Process instruments and control valves are scattered throughout the facility, often in hard-to-reach locations. Routine inspection of the physical devices is a challenging job at best. Hazardous locations, bad weather, and difficult process environments can make the task doubly difficult or even impossible while the process is running. Quite often, poorly-performing instruments and valves are simply ignored until the next plant shutdown or until they fail.
Even with a distributed control system (DCS) in place, process problems, too, go unseen. Heat exchangers foul, processes are unintentionally left in bypass, controls are left in manual, and the performance of the entire plant degrades. Quality, reliability, and efficiency drift off target.
Reduced staffing levels have exacerbated the problem for many plants. The remaining skilled workers are being stretched thin. Plant resources dont have time to do everything: they can only focus on the most important tasks those that will deliver immediate improvements. But how can you determine which tasks are the most important? By prioritizing items before a review.
Diagnostics can help to identify symptoms early. Corrections and adjustments can be prioritized in an organized fashion, reducing trips to the field. Early diagnosis and correction can have a large payback from:
- Upgraded process efficiency and quality;
- Increased reliability, fewer unplanned failures;
- Enhanced maintenance planning; and
- Improved process stability, reduced firefighting.
Opening the door
Getting diagnostics from existing analog instruments may be easier than you think. Your control system is already gathering real-time data for instruments and valves. Using industry-standard OPC communications, you can pull real-time data directly into the Performance Supervision System (PSS) (Figure 1).
Figure 1. With todays computing power, a single PSS computer can easily analyze thousands of sensors and valves.
The PSS works in real-time taking process data and performing analysis and diagnostics. The PSS can monitor the plant 24 hours a day. With todays computing power, a single PSS computer can easily analyze thousands of sensors and valves.
Many plants have already invested in process historians. Most commercially-available process historians support the OPC Historical Data Access (HDA) standard. OPC-HDA allows the gathering of real-time data directly from the historian, with no additional load on the control network.
Adding real-time diagnostics may require some upgrades to the historian and network architecture. Real-time diagnostics perform best when data are gathered at fast sampling rates typically once per second. Data collection at these rates is easily achieved with modern control systems and historians. However, older control systems or historians may be challenged to deliver this information reliably, so a capability analysis should be performed first.
Additional diagnostics also may be available from Alarm Management Systems or Condition Monitoring systems. These data may already exist in separate systems. Sharing data between these systems and the performance supervision system may require additional upgrades to plant infrastructure. However, the combined diagnostics can be very powerful.
Once the data have been gathered in a PSS, diagnostics can be performed. The available diagnostics fit into three general categories:
- control; and
A great deal of process information can be learned by analyzing real-time data streaming from a DCS. For example, oscillation detection can be used to find and eliminate sources of process variation. Process oscillations often start in one area of the plant, but propagate throughout the facility, increasing process variability and reducing efficiencies.
The performance of process plants changes over time. As equipment fouls, degrades, or fails, its sending out telltale warning signs. Process dynamics change, valves start running full-open to meet increasing demand.
As the process evolves, increasing production rates and changing product formulations, process performance also changes. A PSS looks at both the short-term and long-term symptoms, and helps to diagnose the problem.
Closing the gap
One key performance indicator is the Opportunity Gap. This diagnostic recommends shifts in setpoints to drive the plant toward its optimum performance. It works by comparing current setpoints and variability to the desired product specification. As you reduce variability through process improvements, the opportunity gap widens, creating the potential for direct savings.
Figure 2 shows how the opportunity gap was used to increase performance of an oil burner operation. In this case study from a chemical plant in the U.S., energy cost per ton was reduced by 9%, and production rate was increased by 11%.
Figure 2. Reduced costs can be quantified from the process data, such as energy cost and production rate.
Tracing equipment problems
Equipment diagnostics may include indications of sensor or valve problems. Its interesting that these symptoms are often used to diagnose failures of related equipment. For example, spikes in flow measurements have been used to identify damaged pump seals, preventing expensive shutdowns.