A motorised valve used at a refinery exploded at around 3:00 p.m., killing two subcontractors. The refinery was evacuated and production was halted. The refinery operator issued a press release.

The two subcontractors were injecting resin into a sealing clamp on a leaky flange when the attachment rods suddenly ruptured, releasing water from the boiler (70 bar, 290 °C) and causing a steam explosion.

Metallurgical analysis showed that the rods had failed due to caustic corrosion and that they had ruptured at the interface of the nut/flange area (where mechanical stresses are highest).

Examination of the root causes of the accident showed that:

•  The valve displayed a design or manufacturing flaw. As the diameter of the seal throat inside the cuff was too small, the flange was misaligned. Additional mechanical stresses had to be applied to the rods to correct this misalignment.
•  Since 2009, variations in the quality of the boiler water had caused leaks and required the fitting of sealing clamps.
•  A clamp was fitted on the valve in 2011, but it exploded and was out of commission for six weeks in 2012. During these six weeks, it was kept under pressure but at low temperature. The boiler water then came into contact with the flange/sealing clamp connection and formed a corrosive environment.
•  The additional mechanical stresses created during the reinjection work were sufficient to cause the corroded rods to rupture.
•  The sealing clamp was not designed to absorb the additional axial force of the rods when they ruptured (strongback system).

After the accident, the operator inspected the other sealing clamps on the site and found nothing unusual (absence of general corrosion or stress corrosion). However, the clamps had not been subjected to low temperatures.

The operator drew a number of lessons:

•  Visual inspection does not provide sufficient certainty about the condition of the rods.
•  Stress corrosion (mechanical stresses + corrosive environments) can occur very quickly.
•  The condition of the bolts must be assessed whenever resin is reinjected into a sealing clamp.
•  The rods of the flanges must be thoroughly examined if white deposits are found on them.
•  A list and log of operations performed on sealing clamps should be drawn up.

Lastly, sealing clamps should be viewed as temporary, not permanent, repairs.