Explosion at an ammonia plant

A release of "synthesis gas", containing primarily hydrogen occurred at the ammonia synthesis unt of a fertilisers plant.
The location of the leak was a valve within the ammonia synthesis column. The gas, released at a pressure of 220 bar and a temperature of 120°C, ignited spontaneously, forming two flame cones directed along the pipe connected to the valve. The pipe did not resist the heat from these flames and 15 minutes later burst under internal pressure freeing its contents, which in turn exploded.

The quantity of gas released during the accident was estimated at 1.45 tonnes. The gas continued to burn for more than two hours, given the quantities found at the site when the fire broke out. The consequences of this accident however remained confined to the zone adjacent to the valve. Two workers were slightly injured when they were trying to isolate the valve.

This accident was caused by a gas leak on the joint between the body and cap of a shut-off valve on a pipe transporting synthesis gas. The valve, in the open position, was operating at full flow. The seal between the two parts, held together by 12 bolts, was applied to a metal-on-metal contact. Inspection of the valve indicated that gas had leaked in two spots prior to ignition, meaning that two distinct flames were produced.

The INITIATING CAUSE was the loss of confinement of a shut-off valve, due to inadequate sealing of th flange surface.

The investigation revealed several anomalies:
(1) inadequate clamping of nuts, the subcontracted firm did not follow any pre-established set of procedures or specifications in conducting this operation.
(2) degraded surface of the valve seal surfaces: a sizeable lateral surface groove, presence of particles of a shape and composition analogous to those of shot blasting material used to clean metal surfaces.

The ROOT CAUSES were inadequate execution of maintenance on the valve and lack of proper management of contractors’ operation by the site operator.

shut-off valve,
body-cap joint,
process gases feed line

The valve had been operational since 1975. In October 2002, as part of a maintenance procedure, the cap (i.e. the upper part of the valve) had been separated from the body (i.e. the lower part) and some of the valve's internal parts had been isnpected. This procedure was performed by a subcontractor.

DESCRIPTION OF THE PLANT
The plant primarily produced ammonium nitrate-based fertilisers, and part of this process implied synthesising ammonia. The particular unit affected by the incident was the ammonia synthesis column.
The plant was located within an industrial complex, a site under jurisdiction of the SEVESO directive. The closest residences was at a distance of 500 m from the scene of the accident.

Two light injuries: the two technicians were trying to isolate the valve when the pipe burst. The pressure surge generated by the explosion was strong enough to knock one of them to the ground and send debris flying, a piece of which struck and slightly injured the other technician.
The consequences of this accident however remained confined to the zone adjacent to the valve. 1.45 tonnes of synthesis gas were freed and since hydrogen represents 75% of the gas composition, this incident was the equivalent of losing approximately 1.2 tonnes of hydrogen.

Maintenance work at installations generates specific risks. These risks need to be analysed to identify best-suited means of their prevention or minimisation. On top of that, maintenance operations are most often outsourced to subcontractors, and this increases the probability that something will go wrong.
The investigation carried out by the HSE unit exposed the following:
(1) The operator did not consider the valve to be a critical element for plant safety.
(2) The subcontractor selection process seems to have been based on price criteria rather than the level of competence and experience demonstrated in maintaining this type of valve.
(3) The operator provided no technical information concerning the valve to the subcontractor.
(4) The subcontractor was uninformed of valve characteristics and did not keep any kind of log for monitoring the operations already completed.
(5) The operator did not inspect work performed on the valve.
(6) The particles detected on seal surfaces were like materials used when shot-blasting metal surfaces. Their origin remains unknown but were undoubtedly deposited prior to the maintenance work of 2002.
(7) No log was kept regarding the amount of torque applied to the joint fastening bolts. It is therefore impossible to determine whether the valve had been lifted with the right clamping force.

The plant operator must pay careful attention to the choice of contracted parties by ensuring that they have the requisite level of competence, training and qualifications to successfully undertake the planned works. A risk management effort on the part of the plant operator is necessary and implies the following: preliminary risk analysis, qualification procedure for all subcontracted entities, preparation of the maintenance intervention (detailed description of works to be performed, equipment specifications, documentation, procedural information, etc.), traceability of the works undertaken, and acceptance conditions by the operator. documentation, procedural information, etc.), traceability of the works undertaken, and acceptance conditions by the operator.

A similar accident occurred on April 24, 2006, where the inadequate tightening torques on the flange bolts were the cause of the accident (see HIAD_91).

The conclusions of this investigation were discussed with the operator and subcontractor assigned the valve repair job.
Corrective measures were laid out with the objective of improving plant procedures.
The company repaired the plant as pre-incident, however, the valve involved was replaced by a different manufactured type.