Imagine that your power plant is about to have a scheduled outage. As the unit is ramped down and feed water control is taken over by the by-pass valves, you discover that the control valves refuse to close upon receiving orders from the level control system to do so. This is now the last straw for the operators who also have been fighting stability problems with these valves over the past several years. What do you do?
Well, if you know Emerson variability consultant, Eric Ascoli, you contact him. You may recall Eric from a prior post on stability problems at a sugar mill. He shared this story with me.
Instead of continuing with the shutdown, the station had to run at 20% power production for 12 hours costing them hundreds of thousands of dollars while the situation was diagnosed and corrected. A manual unit trip was not an option. The problem was aging pneumatic instrumentation that had locked up and blocked the valve positioner’s operation.
Eric worked with a pneumatic specialist from Proconex, the local business partner for the power station. Their findings were that the operation of the pneumatic trip valve was not completely understood and its adjustment was slightly off. Also, the combined level control valves had a very large variation in installed valve gain and the unbalanced and aggressive controller tuning caused the instability the operators had been experiencing.
The challenge was to find a solution that would remove completely the possibility of such an event from happening again. It involved a short-term fix (servicing and adjustment of the pneumatics and modification of the characterizing functions for the valves) in preparation for the imminent scheduled power up. Additionally, Eric corrected the level controller tuning by using Lambda tuning after he analyzed and evaluated the process gain and empirically defined other important process parameters.
Their longer-term recommendations were to install digital valve positioners to replace the aging pneumatic ones. The same split-range control strategy would be maintained, but the valve performance would be improved through better positioning accuracy and dynamic behavior. The installation would be simpler and less prone to maintenance issues because I/P (current to pneumatic) converters could be removed. An even longer-term solution would be to replace the two split-ranged valves with one single start-up control valve to eliminate any crossover interactions.