The Prince William County Service Authority (PWCSA) owns and operates the 18-million-gallon-per-day (mgd) H. L. Mooney Water Reclamation Facility (Mooney WRF). The primary means of biosolids disposal at the Mooney WRF is by incineration in a 42 dry ton per day fluidized bed incinerator
(FBI). The FBI had been operating for more than ten years without any safety-related problems. However, in February 2007 an explosion occurred in the ductwork downstream of the FBI, resulting in extensive damage to portions of the refractory-lined steel ductwork, supports, air pollution control
system and controls. Fortunately, there were no injuries to personnel on duty. At the time of the incident, the incinerator was being warmed in preparation for introducing biosolids. Upon introduction of biosolids, abnormal pressures developed in the freeboard section of the FBI and the
control system automatically shut the system down twice. The incident occurred shortly after the third attempt at restarting the FBI. Mooney WRF operations and maintenance and engineering personnel immediately secured the FBI and began investigating the cause of the damage. The FBI vendor
and two independent incinerator consultants were called in to render their opinions as to the cause of the incident and to propose hardware, controls, or procedural changes that would prevent further incidents. Several theories were advanced, including: explosion of combustible gas and over-pressurization
due to rapid expansion of water into superheated steam. However, no theory was conclusively proven. Alternative methods of resolving the problem were considered, namely: installation of explosion relief vents, changing of start-up procedures, and automatic operation of the incinerator biosolids
feed valves. Due to the numerous possible explanations for the incident, PWSCA commissioned an industrial safety specialty firm to conduct a Process Hazard Analysis (PHA) review of the FBI and its subsystems. An industrial safety specialist led a multi-disciplined team consisting of Mooney
WRF operations, maintenance and engineering staff and its incineration consultants through a rigorous 2-day analysis period. This paper will focus on the procedures followed in the PHA study, to include: Identification of hazard scenarios,
based on credible causes Potential consequences of hazard scenarios Safeguards for hazard scenarios Ranking of risks to account for severity and likelihood of the hazard scenarios
and to determine whether further risk reduction measures are warranted The study identified thirty-one risks, eight of which were categorized as very high risks, with associated recommendations to reduce those risks. PWCSA ultimately implemented several of
these recommendations and there have not been any further incidents.
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