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

COMPETENCY AND COMMUNICATION - THE ESSENCE OF PROCESS SAFETY

 At 9:20 a.m., a rupture disc of a reactor used to produce an organomagnesium compound burst when the reactor’s internal pressure rose too high. The incident was caused by a nonconforming mixture that had formed in the reactor. First, the ambiguous instructions led a technician to add an insufficient amount of initiator. Then, seeing that the reaction had not yet started, a second technician added more reagents. The process sheet indicated that the reagent could be added after, but only after receiving the supervisor’s approval. 

The operation took place on a Saturday and the chemical engineer belatedly informed the on-duty engineer. This lack of communication between the workers of both shifts and the technicians’ lack of experience are what set the stage for the incident. 

The operator subsequently implemented a number of changes : tracking of technicians who are accredited to carry out synthesis operations has been reinstated ; the process sheet now indicates the amounts of reagent to be added and includes hold points for the start of the reaction ; the reaction may no longer be carried out over the weekend and it must be scheduled at the beginning of a shift so that workers may monitor it from start to finish. In addition, the operator conducted an in-depth review of organomagnesium compound synthesis in order to establish production standards and problem-management guidelines applicable at all its similar production sites.

Source:Aria database

PRECAUTIONS FOR TANKS WITH INBUILT HEATING COILS

 PRECAUTIONS FOR TANKS WITH INBUILT HEATING COILS:

Design of facilities: 

Three interacting elements need to be taken into account when designing installations, particularly in terms of their compatibility with the operating conditions: - the nature of the products stored: flammable or non-flammable, likely to generate decomposition products that are themselves flammable or even explosive, requiring or not requiring temperature control; - the materials used for the storage tanks: flammable or non-flammable, and must be resistant to the products stored; - the heating elements: essential or non-essential, electric or using a thermal fluid (steam, hot water, heat transfer fluid, etc.), sized for a specific heating power. 

Reliable control equipment: - products are present in the tanks to prevent heating if tanks are empty (level and weighing sensors, etc.) - temperature of the thermal fluids used, the products contained in the tank, the atmosphere in the tank, etc. (appropriately positioned probes and sensors); - heating is controlled by the presence of product and by temperature control; - no contact between the heating elements and the tank’s walls; - monitoring of all of the installation’s operating and safety parameters. 

Regulated operating conditions: - preventive maintenance of facilities and equipment to prevent failures; - operating and intervention instructions (concerning the start-up of facilities in manual or automatic mode, shut-down of heating systems before intervention requiring tank emptying, after all interventions, etc.) - training of personnel in procedures and operating instructions; - monitoring and supervision of the installations during operations (the night prior, at night, early in the morning); - verification that all instrumentation and control channels are operating correctly, that the servocontrol and command logic is functioning correctly, and verification of automatic operation programmers.

Source:Aria database

ARE YOUR INTERLOCKS TESTED?

A fire broke out in a chemical warehouse that was inactive at the time. The flames had spread through the area used to store acids, bases and peroxides and included 28 storage tanks. This area is separate from the flammable products area. The site's Internal emergency Plan (IOP) was initiated. 

Six tanks containing hydrochloric acid, potash, flocculant, alkaline solutions and soda were destroyed, and the flames damaged five others. Overheating of a heating element in an empty soda tank over several hours caused the incident, as the product low-level detector had not switched off its power supply. The energy produced melted and then ignited the tank filled with HDPE (high-density polyethene). The material of which tank is made of then also fuelled the fire. The operator implemented the following preventive measures: - no heating device on product storage tanks not requiring it; - preference is given to steel or stainless steel tanks with electric or hot water heating; - the new plastic tanks are heated by hot water only; - the heating power must not allow a tank to ignite, - the reliability of the controller used to regulate the tank heating systems must be improved.

Source:Aria database

ARE YOUR TEMPERATURE SENSORS MEASURING PROPERLY?

 An explosion occurred in a metal bitumen tank (15 m high) in a company that manufactures, stores and distributes bitumen emulsion. The top of the tank was blown off and landed more than 45 m away on the grounds of the neighbouring facility. Upon arriving at the site, the operator switched off the electricity to the entire facility. The site was shut down. The tank in question was destroyed, and two other tanks were damaged in the explosion. The amount of bitumen in the tank was less than indicated by the sensor (10 tonnes) and insufficient to cover the resistors completely. The system began heating the fluid and caused the temperature in the tank to rise because the temperature sensor was not immersed in the bitumen. The heating system thus operated at its maximum level. The temperature rise caused a fire to break out, which ignited the combustion gases. The course of the accident implies that the level sensor had failed.

Source:Aria database