fire on a flange of a heat exchanger

During normal operation, process gas leaked from the main flange of the heat exchanger of a distillation unit. This unit was part of the light oil desulphurisation facility of a refinery.
In the heat exchanger, a feed stream consisting of hydrogen and naphtha was flowing in the tubes, while in the shell a mixture of processed gas was flowing (hydrogen, light oil). The flange released process gas, which ignited in contact with the hot flange surface, resulting in a fire.

The fire was detected during personnel inspection. The facility was stopped and the fire extinguished by the plant fire department. No injury and negligible property damage.

The INITATING CAUSE was a loss of torque of the bolts of the flange, caused by repeated and out-of-design cycles of surface cooling and heating.

After a periodic repair of the constant pressure distillation unit, the startup team temporarily removed the rain shield from the heat exchanger flange joint, to be able to perform the hot bolting procedures, following by a leak detection. The shield was basically a curved metallic sheet covering the upper part of the flange.
The startup team forgot then to reinstall the rain shield. Since this action was not part of the checklist, it remained undetected, and the maintenance team was unaware of its absence.
From approximately 2 months, heavy rainfall interacted directly with the flange lateral surface and bolts. The resulting cyclic cooling and heating caused a relaxation of the bolts force and the reduction of the pressure on the gasket surfaces, which allowed a leak.
The auto ignition temperature of the pipe side (hydrogen, naphtha) was 200–316 °C and on the shell side (hydrogen, light oil) was 250–316 °C. It is plausible to assume that the ignition source was delivered by the hot surface of the flange, definitively above 300 °C.
The ROOT CAUSE was a deficit of safety design, which did not recognise the critical role of the weather protection in guaranteeing normal operation experience of the flange.

bolts,
flange,
heat exchanger

The distillation unit was part of the light oil desulphurisation facility of a refinery.
In its heat exchanger, the feed stream consisting of hydrogen and naphtha was flowing in the tubes, while in the shell a mixture of processed gas was flowing (hydrogen, light oil).
On the tubes side, the normal operating parameters were 3.11 MPa and 524 °C.
On the shell side, they were 2.41 MPa and 385 °C.

DETAILED CHRONOLOGY OF THE EVENT
22:50 (20 Aug) While inspecting the plant, an operator discovered a fire at the heat exchanger flange joint.
22:50 – 23:05 The operator immediately notified the section chief and the direct supervisor.
23:05 The operating department placed an emergency services call.
23:08 The operating department confirmed that the fire had been extinguished.
23:30 Public fire brigade entered the site.
00:00 (21 Aug) The operating department completed a shutdown of light oil supply to the light oil desulphurisation unit (stop to the feed of light oil).
02:41 Gas detection was performed on the heat exchanger flange joint; no leak was detected.
03:00 Public fire brigade withdrew from the site.
03:20 The operating department performed a re-torque (increase tightening) of the flange joint.
04:05 Gas detection after the re torque confirmed no leak.

1. Elements protecting from weather, such as weather seal, covers or rain shields contributed to the safe operation of the components, and therefore should be considered as part of the safety design and operative strategy. Their correct installation shall be ensured by dedicated inspections and checklists.
2. Particular attention is required when such protections are installed on high temperature, high pressure flange joints such as those of heat exchangers, and reactors. If the weather protection is not effective, the action of weather elements can influence the lifetime of components, as well as their confinement capacity, with additional fire- and explosion-related hazards. An effective protection from weather effects shall be part of the risk assessment of the facility.
3. More in general, little design attention has been given to weather protections, aiming at maximising their beneficial role to lifetime and risk prevention. There is a lack of unified understanding of the required function, appropriate structure, and inspection/management methods. The industry should re examine terminology and classifications, to achieve common vocabulary and to support common design.

As immediate measures, the bolts were re-torqued with an increased tightening force and the rain shield re-installed.
Inspection of the gasket for possible damage was planned for the next periodic repair (two years ahead), when the heat exchanger is planned for dismantling.
Before that time, it was decided to execute a daily gas leak detection.

Operating team was informed on the importance of the rain shield and a check on its re-installation became an item of the periodic repair checklist.
A rain shield inspection became part of the overall inspection executed before and after completion of the facility start- inspections (once per half year).
All the 38 other heat exchangers of the plant were inspected to ensure that they had rain shields installed and their flanges checked for possible leaks.