Hazardex 2019 Conference Presentations - Hazardous Area Classification Chemical Plant Case Study
Author : Carolyn Nicholls - Director & Process Safety Expert, RAS Ltd.
19 November 2018
It is important that care is taken when determining hazardous zones. The consequences of applying the wrong technique are clear: hazards can be underestimated and inadequately controlled, or can be overestimated resulting in prioritisation in the wrong places. This paper explains methods to overcome this challenge using a case study from a chemical manufacturing site currently revising its hazardous area classification.
Workplaces that handle dangerous substances with the potential to form an explosive atmosphere fall under the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR). These regulations require workplaces to be classified into hazardous and non-hazardous areas, with the aim of ensuring potential ignition sources are properly controlled so as to reduce the risk of workers to as low as reasonably practicable.
The regulations do not provide a methodology for how to classify these zones. As a result, a number of organisations have developed their own methodologies to do this, the most notable of these being the British Standards Institute and the Energy Institute.
For accidental releases, both the British Standard and Energy Institute methodology follow the same core steps:
1. Select an appropriate hole size.
2. Estimate the size of the LFL cloud from that hole size.
Look-up tables in both methodologies may be used to accomplish step 1.For step 2, however, there are a number of challenges that may not be clarified depending on the site and its processes:
• Neither methodology provides guidance on how to adequately estimate the size of pools from releases.
• The British Standard methodology is straight forward for gaseous releases, but falls short for liquid releases, particularly in how to determine the flash fraction at the release point.
• The Energy Institute methodology is primarily written for the petroleum industry. The look-up tables allow hazard radii to be selected; however, these are based on the dispersion modelling of petroleum products. Additionally, the dispersion modelling is carried out at an operating temperature of 20 °C, which may not be applicable to all releases.
For some chemical sites, these limitations can mean dispersion modelling is the only way to accurately determine the size of hazardous zones from releases. Where sites handle unusual materials, especially at high temperatures, there is a challenge in what approach to take. This can be especially problematic in cases where these chemicals are unavailable in modelling software.
In summary, the key findings and method include:
• Use of both point source and risk-based approaches provided in the Energy Institute methodology in combination to determine the hole size for releases.
• Use of dispersion modelling inputs provided in the Energy Institute methodology.
• Use of surrogate materials that are selected based on similar key material properties justified by comparison of consequence modelling results.
• Identifying the key assumptions for modelling liquid pool diameters.
Carolyn Nicholls is a process safety specialist and a Director of RAS Limited, with more than 10 years’ experience of the risk and hazard management industry. She has worked with a number of UK COMAH sites to develop safety reports and provide support in all aspects of risk management.
She leads the RAS teams of risk and hazard management consultants and has been instrumental in creating the company’s assessment methodologies. Her particular area of interest is the demonstration of ALARP, an often misunderstood concept.
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