Managing explosion risks in the workplace starts with a thorough explosion risk assessment. The first step in this assessment is to evaluate the likelihood and persistence of an explosive atmosphere. This evaluation must be meticulously documented in the Explosion Protection Document, which outlines the assessment process and findings.
In this article, we will delve into how to classify hazardous area zones, the purpose of zone classification in explosion risk assessments, and the proper method for indicating zoning on workplace drawings. We will also discuss the critical factors and definitions necessary for managing hazardous areas effectively.
Likelihood and Persistence of Explosive Atmospheres
How are areas classified based on the frequency and duration of the occurrence of the explosive atmosphere?
Areas are classified into zones based on the frequency and duration of the occurrence of explosive atmospheres. This classification system helps in identifying the level of risk associated with each area and in implementing appropriate safety measures. The ATEX Workplace Directive 1999/92 outlines the following zones:
- Zone 0: An area where an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapor, or mist is present continuously, for long periods, or frequently.
- Zone 1: An area where an explosive atmosphere is likely to occur in normal operation occasionally.
- Zone 2: An area where an explosive atmosphere is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
- Zone 20: An area where an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, for long periods, or frequently.
- Zone 21: An area where an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.
- Zone 22: An area where an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
How is the likelihood and persistence of an explosive atmosphere assessed?
The assessment of the likelihood and persistence of an explosive atmosphere involves analyzing the potential for explosive mixtures of gases, vapors, mists, or dusts to be present in the workplace. This evaluation takes into account several factors, including the properties of the substances involved, the environmental conditions, and the processes that may release these substances into the atmosphere. The primary goal is to determine where and when explosive atmospheres might form, and how long they are likely to persist.
Purpose and Legislation
What is the purpose of zone classification in the context of explosion risk assessment?
The purpose of zone classification in explosion risk assessment is to systematically identify areas where explosive atmospheres are likely to occur. This classification helps in implementing effective safety measures by categorizing areas based on the frequency and duration of explosive atmospheres. By doing so, it ensures that appropriate precautions and safety protocols are in place, reducing the risk of fire and explosions. Zone classification also guides the selection of ATEX-compliant equipment, ensuring that all tools and machinery used in these areas are suitable for the specific level of risk.
What legislation defines the zones for explosive atmospheres in the workplace?
The ATEX Workplace Directive 1999/92/EC defines the zones for explosive atmospheres in the workplace. This directive sets out the minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres. It categorizes hazardous areas into different zones based on the likelihood and persistence of explosive atmospheres, ensuring that each zone is appropriately managed to prevent accidents. This legislation is crucial for standardizing safety practices across workplaces handling flammable substances.
Normal Operation and Catastrophic Failures
Does area classification apply to catastrophic failures?
Area classification does not apply to catastrophic failures, such as the rupture of a storage tank or a pneumatic conveyor. These events are beyond the scope of normal operation and are considered exceptional occurrences. Instead, normal operation includes all conditions within the design parameters of the plant, such as start-up, shutdown, and the failure of fragile components.
What does “normal operation” include in the context of area classification?
In the context of area classification, “normal operation” encompasses all situations within the designed operational limits of the facility. This includes routine processes, maintenance activities, and foreseeable malfunctions of equipment. It covers the entire range of expected operational states, ensuring that safety measures are in place for all typical scenarios within the plant’s operating parameters.
Risk Assessment and Zone Classification
The risk of explosion hazard increases with the likelihood and persistence of an explosive atmosphere. In the ATEX classification:
- Zones 0 and 20 indicate the highest risk, where explosive atmospheres are present continuously, for long periods, or frequently.
- Zones 1 and 21 represent areas where explosive atmospheres are likely to occur occasionally during normal operation.
- Zones 2 and 22 indicate the lowest risk, where explosive atmospheres are not likely to occur during normal operation, but if they do occur, they will persist for a short period only.
The directive uses terms like “continuously,” “long period,” “short period,” “frequently,” and “occasionally” to describe the duration and frequency of explosive atmospheres. These terms are considered vague because they do not provide specific time frames, making it challenging to apply them directly without further guidance.
What guidance is available for zone classification regarding duration and frequency?
Guidance for zone classification regarding duration and frequency is commonly accepted as follows:
- Zone 0 and 20: T > 1000 hours/year or T > 1 hour/shift
- Zone 1 and 21: 10 hours/year < T < 1000 hours/year
- Zone 2 and 22: 1 hour/year < T < 10 hours/year and T < 1 minute/shift
These guidelines help to quantify the vague terms used in the directive and provide a clearer framework for classification.
What factors influence the likelihood of occurrence of an explosive mixture outside equipment?
Several factors influence the likelihood of an explosive mixture occurring outside equipment, including:
- The type of substance involved (gas, vapor, mist, or dust).
- The frequency and duration of substance release.
- Environmental conditions, such as temperature and humidity.
- Ventilation levels, which can disperse or concentrate explosive atmospheres.
- The presence of ignition sources, such as electrical equipment, static discharge, and hot surfaces.
By considering these factors, workplaces can better assess and manage the risk of explosive atmospheres forming outside equipment.
