Bringing Nuisance Alarms Under Control
When operators have to answer 5,000 alarms in 18 hours, it’s time to attack a huge problem.
David Gaertner -- Control Engineering, 3/1/2007
While process alarms are critical to the safe and profitable operation of a power plant, too many and too frequent alarms can have the opposite effect. To reduce alarm floods that seemed to be occurring with increasing frequency, Arizona’s Salt River Project (SRP) implemented a very effective alarm management solution at two of its generating stations, Navajo and Santan.
Its Navajo Generating Station on the Navajo Indian reservation near Page, AZ, operates three 800 MW coal-fired units. Its Santan station in Gilbert runs nine combined cycle units, providing a combined total of about 1,100 MW.
SRP began consolidating aging analog plant controls into a single, modern digital control system in 1990, based on the Foxboro I/A Series DCS from Invensys Process Systems. In 2006, I/A was incorporated on the newly installed Santan units. The I/A Series system now encompasses more than 40,000 I/O points. The benefits of switching to digital control have been dramatic and evidenced almost from the start.
“Dispatchers could not believe what they saw when the unit took a 400 MW runback at an average of 200 MW per minute. The boiler and turbine parameters were not only in line, they were 'flat line.’ Anyone with any experience operating a once-through unit knows that this is remarkable,” said SRP dispatchers, commenting on the startup progress at the Navajo plant. One benefit of such a powerful control system is the ability to generate alarms notifying operations of condition changes, but it wasn’t long before their number and frequency threatened to overwhelm operators.
“We were getting alarm horns all the time--at startup, shutdown, and day-to-day operation. In one 18-hour period, operators were confronted with 5,000 alarms, every one of which required intervention of some sort, and 98% were designated top priority. The plant had to designate an operator just for alarm management,” says Ron Bewsey, I&E supervisor and I/A administrator, commenting on the extent to which the lack of alarm management was jeopardizing plant efficiency.
SRP applied alarm management software from Invensys. SRP has already implemented this at all of Santan’s nine units, is ready to go live on one more Navajo unit, and has a proven execution strategy to apply the solution across both plants.
“Now we have valid alarms that mean something to operators so they can respond appropriately. This increases efficiency and reliability. Operators now see the alarms as a tool they appreciate, instead of a necessary annoyance,” says Bewsey.
How to add software
Starting with the two units at Santan station, SRP assigned an instrumentation and control specialist and a seasoned control room operator to work with two Invensys engineers full-time on implementing alarm management in three phases. In the first phase, the team studied alarm system performance in depth and produced a report which established performance targets based on EEMUA (Engineering Equipment and Materials Users Association) best practices, and described the system improvement methodology.
The second phase focused on performance improvement. This phase included development of an alarm system philosophy, alarm rationalization, execution of alarm system changes, and a human machine interface (HMI) that enables plant personnel to visualize alarm system performance and history.
Related documentation described the causes of each alarm, consequences if that alarm is ignored, corrective actions, and rationale for each. The documentation specified detailed work processes that guide operators, technicians, and engineers.
SRP’s Alarm Management Team prioritized alarms on a basis of economics, safety, environmental severities, and time needed to respond. Those in priorities one, two, and three would be sent to the operator. Those coded as priority four would be sent to maintenance for diagnoses and repair. The priority fives would just go to the historian for record purposes; but not to the operator. These procedures are in accordance with SRP’s new alarm philosophy.
Prior to the project, 98% of alarms were assigned priority 1, with 2% spread across the rest of the priorities. After the documentation and rationalization, only 11% were at priority 1, 14% priority 2, and 75% priority 3. The new priorities are programmed into the I/A control system, which manages alarm routing to appropriate locations.
Implementing the alarm system design at all nine Santan units has reduced the frequency of alarms dramatically. Where plant startup had previously taken two people up to four hours, now one operator can start up in less than two hours.
“We can now attend to each alarm instead of just hitting the 'acknowledge button,’ and we have more time to concentrate in improving the performance of the units rather than just trying to stop the noise,” says Rees Scott, control room operator.
In addition to making things “a whole lot quieter,” eliminating nuisance alarms freed operators to take positive actions. In addition to signaling problems, alarms might also be used to notify an operator of an opportunity to increase plant efficiency, for example, by decreasing heat rate.
At Navajo station, results may be even more dramatic. Once the Unit 1 project is complete, the process will be repeated on the other two units and the scrubbers as well.
The third phase will continue to optimize the alarm system and cut more alarms. Phase 2 will be completed by implementing advanced alarming, which will “focus on reducing alarm floods,” said the SRP Alarm Management Team.
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