Valve Positioners 1

[sysengineer]

Without having practically worked on valve positioners before I would appreciate if someone could confirm my understanding.

A valve positioner will convert an electrical signal to 0-100% of the full pneumatic supply pressure i.e. if the pressure to open/close the actuator is 5 Barg - 0% of the electrical signal will apply 0 Barg to the actuator and conversely 100% of the electrical signal range will apply 5 Barg to the actuator? So to calibrate the positioner/actuator system it is important to know the pneumatic pressure required to open/close the actuator and completely overcome the force of the spring (assuming spring return).

Also, if 0% electrical range relates to 0% output pressure then when the electrical signal rises surely there is a minimum pressure to overcome the spring? In which case a deadband would exist which I'm not sure how you would overcome. I expect that this is where the feedback arm comes into play so the output pressure is increased to allow the actuator to lift of the seat. However if the output pressure at 0% electrical signal range was just below the minimum pressure to lift off the seat then surely there could be issues with maintaining a tight seal when the valve is closed?

I look forward to hearing your response

Thanks
Dan

 

[zdas04]

The system after the positioner is a dead end. If I open the supply to that dead end 0.00005% (assuming the pilot plug actually comes off the seat), then sooner or later the end device would be 100% open. The only lever you have access to is changing the pressure in that piece of tubing. Pressure instruments have a good deal of hysteresis (i.e., the amount that a previous state will affect the new state) and latency, so control schemes that try to control a process by controlling the magnitude of the pressure to the valve are pretty much doomed to spectacular failure.

More successful applications use timed bursts. For example, if the process needs the valve to move towards open, then open the solenoid for 0.5 seconds, and disable it for 60 seconds. Repeat until you are in the dead band. If it needs to move towards shut, open the vent solenoid for 0.25 seconds and disable it for 10 seconds. Repeat until the process is in the dead band.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

 

[sysengineer]

Hi Dave thanks for the reply. Are you familar with valve positioners such as Fisher DVC6200 etc? I understand what you are saying but you are talking about on/off deadband control with a solenoid. The devices i'm talking about are closed loop pneumatic amplifiers for industrial process valves and they amplify electrical signals to pressure to wn actuator.

 

[danw2]

> A valve positioner will convert an electrical signal to 0-100% of the full pneumatic supply pressure i.e.

Correct. A positioner will apply whatever pressure (up to the limit of the supply pressure) is needed to get the position requested by the control signal.

> If the pressure to open/close the actuator is 5 Barg - 0% of the electrical signal will apply 0 Barg to the actuator and conversely 100% of the electrical signal range will apply 5 Barg to the actuator?

Yes, although the action can be reversed in the setup of the positioner, if need be.

> So to calibrate the positioner/actuator system it is important to know the pneumatic pressure required to open/close the actuator and completely overcome the force of the spring (assuming spring return).

Well not really. The positioner needs sufficient supply air pressure to do its work, which is indicated the pressure gauge on the air supply regulator. The actuator/spring combo is typically designed to work at some given pressure that is stamped onto the valve's metal ID tag as 'operating pressure'.

> Also, if 0% electrical range relates to 0% output pressure then when the electrical signal rises surely there is a minimum pressure to overcome the spring?

Yes. But the positioner takes care of supplying that pressure.

>In which case a deadband would exist which I'm not sure how you would overcome. I expect that this is where the feedback arm comes into play so the output pressure is increased to allow the actuator to lift of the seat. However if the output pressure at 0% electrical signal range was just below the minimum pressure to lift off the seat then surely there could be issues with maintaining a tight seal when the valve is closed?

The setup/calibration routine that 'smart' positioners go through takes into account getting valve closed.

The DVC6200 manual is worth a read.
Link

 

[sysengineer]

Thanks danw2 I understand that modern 'smart' positioners have auto-calibration setup features which automate this process however I would still be interested to know how this 'deadband' is overcome on a traditional analogue positioner. If say the minimum pressure to overcome the spring was 0.5Barg and the max output pressure is 5Barg then the actuator would not move until 10% of the electrical signal is reached (5.6mA on a typical 4-20mA signal range).

 

[xnuke]

Here's my understanding:

A positioner is really a high-gain controller that eliminates or reduces valve position error. For example, the reference position is given by the 4-20 mA signal; the position of the valve is then detected and subtracted from the reference position to generate an error. The error signal is then converted by the high-gain controller into movement of a high-volume relay or spool valve that admits more or less air to the actuator to move the valve to the desired position. This allows the positioner to deal with overcoming spring pressure, static friction, backlash, and deadband, etc., and the controller sending the 4-20 mA position command does not have to be concerned with these issues. It issues the command for position, the positioner makes sure the valve moves to that position.

