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Investigating the Source of Process Failures
Rainer Semmler, Notified Expert, TÜV SÜD Chemie Service GmbH. Chemical companies are always engaged in improving their safety measures by analysing the risks associated with their business. HAZOP (Hazard and Operability Study for Safe Chemical Plants) analysis for risk qualification offers the opportunity of systematically and proactively improving plant safety and reliability. In this article, TÜV SÜD Chemie Service shows how to successfully apply and implement the HAZOP technique in practice.

The fact that accidents and incidents still occur in German chemical companies in spite of the high level of safety demonstrates how important it is to work continuously on developing the best possible safety concept. From inattentiveness and interference by unauthorised parties to technical faults in process control and chemical systems, there are many causes of unplanned events.

Early this year, a mix-up with extensive consequences took place in the electroplating shop of a company for safety systems in North-Rhine Westphalia, Germany. The workers there accidentally confused the tanks of sodium hypochlorite, commonly known as bleach, and hydrochloric acid when the substances were delivered. According to ZEMA, GermanyÊs Central Reporting and Evaluation Office for Hazardous Incidents and Incidents in Process Engineering Facilities, the workers filled around 380 litres of sodium hypochlorite into the hydrochloric acid storage tank. The two chemicals reacted, causing chlorine gas to escape from the air bleed valve of the storage tank and spread through the Investigating the Source of Process Failures Chemical companies are always engaged in improving their safety measures by analysing the risks associated with their business. HAZOP (Hazard and Operability Study for Safe Chemical Plants) analysis for risk qualification offers the opportunity of systematically and proactively improving plant safety and reliability. In this article, TÜV SÜD Chemie Service shows how to successfully apply and implement the HAZOP technique in practice. company through the air-condition system. Thirty-nine members of staff working in the company suffered injuries caused by the toxic gas. The fire brigade had to be called to get the situation under control.

In 2011 alone, ZEMA registered 18 reportable incidents throughout Germany. Its most recently published Annual Report for 2009 gives a detailed summary of the circumstances, causes and effects of the incidents that were recorded in that year. It is noticeable that incidents are frequently triggered by unplanned events, which prevent a certain process from running as planned. Depending on the situation, if no effective safety strategies are in place the process may develop a dynamic of its own.

Better safe than sorry
Since the hazard potential of process systems and the physical properties of materials have become increasingly complex, the causes of incidents are more and more difficult to grasp. In addition, chemical companies and plants differ in size, technical infrastructure, production processes and many other details. Not least for this reason, new approaches to risk qualification and classification have been developed on the basis of failure probabilities and the existing risk assessment techniques have been further refined. With the help of HAZOP, standing for Âhazard and operabilityÊ, studies possible risks faced by a company or involved in plant modifications can be predicted, their causes and effects assessed and effective counter measures developed.

The essence of the HAZOP technique is a methodological discussion of possible deviations from the specified condition led by a multi-disciplinary team of experts. The multi-stage analysis and assessment of the operating process takes into account technological and organisational hazards, potential human error and external influences such as extreme weather conditions or transport accidents on the companyÊs premises. To this end, an experienced TÜV SÜD facilitator leads a multi-disciplinary team of internal and external experts which discuss various scenarios on the basis of a number of commonly used guide word-parameter pairs and common interpretations of them. This permits targeted differentiation to be made between safety-related measures that are actually required and those that are less useful and enables a clear list of priorities and a comprehensive safety concept to be drawn up on this basis.

"What if" - The human factor within the scope of risk qualification
What if a V1 safety valve fails to close? What chain of events must be expected? The expertise of the participants in the HAZOP study and the selection of the facilitator are critical for the efficiency and effectiveness of the discussion and the HAZOP process. Practice shows the optimum team size to vary between six and ten experts.

The facilitatorÊs capability to ensure systematic and well-structured team meetings is as critical as his/her talent to stimulate the imagination of the team members to think of possible scenarios and new approaches to solutions. Other systematic methods, including the Failure Mode and Effect Analysis (FMEA), Event Tree Analysis (ETA) or Fault Tree Analysis (FTA), can be used to round off the facilitated expert discussion.

The process of a safety assessment can be illustrated by the example of a fermenter. In step one, the plant is divided into various functional units based on design drawings, documents and process components. The expert team then looks at one specific sub-system - for example, gas production in the fermenter - and examines the individual process parameters including pressure, temperature, filling level, pH value and cooling. Deviations from the required value are determined on the basis of a defined keyword list and their possible causes discussed. What can cause the pressure in a line to be too low, too high or drop entirely? The investigation reveals that this situation can be caused by several events, for example failure of a pump or closing of a valve. However, a drop in pressure may equally be caused by power failure or leakage.

Once the possible causes have been identified, the members of the HAZOP team focus on the potential consequences of the deviation. A detailed description of the chain of events to be expected permits solid assessment and evaluation of failure relevance. The facilitator must find possible safety-related events and guide the participants to new ideas and solutions. This approach enables the development of effective countermeasures for different causes, which effectively prevent possible disturbance or where this is impossible, restrict their impacts. Depending on the situation, these measures may be design or technology-related or organisational.

In the above case, which resulted in a release of chlorine gas, a relatively simple design- and technology-related measure can help to prevent tanks from being filled with the wrong substance in the event of similar confusion in the future.

The connections of the two storage tanks are equipped with electrical valves that are locked via pH measurement. This ensures differentiation between acids and alkalis and excludes accidental mixing of these two substances.

Owing to their comprehensive, well-structured approach, HAZOP studies are ideally suited for risk prevention. TÜV SÜD Chemie ServiceÊs experts can work with the respective companies to reliably identify process-related weaknesses. To this end, HAZOP studies should be repeated at regular intervals such as every five years, and applied as early as possible in the process - in particular in plant planning or expansion, modernisation or modification.

The owners of chemical plants benefit from a higher level of safety, increased reliability and a profound basis of information for investments. Apart from this, HAZOP documentation provides a detailed overview of the safety standard of chemical plants, so that an effective and established HAZOP study can have positive impacts on approval and licensing procedures (e.g. as per BImSchG) and the terms and conditions of insurance.