ARE YOUR FLANGE TIGHTENING TORQUES PROPERLY SET?

While re-starting an ammonia production unit in a chemical plant following a technical breakdown that lasted one hour and 30 minutes, a synthesis gas leak (50% hydrogen, methane, nitrogen and 17% ammonia) ignited on a flange (O-ring joint) located just after the synthesis reactor. The operator triggered the emergency shutdown after the unit’s alarms were triggered, isolating and depressurising the synthesis cycle. Steam was sprayed on to the leak using mobile fire nozzles to extinguish the fire that was brought under control 55 minutes following its start. Material damage was estimated at 60 K euro The blaze did not damage the reactor that was protected by a deflector. Operating losses stood at 300 k euro. 

The tightening torques of the bolt on flange where the leak occurred is responsible for the accident: they were not adapted to the exceptional conditions prevailing during the accident (significant temperature difference between the bolt and the flange resulting from the abrupt temperature variation due to the short technical breakdown.) The implemented corrective actions involved redefining the tightening torques, improving the tightness of the drip collar on the flanges (defective at the time of the accident), designing a nitrogen injection system in drips collars and installing a steam injection crown to protect the bottom of the synthesis reactor.

Source:Aria database

ARE DOLLARS TAKING PRECEDENCE OVER OVER PROCESS SAFETY?

Shortly after midnight, a highly flammable gas leaked in an ammonia manufacturing unit of a chemical plant producing nitrogenous fertilizers. The gas, mainly comprising hydrogen, leaked from a valve at a pressure of 220 bar and a temperature of 120°C. It spontaneously ignited forming a fiery stream directed towards the pipeline carrying the gas. Due to the heat, the pressure in the pipeline increased; the top portion of the pipe ruptured releasing the gas that exploded. The alert was sounded and the police implemented an emergency plan: traffic was stopped in the roads in the vicinity of the plant and resident were asked to stay indoors. The rest of the gas burnt at the rupture point for 2 hours. The leak was plugged and the fire brought under control at 2.45 am. Human casualties included 2 employees who suffered mild concussions and sustained cuts. They were treated onsite. The plant operations were stopped during the enquiry. 

The accident occurred due to a gas leak at the joint of a valve (in open position and at maximum flow). There was no leakproof joint between the two metal parts in contact. The internal metal surface of the valve was not compliant with the specifications in terms of quality. The valve, replaced in 2002, was not considered to be an element critical to the safety even though the process was classified in the high-risk category. Only price was taken into consideration while selecting the maintenance supplier. There was no exchange of information between the operator and the subcontractor during the operation and no acceptance of work was done (especially inspection of the valve bolting). The operator revised procedures, optimised management of maintenance and subcontractors (qualification, information, follow-up, etc.). The emergency operation conditions were also reviewed (appropriate number of trained staff even during low activity periods).

Source:Aria database

COMPETENCE AND CULTURE - GO HAND IN HAND

During a maintenance operation in a refinery, two operators opened the wrong purge. Four hundred litres of naphtha spilled out and caught fire. The operators were not qualified to perform this task and, like 10% of their colleagues, did not follow the operating procedure. The authorities noted persistent shortcomings in the facility operator's safety culture.

Source:Aria database  

THE IMPORTANCE OF OPERATIONAL READINESS REVIEWS - ARE YOU WALKING THE LINE?

 Leak of boron trifluoride (ARIA 51230) Five employees were hospitalised following the release of 15 kg of BF3 during the restart of a petrochemical site. A valve had remained open during a leak test performed the previous day. The operating procedure did not specify the position of the valve (not identified in the DCS) upon completion of the test.

Ammonia leak incident:

When a service provider was filling a tank, 2.1 m³ of NH3 was released via a drain. The day before, the technician preparing the operation was interrupted to take an urgent sample. The service provider's procedures did not provide for an inspection of the installation.

Source: Aria database

ARE YOUR HAZOP STUDIES LOOKING AT HUMAN ERROR AND ITS EFFECT ON AUTOMATED SYSTEMS?

 An exothermic runaway chemical reaction abruptly occurred within a production facility when an operator initiated an automated sequence to add water inside a multi-purpose batch reactor. The reactor suddenly experienced a pressure build-up due to its foaming content, but the automated control system was not able to regulate it. The glass reflux condenser burst while the rupture disc remained unbroken, given that its bursting pressure had not been reached. The irritating HCl vapours released in the facility was evacuated outside through the ventilation system. These vapours affected 7 people in the neighbourhood, 2 of whom would be kept in hospital overnight for observation. 

The investigation conducted revealed that the sudden exothermic reaction followed a 30-litre spill of water into the reactor instead of the 3-litre quantity indicated in the procedure. The 32-litre water tank was connected to the reactor via a pipe fitted with 2 valves. The first valve, activated by the automated control system, normally delivers 3 litres of water, while the second manual valve is supposed to stay in the closed position at the beginning of this water addition sequence. During the accident however, the manual valve was left open, and this oversight led to quickly draining the 30 litres water content of the tank into the reactor. The exothermic reaction was triggered, and the sole control system designed to prevent chemical runaway was unable to function properly, since the system was designed to provide a control of the water added to the reactor through the closure of the second valve.

Moreover, the manual valve, which was not equipped with an open/closed position indicator, could not be easily reached by the operator, making it difficult to control. This risk of exothermic reaction had been identified during the process safety study (i.e. the Hazard and Operability Study, or HAZOP), but at the time of the accident only a call for procedural improvements had been issued. The company limits the maximum volume of water which can be added at one time in the reactor and improves the automated control system. The safety studies (HAZOP) are updated for all exothermic reactions carried out on in this apparatus. A particular attention is paid to consequences of operating failures and a balance is found between risk control measures and the potential severity of the consequences.

Source:Aria database