Two-wire 4-20mA specs / expectations below 4 mA

[schlebda]

Is there an industry standard or performance expectation for 2 wire 4-20mA output devices (actuators, positioners) for signal levels below 4mA? For example, many field transmitters incorporate special signal indications outside the 4-20mA interval for alarm levels or diagnostic indications. And I am aware that for valve positioning driving sometimes DCS systems incorporate driving below 4mA (or above 20mA) to assure valve seating at the endpoints.

Thanks in advance for your comments and replies

 

[danw2]

The German association, NAMUR, advises process instrument manufacturers. Those manufacturers adopting NAMUR's
recommendations use 3.6mA for the fail safe 'fail-low' signal (I believe 21.0 mA is the fail-high level). Their NE-06 recommendation covers analog control signals.

http://www.namur.de/start/?&L=2

NAMUR operation is usually an option for the American manufacturers.

For instance, Honeywell's pressure transmitters use 3.8mA as their fail safe 'fail-low' level and 20.5mA when the
NAMUR option is not part of the device.

I don't recall that ISA's 4-20mA standard ANSI/ISA-50.00.01-1975 (R2002) addressed fail safe levels, but I could be wrong.

 

[KiwiMace]

I think Danw2 has answered your question, but I am a bit dubious about the 'assure valve seating' reason.

The transmitter, controller, i/p, positioner and valve should be calibrated such that if 4mA is closed at the controller, then it is also closed at the valve.

CV's are not IV's. If it has to be 3.6mA closed, then tell the operator to get off his behind and shut the isolation.

 

[ilan7]

No "Standard" for less then 4 ma. For valves, nothing less then 4 ma, but you can get more information from the positioner using HART or other inteligent communication if the positioner equipped with the said feature.
For transmitters, usually when 4 or 3 wires transmitters there are a loy of options - not really standard. we use gas detectors and fire detectors when the 4 to 20 mais the instrument readout and there are few step of lower reading for transmitter problem indication.

 

[powerjunx]

ilan7 has the point...While using HART you may have your 4ma as upper scale.
Pertaining to I/P valve, i guess your talking about the corresponding stroke-mA relation where you want to adjust the valve to achieve valve seat tight close or tight open feature like (siemens) SIMATIC PS2 Series. It is possible to trim the valve stroke to a certain degree but never 4mA.


"..the more, the merrier" Genghis Khan

 

[schlebda]

Thanks for your input on this topic. Yes, apart from the valve positioners users that take advantage of the embedded positioner features to achieve full valve seating, I wanted to learn if there was a standard or accepted pratice for those people that choose 3.85mA, or 3.80mA, or 3.75mA or 3.60mA to achieve the same purpose. (The lower you go the better you feel about the sealing but it's more likely that you'll power down the positioner.)

 

[JLSeagull]

These years we mostly use digital valve positioners for the transducer. The transducer includes feedback from the valve. The pneumatic signal may be adjusted from zero psig to the supply pressure to move the valve to a travel location corresponding to the 4-20 mA signal. Thus you could regard 3.95 mA and 20.05 mA outside the range of the output signal.

 

[powerjunx]

" choose 3.85mA, or 3.80mA, or 3.75mA or 3.60mA "

schelba, so far, these signal were used specifically by the manufacturer standard test signals to determine the functionality/operational test of their products especially on fail-safe test to verify and bring the valve to a certain intended position such as tight closing/open upon failure. Note, these test signals varied on different products and i'm certain your proposed signals would be a recommended test signal of other OEM.

Since most of positioner are interactive,i usually use 3.75mA/20.25mA scale to finely calibrate this device for zero/span. i don't have a hint if some instrument Tech'n uses same course as mine.

(By the way, for your reference it's (Siemens)SIPART PS2 not SIMATIC.. i beg your pardon)



"..the more, the merrier" Genghis Khan

 

[KiwiMace]

If I send a few people out into the field to calibrate a loop, and one guy does the transmitter, one the valve, and so on for every component, and they all use rules of thumb about what the real 4-20mA should be... where does that leave my loop at the end of the day?

