Fire In the hydrodesulphurization unit of a refinery

The event occurred at the charge heater (also called a 'fired heater') of a vertical cylindrical hydrodesulphurisation reactor.
The heater lost hydrogen flow briefly while the unit was being changed over from recycle to once through hydrogen flow. This operation, performed at start up, aimed at replacing the recycled hydrogen-oil mixture usually used with pure hydrogen.
As the hydrogen flow was such a large percentage by volume of the total flow to the exchangers and furnace, the main part of the mass flow at the furnace outlet was lost while the exchangers and heater tubes filled with oil.
During this period the outlet tube in one pass overheated and ruptured. The operating procedures did not warn that the furnace fires should be reduced or stopped when making such a changeover.
Skin thermocouples were added to the heater outlet tubes at a later date, to monitor local temperature and spot in time temperature run away.

The INITIATING CAUSE was the overheating of a tube of the heat charger, which then failed due to creep.
The ROOT CAUSE is related to shortcomings in (1) the operating procedure, which did not prescribe a that the furnace fires had be reduced or stopped when making such a changeover in fluid, and (2) and of the monitoring system, which was not measuring the temperature of individual tubes.

pipe, heat charger, heat exchanger

The event happened at stat up, during one-off procedural step condisting in a flow of pure hydrogen in palce of a mixture of oil and hydrogen.

DESCRIPTION OF THE UNIT
The 'heat charger ' in this incident of 1970 is also called in more recent plant the 'fired heater.
In a preparatory step before the fixed bed where the desuphurisation reaction takes place, "...the liquid feed is pumped up to the required elevated pressure and is joined by a stream of hydrogen-rich recycle gas. The resulting liquid-gas mixture is preheated by flowing through a heat exchanger. The preheated feed then flows through a fired heater where the feed mixture is totally vaporized and heated to the required elevated temperature before entering the reactor and flowing through a fixed-bed of catalyst where the hydrodesulfurization reaction takes place."


[From the Wikipedia term ‘Hydrodesulfurization’ https://en.wikipedia.org/wiki/Hydrodesulfurization]

Only damage to equipment.

This is a case of a material failure in a technical system, due to shortcomings in operating procedure and lack of a monitoring system, able to inform the operator with enough details on deviations from the expected local temperatures.
(1) Detailed and effective operating instructions should cover also operations which are performed only one off, or at the start/stop of the operative cycle.
(2) Tube skin thermocouples are often used in these systems, because they are the most direct measure of the risk of creep failure on furnace tubes. It is also good practice to provide redundancy, with multiple installation on each tube/pass because skin couples have a tendency to detached from the tubes.

It is unknown if the lesson learnt shortcomings identified by this accident and its related lesson learnt were transformed into improvements of procedures and monitoring systems.
Fires due to overheating and failure of tubes in a fired heater (' heat charger') occur regularly (see HIAD event 656 and for more recent cases events 1168, 1169 and 1178)