Category : Incidents
Summary: The following article is a part of the National Board Technical Series. This article was originally published in the January 1987 National Board BULLETIN. (3 printed pages)
In the fall of 1986, the National Board received a request from a member jurisdiction for assistance in the investigation of the catastrophic failure of a jacketed steam-cooker vessel located in an animal rendering plant. The National Board was advised that although the failure was contained within the vessel, one life was lost. A member of the National Board staff was immediately dispatched to assist the Chief Inspector in this investigation. It was learned that prior to the accident, the vessel had been removed from steam service in an effort to find a suspected leak. The vessel was cooled down and pressurized with a reported 125 psi of air. Subsequently, one of the plant's maintenance personnel entered the inner vessel through the lower entryway and proceeded to inspect for leaks. One leak was apparently located near an upper dome as the maintenance person progressed to that portion of the vessel. While preparing this dome area for repair welding, a helper stationed outside of the vessel reported that a violent explosion occurred, which almost caused him to lose his balance on the catwalk. The helper ran for assistance in an attempt to rescue the person inside the inner vessel. However, the explosion was fatal because of massive damage to the person's chest when thrown against the upper dome region of the vessel.
Various information was obtained by both the jurisdiction and the National Board through interviews and outside inspections, which resulted in the following determinations:
- Jacketed cookers in the plant had been operating at approximately 90 psi steam.
- The vessels were not individually protected with safety valves.
- The vessel that failed was originally fabricated in 1958 and appeared to have several areas previously repaired by welding. Although this member jurisdiction required by law that all repair welding was to be carried out by holders of a National Board "R" Certificate of Authorization, there was no apparent evidence that this requirement had been met.
- At some point in time, the inner head-to-shell weld had been repaired by a lap-seam patch, which appeared to be the initial point of failure. Apparently no one in the plant knew by whom or when this repair had been made.
- The 125 psi air pressure reportedly applied to the jacket was apparently in excess of the maximum allowable pressure to be applied to this vessel, if the vessel was in a new and non-corroded condition.
- The vessel's inner shell had eroded from .75 in. wall thickness to approximately .25 in. wall thickness without a corresponding reduction of allowable working pressure.
After evaluating the circumstances and observations, the jurisdiction required the plant to be closed until the overall safety of the plant and its personnel could be assured.
In the meantime, preliminary calculations prepared by the National Board representative indicated that perhaps problems equally as great might exist with the other cooker vessels in the plant. The National Board responded immediately and suggested the jurisdiction require ultrasonic thickness readings be taken on the inner shells of the remaining cookers before placing these vessels back into service. The jurisdiction complied and subsequent examinations indicated that based on the actual thickness readings obtained, none of the vessels were acceptable for the 90 psi at which they were operating. Furthermore, three vessels did not calculate to a positive pressure. As a result, the National Board was later advised that all cooker vessels in the plant would be taken out of service and replaced with acceptable equipment.
The jurisdiction contacted another rendering plant within the state and requested that thickness measurements be taken. Based on the results of that examination, it was found necessary to remove seven jacketed vessels from service. The jurisdiction is now expanding this program in an attempt to determine the full depth of the problem.
The reaction resulting from the rendering process does cause the erosion of metal from the inner shell. In addition, the type of product being rendered has a definite effect on the rate of speed at which the metal is eroded. This wear must be tracked. Ultrasonic measurements may be taken or holes may be drilled to obtain actual thickness to assist in determining new allowable working pressures and the rate of erosion. With the knowledge of probable erosion rates, a significant inspection program can be structured. However, if inspectors provide inadequate or visual external-type inspection only, the rate of erosion patterns cannot be set. When erosion rates are not determined, the safety of the vessel is undetermined and plant personnel may be in serious jeopardy.
Jurisdictional inspectors and insurance inspectors must realize the critical need for a thorough inspection of this type of equipment. Inspectors are charged with carrying out responsibilities and duties to the best of their individual abilities, regardless of whether or not the surroundings are comfortable. Accidents don't pick the best locations or circumstances in which to occur; they generally happen where two or more problems exist. Don't let inadequate inspection be one of those problems.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.