Calibration of Control Valve Positioner: The Process Discussed
November 23, 2020
Control valves are the devices that monitor and control the process parameters like fluid flow, pressure, etc. The function of control valves relies on a component called the positioner. Although the open/close operation of a control valve can be done using a controller, its position cannot be confirmed without a positioner. In order to offer accuracy to the control valve’s operation, the position of the control valve must be accurate. The position of the control valve is controlled by the positioner. The positioner senses the position of the control valve via a feedback mechanism. The calibration of the control valve positioner is a crucial process and must be done properly for the desired results. Do you know how to calibrate the control valve positioner? If not, then the post leads you through the important steps of control valve positioner calibration and more.
Construction of a Positioner Assembly
Before going into the calibration process, it is important to understand the design of a positioner assembly. The following pointers will help you understand it better.
- The positioner assembly of a control valve consists of a flapper-nozzle, cam linkage, summing beam, and spring and a diaphragm interlinked together with the help of mechanical linkages.
- The summing beam is connected between the cam-spring linkage and the bellow.
- The action of cam-spring is directed by the pressure applied by the flapper-nozzle, which eventually triggers the motion of the summing beam.
- The diaphragm displays the applied and exerting pressure on/by the flapper assembly.
An Overview of Step-by-Step Calibration of Control Valve Positioner
The positioner is responsible for the movement of the flapper. The summing beam is directed in the reverse or forward motion, which acts as the feedback mechanism to the positioner. Since the positioner and feedback mechanisms are assembled in a complete loop, there are two parts of the calibration process of a positioner, namely, zero adjustments, and span adjustments. Let us discuss the steps involved in the calibration process.
The zero adjustment is done by adjusting the nozzle pin. The following steps are involved in zero adjustments.
- The measurable pressure range is shown on the supply gauge. Move the flapper assembly to the mid-level of the force beam on the forward movement side.
- Set the input value to the lowest reading value.
- Further slowly adjust the nozzle pin to get zero value.
- Then slowly increase the input pressure value to 3psi, and check if you get zero output. If yes, then the zero is adjusted precisely.
The span adjustment is done by moving the flapper assembly to a specific level. The following steps are involved in span adjustments.
- Increase the input pressure slowly till it reaches the maximum pressure value. The input pressure value to the positioner is commonly 15 psi.
- At 15 psi input, check for the maximum output of 20 psi.
- To adjust the span, adjust the flapper assembly by moving the force beam. Ensure that the zero adjustments are made so that the force beam remains at the mid-level only.
- Move the flapper assembly to the lower value if the saturated output value is in the minimum measurable range. Move the flapper assembly to the higher value if the output value is saturated to the maximum measurable range.
This completes the calibration process for a positioner of the control valve. However, the calibration of the positioner has to be precise, therefore, it is essential that it is performed by experts. Thus, you must partner with experienced calibration services like The Transmitter Shop. The company offers calibration and maintenance services. Also, you can buy the control valves from various trusted suppliers to ensure the quality and function of these products.