Toxic gas management: a review of the strategy behind BS 60080:2020
16 December 2020
On premises where toxic gas is used, stored or created, a Gas Detection System may be required to protect the occupants or people on site. In many cases, a Fixed Gas Detection system is required. BS 60080:2020 has been recently published to provide standard guidance to Design Engineers and relevant parties involved with these systems. This paper reviews the strategy behind the standard’s approach to addressing the Toxic Gas risk.
Figure 1 – Taken from page 38 of BS 60080
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The location of field devices (Gas and Flame Detectors) is challenging and the response by the oil and gas industry was to develop in-house Engineering Technical Practices or standards (ETPs) to fill the gap between legislation/regulations and method for design. Understandably, these have primarily focused on flammable atmospheres.
The petrochemical industry’s approach to toxic gas has been limited to the specific risks (mostly H2S) with a prescriptive approach.
The strategy to protect against flammable gas accumulations is based on UK HSE publication OTO 93-002 which is addressed in various ETPs with Performance Targets. These are assigned by design engineers to address the appropriateness for each area. The advantage to this scientific approach is the empirical evidence of speed and overpressure.
The empirical evidence for toxic gas is more problematic. The UK HSE publication EH40/2005 Workplace Exposure Limits gives guidance on Workplace Exposure Limits which ‘are concentrations of hazardous substances in the air, averaged over a specified period of time, referred to as a time-weighted average (TWA).’ [UK HSE publication EH40/2005 Workplace Exposure Limits – para 3 (page 6)]
EH40 identifies the toxic substance and the short/long term exposure limits. However, it does not provide specific information about gas cloud size and the concentration that might cause immediate harm (immediately dangerous to life or health). For the majority of toxic gases, there is scarce comprehensive empirical data about each, and the environments of application are not repeatable for reasonable study. In light of the lack of demonstrable evidence, the question remained, ‘What is the best strategy for deciding the location and number of Toxic Gas Detectors?’
Acknowledging the undefined variables, the new British Standard recommends developing the strategy around providing time to make the occupants safe. See Figure 1, taken from page 38 of BS 60080, which attempts to illustrate the response time versus toxicity on a graph.
The reader may notice that there are no times or values indicated on the graph. Those values are to be defined by the Design Engineer prior to the placing of gas detectors by understanding the site and meeting with site representatives who understand the risk, mitigations and operational procedures.
Identification of the risk or hazard is the first step. This is normally the result of a study or report where the risk of toxic gas has been identified. The risk assessment process is illustrated in BS 60080 with a flow chart that seeks to remove the risk if possible, what risk remains will require additional controls to reduce the risk as reasonably practicable. A Fixed Gas Detection system may be an additional method of control or layer of protection. It is important to understand that in the moment the Gas Detection System has detected gas, all the other control measures have failed and the site is now having a life-threatening incident. The author encourages sites to take an aggressive approach to control measures prior to the installation of a Fixed Gas Detection System.
Table 1 – Sample of Toxic Gas Performance Targets
Toxicity – how much is too much?
As stated earlier, EH40/2005 Workplace Exposure Limits gives guidance on Workplace Exposure but does not provide the values that are immediately dangerous to life or health (IDLH). Material Safety Data Sheets from the site provide some information, however, even if the IDLH Value can be identified, the toxicity of the gas may be altered by combining with other gasses in the environment. Gas detectors normally measure the toxicity in parts per million (ppm). The gas detector setting must be decided by the end user and should provide a margin of error. If the harm caused by the gas is deemed to be a greater risk, then lowering the Action Required threshold will be required.
Inventory and supply
The amount of toxic gas will impact on the risk assessment, as the inventory may range from a single bottle to a large tank. The designer needs to understand how an uncontrolled loss of containment (LOC) will impact the environment. The pressure of the supply will also impact how much gas will be introduced to the environment during LOC. Where the supply is external to the area being assessed, some form of supply pipe/hose integrity check should be undertaken to protect from LOC.
Understanding the physical space of the site impacts how the risk can escalate. For example, in an area below the floor level with a gas that is heavier than air, the gas has no escape, whereas an external installation with a lighter than air gas may allow the gas to escape. Also understanding the purpose of the areas will provide greater awareness of the occupants (i.e. designated access routes will be populated with a greater number of potentially untrained people then a workstation that is manned 24/7).
The occupants- who are we trying to protect
BS 60080 goes into detail of the expected occupants of the area at risk. How much knowledge or training each occupant has will impact their response and time required to escape. How long they occupy the area will also feed into deciding who is at risk. It is important to understand that if an area is not normally occupied it may not require gas detection. Understanding the means of escape and procedures will inform the time required to act in the event of an alarm.
Figure 2 – 3D Layout of a Toxic Gas Risk
A site may initiate passive or active systems and procedures to reduce or eliminate the risk. Passive mitigation is part of the installation that is normally structural or part of the process equipment. If passive mitigation is included, then regular inspections should be part of the ongoing management. Active mitigation is an activity or system that is undertaken to prevent the gas becoming a risk or a system that once activated prevents the risk from escalating.
