Basic steps for checking normally closed valves with a multimeter
Valves, the actuators that operate the valves, and the electronic circuits that control the valves are all prone to deterioration after installation. Valve seats can wear due to repeated operation of the valve and the flow of liquids and gases. Valves may be operated tens of thousands of times each year, causing bolts to shift, springs to weaken, and mechanical linkages to loosen. The value of electronic components changes over time. All of these can cause the valve to fail to fully open or close, close prematurely, or work erratically. This "calibration drift" can cause the valve to not control gas or liquid regulation well.
In order to ensure that the valve works correctly, it is necessary to regularly check the positioning of the electronic valve. However, inspections need to be completed quickly to minimize downtime. If calibration drift is detected, the valve positioner must be recalibrated immediately. A better tool for this purpose is a handheld tool that can test and recalibrate electronic valve positioning, such as the Fluke 789 ProcessMeter. The tool provides a signal output that excites a controller connected to the valve positioner input and can incrementally and continuously adjust the output current, so the linearity and response time of the valve can be checked.
Basic steps for checking normally closed valves with a multimeter:
1. Set the ProcessMeter to output mode and use 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 the process multimeter to the valve positioner input terminals.
5. To determine whether the positioner fully closes the valve at 4 mA, use the keys to adjust the output current to 4.0 mA. The valve should be closed.
6. While observing whether the valve moves, press the Coarse down arrow button once to reduce the current to 3.9 mA. There should be no movement in the valve.
7. When setting the point at which the valve begins to open, ensure there is no reverse pressure on the actuator (the pressure that holds the valve closed when the controller input is 4.0 mA). With spring-closed 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 not any reverse pressure in the closed setting, the starting point at which the valve opens can be set between 4.1 and 4.2 mA.
8. Check that the valve is open, press the up arrow button of Coarse, and start adjusting from 4.0 mA. Each time the Coarse up arrow button is pressed, the current increases by 0.1 mA. The zeroing function of the valve positioner should be adjusted to set the valve to the corresponding closed mode.
9. To check the valve's fully open position - 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 as small as possible and can be adjusted using the span adjustment on the positioner.
10. Use the Coarse control button to adjust the current up or down between 20.1 mA and 19.9 mA. The valve stem should not move between 20.1 and 20 mA, and should move slightly between 20 mA and 19.9 mA.
11. In most valves, the zero and span set points of the valve controller interact with 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.
