Leakage from a flange of a hydrogenation unit

The event occurred at the flange of a heat exchanger of a lubricating oil hydrogenation refining unit.
It started with abnormal noise the bearing of the raw material feed pump of this unit. To replace the pump bearing, a shutdown of the hydrogenation unit was started: the raw oil feed was stopped, the main burner of the furnace was extinguished, and the plant was cooled while hydrogen was circulated inside the unit.
Approximately one hour into the shutdown process, a small leak of hydrogen, hydrogen sulphide, and lubricating oil was observed from the flange area of the reactor outlet heat exchanger.

The leak point was later identified on three of the 24 bolts, based on the trace left by the gas on the flange.

The INITIATING CAUSE was a loss of tightness of some of the bolts of the flange, due to uneven and inadequate torque during cooling down.

No abnormal strain was detected and no damage of the gasket observed. No cracks, corrosion, galling, or other damaging defects were found.
No leak had occurred on the accident equipment during earlier shutdowns. Moreover, the cooling was performed at 30 °C/h, slower than the standard limit of 50 °C/h. However, the oil circulation had been stopped when starting the shutdown, and this resulted in a larger temperature gradient compared to previous start-up and shutdown experiences.

The root cause was found in fact that the flange had been tightened manually. The tightening procedure of the leaking flange included hot bolting, as all equipment operating above 200 °C. However, the flange belonged to a class of components for which quantitatively tightening management was not required.
This caused uneven tightening forces between individual bolts. When the tightening force decreased during rapid cooling, the pressure on the gasket faces could not be maintained, causing a loss of confinement.

bolts,
flange,
heat exchange,
oil-lube hydrogenation

The event occurred i nthe middle of a shutdown due to feed pump malfunctioning. The flange was operating at 309°C before shutdown, and started leaking at 220°C.

In this heat exchanger, the reactor products (lubricating oil, hydrogen, hydrogen sulphide) were exchanging heat with the hydrogen coming from the hydrogen recycling unit.

Negligible material damage

The depressurisation of the unit was immediately carried out, and the fire department was notified.

The KHK lessons learnt were:
1. Bolt tightening on high temperature/high pressure flanges is a critical safe element. Inadequate tightening can cause an immediate leakage, and if the internal fluid leaking is flammable or toxic, the consequence can be fire, explosion and/or poisoning.
2.Hand tightening will always produce an uneven clamping force among bolts. The tightening method must be selected according to the equipment’s condition (high temperature, high pressure, large diameter, difficult access, etc.) and a quantitative tightening control should be applied.
3. Temperature gradients must be considered both during heating (hot bolting) and cooling. These gradients affect gasket elastic plastic behaviour and relaxation, which in turn influences the required clamping force.
4. High temperature flange bolts require special attention, on bolt hole cleanliness, seizing, bolt deformation and creep; appropriate replacement must be performed when necessary.
5. Historical accident cases show that even when quantitative tightening was performed, insufficient actual clamping force still caused leaks. Verifying that the tightening is truly adequate is the final line of defence.
6. Regular inspection of bolt tension, especially on equipment that is difficult to access, is essential to prevent recurrence.