VALVES, the actuators that move them, and the electronic circuits that control them, are all subject to the effects of aging soon after they are installed. 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.
To keep a system operating properly, a good preventative maintenance programme that mandates periodic checks of valve positioners is required. These checks need to be conducted quickly to minimize down time. 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.
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. In addition, the process meter must continuously adjust the source current in a ramping or stepping fashion, allowing the user to check the valve’s linearity and response time.
In most valves, there is an interaction between the zero and span settings of a valve controller. Therefore, it is best to ensure proper valve position adjustment by repeating the test of the fully closed and fully open positions until no further adjustment is necessary.
Safety note:
Always ensure that the personnel responsible for the process you are working on are informed of your intentions prior to making any checks of valve operation. Be alert when touching any moving machinery.
