To keep a system operating properly, a good preventive maintenance programme that mandates periodic checks of valve positioners is required, says industrial product supplier Comtest Group.
This is because valves, the actuators that move them, and the electronic circuits that control them, are all subject to the effects of ageing soon after they are installed.
The company explains that the valve seat wears not only from the repeated seating of the valve, but from the liquid or gas that passes through it. “Depending on the application, a valve can be stroked from hundreds to tens of thousands of times over a one-year period.”
This amount of mechanical motion inevitably causes screws to reposition, springs to weaken and mechanical linkage to loosen. In addition, electronic components change value over time.
The results are valves that do not fully open or close, close prematurely, or operate erratically and cause improper regulation of the gas or liquid under its control. This is more commonly referred to as “calibration drift”. As such, valve positioner checks need to be conducted quickly to minimise downtime.
“When these checks reveal calibration drift, recalibration of the electronic valve positioner must be performed quickly. With the varied locations in which a valve can be installed and the difficulty in removing it, the equipment used to perform the checks must be brought to the valve positioner itself. Therefore, this ‘in-field’ tester must be portable, easy to use and rugged,” Comtest notes.
Further, the tester, with signal sourcing, must simulate a controller connected to a valve positioner’s input. Through the controls on the ProcessMeter, the operator can set the positioner’s input current to a specified level and visually inspect the reaction of the valve’s position using the mechanical position indicator, the valve stem position or flow indicators.
Additionally, the ProcessMeter must continuously adjust the source current in a ramping or stepping fashion, allowing the user to check the valve’s linearity and response time.
Comtest cites an example to illustrate how to conduct these checks. “This example only demonstrates the basic principles in making position checks on a valve positioner. Manufacturer’s specific instructions should always be consulted for proper and appropriate valve positioner testing and calibration.”
General Steps for Checking Valve Positioning
Comtest notes that the first step is to set up the ProcessMeter in the sourcing mode using the appropriate range of current for the positioner.
“The tester uses a separate pair of jacks to source current. First, connect the test leads into the 24 V loop power mA output jacks. Next, select the 4-20 mA range by moving the function switch from off to the first mA output position. Now the user is ready to connect the tester to the input terminals of the valve positioner.”
With the test equipment properly set up and connected to the positioner, it can be determined if the positioner fully closes the valve at the 4 mA input current level, says Comtest.
“Using the push buttons on the ProcessMeter, adjust the source current to 4 mA. Now, while watching the valve for any movement, press the coarse down button once to decrease the current to 3.9 mA. “There should be no movement of the valve,” the company explains.
It adds that, in setting the point at which the valve starts to open, one would want to be sure there is no counter pressure by the actuator against the force holding the valve closed when there is 4 mA on the controller’s input.
In a spring-to-close valve, there should be no pressure on the diaphragm. With a double acting piston actuator, there should be no pressure on one side of the piston. Comtest notes that the user may want to set the start of opening between 4.1 and 4.2 mA to get that insurance at the closed setting. “To check the opening of the valve, press the coarse button up from 4 mA.”
The ProcessMeter will increase 0.1 mA for each press of the coarse button. The user should adjust the zero adjustment on the positioner to set the valve for the closing they desire.
“The next check is with the valve at the next extreme: fully open. This is referred to as a span position check. Using the range buttons on the ProcessMeter, adjust the source current for a 20 mA reading and allow time for the valve to stabilise. While watching or feeling for valve movement, press the coarse up range button once to 20.1 mA,” says Comtest.
It states that, in most valves, there is an interaction between the zero and span settings of a valve controller. Therefore, it is best that the user ensures proper valve position adjustment by repeating the test of the fully closed and fully open positions until no further adjustment is necessary.
“For valves with linear action, linearity can be checked by setting the tester to 4 mA and then, using the % step button, step the current to 12 mA (50%) and confirm the valve position indicator is at 50% travel. “If the valve is of a non-linear type, refer to the valve manual for proper operation,” says the company.
Comtest stresses that – as demonstrated by the aforementioned example – the ProcessMeter comprises all the necessary tools for checking and recalibrating electronic valve positioners. In addition, the tester not only simulates a current loop transmitter, but it is a measurement tool as well.
“Along with the normal digital multimetre measurements (direct current (dc)/alternating current (ac) volts, dc/ac current, and resistance), the tester can also measure frequency. “Additional features include minimum and maximum, relative measurements, diode test and auto hold. A feature-rich product designed specifically for the loop process technician, the ProcessMeter also complies with IEC 1010-1 standard for CAT III 1000 V environments,” Comtest concludes.