Explosion in a process gases line of a fertilisers production plant

A gas release occurred in the building containing the compressors for the recirculation of process gas of a plant producing fertilisers. The process gas was a mixture of hydrogen and nitrogen. It occurred at the outlet valve of a compressor, which had been identified as defective several years before, but had been kept in place. Its poor design and ageing eventually caused the gas leakage in the building. During repair works aiming at replacing it, it failed open releasing a large quantity of process gas and enabling the formation of an explosive gas mixture. The ignition took place 9 minutes after the release, so that the mixture could form not only in the compressors building but also diffuse into the neighbouring power supply room and the carbon synthesis workshop, before it ignited.
Explosions occurred both in the power room and the compressors building. Due to uneven gas distribution, the latter had the highest concentration of flammable gas and experienced the greatest explosion pressure, resulting in the most severe damage. The incident caused 5 deaths, 1 serious injury, and 25 minor injuries to employees.


The DETAILS OF THE SEQUENCE are not completely clear, probably due to a non-optimal translation from Chinese.
On the day of the explosion, an abnormal noise was noted from the outlet valve of one of the recirculation compressors. The responsible team started troubleshooting works, consisting of (a) switching the operation from the affected compressor to another one, (b), shutting down and depressurising the affected compressor and troubleshooting both inlet and outlet valves. After restarting the compressor, the outlet valve was still noisy. The same operations were repeated. Since both inlet and outlet valves were found leaking, attempt was done to better close the outlet one. This manual operation caused valve’s full opening and the release of the whole compressor content into the building.

the INITIATING cause was a leak of a mixture of hydrogen and nitrogen from a defective valve, facilitated by a wrong manual operation.
IGNITION SOURCE were electrical sparks generated by an air conditioning unit still operating when the plan was shut down. Since the ignition occurred several minutes later, the hydrogen had the time to form a large flammable cloud within the building. The delayed ignition very possibly led to a detonation.

The ROOT CAUSE(S) was a lack of safety culture, awareness and technical knowledge throughout all the hierarchical levels. The failure to repair/replace was the consequence of this, worsened by a lack of professional approach to troubleshooting and operative procedures. From the scientific article by Xiu Jiyau, it appears that the compressors were not (well) de-pressurised, and definitively not purged before starting repair activities, sot that a considerable amount of process gas with a potential to form large explosive mixture was still present I the equipment.

process gas line, (probably recirculation loop compressor)

since April 1999, abnormal opening and closing of the outlet valve of the one of the compressors had been observed, but the true cause could not be found, and the fault could not be eliminated. On top of that, when the valve was in the closed position, continuing to operate the valve disc in the closing direction would cause the valve to re-open instead; the tightening screws were loose, and there were scratches on the worm gear limit screws and the pressure cap limit points.
Some employees had drawn a closing indicator line on the valve with a colored pen; the valve had to be precisely aligned with this line when closed, and if it exceeded the line and was then tightened, the valve would reopen.

Five fatalities, 25 injured (one severely).
The 100-meter-long plant building had been blown in two, and the circulating pump room and adjacent high-voltage power distribution room had completely collapsed.
The explosion affected an area of about 680 m2, causing 286k Euro of damage.

The failure to realise the importance of the role to guarantee safety of specific components is at the base of this incident. The plant allowed that a malfunctioning compressor valve could remain in operation for years and left the operative team to find out ‘creative’ ideas to compensate for the malfunctioning, not realising how this would imply deviation from the risks assessment ruling plant’s safety.
This left the personnel to develop off-the-record practices, which were not carved in the existing operative procedures (the mark on the valve beyond which it would start again to open). In moments of operative stress (e.g. urgency due to emergency), without proper training and drills, even the experienced employee will forget these off-the-record rules.

The investigation found out a severe inadequacy of the company’ safety management. A lack of safety culture and of technical knowledge permeated the whole plant operative structure. The following specific findings deserve to be mentioned:

(1) Safety awareness of the managers and part of the employees was poor. Since 1999, the abnormal opening and closing of the failed outlet valve was known, but the workshop failed in effectively solving the malfunction or taking the required preventive measures.

(2) Rules and regulations were not strictly enforced. The plant requested all equipment to meet the "four understandings and three skills" requirement (i.e., understanding the structure, principle, purpose, and performance; being able to use, maintain, and troubleshoot). However, from the plant headquarters and equipment department to the workshop employees, no one understood the role, the design and structure of the failed valve. Therefore, effective maintenance of the valve was impossible. In December 1999, after the machine overhaul, the workshop did not organize a step-by-step acceptance inspection as required by technical regulations, and the equipment department did not conduct a thorough review. The damaged valve was put back into use without being replaced, causing the valve's malfunction to persist for a long time.

(3) Third, the company’s safety management was degrading with the time. The investigation found that due to enterprise restructuring, the full-time position of the safety officer was abolished. The restructuring plan proposed by the factory also included the idea of abolishing the security department.