CONFUSION DURING A MAJOR LOPC CAUSES WRONG ACTION TO BE TAKEN

In a petrochemical complex, an ethylene (a highly flammable gas) compressor was in stable operation when a sudden drop in pressure occurred at 5:33 p.m., accompanied by a loud noise. The gas detectors in the zone became saturated, and a 2nd level alarm was triggered in the unit’s control room, as well as in the control room controlling the nearby compressor and at the safety station. The compressor began to vibrate. Its motor stopped but not its ethylene supply.

Confusion over which compressor was leaking
Not knowing which compressor was causing the leak, the shift crew of compressor No. 1 contacted the crew in charge of compressor No. 2. The latter crew persuaded them it was compressor No. 2 that was to blame. Crew No. 1 left their control room to help them. The alarms and parameters indicating the malfunction of compressor No. 1 (pressure drop) were not taken into account since the control room had been deserted.

The two shift crews, joined by the internal fire brigades, approached the area of the accident but were unable to enter due to the deafening noise. They encountered a flammable cloud of ethylene measuring 4 m high x 100 m, with visible droplets. The firefighters protected the nearby units with water curtains. Around 5:45 p.m., the shift leader 1 consulted with the shift leader in charge of the neighbouring unit. A leak on the ethylene supply network was suspected. Two operators, equipped with hearing protection, moved through the cloud, protected by a water curtain, to reach the network’s manual shut-off valves. They were able to close the valves manually, ending the leak. The cloud rapidly dispersed. The operators returned to the control room and closed the local supply valve of compressor 1. 8 t of ethylene (354 kg of flammable mass) was released in 21 minutes.

ROOT CAUSE: Valve ejection due to incorrect tightening and a non-compliant seal
The hatch and the valve porthole of the second stage of compressor 1 were found 6 m away. One stud from the hatch was severed (sudden brittle-type rupture), while the other five studs remained in place, three of which were missing their nuts and exhibited torn threads. These studs are compliant but had never been replaced since the compressor was commissioned 16 years ago. An expert assessment showed that the valve’s copper seal was not annealed at the time of its installation, contrary to procedure and the other seals on the equipment. It was therefore not as flexible and less able to absorb stresses. This defect, combined with a bolt tightening error on the verge of plastic deformation, led to fluttering in the stack and its rupture.

If the emergency stop had been activated, the leak could have been stopped more quickly as it shuts down both the motor and the ethylene supply. The operators believed that the motor shut-down because of vibration was sufficient. After the accident, the compressor was equipped with an emergency stop triggered by gas detection. However, the manual emergency stop remains in operation should this detection system fail.

Source: Aria database

INADVERTENT CHEMICAL ADDITION DURING CLEANING CAUSES EXPLOSION

 An explosion and fire occurred at night in a workshop set up to synthesise toluene diamine (TDA), by means of hydrogenating dinitrotoluene (DNT) in the presence of Raney nickel, during a scheduled maintenance downtime. In-house fire-fighters brought the fire under control within 35 min; in the meantime, four employees required hospitalisation. One of them, who was handling the valves to wash the hydrogenation reactors with isopropanol, sustained burns over 40%-50% of his body and died 15 days later. The workshop was completely destroyed. The reactor burst; the bunker housing the workshop was deformed due to the combined action of the blast wave and sprayed fragments; the reinforced concrete wall was ripped open, with rebar twisted; and the control room was heavily damaged. Glass panes were broken over a 50- to 100-m radius, while the distillation unit juxtaposing the bunker was damaged and allowed isopropanol and TDA to escape, adding fuel to the fire. Buildings belonging to the neighbouring industrial facility located 150 m away suffered deformations to their lightweight structures. 

According to the investigation conducted, this explosion resulted from injecting pure DNT into one of the reactors washed by the circuit used at the time of production startup. Two valves connected in series equipping this DNT feed line were found partially opened (at 10°) after the accident, most likely allowing 500 to 700 kg/h of product to flow into the reactor. The heat release upon hydrogenation of a small quantity of DNT probably triggered the sudden decomposition of the remaining DNT, while abruptly reheating the reaction medium. Corrective measures were adopted to prevent routing pure dinitrotoluene into the reactor. These were elimination of the DNT intake line on the injection tank, addition of two automatic on-off valves on the mixing tank’s DNT feed line, closure of the link (by an automatic on-off valve) between the mixing tank and the injection tank during the reactor washing

ARE YOU PREPARED FOR AN OFF SITE EMERGENCY?

