Overview
During start-up operation of a high-temperature, high-pressure plant using hydrogen, hydrogen gas leaked from the flange of a heat exchanger and a fire occurred. The leakage occurred for two reasons:
- Insufficient tightening torque control was carried out during hot-bolting and an unbalanced force was generated across the bolts.
- A temperature rise was induced across the heat exchanger as a result of a revamping activity, during a turnaround shutdown.
Background
Hot-bolting: In equipment and piping that operate at high temperatures, as the temperatures rise, the tightening force decreases, thus re-tightening of bolts is necessary. This work is called hot-bolting. The design conditions of the evaporator where the fire occurred were 2.4 MPaG, and 500 °C at the shell side, and 2.8 MpaG, 300 °C at the tube side, and it required hot-bolting.
Sequence of Events
- A turnaround shutdown of the benzene manufacturing plant was carried out in June.
- A gas-tightness test with real gas (hydrogen and another gas) was completed in September, and normal plant operation was initiated.
- During warm-up operation of the benzene manufacturing plant, which uses hydrogen at high temperatures and pressures; hydrogen gas leaked through a heat exchanger flange, and a fire occurred. At the plant, benzene was produced by a dealkylation reaction using toluene and C9 aromatic hydrocarbon as raw material.
Causes
- There was unbalanced tightening torque among bolts of the flange.
- The shell-side inlet temperature rose from the previous 400 °C to 420 °C as a result of an energy-saving revamping.
- A decrease in tightening torque was locally generated by a difference in thermal expansion between bolts made of stainless steel and the flange made of carbon steel. Thermal expansion of stainless steel is greater than that of carbon steel.
- The ignition source was considered to be static electricity generated at leakage.
- Quantitative control of tightening torque of the bolt was insufficient, and there was a local shortage of initial tightening torque.
- It is speculated that there might be an insufficient work standard for hot-bolting or imperfect execution of hot-bolting.
- The temperature rose as a result of an energy-saving revamping, and the effects might have been overlooked.
Lessons Learned The site initiated the establishment of a field patrol and strengthened their onsite monitoring of mechanical equipment during a plant-wide temperature rise.
- Quantitative control of tightening torque on bolts should be carefully executed at all high-temperature and high-pressure plants.
- A plant must pay close attention to both the chemical impacts and the mechanical impacts of an unexpected temperature rise during start-up operations.
Several factors are important for flange use in applications of this type. Bolting patterns/torque order need to be considered in engineering, training, stewardship, and maintenance aspects of the specific operations. Cold bolt torque requirements should be developed so hot bolting is not required or relied upon for start-up. Flange stress from an increased or decreased operating temperature needs to always be checked. Substantial industry knowledge and practice exist for flange use to ensure that such operations are conducted safely.