Originally issued: February 8, 2022
Shaun Williamson, P.L.(Eng.), CFSE
Director of Engineering
Last Line of Defense
Fire and Gas Systems (FGS) are often considered the last line of defense in the safeguarding strategy when other preventative safeguards have failed. These systems allow us to breathe easy when walking around and working at facilities that process hazardous materials. Unfortunately, many incident investigations show us that often FGS have failed to provide the preventative and mitigative measures when needed. Without the mitigative effects of FGS, small releases or fires can quickly escalate and result in much greater consequences.
Historically, most effort in a safeguarding strategy has been geared towards preventative safeguards. At first glance, this may appear a sensible approach. Why not focus our attention towards keeping product in the pipe rather than worrying about how to handle the situation if it gets out? However, further analysis shows that in many cases, preventative safeguards may not have been able to prevent leaks or fires based on the failures that occurred. Pump and compressor seals, valve seals and piping leaks occur and this is not always detectable using preventative safeguards. Despite our best efforts to properly install and commission equipment, bolts may not be properly tightened, sealing surfaces may loosen or leak once process pressures and temperatures increase or decrease. For all these situations, we rely heavily on FGS to keep us safe.
Once installed, fire and gas detection can appear dormant, with no change in values like we see with process measurement. Certain gas detectors such as catalytic bead and electro-chemical technologies make use of sensors that degrade over time and that may be poisoned by atmospheric contaminant. Therefore, it is important that test intervals are appropriate for the technology and not extended beyond reason.
Hardware reliability can be a factor to consider relating to detector effectiveness. Newer technologies often include enhancements that result in greater reliability and stability over longer durations. Often test schedules can be extended and coverage improved by taking advantage of the latest innovations. Selecting hardware that comes with IEC 61508 SIL certification is another way to ensure a higher grade of quality in the manufacturing process.
The most important factor to consider regarding FGS effectiveness is to ensure proper detector coverage. Fire and gas detectors are unlike preventative safeguards since they do not have direct connection to what they are monitoring in the way process sensor would. Fire and gas sensors only do their job if they are placed in the correct location. If a flame sensor is not pointed where the flame occurs, it does not alarm. Perfect coverage is not feasible and more coverage often comes at a cost from more detectors. Therefore, our best efforts are needed to predict and prioritize detection where fire and gas events may occur, in areas with the greatest risk of escalation, and where detectors are still accessible for maintenance.
Another important consideration is the effectiveness of mitigative actions. This often includes a combination of annunciation systems, process interactions and in some cases flame suppression. Annunciation also requires care and attention to ensure the hardware works, and sufficient audible and visual coverage is provided.
When properly designed, commissioned and maintained, FGS can be highly effective at preventing small gas leaks from escalating into fires and explosions. They also support effective evacuation, reduce inventory of hazardous products and remove many ignition sources.
A Troubled History
Hopefully by this point it is agreed that FGS plays a very important role in a safeguarding strategy. With this in mind, it can be helpful to look at our past to see how well these systems have been performing. Industry findings published following incident investigations of past major accidents show many examples in which properly placed or functioning FGS may have contributed to the prevention or mitigation of the incident. Industry can learn from these unfortunate events and apply learnings to their new and existing facilities. There is an opportunity to improve how FGS have been implemented which may come with an extraordinary financial benefit if a major accident can be avoided or mitigated.
Case Study example
One example of a major explosion to consider is the 27-year-old Philadelphia Energy Solutions Refinery explosion in 2019. The initiating event is believed to be a piping elbow rupture resulting in a toxic hydrocarbon release. The subsequent gas cloud which was not detected by gas detection ignited a couple of minutes later, resulting in a large fire. Quick action by operations allowed for significant toxic product depressurization before a BLEVE occurred, rocking the facility and sending a large section of the vessel across the river. Thankfully, there were no fatalities. Following this accident, the company laid off 1,000 workers and filed for bankruptcy.
Had gas detection been able to detect this event prior to ignition, it’s possible ignition sources could have been isolated to prevent or at least delay the ignition. Earlier product de-pressurization may have helped to avoid the eventual BLEVE or reduced the size of toxic gas released by enabling the water sprayed neutralization system to be activated sooner.