A power failure occurred in a refinery, as a result of the failure of the main power line during maintenance. This led to an emergency shut down of the whole plant. The automatically operated safety systems started working : large quantities of products were dumped in the flare and were burnt off. Safety valves opened and released gasses to the atmosphere. Personnel and people working at the refinery were evacuated and only emergency staff remained at the plant.

Information at the central operating desk about what was going on in all the components of the plant was sparse. In the first hour after the incident it was not known which safety valves were opened and which products were vented. That information became available bit by bit.

One of the safety valves that opened released an amount of 70 kg H2S into the atmosphere. The release point is situated at about 40 m above ground level.

After 5 min, the cloud of H2S formed reaches a downwind distance of about 3 km with a concentration valued at nearly 10 ppm 3 m above ground level.

Driven by a wind from the south-south-west at 45 km/hr, the cloud proceeds over the western part of the province of Brabant and after about 70 min has reached the city of Dordrecht, 50 km from the refinery. Concentrations of H2S in the cloud are about 0.06 ppm, still well above the smell detection level .

No warning of the H2S spill was issued, partly due to a lack of information at the plant, partly due to a lack of communication between Belgium emergency services and the Dutch authorities.

A population of about 100.000 people was in the path of the cloud and potentially affected by it. An estimated several hundred people were affected by the H2S and experienced nauseous ness, and respiratory problems. 57 people needed medical care.

However the Dutch emergency services were not prepared to deal with the situation, due to lack of information about the event and its possible consequences. This in turn led to insecurity and a loss of confidence in the capacity of the government to deal with incidents like these.

Source:Aria database

DO YOU CONSIDER ACCIDENTAL REVERSAL OF PNEUMATIC HOSE CONNECTIONS TO CONTROL SYSTEMS DURING HAZOP?

 A reactor exploded in a fine chemicals plant during the chlorination of an alcohol by thionyl chloride (SOCl2 ). The relatively non-exothermic reaction took place in a solvent medium (1,2 dichloroethane or DCE), under a slightly lower pressure and a temperature of 70°C maintained by means of steam injection. The reactor initially contained the SOCl2 in solution in the DCE, with the alcohol being added under close monitoring for 30 hours. 

At the time of the accident, the reactor was being fed for three hours by successive 200- litre loads of alcohol, with the first injection still incomplete. Monitoring performed by two technicians, one of whom was a trainee, included an hourly reading of both the temperature and pressure drop; no anomaly had been observed until that point. Upon hearing a noise accompanied by a break to the protective disc on the glass column connected to the reactor and noticing smoke around the disc joints, the technician turned the feeder control box selector switch to the “off” position. As he closed the alcohol feed valve and was making his way to the valve used to shut down steam injection, he spotted that leaking on the column was becoming more persistent. He immediately left the unit, requesting that a co-worker follow him out — at which point the explosion happened. A rupture disc calibrated at 0.3 bar and the glass fixtures on top of the device burst. The explosion or toxic gases emitted once the equipment had broken killed the trainee technician, who did not exit the premises quickly enough. 

The feeder was equipped with two valves. The upper one (loading side) was found in the closed position while the lower valve (reactor side) was open with a reversal of the pneumatic control hoses. These recordings supported the hypothesis of an accidental addition of water into the reaction medium via the feeder. The laboratory simulation of such an addition found that the SOCl2 hydrolysis with the formation of SO2 and HCl led to a sudden pressure rise. 

Source:Aria database

ARE YOU MONITORING SUPPORTS FOR THEIR INTEGRITY? ARE YOU PROPERLY CONGURING ALARMS?

In a refinery, an alarm in the control room informed the operators of a fire in the distilling unit. The unit’s emergency shutdown procedure was initiated from the control room. The internal fire-fighting resources were initiated at to extinguish the fire and cool down certain installations in addition to the fixed installations at the site. No injuries were reported on or off the site. The distillation unit was partially destroyed over an area measuring 50 m x 50 m, and flaring episodes were required.

A petrol leak was discovered on a 3” diameter pressure testing pipe of a flowmeter on a hollow tubular support. The operator had visually noted corrosion on the support already 3 years earlier. Replacement of the support was planned to take place during the regulatory shut-down period but was not performed. The ignition source was not precisely identified. Before the fire started, an alarm corresponding to the 20% lower explosive limit had been triggered 6 times, without the operators noticing. As some of the units had still been shut down, the alarms dedicated to the unit in operation were filtered. This filtering arrangement masked the display of the fire and gas alarms and only displayed those pertaining to the unit’s processes that had been restarted. The flashing light visible in the control room was considered a “process” alarm, knowing that such signals are not explicitly dedicated to fire and gas alarms.

Source: Aria database