack melted off.
Now I do a field check on my loops to prove that there are no low voltage issues.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[moltenmetal]

A start to waross: people often forget how loops actually work. If there is too much resistance or individual loop devices need too much voltage, the loop won't be able to deliver its full current. That failure you describe is a failure mode that I've never heard discussed in a Hazop- and one I'll try not to forget!

 

[MaintEngnr]

Thanks for the input. I went back and carefully reread the manual and it says that the DVP is designed for use with all control systems and that it can withstand up to 30 V from a current source. Also I confirmed that 9V is the compliance voltage. Being such a small facility, it is hard to have someone who is strictly a I&C expert so this forum is a great resource.