For example, passive mitigation may have vapour sealed doors (fire doors) and fire stopping which prevent the spread of gas beyond the area. An example of active mitigation is powered ventilation (HVAC) which circulates, dilutes, or extracts the air to prevent the accumulation of gas. For all mitigation, verification or regular inspection is required to include these into the mitigation for the risk assessment. Note: any active system will also need to work in fail-safe mode, i.e. the active system will engage in a fault condition.
A more detailed explanation of risks and mitigation are unpacked in Table 1 on page 39 of BS 60080. The designer should document the agreed risk and validation as a basis for the Performance Targets.
The performance targets are set based on the risk (who or what is being protected), environment and mitigations as reported in the earlier documentation. Performance targets are a set of detection goals against which success of detection can be measured.
The targets will state:
· Defined areas of detection or mapping zones (graphical plan)
· The maximum a gas cloud can grow before being detected
· The percentage of the mapping zone that is the target coverage
· The set points for the gas detectors
It is at this stage, after the strategy has been decided, that the engineer can begin to design the gas detection system.
The design engineer should take one of three approaches:
· Prescriptive: placing gas detectors to an agreed rule set.
· Volumetric: detecting a certain size of gas cloud in a defined area.
· Scenario based: designed around a specific risk.
BS 60080 goes into greater detail in section 7.6.3 to 7.6.5, the following is a brief summary.
Based on previous evaluation and documentation that will provide rules for the areas at risk. This approach is attractive because of its simplicity. However, unless the site has supporting documentation to support the strategy and the Design Engineer can verify that the application fits into the criteria the location design may be inadequate. This strategy does not normally require any verification software.
Concentrating on the defined area that requires protection. Because the risk gas is three-dimensional, volumetric mapping will require a 3d model of the site area, to visualise the area and audited by the appropriate software. The advantages and disadvantages are listed on page 43 of BS 60080. The strength of a volumetric approach is the focus on protecting the occupants and excluding subjective factors which are likely to change.
This approach is centred on the risk and how the gas might migrate in predefined scenarios, similar to Quantitative Risk Assessments. It sometimes integrates dispersion modelling or Computational Fluid Dynamics (CFD). Again, the advantages and disadvantages are detailed on page 44 of BS 60080.
This approach carries more risk than the other two detailed above, as it runs multiple scenarios based on assumptions. When protecting individuals from harm or life-threatening conditions, this approach cannot demonstrate that the design is suitable or sufficient as the conditions of the assumptions may have changed.
Gas Detection location design
At this stage the engineer will be able to design the specific gas detector locations based on the risk assessment and agreed design strategy.
The full title of BS 60080:2020 is “Explosive and toxic atmospheres. Hazard detection mapping. Guidance on the placement of permanently installed flame and gas detection devices using software tools and techniques” which isn’t very captivating, but it does mention software tools. In relation to toxic gas, mapping software is primarily for verification of the proposed design. It also provides the opportunity to try different designs. Assessing the coverage is key to demonstrating that the system is suitable and sufficient.
Once the design is completed, a form of report should be provided to demonstrate compliance, including:
· Review of Gas Detection Risk Assessment
· Recommended Performance Targets
· Areas of Detection (details of risks / mitigations / performance targets) with tables and illustrations to demonstrate detector locations and coverage
Gas Detection system management
Randall Williams, Proeon Systems
And finally, while the design of a fixed gas detection system is demanding, installation and ongoing management of the system is required to demonstrate that the system is appropriate to address the risk.
· Installation and commissioning (Note: reverification may be required if devices cannot be fitted to the original design).
· Maintenance and Calibration – the frequency will depend upon the environment and detector technology, but the frequency should be at least every 12 months.
· Testing control panel functionality – if this includes executive action (active mitigation) then that functionality should be included.
· Management of change – when a process or significant change takes place in an area of risk, the gas detection system should be reviewed for adequacy.
· Regular reviews – most sites are active with continual changes of process and procedures. If there is no review as part of the management of change, then the site should be reviewed with a frequency of at least every five years or after any significant changes.
Legal obligations and responsibilities
BS 60080 recommends ‘the Fire & Gas mapping report should be reviewed regularly.’ BS 45444 (Toxic Gas) specifies that gas detector location design requires expertise and the advice of experts having specialist knowledge. The Management of Health & Safety at Work Regulations 1999 requires that any gas detector location design be undertaken by a competent person with sufficient training and experience or knowledge. These are the legal obligations of the ‘Responsible Person.’ In many cases the Responsible Person (the person who is in control of the building) will not have the training, skills or experience to design or review the Gas Detection Locations as required by HSE.
In conclusion, unless your site has the expertise, specialist training, experience and knowledge then it would be wise to contact a suitably qualified business to ensure that the design is correct and appropriate, protects personnel and supports your company and the Responsible Person with their obligations.
About the author:
Randall Williams (BSc, GIFireE) has been working in the fire safety industry since 1988 as a Fire Alarm Engineer and Fire Risk Assessor. Randall was a member of the BSI Committee which drafted BS 60080 and he leads the Fire & Gas Team at Proeon Systems Limited in Norwich, (UK). Randall is a Graduate of the Institute of Fire Engineers (GIFireE) in 2013 and graduated with BSc Fire Protection Management & Technology from California State University, Los Angeles, California in 1986.
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