The decision to open a silo valve, made based on conflicting computer information, started a chain of events that ultimately caused an explosion in the cascade building at the Deely Unit 1 plant, according to an analysis completed Friday.
The plant is scheduled to return to service tomorrow, Oct. 12.
Seven recommended changes to policy, procedures, training and design were recommended in the report; the most critical changes have already been made in advance of the unit returning to operation on Saturday, said David Herbst, Senior Vice President of Power Generation.
The decision to open the valve was made by two veteran CPS Energy plant operators, said Herbst, and was supported by the information they had at the time and their past experiences.
“There were also several contributing factors that will be remedied by the changes recommended in the report,” Herbst said. “That’s why a thorough investigation and assessment is so important after an incident like this.”
Within 24 hours after the explosion, a team of veteran engineers was tasked to investigate, even as employees at the plant assessed damage and made repairs quickly to return the plant to service.
“It’s critical that we understand exactly what happened, so we can then determine what policies and procedures, if any, need to be updated,” Herbst said. “This is an incredibly dangerous business. Our employees make life and death decisions every day, and it’s imperative that they do so under the safest conditions possible.”
In this case, a pair of sensors on either side of a valve that allows coal to drop from a silo into a coal mill below, where it is dried and pulverized, alerted the operator that there was no coal flowing through the silo. Those alerts triggered an automatic closing of the valve.
Another sensor indicated that the silo was half full. After consulting with a supervisor, the operator reopened the valve.
The silo in fact was empty, and with no coal in the mill, the temperature shot up, starting a fire. It was quickly detected, and the fire suppression system was deployed as it should be. Fires within a coal mill do occur, although infrequently, and typically cause no significant damage.
Because the mill was empty and hot, the water created a powerful burst of steam. Flames and gasses traveled rapidly up into the silo through the open valve, where it ignited coal dust within the silo. The fire and accompanying pressure wave continued to travel up into the cascade housing, disturbing the coal dust there and igniting it. Coal dust is incredibly combustable, like gunpowder.
Because the cascade housing is essentially sealed, leaving the fire and gasses nowhere to go, it exploded, damaging the housing and slightly injuring two employees from falling debris.
The plant was powered down and the fires extinguished. A third employee, who assisted in the firefighting, reported a sore throat from smoke inhalation two days later.
The faulty sensors showing the silo to be half full of coal were determined to be a contributing factor to the explosion.
The sensors had been listed for repair, but because they weren’t recognized as a potential hazard in and of themselves, the work had not been prioritized.
The report also recommends changes to coal dust cleaning procedures.
“This is the kind of analysis necessary to make changes in the wake of an incident like this,” said Herbst, “and it’s something we do any time we have an incident.”
The CPS Energy team worked with outside consultants who have investigated similar incidences, as well as an engineer from another utility, “who brought an outside perspective to the team” he said.
Herbst has worked with staff to update training, plant and coal yard procedures and beef up communications between departments. Additionally, the prioritization process is being scrutinized, as was the design of the cascade housing.
CPS Energy is also sharing its findings with other utilities. An engineer with an out-of-state utility recently contacted CPS Energy, looking to learn how he might proactively implement the report’s recommendations.
None of the potential changes and upgrades will be impacted by the recent rate request reduction, Herbst emphasized.
“Safety is non-negotiable,” he said.