Valve position feedback is critical to the operation of a control valve. A Fisher Digital Valve Controller, the DVC6200 (HART or Foundation Fieldbus) can detect position feedback problems and automatically revert to I/P [current to pressure] transducer mode to keep the valve operational. This avoids costly unplanned spurious trips, which are critical for plant operational performance.
The DVC6200 can change from Travel control mode to pressure control mode based on following criterion.
- Tvl Dev Press Fallback—When the difference between the travel target and the actual travel exceeds this value for more than the Tvl Dev Press Fallback Time, the instrument will disregard the travel feedback and control based on output pressure.
- Tvl Dev Press Fallback Time—This is the time, in seconds, that the travel target and the actual travel must be exceeded before the instrument falls back into pressure control.
Fallback Recovery—If the instrument has fallen into pressure control mode and the feedback problem is resolved, recovery to travel control can occur automatically or with manual intervention. To return to travel control when Manual Recovery is selected, change the Fallback Recovery to Auto Recovery, and then back to Manual Recovery (if desired).
Summary
DVC6200, the Travel / Pressure configuration parameter allows for choosing the control mode:
- Travel
- Pressure
- Travel with Pressure Fallback (Auto Recovery)
- Travel with Pressure Fallback (Manual Recovery)
The benefit is that this avoids plant trip due to failure of travel Sensor—maximizing plant availability. This capability can be extensively can be used in HOT cutover applications—saving considerable operating costs.
Frequently Asked Questions
Q: During initial calibration, does the DVC6200 calibrate the valve to operate for both Travel and Pressure Control mode or an extra step to re-calibrate as Pressure Control?
A: No, a single calibration captures both, enabling DVC6000/6200 to operate Travel (Default mode) and reverts to Pressure Control mode, when configured.
Q: Any use cases where travel sensor failures lead to Pressure Control mode?
A: In one of the refineries, the travel sensor linkage of a DVC6000 (older DVC version) was broken. A travel deviation alert was sent to the AMS Device Manager station and operator noted that DVC6000 was automatically reverted to Pressure Control mode—the DVC6000 had linkages to valve stem / shaft.
Q: What criteria sets within the DVC to detect travel sensor failure and switch to Pressure Control mode?
A:
- Travel Deviation Pressure Fallback
- Travel Deviation Pressure Fallback time
Please refer to Instruction Manual D103605X012 for setting parameters in detailed setup.
Q: If the DVC6200 is configured for auto recovery, will it revert to Travel Control once the travel sensor issue has been rectified or its limits are back to normal range?
A: Yes. No other parameters are required to be configured.
Q: In case of a sensor failure, is change of Travel to Pressure Control mode bumpless?
A: Yes
Q: Is Travel / Pressure Fallback a default feature of a DVC?
A: Any DVC supplied by Emerson with AD, PD, ODV will have default Travel control with Pressure fallback, auto/manual recovery option.
