Figure 6 shows a capture from an automated performance evaluation of proportional-integral-derivative loops at one site. The program identified several loops (highlighted with a red background) as having poor performance. Prior to the automated analysis, neither operations nor controls personnel had flagged any of the loops because their poor performance wasn't causing any noticeable problems in the unit. Addressing only one of those loops, PDIC-1318, to improve its performance, reduced steam consumption by over 1,000 kg/hr.
Another activity that can achieve lasting benefits is to review the control structure for ways to reduce energy consumption while meeting all processing objectives. For example, many sites now tie distillation-column reflux flow rate to the feed flow rate instead of holding reflux flow constant. A similar control strategy for the reboiler duty, varying heat input with column throughput, may suit some columns. Operations and process control staff can work together to identify such opportunities at most sites.
Keeping assets, especially rotating equipment and heat transfer units, in peak condition is crucial for energy efficiency. Often it's possible to perform important cleaning activities while the plant is in operation rather than waiting for the next shutdown or turnaround. The question then becomes when is the right time?
Changing stream properties and flow rates can make it difficult to know the underlying performance of exchangers. Simply tracking the overall heat transfer coefficient isn't sufficient, as the trend chart from an operating process (Figure 7) shows. Relatively sharp changes in the UActual value didn't indicate rapid fouling, a conclusion borne out by comparing the changes during the same period of the UExpected value, which compensates for the process variations, and the calculated fouling factor, which displays the ultimate performance of the exchanger on the right-hand y-axis.
Many installations now rely on real-time monitoring to track in detail the underlying condition of critical heat exchangers in fouling services. This very effectively identifies when cleaning is cost-effective.
Tracking allowed for cleaning operations to be scheduled at the right times, as evident from the sharp drops in fouling factor at the beginning and again at the end of the time period shown in Figure 7.
AN INDIVIDUAL ROLE
Just as each person and job function at a process plant plays a part in ensuring production targets are achieved safely, everyone has a similar opportunity to impact energy efficiency. However, people must understand their roles, get objectives and instructions on how to achieve them, along with the required time and resources.
A comprehensive energy management program will address efficiency in all stages of the plant's lifecycle and for each role. It can provide significant and lasting improvements.
JESUS VALLEJO is performance service director for Honeywell Process Solutions in Madrid, Spain. E-mail him at Jesus.firstname.lastname@example.org.