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.

 

[danw2]

I need to correct my previous sloppy generic answer:

>If the pressure to open/close the actuator is 5 Barg - 0% of the electrical signal will apply 0 Barg to the actuator and conversely 100% of the electrical signal range will apply 5 Barg to the actuator?

Technically, no, the 0% electrical signal (4.0mA) will be 3 psig for a typical operating range of 3-15 psig. The actuator spring is typically preloaded and does not 'start' from zero psi, but from some elevated 'live zero' pressure.

The non-smart analog positioners have an adjusting screw for closed position and another for open position so one can tediously iterate back and forth with minor adjustments until the positioner is calibrated for that particular valve/spring/llnkage like shown in the graphic below.

 

[sysengineer]

Ok this makes more sense. One thing I would debate is the preloading of the spring to 3-15 psi, I see a lot of that on valve data sheets for electro-pneumatic positioners but I thought people were confusing it with the 3-15 psi control signal now replaced by a 4-20mA loop current.

In other words 3-15 psi is only be a control signal and the valve acutator output would be amplified to up to whatever the max operating pressure of the actuator was. It seems hard to believe that 15 psi would overcome the spring tension on larger valve actuators. I saw a DN50 globe valve with a spring return diaphragm that required something near 5 Barg (72.5 psi) to overcome spring tension! I suppose the diaphragm area would have to be huge to provide enough force at 15 psi.

 

[Multikraai]

I think the OP means I/P covertor instead of a valve positioner. In my opinion a valve positioner has a mechanical feedback arm connected to the valve stem. The positioner puts out whatever air pressure required to give the valve stem the % of travel governed by the % of input signal. Of course the pneumatic output of the positioner is limited to the air supply pressure.

 

[sysengineer]

No I mean positioner hence why i said positioner and referred to the DVC 6000

 

[danw2]

>I saw a DN50 globe valve with a spring return diaphragm that required something near 5 Barg (72.5 psi) to overcome spring tension! I suppose the diaphragm area would have to be huge to provide enough force at 15 psi.

Indeed, it would. Hopefully, somebody would suggest using a higher air supply pressure, like 80 psi, on a reasonably sized actuator.

There is nothing sacred about 3-15. It was only an example. It is very common to use supply pressures greater than 18-20 psi needed for a 3-15 spring.

 

[Multikraai]

Sorry Dan, my mistake. I saw the video of mounting a DVC 6200 to a valve and noticed that is has a non contact way of position feedback instead of some kind of mechanical linkage. At the end of the video the voice-over says: "now your ready for calibration". To my knowledge all modern digital positioners heve on-board routines to do this completely by themselves at commissioning. So I was only confused by your worries about pressure calibration, dead bands and overcong pre-tension of springs.

 

[phteven]

Just thought I'd do a late response to original post:
Obvious aside (every valve unique), a valve should be checked out in shop conditions before installation. So as to a standard DVC positioner (for example) using 4-20mA signals and a valve requiring 15psi for full stroke (the force required to move the spring is usually stamped on nameplate - either the spring rate or a springe range) can be set in shop so that approx: 4.08ma signal begins actuation (just example anything can be done). Valves can be adjusted for how tight the seal is...too tight will cause a worse seal and vice versa, smart positioners like DVC's have auto tune capabilities that find 'stops' and adjust accordingly. When 4 mA is applied a seat leak test should be recorded as not every valve will fully seat (depends on application ex.low Cv typically low SCFH leakage). But for simplicity doing an auto-tune with valve in place will set-up a good fully open/close calibration.

FYI as to feedback arm (positioner):
Most new smart positioners will use some sort of Potentiometer connected to arm (non-contact using magnet). As the actuator stem moves the pot voltage changes giving the positioner its location. Driving valves to there 'stops' (open and close limits) will give the voltage span to the positioner allowing calibration.

NOTES: If a spring has a rate of 7psi it will take 7psi to compress 1 inch. The positioner increases the output pressure as the electrical supply increases and the spring begins to compress moving stem and feeding back millivolts on pot giving positioner error to stop increasing output pressure and stabilizing. Every valve may be different obviously some split ranged some not closing fully for freeze protection.