Process hazard analysis for a fluidized bed incinerator
Gaudes, R.J.1, Bennett, S.M.2 and Sapienza, F.C.3, 1Camp Dresser & McKee, Ireland, 2Prince William County Service Authority, USA, 3CDM, USA
(free)
The Prince William County (Virginia, USA) Service Authority owns and operates the 68 megalitre-per-day H. L. Mooney Water Reclamation Facility (Mooney WRF). The primary means of biosolids disposal at the Mooney WRF is by incineration in a 38 dry tonne per day fluidised 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 on two occasions. 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 independent consultants were called in to render their opinions as to the cause of the incident and to propose changes that would prevent another such incident. Several theories were advanced, including: explosion of combustible gas and over-pressurization due to rapid expansion of water into superheated steam. Alternative methods of resolving the problem were considered, inclulding: 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-disciplinary team consisting through a rigorous analysis over a three-day period.
The study identified twenty-eight risks, eight of which were categorised as very high risks, with associated recommendations to reduce those risks. PWCSA has already implemented several of these recommendations.
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
KEY WORDS
Biosolids, Hazardous, Incineration, Process Hazard Analysis (PHA), Safety
