Hydrogen leak in a refinery

The leak occurred at an heat exchanger of the hydro-isomerisation unit of a refinery.
Following the observation of a pressure drop between two heat exchangers, controls were carried out on site. A noise was heard, indicating a gas leak, but its location could not be not identified. The gas detectors installed on the unit did not activate.

The access to the unit was forbidden and emergency services and the fire department alerted. The unit was decompressed using the safety flare. When the pressure was sufficiently reduced, two operators looked for the origin of the leak. They located it on a flange at the level of a heat exchanger/beam gasket. The line was depressurized under nitrogen. The flange was leaking to the a pressure drop accompanied by a rise in temperature between the 2 exchangers, causing fragility to the flange assemblies. The hydrogen leak is estimated at 100 kg.

When the bundles of the 2 exchangers were dismantled, it was then noticed that the baffles were mounted upside down on the 2 exchangers (baffles not mounted in the direction of fluid flow: baffles located on the outlet and not on the inlet of the load). Maintenance on these exchangers had been carried out during the previous major shutdown. The deflectors were deformed at the tube plate. In addition, an imprint of the outlet line of the heat exchanger on the deflector was noted. The deflectors are repositioned in the good direction and reintroduced in their respective heat exchanger.

The INITIATING CAUSE was the loss of sealing capacity of a gasket on a flange of the heat exchanger.

The baffles inside the two heat exchangers were wrongly mounted after a maintenance step during the last shutdown. This had probably an impact on the behaviour of the connecting pipe between the two heat exchangers and lead to a weak point on the gasket.

The ROOT CAUSE was wrong installation due to inadequate application of the maintenance procedures.

flange

Maintenance on these exchangers had been carried out during the previous major shutdown. It is probable that the baffles in the two heat exchangers had been re-mounted wrongly in that occasion.

DESCRIPTION OF THE UNIT
A typical isomerisation plant includes feedstock pretreatment, a fixed-bed reactor with a catalyst (like zeolite or chlorinated alumina), and product separation. The heat exchangers are part of the feedstock pretreatment. The most common exchangers are of the shell-and-tubes type, their baffles are mounted inside the shell, positioned around the bundle of tubes. Their role is to maximise the heat exchange by directing the flow of the shell-side fluid across the tube.

DESCRIPTION OF THE ISOMERISATION PROCESS
Hydro-isomerization is a chemical process that converts straight-chain hydrocarbons (n-alkanes) into branched-chain isomers using hydrogen and a catalyst. It is required or upgrading fossil and renewable feedstocks into the higher-quality fuels used for transportation. It improves its properties like cold flow properties, viscosity, and the cetane number. It involves the catalyst first dehydrogenating the n-alkane into an alkene, which is then protonated to form a branched carbenium ion, and finally hydrogenated back into a branched alkane (https://en.wikipedia.org/wiki/Isomerization).

There was some damage intenal to the heat exchangers, but the monetary cost of it is unknown.

Mishaps during preventive or corrective maintenance of industrial facilities, followed by accidents during start-ups and operations are a frequence occurrences. An intrinsic factor to them is the fact that the facility operators contract maintenance activities to other companies. Due to the complexity of industrial plants, there are often onsite several employees of different contractors, which could have different safety culture and practices. An effective flow of information on site safety rules and on technical procedures and know-how, including the historical performance of the facility components, requires a lot of attention. It demands as well careful supervision and the identification of well-designed checks on the soundness of the maintenance work.
It is not surprising that often things go wrong in these occasions. For this event, it would have been easy to assign the root cause to a human error. It speak well for the safety culture of the plant operator, the adopted measures aiming at improve the procedure describing how to do the work.

The tightness of gasket on the affected flange was re-assessed.
The plant operator established a control procedure for the reassembly of the exchanger.