Issues regarding the calibration of pneumatic control valves

The valve shall be considered closed/open at 4mA/20mA

Both the signal and the opening degree need to be considered. The instrument zero point refers to the output position when the input is zero (the position at which the output or indication falls within the output or indication range when the input is at the lower limit of its range). A valve positioner is essentially a follow-up control system in which the valve position serves as the controlled parameter, while the signal acts as the setpoint parameter. The control law of traditional positioners is proportional control, while most intelligent positioners use proportional-integral control. When the positioner and actuator are used together, the relationship between the positioner’s output and the signal often does not remain a perfect one; it can only be used as a reference. Because, in the process of a pneumatic control valve going from fully closed – with the valve disc in contact with the seat – to fully open – with the valve disc completely separated from the seat (or the reverse process), the relationship between the valve position and the driving force becomes highly unstable. Therefore, the zero-to-full-stroke check of pneumatic control valves, as well as the stroke check, are two separate specifications in GB/T 4213-2008 \"Pneumatic Control Valves\".

What if 4mA/20mA correspond to the valve being closed/open?

Wrong, how to zero the setting so that 4mA/20mA corresponds to the valve being open/closed?

You can try using 3MA; normally it’s 4MA with everything turned off. Check what the feedback is – if it’s too high, and using 3MA still results in a higher value, then lower the zero point of the locator

Just look at the locator’s instruction manual to find out

This post was last edited by Hao Liang Zhi Le on 2022-3-26 at 10:07. Is it a simple positioner, or a positioner + control valve? ?

It depends on the operating mode of the control valve. For air-operated valves, 4mA corresponds to full closure while 20mA corresponds to full opening. If it’s not certain whether the valve has actually closed completely, the signal generator can be set to a value below 4mA (in my opinion, simply disconnecting the signal generator is the most straightforward approach). For air-shut valves, it’s the opposite: 20mA corresponds to full closure and 4mA corresponds to full opening. To confirm that the valve has indeed closed completely, the signal generator should be set to a value above 20mA and observed accordingly- -

This post was last edited by white-cg on 2022-3-28 08:11. If there is no positional feedback signal in analog form (in mA), or if it is not used for control, then it is not included in the interlock system. I suggest the following: 1. For a direct-acting system (where a signal of 4 mA ↔ 20 mA corresponds to the valve being open ↔ closed), the valve should start operating at a signal level of 4 mA, and it should be completely closed when the signal level is ≤ 20 mA (for example, 19.8 mA)； Valve with a high pressure difference: the valve opens when the signal is 4 mA; it also opens at 4 mA (for example, 4.5 mA), and it is fully open when the signal reaches 20 mA.