Following tricky instruments
The most important diagnostics are typically those that signal an imminent failure. Usually these include: communications, e.g., lost signal; variability; noise band; signal spikes; and sensor at limits.
Diagnosing instrument failures quickly helps to ensure safe, reliable, and efficient plant performance. Typical failure modes of instruments exhibit intermittent symptoms, such as spikes in the signal, periods of high noise, or periods of very low noise. These intermittent symptoms will often go un-noticed. A PSS can quickly identify these issues, and elevate their priority so that safety or quality issues can be averted.
Control valve diagnostics are at least as important as those from sensors. Because valves are mechanical devices, they are prone to mechanical failure. Control valves also have a direct impact on process results. Some of the most important valve diagnostics include: percent of time at limits; stiction analysis; hysteresis; valve travel; and valve reversals.
These control valve diagnostics indicate the condition of the valve. In the case of valve travel and valve reversals, the diagnostic can also tell you about the quality of controller tuning. A PSS can quickly identify which valves in your plant are at the greatest risk for failure.
Control loops, once governed by simple pneumatic tubing, are now complex networks, integrated with software, configuration, and custom programming. There are many ways for a control loop to fail, and many more ways for control loops to degrade process performance. Keeping a close eye on the right symptoms will help to ensure that the control system performs its best.
A performance supervision system will monitor the basics of control performance, such as:
- Time in normal mode;
- Integral of absolute error (IAE);
- Service factor;
- Robustness of controller tuning;
- Response time; and
- Setting time.
Normal mode is the opposite of manual mode. In normal mode, a loop is operating in automatic. If an operator takes the loop off automatic its because he doesnt trust it. Of course, there may be some reason why the control engineer didnt design an automatic loop; sometimes this is worth investigating. IAE measures the effectiveness of the tuning to hold the process variable at setpoint. Service factor measures the percent of time that the entire loop (sensor, controller and valve) is working properly. Robustness is the ability of a loop to respond to changes over a wide range of operating conditions. Response time measures the dampening ability of a loop. Settling time is a measure of how quickly the process will return to setpoint after a setpoint change or process upset.
More advanced systems can automatically identify process dynamic models, based on operator setpoint changes. With these models, it is straightforward to identify opportunities for better tuning and improved stability. For plants that have limited process control resources, a PSS goes a long way to improve control performance.
This study compares the costs of a PSS with the costs of new digital instrumentation. The study analyzes a large process plant, containing roughly 1,000 control loops. To simplify the study, and to remain conservative, the costs of DCS components, such as upgrading I/O, software and other components, are not included.
The cost of a digital field bus device can run as much as $1,000 more than a traditional analog device. To be conservative a $500 premium per device is used. If a typical control loop has one transmitter and one valve then a low estimate for the cost is $1,000 per loop. The cost premium for the interface device, field bus wiring and software to interface with the device isnt considered.
The cost for the performance supervision software is approximately $100 per loop inclusive of all costs. The cost for OPC server software is estimated to be $15,000 per server which can easily accommodate up to 2,000 control loops. The per loop cost of the OPC software is estimated to be in the range of $7.50 per loop.
A plant shutdown, which would be required for a field bus conversion, could add several hundred thousand dollars in lost production opportunities to the field bus installation. Costs are summarized in Table 1.
Table 1. The cost for a plant with 1,000 control loops shows that the project cost is reasonable for the benefits attainable.
Getting ahead of the curve
Existing DCS systems and analog instruments can provide diagnostics. A PSS gathering real-time data can analyze and prioritize diagnostics for equipment, controls and process. Available diagnostics include diagnostics for the process, the equipment, and the controls. The cost of a PSS is one-tenth the cost of conversion to digital field bus instruments.
George Buckbee is director of product development for ExperTune, Inc., Hartland, Wis., e-mail him at email@example.com.