Basic steps to check normally closed valves with multimeter
Valves, the actuators that operate the valves, and the electronic circuits that control the valves are all susceptible to degradation after installation. Valve seats are subject to wear due to repeated operation of the valve and passage of liquids and gases. Valves can be operated tens of thousands of times a year, causing bolts to shift, springs to weaken, and mechanical linkages to loosen. Electronic components change in value over time. All of these can cause the valve to not fully open or close, to close prematurely, or to work erratically. This "calibration offset" can cause the valve to not control gas or liquid regulation well.
In order to ensure the correct operation of the valve, it is necessary to regularly check the positioning of the electronic valve. However, inspections need to be completed quickly in order to minimize downtime. If calibration drift is found, the positioner must be recalibrated immediately. A good tool for this purpose is a handheld tool capable of testing and recalibrating electronic valve positioning, such as the Fluke 789 ProcessMeter process multimeter. The tool provides a signal output to energize a controller connected to the input of the valve positioner to continuously adjust the output current incrementally, so the linearity and response time of the valve can be checked.
Basic steps to check normally closed valves with a multimeter:
1. Set the ProcessMeter to output mode with the appropriate current range for the positioner.
2. Insert the output current test lead into the mA output jack.
3. Move the rotary function switch from the OFF position (OFF) to the first mA output position above to select the 4~20 mA range.
4. Connect a process multimeter to the input terminals of the valve positioner.
5. To determine if the positioner fully closes the valve at 4 mA, use the key to adjust the output current to 4.0 mA. The valve should be closed.
6. While watching the valve move, press the Coarse down arrow button once to reduce the current to 3.9 mA. There should be no movement of the valve.
7. When setting the point where the valve starts to open, make sure there is no back pressure on the actuator (the pressure that keeps the valve closed when the controller input is 4.0 mA). With spring-to-close valves, there is no pressure on the diaphragm. For a double-acting piston actuator, there should be no pressure on one side of the piston. To ensure that there is no back pressure at the closed setting, the starting point at which the valve opens can be set between 4.1 and 4.2 mA.
8. To check that the valve is open, press the Coarse up arrow button to adjust from 4.0 mA. Each time the Coarse up arrow button is pressed, the current increases by 0.1 mA. The zero function of the valve positioner should be adjusted to set the valve to the appropriate closed mode.
9. To check the fully open position of the valve - called the span position check, use the Range button to adjust the output current to 20 mA and wait for the valve to stabilize. While watching and feeling the valve move, press the Coarse up arrow button once to set to 20.1 mA. Valve movement should be minimal and can be adjusted using the span adjustment on the positioner.
10. Using the Coarse control button, adjust the current level up or down from 20.1 mA to 19.9 mA. Between 20.1 and 20 mA the valve stem should not move and between 20 mA and 19.9 mA the valve stem should move slightly.
11. In most valves, the zero and span set points of the valve controller affect each other; therefore, it is best to repeatedly test the fully closed and fully open positions and adjust the valve position correctly until no further adjustment is required.
12. For linear action valves, the linearity can be checked: Set the ProcessMeter to 4 mA, use the % step button to step the current to 12 mA (50%), and confirm that the valve position indicator is at 50 % travel position. If the valve is of the non-linear type, refer to the valve manual for proper operation.
13. To check whether the valve works smoothly, turn the rotary switch to output mA, then select linear slow change. Ramp the mA signal for a few cycles with the process multimeter while watching or feeling for any unusual movement of the valve. At any step position where the linear change is slow, the valve should not oscillate or swing, and there should be no sluggish movement. Set the valve controller gain to the point that gives the best response between the above two conditions.
