VT-MVTW-1-16 D DCS Control System

VT-MVTW-1-16 D will be connected to a device on the control valve known as a positioner, which is more complex than an I/P sensor. Besides converting the 4-20MA signal into air pressure, the positioner actively checks the position of the valve stem to ensure that it is accurately placed for any given 4-20MA command signal. To verify that the control valve complies with the command signal across the whole 4-20mA signal range, the technician employs a loop calibrator. To provide power within a loop to an indicating instrument using a known current signal, a loop calibrator can be set up to simulate the behaviour of a 2-wire transmitter powered by a Lobo.

VT-MVTW-1-16 D does not require the transmitter since the calibrator substitutes it. It is worth noticing that the calibrator operates as an active current source and not as a passive load, unlike in the previous scenario. Moreover, the calibrator not only supplies information (in other words, it regulates the current) but also offers energy to the circuit. The DC supply present in the controller is not used for loop power because the “source” mode calibrator provides the needed power to conduct the current through the 250 ohm resistor. A common application of the loop calibrator in “source” mode is to evaluate a control valve for accurate calibration, quick response time, and friction measurement.

The VT-MVTW-1-16 D circuit configuration can be a valuable tool for instrumentation technicians when testing the response of control valves due to its ability to finely tune the signal while directly observing the valve’s motion. For instance, in cases where a control valve is presumed to have excessive friction in its moving parts, a technician can test the valve by gradually increasing and decreasing the source current of the cyclic calibrator in small incremental steps. Significant alterations in current will result in the valve overcoming friction and moving, but if friction surpasses the incremental force generated by pressure changes, minor modifications will hinder the valve mechanism’s movement.

Testing of the transmitter cables and controller inputs to verify that any 4-20MA signals sent by the transmitter are accurately received and displayed by the controller is facilitated by the analogue transmit feature of the VT-MVTW-1-16 D. This procedure is typically carried out on a recently installed control system during the commissioning process, prior to initiating the control process. Its purpose is to confirm the accurate functionality of the controller’s process variable inputs, 24 VDC power supply, and transmitter cabling. An instrumentation technician usually tests the controller’s PV displays and alarms to confirm effective operation while simulating various current values (e.g., 4MA, 8MA, 12MA, 16MA, 20MA) using the calibrator in “analogue” mode.