Branden Powell is supervisor of environmental process for City Water, Light & Power. Mike Arpin is area manager for SUEZ – Water Technologies & Solutions. Powell can be reached at [email protected]. Arpin can be reached at [email protected].
undefinedIn July 2018, Springfield, Ill., encountered a classic chemical upset. The utility supplies the city of Springfield with electric power generated from four coal-fired steam-turbine generators. The boilers operate with water from a nearby lake, which also provides drinking water for the city. An existing demineralizer trailer that serviced the utility’s 200-MW Plant 4 generating station had exhausted, causing a release of silica that contaminated the system.
Fortunately, the utility was able to identify the source of the silica early and take immediate corrective action to mitigate the problem—contacting SUEZ Water Technologies & Solutions, which delivered and put into service a new demineralizer water truck by early the next morning. Within 24 hours all water had returned to optimum conditions.
During the ordeal, the boilers in service saw minimal negative effects, and the critical benefit was that the utility, led by Branden Powell, supervisor of environmental process, caught the Plant 4 demineralizer truck exhaustion in time before serious damage could be inflicted—thanks to the lessons learned from battling a multi-month silica contamination problem that occurred only a year earlier with a separate boiler unit.
A Complex Series of Problems
Beginning in the spring of 2017, the utility experienced the first of several silica contamination issues with its Unit 3 boiler, a chain of complications that merged together to form a larger and difficult-to-detect problem that Powell later described as a “perfect storm.”
The troubles initially started following the utility’s spring outage, where Unit 3 underwent four weeks of scheduled maintenance to prepare it for continuous service during the summer peak demand season. During this time, two serious dents were found on the leading side of the system’s reheat pendants due to heavy falling slag. Repairs were needed to address tube leaks and welding was required in the boiler fire box to repair multiple tube failures. However, during this work the utility also discovered a major problem during a boiler inspection that changed the course of the entire outage. The boiler coutant slope on both sides was found to be severely damaged, requiring extensive welding to repair more than 400 water tubes, extending the outage to 10 weeks.
Following this work, the utility expected to see elevated silica readings in the cycle when the unit was brought back into service, as patch welding is a cause for silica ingress. However, that expectation effectively masked the real source of silica in the unit following the outage.
When the utility started up the Unit 3 boiler, they found high silica concentrations even for a startup following a major outage. Also surprising was the high cation conductivity readings that were observed. The utility then collected samples for sodium and magnesium from the cycle system. The results showed that magnesium was beyond calibration, leading to a positive identification of a condenser tube leak. Since the condenser had been checked for leaks during the outage, the utility surmised that the leak most likely occurred during startup.
Demineralizer Problems
After the boiler unit was removed from service, repaired and placed back in service, the silica contamination problem persisted. Over the ensuing months, the utility’s Environmental Process team conducted numerous investigations and maintenance procedures as they tried to determine the source for the silica contamination. Many false hopes were encountered along the way, but a break finally came in mid to late August.
Makeup water previously was supplied to the condensate storage tanks by a combined reverse osmosis and demineralizer high-purity water system. When the boiler blowdown was open, makeup water also could be sent directly to the cycle as needed. Demineralizer polishing was accomplished by either a demineralizer water trailer or four mix bed jumbo vessels. The utility discovered that the system was not keeping up with makeup water demand.
Previously, following the initial spring outage, the utility had elected to open the boiler blowdown in full as a strategy to reduce silica contamination. With the blowdown fully open, the boiler uses a larger volume of makeup water, and if this water is not high purity, silica leakage hampers any removal of silica from the cycle water that would be accomplished from the blowdown completely open.
In retrospect, they realized that the blowdown had been fully open during a demineralizer exhaust (occurring overnight and discovered in time), which took place shortly after the condenser leaks were repaired. Moreover, the plant’s mix bed vessels also experienced a series of exhausts over the course of the Unit 3 boiler difficulties. These vessels provide a higher water production compared to the demineralizer water trailer, but have less safeguards against any exhausting resin beds. When a jumbo vessels exhaust is not immediately caught, it will continue to leach silica into the makeup water being supplied. These demineralizer exhausts were discovered too late before damage was inflicted.
Lessons Learned
After recovering from these problems, the utility implemented several changes based on what it had learned. Among the measures that were adopted, the utility employed new precautions to monitor demineralizer performance more closely when the boiler blowdown was open, as a demineralizer exhaust occurring while the blowdown is fully open will cause any poor-quality makeup water to go directly to the condenser and eventually the boiler. The utility also put into place a better system for detecting chemical excursions, including proper communications on the part of operations and management for taking the necessary corrective action.
In July 2018, the new operational changes enabled the utility to catch the Plant 4 demineralizer truck exhaustion early. In less than 24 hours, SUEZ had delivered a mobile demineralization water trailer and was supplying purified water to the Plant 4 boiler. By responding in an expeditious manner to this emergency, the team was able to quickly decontaminate the Plant 4 storage water, allowing the power plant to remain online and prevent any further revenue loss.
Following the emergency mobile response, the utility now has VersaFlo mobile demineralization trailers in place as a long-term treatment solution servicing three separate power generating locations. The solution includes InSight remote monitoring on every trailer, which enables the utility and SUEZ to monitor demineralization water quality in real time. This feature provides analytics to prevent any future undetected silica contaminations, allowing the utility to focus on its core competencies. The solution is anticipated to save $50,000 per year over the life of a five-year contract.
