Fire in an ammonia production plant

The accident took place in a tank-type pressurised separator on the synthesis gas line of an ammonia production plant. The pressurised separator operates at nominal values of 25 bar and 5°C. It contains a mixture of hydrogen and nitrogen (75%/25%). It is located slightly upstream of the synthesis gas compressor (on the low pressure side) that was shut down. The connector connecting the separator to its level measuring instrument is located outside, near the building containing the compressors. The connector is thermally insulated (gas inside at 5°C) over its entire length.

The leak took place at the point of connection between the separator and its level measuring instrument.
1. First a compressor on the synthesis gas circuit was shut down following a malfunction.
2. When the compressor stopped, there was a slight increase in pressure (25 -> 29 bar) upstream (at the low pressure end). Normal pressure is roughly 25 bar, and the system is protected by a 30 bar pressure relief valve.
3. The pressure of 29 bar, although not too high for the installation, caused the connecting pipe to break because of heavy wear resulting from undetected corrosion under the insulation.
4. A leak of hydrogen (25 bar) occurred and caught fire. A flame jet approximately 10 metres high then damaged various surrounding installations (domino effect). Equipment containing oil that became engulfed by the flame jet also caught fire.

The flame jet was extinguished by closing a valve located upstream of the hydrogen leak. Once the valve had been closed, the flame jet gradually became smaller and smaller until the hydrogen from the installations between the valve and the leak had burnt off completely.
The company’s emergency response team fought the (oil) fire and cooled the equipment near the flame jet so as to minimise damage.

Although no one was hurt in the accident, the damage caused (solely on the company’s premises) was significant, and the plant had to be shut down for several months for repairs. Approximately 700 kg of hydrogen was estimated released.

INITIATING cause was failure of a pipe below the nominal operative values due to undetected corrosion.
ROOT CAUSE was a lack of an effective procedure for preventive inspection of damage.

pressurised separator, synthesis gas line, ammonia reactor

First a compressor on the synthesis gas circuit was shut down following a malfunction. When the compressor stopped, there was a slight increase in pressure (25 -> 29 bar) upstream (at the low pressure end).
Normal pressure is roughly 25 bar, and the system is protected by a 30 bar pressure relief valve.

Although no one was hurt in the accident, the damage caused (solely on the company’s premises) was significant, and the plant had to be shut down for several months for repairs. We estimate that roughly 700 kg of hydrogen was released during the leak.

The accident happened due corrosion which could not be detected because not visible. Probably, the regular inspection consisted only in a visual control of the state of integrity of the pipes. The corrosion which led to the leak was not visible because under layer(s) of thermal insulation. This is a recurring case in high temperature processes and would call for an inspection methods able to detect degradation under these circumstances.
More in general, corrosion starts usually from inside pipes and components and may not be detected from outside until the pipe wall became so thin that leak can start at every moment.

For a complete (older) review of hazards related to hydrogen in ammonia plants, including corrosion and its interaction with operative mechanical loads (Stresses) see references.

The following improvements were implemented:
1. Improvement of the system for inspecting insulated pipes;
2. Installation of an automatic fire extinguishing system in the compressors;
3. Moving equipment containing oil away from the compressors.