fire on a flange of a hydro-desuphurisation unit

The event affected on a hydrodesulphurisation unit of a refinery.

Hydrogen-rich gas was accidentally released from a flange. during maintenance works. Workers were busy in removing a partition plate installed on the flange. Approximately 350 Nm³ leaked and ignited.

The flange was located on a pipe supplying the gas generated in the desulphurisation section of a catalytic reformer to other hydrogenation desulphurisation units.
The flange was installed downstream of the flange at the outlet of the reformer. This flange was closed but leaked. On top of that, the reformer was in function, and was supplying the pipe with 2.3 MPa of process gases to the valve.

To fight the emergency, the piping system was isolated by closing the main valves on the downstream sides and nitrogen was injected near each main valve. Cooling water was sprayed around the incident site.

The INITIATING CAUSE was a leaking flange, which allowed high-pressure hydrogen (2.3 MPa) to reach the partition plate mount on the downstream valve, which workers were dismantling to remove the partition plate.

The ROOT CAUSE was a combination of factors:
(1) The supervisors of the works assumed erroneously that the whole section of the pipe had been depressurised and purged.
(2) The assumption was based on previous experiences, when the pipe had been always purged, and on the trust which supervisors had in their mutual knowledge.
(3) A lack of a safety check of the instruction procedure, which did not require the measurement of the pipe conditions before starting the works.
(4) The use of the wrong gasket, which could not bear the operative pressure and failed when the flange was opened.

No injury, The actuator (the drive unit) of an electric valve burnt out.

To fight the emergency, the piping system was isolated by closing the main valves on the downstream sides and nitrogen was injected near each main valve. Cooling water was sprayed around the incident site.

The event, with limited consequence, is a good example of how personnel assumptions and lack of accurate instructions can create a hazardous situation.
The ongoing maintenance operations were regulated by an instruction manual. However, the manufacturing staff member who instructed the crew responsible for the execution of the operations mistakenly believed that the pipe in question was not in use. The reason for this mistake was that during previous operations that section of the pipe had been always purged, and when the plate to be removed had been installed, few days before, he had witnessed that the pipe was properly purged. He did not aware that that pipe had been used again because the reformer was in back in function.
The shift supervisor was aware that the reformer was in operation and that there was pressure due to hydrogen on the upstream side of the partition plate. However, the supervisor assumed that the manufacturer staff member had checked the situation.

The fact that the instructions manual did not include any check of the condition of the pipe (pressure, temperature, etc.) did not help. Therefore, the overall operation safety system was not able to prevent misunderstandings by the workers in charge.

An accurate information sharing and a thorough confirmation of plant status are essential for safe operations and should take place before the execution of any repair and maintenance work. In addition, the procedures should foresee a dual person verification checklist.

The instructions manual was modified by requiring:
(1) The record of fluid type, pressure, and temperature on both sides of the partition plate before any insertion or removal.
(2) A senior supervisor to review all pre check items before work starts.
(3) Leakage checks on all valves.
(4) A safety review meeting with relevant departments before proceeding and reflect the outcome in the procedure.

Moreover, the type of gaskets to be used was reviewed, aiming at avoiding low pressure sheet gasket.

Finally, management and plant personnel were informed of this case and training as organised to ensure its full understanding.