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Determining safety distance in process design

Safety distance determination is a key design issue that may have a dramatic impact on a refinery construction project

Amec Foster Wheeler
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Article Summary
On 9 July 1976, one kilogram of 2,3,7,8 tetrachlorodibenzodioxin (TCDD) was released through a rupture disk at the ICMESA plant in Seveso, Italy. That was not only the day when the world faced for the first time the hazard of a toxic cloud potentially spreading over the whole community, but it was also the beginning of a huge change in the regulatory and methodological approach to process safety. Seveso Directives I (1982), II (1997) and III (2012) have introduced the concept of risk in the industry and have addressed the quantitative risk assessment (QRA) approach for siting of potentially hazardous installations.

Previously, a prescriptive approach was the general method used to manage safety and occupational aspects of the industrial world. The methodological change was progressively reflected in all of the safety and occupational health laws of the European Union. Through New Approach and Global Approach, the European Commission in 2000 also introduced individual responsibility for the site owner to provably certify the acceptability of risk. In the industrial sectors potentially affected by major hazards, such as the oil and gas and petrochemical/chemical industries, this process has been implemented relatively more quickly than in others, due to the industries’ cultural background and their high potential hazards. The need to minimise risk and a progressively growing consciousness about “friendly safety” (Kletz, 2010) have led to the adoption of techniques and methodologies which are capable of reducing post-incident measures and able to develop increasingly sustainable approaches because of their inherent low hazard and potential for harm. The key concept of ‘inherent safety’, which had been introduced several years earlier (Kletz, 2010) is “the limitation of effects by changing designs or reaction conditions rather than by adding protective equipment that may fail or be neglected”.

QRA studies in the industry have traditionally been implemented as separate, stand-alone tasks, often not synchronised with design development. A possible outcome of this for the design team is to be delayed while implementing suitable design and layout changes, which generally results in significant addition of protective measures, a non-harmonised approach, a very significant impact on project cost and, last but not least, an ineffective achievement of safety targets. This is often the case with plant/equipment siting. The traditional approach consists essentially of the adoption of prescriptive distances, which may in fact be unsafe, or which may lead to the available space being used in a less than optimised manner. Amec Foster Wheeler’s experience includes a long project execution history, throughout which the necessity to develop risk-based, simplified techniques to identify safety distances between plant units, between main equipment and occupied areas, has increased in importance. This article describes this evolution and presents a state-of-the-art, quantitative risk assessment approach to safety distance determination.

Background of the methodology of the separation distance assignment      
Early guidance about safety distances was given by Armistead (1952), Backurst and Harker (1973), and Anderson (1982). In 1976, the Dow Chemical company included safety distances in its Fire and Explosion Index (FEI) Guide. Developed in the 1980s, the Mond Fire Explosion and Toxicity Index method is an extension of the original Dow Index method. Exxon (1998) issued some safety design standards which specified prescriptive values for layout spacing. Similar separation distance tables have been given by Mecklenburgh (1985) and Industrial Risk Insurers. Mecklenburgh also carried out a categorisation of the most important hazardous scenarios to be used in support of plant layout.

Prescriptive separation distances for small and large tanks containing flammable liquids were given by the Health and Safety Executive in 1998 and, for LPG, in 2013. The US Center for Chemical Process Safety (CCPS) (2003) has provided typical separation distances between various elements in open-air process facilities. These tables are based on historical and current data from refining, petrochemical, chemical, and insurance sectors. The data were developed based on experience and engineering judgment and, as clearly stated in the CCPS textbook, not always on calculations.

On the other hand, risk- and consequence-based methods have increased in importance and this has been progressively reflected in codes and standards. In 1996, the International Atomic Energy Agency (IAEA) released a comprehensive paper dealing with risks to public health from fires, explosions and releases of toxic substances outside the boundaries of hazardous installations due to major accidents in fixed installations with off-site consequences; maximum distances and areas of effect are given on the basis of the classification of substances by effect categories. The IAEA in 1999 also issued a specific paper on safety distances relative to hydrogen according to effects analysis. API 521 (2008) provides guidance for predicting the distance to flammable concentration limit following a gas momentum-driven release; this formula has been reviewed recently by Benintendi (2010) as a more accurate approach to identifying hazardous areas. The same standard includes a method to determine flame radiation to a point of interest.

The European Industrial Gases Association’s report Determination of Safety Distances (2007) provides the basic principles for calculating appropriate safety distances for the industrial gas industry. The well-known US Environmental Protection Agency’s Risk Management Program Guidance for Offsite Consequence Analysis (2009) provides guidance on how to conduct the offsite consequence analyses for risk management programmes required under the Clean Air Act. This guidance identifies distances to specific toxic, flammable and over-pressure endpoints, based on substance characteristics and on release models. Also, Factory Mutual (2012) states the necessity of identifying separation distances accounting for specific hazard factors and provides some quantitative graphs for outdoor chemical processing equipment.

Finally, ATEX Directive 1999/92/EC (2000) requires hazardous area classification, which consists of the sizing of areas where explosive atmospheres can exist, which is indirectly a safety distance assessment. The hazardous area classification primary standard is BS-EN-60079-10-1 (2009), which, for gases and mists, is based on the calculation models provided by Cox, Lee and Ang (1993), Iving et al (2008).

Safety distance as part of inherently safer design
Identifying safety distance through a risk-based methodology is considered to be a part of inherently safer design philosophy. In 1990 Englund developed a section of his Chemical Hazard Engineering Guidelines dealing with separation distances within an inherently safer design procedure. In a recent book, Process Plants: A Handbook for Inherently Safer Design (2010), Kletz said, “The essence of the inherently safer approach to plant design is the avoidance of hazards rather than their control by added-on protective equipment.” Properly assessing the outcome of an incident scenario, conservatively identifying its extent and, finally, accounting for these data to arrange plant layout, minimising in this way the likelihood of any impact, can be considered consistent with Kletz’s statement. His inherent safety approach includes the following elements, to be addressed early in a project phase:
1. Intensification or minimisation
2. Substitution
3  Attenuation or moderation 
4. Limitation of effects.  
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