In a post last week, Valve Travel Control-Pressure Fallback, Emerson’s Riyaz Ali shared the process of how a control valve with a digital valve controller could fall back to standard pressure control upon an issue with the travel sensing mechanism. We both received a great email with some follow up questions that I thought would make a good follow on blog post.
The first series of questions were, “What about friction? How friction affects the transition from travel to pressure control? Valves with high friction are not available for this pressure control?”
Transition from travel to pressure control is bumpless. It should not affect transition. However, it is correct that for a high-friction valve, performance may deteriorate, in Pressure Control mode (as I/P [current to pressure] mode) vs. Travel Control mode (positioner mode). But still the valve will control and maintain availability to the process loop, until the plant staff has the next opportunity to correct the travel sensor or linkages issues.
The general theory of control very well respect the positioner (with travel linkages), a better choice for control, vs I/P (without travel linkages), when it comes to a process control loop. Therefore, it is established fact that a control valve working in I/P mode will have a little degraded performance vs. a control valve working with positioner, specifically when the valve has higher friction. Because a positioner, not only positions the valve, but also provides a boost to pneumatic output. In conventional positioners, calibration is done based on input current (mA) vs. valve travel (in).
Now talking about a microprocessor-based device like Emerson’s Fisher DVC 6200 with travel / pressure control mode, during calibration three parameters are considered—namely input current (mA), output pressure (psi) and actual stem travel (in). For an example, if 12mA current expects 50% valve travel, assuming 3-15 psi spring range ideal condition with no friction, a pressure of 9.00 psi will be tabulated in memory during calibration. During initial set up, a wizard DVC calibrates the valve based on actual travel from movement sensor and actual pressure from the built-in pressure sensor.
Unlike a conventional I/P transducer, the DVC being a microprocessor-based device, make checksum calculations on both travel and pressure when the controller input signal is changed. In the example above, if at 12mA, the DVC does not see 50%, it tries to adjust the pressure to bring the valve travel based on 12mA and vice versa. Since the DVC, being a smart microprocessor-based device, it captures best results in NVM (Non-Volatile Memory) of the microprocessor, until the next calibration.
If the DVC is mounted on a high-friction valve and if the travel sensor fails, it will still control the valve based on the input signal (mA) vs. pressure value (psi) in the NVM. It will be better than the conventional I/P transducer, where it does not have values in NVM.
I can’t share the questions referencing a specific installation but can share Riyaz’ concluding remarks.
The Pressure Control mode has been introduced a long time ago and many end users have adopted into their work practice and implemented in plant operations. Here’s an example from one international oil company. “Complete compliance to IOC [international oil company] clause, requiring that positioner should revert to I/P mode or pressure control for actuator positioning if position feedback is lost, either through failure of the mechanical linkage or through failure of the position sensor.”
Visit the Digital Valve Controllers section on Emerson.com for more on how these controllers provide accurate control valve performance and fallback operation to conventional I/P control in the event of issues with travel sensors and linkages.