Each boiler design presents its own challenges to ensure reliable operation, and some are more sensitive than others. A failure occurred in the finned, carbon steel tubing in the Waste Heat Boiler Coil at a large North American oil refinery. After witnessing several visible steam leaks in the hot bank, the coil was shut down. Testing identified 29 leaking tubes. Subsequent non-destructive testing indicated up to 58% wall loss in the cold bank tubes. This paper reviews the conditions leading up to those failures in the hot bank and identifies the primary causes and contributing factors, including coil design aspects, operational practices and others. Remedial measures taken to circumvent design issues and best practices implemented will be discussed that can benefit boiler operators in preventing costly downtime and safety hazards from boiler tube failures.
In late 2021, several leaks were observed inside the waste heat boiler coil of the steam reformer furnace at the refinery. The leaks were located in the first row of tubes of the hot bank in the vertically-oriented coil, where boiler water inside the tubes is heated via waste heat of the reformer stack. The waste heat boiler coil has a design duty of 69.59 MMBtu/hr (∼20.4 MW), with design pressures and temperatures of 1010 psig (6.9 MPa) and 700°F (371°C), respectively. Typically, the unit operates at around 907 psig (6.3 MPa) and 535°F (279°C). The nominal dimensions of the coil tubes were specified as 1.9 in (4.8 cm) O.D., with a 0.145 in (0.37 cm) wall thickness. The material of construction specified for the finned tubes was ASTM A106, Grade B plain carbon steel, seamless. The unit was installed in 2000, which indicates a service life of 21 years at the time that the failure occurred.
Waste heat boilers improve plant economics by using reclaimed waste heat to produce steam while cooling a process fluid or gas stream. There are many types, and, depending on desired steam and process outputs, these units can operate across wide pressure and temperature ranges. A detailed breakdown of waste heat boiler types, and some common boiler waterside corrosion issues associated with each, are presented in the paper by Hargrave1.
