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Case Ditails

Scenario
Case Name Rupture of a flush liquid cooler for a mechanical seal after repairing a high-temperature pump
Pictograph
Date November 11, 1997
Place Sakai, Osaka, Japan
Location Refinery
Overview A mechanical seal flushing cooler in a pump ruptured at a high temperature. Usually, steam is used as coolant. Condensed water of steam generated during repairs evaporated again, the pressure increased because the valve was shut, and a rupture occurred. Any of the valves should have been opened because steam generation could be expected. The instruction for shutting valve was not clear.
Incident A rupture occurred at the bottom of a vacuum distillation column at a vacuum distillation unit of a refinery. The drain water accumulated in a flushing oil cooler tank was heated, evaporated, and generated steam while the bottom pump of the column started after repairs. The steam pressure increased, and the cooler tank ruptured.
Flushing oil cooler: At this unit, coolant oil from outside the plant is called flushing oil, which is supplied to prevent mechanical seals from burning. In the usual operation, vacuum gas oil, which is a distillate of this column, is used as flushing oil. Bottom oil is used only for starting. The cooler for cooling bottom oil is cold a flushing oil cooler.
Processing Manufacture
Individual Process Distillation
Substance Water, Fig2
Fuel oil
Type of Accident Burst
Sequence 14:00 on November 11th, 1997; as oil had leaked from a shaft seal part of the bottom pump of the column, operation was switched to a spare pump. At this time, steam continued to be supplied to prevent bottom oil in the flushing oil cooler from solidifying.
About 20:10; after repair work, the main pump and the spare pump were both operated in parallel, gradually switching to only main pump operation; then the situation had almost returned to usual one. While the pump stopped, steam for preventing solidification of bottom oil in the cooler tube was condensed and remained in the flushing oil cooler tank. The more the temperature rose, the more steam was generated from drain water evaporation in the cooler tank, and the amount of discharge increased as well. The shift leader directed the valve to be shut intending to shut the inlet side block valve. However, an operator closed the outlet valve about one-quarter-turn.
About 20:15; drain water in the oil tank was heated and became steam, which caused a pressure increase in the tank. The welded parts of the tank were damaged. The tank, high-temperature oil, and piping, etc. were scattered.
Cause As steam was supplied to prevent heavy fuel oil from solidifying during repairs, drain water that had accumulated in the tank was heated and became steam. The temperature of the inlet oil was 325 °C. As there was a possibility that the pressure of the generated steam would reach more than 0.95 MPaG, which is the inlet pressure of usually received steam, when they shut the outlet valve, the pressure in the tank exceeded the tank strength and it ruptured. The design pressure of the tank was 1.1 MPaG. As steam was supplied for cooling flushing oil, the steam pressure was not supposed to be more than 0.95 MPaG. However, as the inlet valve was slightly open, the pressure in the tank increased. The temperature of the flushing oil inlet was high, which was 325 °C, so steam might be generated rapidly.
The instruction to shut the block valve was not clear, which caused a mistake in operation; shutting the outlet valve in stead of the inlet valve.
Response An emergency shutdown of the unit and pressure releasing. The company's fire fighting team conducted water spraying, etc. The public fire fighting brigade turned out.
Countermeasures If internal pressure is above the atmospheric pressure, a safety device such as a safety valve should be installed. There should be preparations for an emergency shutdown in case there is an abnormal temperature or pressure in the tank. Preventive measures for operation errors should be taken. Thorough explanations of irregular operations and directions on valve operation should be made.
Knowledge Comment Unclear instructions cause an accident. Operators themselves should think about the situation.
Background The instruction for valve operation was not clear and the operator made a mistake. Both of them were human error. When the cooler tank model was changed, there was insufficient investigation on its safety. Besides, the drain valve was left closed and there was no way out for the drain water. That is the reason why steam was generated. There is a question why the accumulation of drain water was not checked before supplying high-temperature oil. When high-temperature oil was supplied, the steam would naturally be generated. The basic problem may be lack of previous preparation rather than the operation error of the valve.
Incidental Discussion The information in "Event" is based on a report. The plant is not shut down by stopping only one pump with a spare. It is questionable why bottom oil was used as flushing oil, in stead of usual vacuum gas oil. It is not known because nothing is written in any of the reports we reviewed. It is difficult to understand the accident.
In normal use of the tank, the pressure of received steam does not increase. Therefore, if there is a guarantee that the pressure of received steam will not increase, a pressure relief valve does not have to be installed. However, because of an operation error, extra costs are needed.
The information in "Countermeasures" is an abstract in a report. The tank where the accident occurred might be maintained as if it was steam piping in usual operation. The basic potential hazards need to be properly understood by managers and engineers, and must be communicated to the operators.
Reason for Adding to DB Example of rupture caused due to a mistake of directions
Scenario
Primary Scenario Carelessness, Insufficient Precaution, Carelessness of Operator, Poor Value Perception, Poor Safety Awareness, Inadequate Risk Recognition, Organizational Problems, Inflexible Management Structure, Insufficient Education/Training, Regular Operation, Erroneous Operation, No Operation, Bad Event, Chemical Phenomenon, Vaporization, Failure, Large-Scale Damage, Rapture, Bodily Harm, Injury, 3 person injured
Sources Japan Industrial Safety and Health Association. Information Center for Safety and Health, Labor accident No.100216, "High-temperature oil spattered when oil tank ruptured due to unusual pressure increase, Japan Industrial Safety and Health Association home page
Fire and Disaster Management Agency. Major oil leakage in a vacuum distillation unit, Accident cases of dangerous materials. 1997, pp.526-527
Japan Industrial Safety and Health Association, details of when the accident occurred.
Number of Injuries 3
Physical Damage Damage to a flushing oil cooler and surrounding non-hazardous piping.
Financial Cost Less than ¥ 10,000 (Fire and Disaster Management Agency)
Multimedia Files Fig2.Chemical formula
Field Chemicals and Plants
Author WADA, Yuji (National Institute of Advanced Industrial Science and Technology)
TAMURA, Masamitsu (Center for Risk Management and Safety Sciences, Yokohama National University)
Scenario