Implementation in Workplace Drawings
Concerning releases of flammable gas, vapor, mist, or dust into the atmosphere from a release point (for example, through vents and failure leaks), one uses as a “starting point” the concept “Grade of release” which is closely related to the zone classification conception. However, the Grade of release is dependent solely on the frequency and duration of the release, and independent of the rate and quantity of the release, the degree of ventilation, or the characteristics of the fluid. The Grade of release is classified/defined as follows:
| Grade | Definition | Time Guidance (hours/year) |
|---|---|---|
| Continuous | A release that is continuous or is expected to occur frequently or for long periods | T > 1000 |
| Primary | A release that can be expected to occur periodically or occasionally during normal operation | 10 < T < 1000 |
| Secondary | A release that is not expected to occur in normal operation and, if it does occur, is likely to do so only infrequently and for short periods | 1 < T < 10 |
The terms “Grade of release” and “Zone” are not synonymous, but in most cases under unrestricted “open air” conditions, there exists a direct relationship between “Grade of release” and “Zone” according to:
- A continuous grade normally leads to a Zone 0 or 20.
- A primary grade normally leads to a Zone 1 or 21.
- A secondary grade normally leads to a Zone 2 or 22.
The general procedure for zone classification can be divided into five steps:
- Obtain process information, such as temperature, pressure, concentrations, and details about plant design and material explosion risk parameters (e.g., flash point, limits of explosion, and layer ignition temperature).
- Determine internal zone classifications as appropriate, where the zone extents will be fixed by the equipment.
- Identify release points and assign grades of releases.
- Consider the relevant effects of ventilation.
- Determine zone numbers and zone extents.
Drawing Hazardous Zones
How should zoning be indicated on drawings?
Zoning on workplace drawings should be clear and precise, using distinct patterns for each zone to ensure easy identification and understanding of hazardous areas. Here’s how to visually represent each zone:
- Zone 0: Use dots to fill the area. This creates a pattern of small, evenly spaced points within the zone boundary.
- Zone 1: Use a pattern of diagonally crossing lines that form vertical squares. This creates a grid-like appearance within the zone boundary.
- Zone 2: Use diagonal lines running in one direction across the zone. This gives a striped appearance within the zone boundary.
- Zone 20: Use squares in line to fill the area. This creates a checkerboard pattern within the zone boundary.
- Zone 21: Use vertical and horizontal lines crossing each other to form a grid. This creates a mesh-like pattern within the zone boundary.
- Zone 22: Use vertical lines only, running parallel to each other across the zone. This creates a series of parallel stripes within the zone boundary.
Additional Considerations for Safety Measures
Hazardous area classification is a complex process that should be performed by a third-party expert. Below, you will find a Q&A list addressing more intricate zoning considerations.
Why is hazardous area classification not suitable for certain high-temperature processes?
Hazardous area classification is not suitable for processes operating above auto-ignition temperatures because the risk of ignition is inherent due to the high temperatures. Instead, safety is managed through robust system integrity and rapid fault detection and isolation.
How should processes handling products above their auto-ignition temperature be managed?
Such processes should be specifically identified in safety cases, and appropriate measures should be implemented to manage ignition risks. This includes ensuring equipment integrity and having rapid response systems in place.
How should hazardous areas in storage compounds be managed?
Storage compounds should be managed by justifying the exclusion of normal road vehicles, using ATEX-compliant vehicles, or implementing stringent control measures. This includes gas detection systems that can shut down engines and isolate ignition sources in case of gas release.
What are the considerations for vehicles with gas detection systems in hazardous areas?
Vehicles with gas detection systems, which are designed to shut down engines and isolate ignition sources during a gas release, should be evaluated for their usefulness in specific applications, despite not claiming full ATEX compliance.
What controls are necessary during the filling and emptying of road tankers with flammable materials?
Controls to prevent the release of gas or vapor are crucial, along with controls over ignition sources. Hazardous areas are present during transfer operations, requiring safe systems of work to manage these transient zones.
How should plant modifications affect hazardous area classification? Plant modifications should consider the impact on existing hazardous area classifications. Passive changes, like new walls or buildings, can influence ventilation and, consequently, the classification of adjacent areas.
Equipment and Standards
How is lightning protection implemented for structures?
Lightning protection involves installing surge protection devices between non-earth bonded cable cores and the local structure. Guidance can be found in standards like BS 6651.
How should specialist vehicles be managed during maintenance operations in hazardous areas?
During maintenance operations, normal zone restrictions may be suspended with proper controls, such as written instructions or a formal permit-to-work system, ensuring the safe use of specialist vehicles like cranes.
Specific Risks and Precautions
What is the role of integrity and detection in fuel and process pipelines for safety?
Ensuring the integrity of fuel and process pipelines and having rapid detection and isolation mechanisms are crucial for preventing leaks from causing fires or explosions.
What are the main ignition sources in normal vehicles?
Ignition sources in normal vehicles include electrical circuits, internal combustion engine inlets and exhausts, electrostatic build-up, overheating brakes, and other moving parts.
What control measures should be adopted for ignition sources during maintenance in hazardous areas?
During maintenance, control measures like supplementary ventilation, portable gas detectors, and inerting of plant sections should be used to manage ignition sources.
