Digital control for valve pneumatic actuators

Edited by the Engineeringtalk editorial team May 22, 2006

Digital valve device controls valve pneumatic actuators in response to commands from a control system and communicates using the Foundation Fieldbus protocol.

The Yokogawa YVP110 advanced valve positioner is a digital valve control device that controls valve pneumatic actuators in response to commands from a Fieldbus control system.

It communicates using the Foundation Fieldbus protocol.

The versatile component reduces transmission lag, improves valve characteristics and response, and reduces flow-pressure variation effects.

It is for use with the firm's Valvenavi software tool for facilitating the process of setting up, tuning and maintaining the YVP110.

Today's field devices incorporate various self-diagnostic features, allowing external conditions to be diagnosed by the devices themselves.

These could include diagnostics of a valve by a valve positioner or checks for clogging in the pressure impulse line by a differential pressure transmitter.

To use these diagnostics effectively, mere transmission of simple process values and status is not enough; more information needs to be acquired from the field devices and presented to the user in a form that is easier to interpret and use.

Valvenavi runs on a PC or as a plug-in for the plant resource management package to facilitate initial adjustment and parameter tuning, display trend graphs and diagnose the valves.

The valve positioner has online and offline diagnostics.

The online diagnostics include the device's self-diagnostic functions plus online monitoring and integration of the valve operation information.

The offline diagnostics, also known as signature functions, include: measurement of the I/O characteristics of the valve (input pressure versus valve position); measurement of the step responses of the positioner; and measurement of the I/O characteristics of the positioner.

The YVP110 contains a compact pressure sensor, and in operation scans the output pressure and measures the valve position.

After scanning and measurement, the software acquires the measured data via the Fieldbus and displays the results as a graph.

The data can be saved in the computer and overlaid on past measurements for comparison to check changes in hysteresis and the slope of the curve.

As a result, anomalies such as looseness of the gland packing, damage to the return spring, clogging and erosion can be detected.

I/O characteristics are measured at the various points in 10% increments as standard, but high-resolution signature measurement at more than 1000 points is available as an option.

Up to two sets of measured signature data can be saved to the device's non-volatile memory.

A typical example of a Valvenavi application is in displaying the step response of a valve positioner system.

The step width and measurement period are set prior to measurement, and the response data measured in the positioner are displayed after measurement.

As with the I/O characteristics, the step response can be compared with past response data, allowing detection of excessive overshoot and limit cycle due to an anomaly in the actuator or positioner.

Up to 600 measurements can be sampled at variable intervals so that various patterns of response can be obtained as required.