Press Release

Press Release October Steam Explosion Caused by Temporary Pressure Spike

Investigation of Incident at Kneeland Street Station Concludes
For immediate release:
  • Department of Fire Services

Media Contact for October Steam Explosion Caused by Temporary Pressure Spike

Jake Wark, Public Information Officer

DFS logo with text reading "Boiler & Pressure Vessel Safety Program"

STOWThe explosion at a Boston steam generating facility last year occurred when a steam pipe fitting failed due to a temporary pressure condition known as a water hammer event, and not as a result of corrosion, degradation, or other system-wide issues that make the facility unsafe, Massachusetts State Fire Marshal Peter J. Ostroskey announced.

The Department of Fire Services investigated the cause of the Oct. 15, 2021, explosion at Vicinity Energy’s Kneeland Street Station pursuant to its jurisdiction over boilers and pressure vessels at power and steam plants such as the one at 165 Kneeland St. The investigation was led by the chief of engineering inspections and district engineering inspectors assigned to the DFS Boiler & Pressure Vessel Safety Program. They were assisted by members of the State Police Fire & Explosion Investigation Unit assigned to the State Fire Marshal’s office and they engaged the metallurgical analysis services of an independent testing, engineering, and consulting firm.

Inspectors found no evidence of criminal conduct related to the incident. Vicinity Energy cooperated fully with inspectors and complied with all orders issued during the investigation. A review found that the site and its personnel were in compliance with Massachusetts laws regarding licensure and certification requirements.

Three people were injured in the 9:15 am explosion, which ruptured a cast iron tee fitting on a six-inch steam pipe that was part of an auxiliary steam piping system for the boilers at the site. The explosion also released asbestos into the facility, requiring a remediation plan to be developed and implemented over the course of several days. In the interim, DFS issued an order temporarily suspending the certificates of inspection for all four boilers on site, halting their use until it could be determined that they were safe to operate. Based on the progress of the investigation, this order was amended on Nov. 5 to allow three of those boilers to resume operations under strict controls and to allow repairs to the auxiliary steam piping system with ongoing documentation of that process. 

Interviews, witness statements, Vicinity records, photographs, observations at the scene, and metallurgical testing and analysis all support the finding that the explosion was accidental and the result of an isolated event rather than a symptom of corroded, degraded, or otherwise compromised components throughout the system. Specifically, steam within the pressurized pipe became trapped by lower-temperature condensate, causing the steam to rapidly condense into water. This created a sudden void within the pipe that surrounding condensate rushed in to fill, creating a rapid rise in pressure that a tee fitting on the pipe was unable to withstand. Certain valves or drains not being properly aligned during the startup or shut down procedure, and/or the absence of an active steam trap maintenance program that would ensure removal of condensate from the steam lines, may have contributed to the cause of the event.

DFS issued an order on Jan. 7 reinstating Vicinity’s certificates of inspection and rescinding all previous restrictions on the plant’s operation. As part of that reinstatement, and to prevent a recurrence of the Oct. 15 incident or a similar event, DFS ordered Vicinity to establish and maintain a steam trap maintenance program, review various operational and personnel procedures at 165 Kneeland St., and demonstrate its compliance by April 1, 2022. If these requirements are not met, the boilers’ certificates of inspection could be suspended again.

The DFS Boiler & Pressure Vessel Safety Program has jurisdiction over non-residential boilers and pressure vessels such as those at power plants, hospitals, nursing homes, and schools. It issues trade licenses to and administers examinations for regulated industry professionals such as power and steam plant operators, boiler and pressure vessel inspectors, and oil burner technicians. To learn more about its work, visit


Media Contact for October Steam Explosion Caused by Temporary Pressure Spike

Department of Fire Services 

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