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ATEX Establishing a Basis of Safety

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"You cannot fall down stairs you don't have". Not our words... But those of Trevor Kletz; renowned Guru in the field of process safety. What did he mean? Well, he was talking about ‘bungalows’ (single-story buildings) and how they relate to INHERENT SAFETY – the very first topic which should spring to mind in any hazard or risk assessment i.e. how can we eliminate the hazard in the first place?

Later, in the third article of this series, we look at the classification of hazardous areas and cite an incident in Mexico involving a water-immiscible solvent (n-Hexane) which entered a town drainage system. Had the manufacturing process been designed to eliminate the risk of using a water-miscible solvent, many of those killed in the violent explosion which occurred would be alive today.


EU Areas and equipment in which combustible gases and/or vapors may form and in which airborne clouds may be produced, fall within the scope of the Dangerous Substances and Explosive Atmosphere Regulations 2002 Statutory Instrument No. 2776. A more detailed account of the principals involved is discussed in International Standard IEC 61241-19. The Regulation (Section 6.4 Risk Reduction) states the following measures, in order of priority, are those specified for risk control:

  •  Reduction of the quantity of dangerous substances to a minimum
  •  Avoidance or minimizing the release of a dangerous substance
  • Control of the release of a dangerous substance at source
  • Prevention of the formation of an explosive atmosphere, including the application of appropriate ventilation
  • Ensuring that any release of a dangerous substance which may give rise to risk is suitably collected, safely contained, removed to a safe place, or otherwise rendered safe, as appropriate
  • Avoidance of ignition sources including electrostatic discharges and adverse conditions which could cause dangerous substances to give rise to harmful physical effects
  • Segregation of incompatible dangerous substances.

 

Dilution

Simple steps, for example, could help eliminate the hazard – like adding water to an alcohol or changing the process operation e.g. method of addition.

Pure iso-Propyl Alcohol (IPA) will form flammable atmospheres in air under ambient conditions as it has a low ash point value, i.e. 12 °C.

Hence, hazardous areas will arise during handling and inside process vessels under normal ambient temperature conditions. But could an IPA-water mixture be used in place of the pure solvent?

Dilution with water results in an increase in the closed-cup flash point value. This arises because the rate of evaporation is suppressed. The graph shows empirical data from which a ‘line-of-best- t’ relationship has been determined:

been determined:

y = 106.7 x-0.4674

where:

y = flash point (°C), and

x = IPA concentration (% v/v)

 

With a 5K safety margin, if the temperature of an IPA/H2O solution can be restricted to normal room temperature (say 25 ºC) or below, hazardous vapor-air mixtures can be avoided by controlling the alcohol content to 15 % IPA by volume or less (equivalent Flash Point = 30 ºC). Clearly, lower concentrations provide an even greater margin of safety

Change in Procedure

When pouring liquids with moderate ash points (e.g. Flavors) in to a heated batch, to preclude the formation of hazardous areas within the vessel, some liquids (Flavors) have to be restricted, dependent on their ash point. To eliminate this, one option is to dilute the flavors with some of the batch liquor in a separate area (i.e. within a ventilated cubicle or fume cupboard), prior to the addition of the (then) diluted mixture.

Occasionally, to achieve the same goal, the batch temperature can be lowered, although this is not always a tenable solution. Increased ventilation offers a third alternative, whereby extraction (LEV) is provided immediately above the point of addition i.e. rather than at (say) ceiling level.

Ventilation

With increased ventilation, the extent of the hazardous area will be reduced. Suitable ventilation rates can also avoid persistence of the explosive atmosphere, thus in influencing the type (and/or extent) of a zone. However, key points need to be considered in the use of ventilation: -

  • Extract discharge point requires consideration
  • Air should be drawn from a non-hazardous are
  • Release conditions must be defined
  • Need to consider changes in gas densities (with temperature)
  • Need to consider ow of heavier-than-air gases
  • Need to consider local obstacles/impediments to air movement

Ventilation is often categorized as follows:

High ventilation (VH) – can reduce the concentration at source virtually instantaneously, resulting in a concentration below the LEL. A zone of small (or even negligible) extent results.

Medium ventilation (VM) – can control concentration, resulting in a stable situation where the concentration beyond the zone boundary is below the LEL whilst the release is in progress and where the hazardous area does not persist unduly, after the release stops.

Low ventilation (VL) – cannot control the concentration whilst release is in progress or prevent undue persistence of hazardous area after release has stopped.

Need to consider local obstacles/impediments to air movement

Hazardous Area Classification

The process of area classification involves the identification of all flammable materials, the identification and grading of all releases of flammable material, the assessment of the level of ventilation and/or housekeeping and the determination of the resulting types and extents of the zones. In turn, the designation of zones enables the correct equipment, practices and procedures to be applied to protect the health and safety of the workers concerned with the facility.

It is important to note that area classification only deals with reasonably foreseeable events and does not consider highly improbable (‘catastrophic’) events. EN 60079-10 section 1.1(d) defines ‘catastrophic’ failures as ‘beyond the concept of abnormality dealt with in the standard’ and lists ‘the rupture of a process vessel or pipeline and events that are not predictable’ as examples.

Thus, a ‘catastrophic’ failure may cause an explosive atmosphere to be present in an area defined by area classification as ‘non-hazardous’, and such situations are subject to a risk assessment by the operator under other legislation.

Quick- fix ‘Gaffer Tape’, often used for a temporary repair, is not a sound engineering solution. Moreover, in many cases, it becomes a permanent fixture! Certainly not an example of catastrophic failure.

Warehousing is not immune to risk either! A recent audit found several contraventions of HSE Guidelines:

  • Shared storage of oxidizing materials and flammable liquids in the building.
  • No provision of natural or forced (mechanical) ventilation (battery charging).
  • Processing (mixing) operations undertaken within a warehousing environment.
  • Limited segregation of operations.
  • An opportunity for the release of gaseous oxidants within the building.

Simple procedures can help hugely; such as protecting containers against banging or other physical damage when storing, transferring or using them and not using wooden pallets or other combustible pallets for storing containers of oxidizing materials and of course, ensure containers are suitably labeled.

In 2004, ICL was fined £400,000 over a factory explosion at Stockline Plastics in Maryhill, Glasgow.

The blast killed nine workers and injured 40 others and was Scotland's worst industrial disaster since the Piper Alpha oil rig explosion and in 1988.

The blast was caused by a build-up of liquid petroleum gas that had leaked from pipes. The pipes dated back to 1969 and were so badly corroded that escaped gas was ignited when a builder flicked a switch in the factory.

The High Court in Scotland was told that the pipework in question would only have cost £405 to replace and that one risk assessment undertaken was carried out by a college student doing vacation work.

 

 

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