Figure 2. Tuning and valve repair significantly enhanced furnace temperature control.
For instance loop FC-16026 was diagnosed as having extremely high valve travel — which causes high process variability, increased valve wear-and-tear and valve mechanical problems. Simply adding a small filter to the instrument signal can address the issue. Process stability improves almost immediately. “In most plants, valve travel may be reduced by a factor of 50% or more through the application of a filter and some appropriate tuning,” notes John Gerry, president and founder of ExperTune.
“When plants first start with performance monitoring, it is quite common to find some ‘low hanging fruit.’ These are problems that have been overlooked because most plants do not monitor some of the most important key performance indicators,” Gerry adds.
Borealis found “low hanging fruit” in the form of underperforming control valves, lack of instrument filters and some controller tuning opportunities.
Uncovering valve issues
The performance supervision system automatically identifies a variety of mechanical control valve issues. These include:
- oversized valves
- undersized valves
- valves with stiction and hysteresis
The standard reports prioritize valve repair issues according to both technical and economic factors. This information helps the team to plan and schedule the appropriate maintenance and to focus on issues to investigate in more detail. In some cases overcoming valve problems requires only minor adjustments, such as recalibration of positioners or adjustments to valve air pressure.
The payout from resolving these issues can be dramatic. At Porvoo, the PlantTriage software detected problems with the feed and dilution steam control valves on the cracking furnace. So, the maintenance schedule for these valves was accelerated. The furnace operation stabilized, reducing energy consumption. Figure 2, which charts a composite metric called Loop Health that is used to measure the overall effectiveness of the control loop, shows the impact of tuning and valve repair for the furnace temperature control. Figure 3 depicts real-time data from the temperature loop. Variability is cut, valve travel is decreased, and the furnace temperature is stabilized. The variability reduction resulted in better quality and capacity utilization.
The undersized valves report identifies process constraints or bottlenecks. In some cases the constraint may be removed with relatively minor equipment changes.
Oversized valves amplify mechanical, tuning and control design issues. Replacing valves or swapping out the valve trim greatly reduces these effects and brings the process back into control.
Figure 4. Effective tuning provided close-to-optimal control of process variable.
The robust controller tuning approach eliminated loop cycling and interactions, adding a great deal of stability to the process. This, in turn, allowed APC to be used to the fullest, actively pushing the entire process toward optimum performance.