 

[JLSeagull]

Calibrate the field devices 4-20 mA. If the control engineer wants to extend the output of the controller this is a different issue.

 

[roydm]

The transmitter is always calibrated
4mA=lower range limit, 20mA=upper range limit.
Most techs I know set the valve to be closed slightly over 4 mA.
It really doesn't matter if they all set it differently because the controller will take care of that.
When the operator sets the controller output to zero in manual the valve must close. (unless it's supposed to be fully open of course)
I have never seen a standard for below 4 mA, I think that is decided by the individual system manufacturer.
Roy

 

[Standingback]

I believe schlebda's question actually raised 2 points

1st, "Is there an industry standard or performance expectation for 2 wire 4-20mA output devices (actuators, positioners) for signal levels below 4mA?"

This answered by Postdanw2, Yes, there is a Standard laid down by NAMUR and Nearly all european, japanese manufacturers adopt NAMUR's recommendations use 3.6mA for the fail safe 'fail-low' signal. These levels are used as common alarms indications, where smart transmitters are transmitting analouge signals (not part of a bus system). If a smart transmitter outputs a "failure" sinal of 3.6mA, then a techie can connect a communication to dianose the fault.



2nd , "I am aware that for valve positioning driving sometimes DCS systems incorporate driving below 4mA (or above 20mA) to assure valve seating at the endpoints.

The calibration of the valve positioner to the valve position will determine the effectiveness of the shutoff and seating at the end points (assuming a healthy plug and seat). Digital positioners can these days "self calibrate" by driving to the physical limits of the valve before referencing them to 4-20mA. They do this by applying minimum and maximun air pressure to the actuator. This ensures that include the maximum force available to them as part of the calibration.

Positioner self calibration is very diferent from a techie calibrating 4-20mA to give a pressure output of 3-15psi (or what ever bench pressure the valve is set to). As typically he/she will use sight and touch to determine the full stroke of the valve.

The practice of driving beyond 4-20mA for this second point ignores the true purpose of the valve positioner.

I trust this helps,
Mlv

http://www.coulton.com

 

[roydm]

Some "Stand Alone" controllers and PLCs are not capable of sending less than 4 mA or more than 20 mA, that's probably the reason instrument techs allowed for this by closing the valve early e.g. fully closed just above 4 mA. Otherwise if the calibration drifts slightly the valve may be just cracked at 0% from the PID.
Besides the mA span of the valve is not that important as the PID function will correct for slight differences.
Roy

 

[garymiles]

Hi. I'm new on this site and was interested by your comments so far.

Regarding signal handling below 4 mA and above 20 mA).. I think the NAMUR NE43 standards are pretty recognised as a definition of diagnostic signal levels, defining under/over range values 3.8mA / 20.5 mA and detected failures 3.6 mA / 21 mA. These values are widely used in Europe, especially with Fail Safe systems.

Being able to respond tp the output query makes me feel old as I learned my work in the 0.2 - 1 bar era.
Most of the comments relating to this are valid but the significance of the 0.2 (3psi) - 1 bar (15psi) -> 4-20 mA has not been mentioned. These values are taken as linear part of relationship between flapper nozzle versus back pressure of pneumatic devices, hence the significance of not overshooting the values in either direction when calibrating them.

In the old days we used to spend hrs setting the closed position of the valve at 3% to ensure tight shutoff, and full open position at 98% to ensure maximum control range.. The advent of DCS and smart positioners etc have lessened the importance of this but I personally still set up valves in this way.. Guess I'm an old fart. hahaha

 

[roydm]

Garymiles,
I qualify as an Old Fart, that's for sure.
When setting the valve positioner if it has gauges I like to see all the actuator pressure bleed off at 0% or slightly above.

 

[powerjunx]

"In the old days we used to spend hrs setting the closed position of the valve at 3% to ensure tight shutoff, and full open position at 98% to ensure maximum control range..,Guess I'm an old fart."
old days, old fart however the context and principle of manual settting of control valves still misunderstood these days.And, to the extent with surging markets of smart positioners, its getting more complex.
Old farts,tech-trend updated..learning never ends.


"..the more, the merrier" Genghis Khan