This accident also highlights that aging facilities are often at greater fire and gas risk due to wear and tear on equipment, and due to outdated quality standards in use during the original construction. In this case, the piping materials composition was no longer considered adequate and should have been replaced. However, the requirement was not taken seriously since the original design was “Grandfathered”.
Issues in Every Phase of the Lifecycle
Incident investigations have highlighted that failure of FGS occurs in many phases of the lifecycle. The following are some common examples:
Design Phase – Lacking design philosophy, improper technology selection, leak sources not identified for detection, insufficient coverage provided, sufficient placement, orientation and configuration requirements not provided to installer, impairments not considered
Construction and Commissioning Phase – Design documents not followed, installation not inspected by designer, detectors placed by electrician without specialty knowledge based on convenience rather than where coverage is best, obstructions not considered, access for maintenance not considered
Operations & Maintenance Phase – Maintenance and testing not performed properly or on time, systematic failures not investigated and corrected, proper training not provided, hardware left in service past design life, nuisance alarms bypassed and disabled, blown fuses and bunt out bulbs not fixed, bypasses left on, facility changes not evaluated for impact on detector coverage.
These observations have led to an understanding that FGS effectiveness requires a lifecycle management approach to ensure the installed system performs the intended action over the life of the system.
Assessing Fire and Gas Risk
Process Hazard Assessments (PHA) have become common place when building or modifying a facility, and when combined with Layer of Protection Analysis (LOPA) are highly effective in evaluating requirements for preventative safeguards. While a PHA can be helpful for identifying fire and gas risk, PHA is not particularly well suited for establishing performance requirements for the FGS. In many cases, this results in detectors being placed with little consideration for risk and rather using rules of thumb and generalist knowledge of designers with varying levels of competency. Without sufficient understanding of risk, this can either lead to an under-designed system with little chance of detecting an event, or an over-designed system that is difficult and costly to maintain.
The ISA solution
Good news, ISA has support available in their Technical Report ISA TR84.00.07 - Guide on the Evaluation of Fire, Combustible Gas, and Toxic Gas System Effectiveness. This technical report provides guidance on a lifecycle approach to FGS management. This includes a process on how to evaluate fire and gas risk and establish performance targets that can be used to provide a risk-based approach to detector placement. Guidance is also provided on a risk assessment process called Hazard Grading. The Hazard Grade can be used to establish performance targets for detector coverage and safety availability. Detector coverage is typically evaluated using software tools or in some cases qualitatively by FGS specialists. High quality installation guidance can then be provided to installers to maximize coverage.
Hopefully this article provided will help readers recognize the importance of FGS, understand some of the issues contributing to past FGS failures, and raise awareness of the approach proposed by the ISA Technical Report as a possible solution. FGS engineering is a highly specialized activity and requires advanced experience and training to do well. Engaging an FGS specialist with demonstratable expertise for new and existing applications to support the engineering and operations teams can go a long way to maximizing effectiveness.
For readers responsible for the fire and gas systems at their operations, we encourage you to contemplate these questions: “How risky is my process and how likely are we to see a hazardous release”, “What is the site FGS Philosophy and has it been consistently applied?”, “Has Detector Coverage ever been assessed?”, “How confident am I that fire or gas events would be detected, and effectively communicated to all parties onsite?”, “How would my company be impacted by an accident like the one discussed?”. If the answers are not known, auditable and defensible, then further action should be considered.
Stay tuned for more articles on FGS where we will dive deeper into these topics and more.
Feel free to reach out to the author for support or to discuss these questions and others in more detail.
> ISA TR84.00.07: Guidance on the Evaluation of Fire, Combustible Gas, and Toxic Gas System Effectiveness > CSB Video on Philadelphia Refinery Explosion: https://vimeo.com/366764121
Originally issued: May 4, 2022
Shaun Williamson, P.L.(Eng.), CFSE
Director of Engineering
Risk Based Detector Placement
This article is a continuation on FGS-BLG-100 which discussed Fire and Gas Systems as a “Last Line of Defense”. For safety critical protection layers, time is of the essence in the event of a fire or gas release. Without mitigative action, a combustible product release can escalate to a fire or explosion. Small fires can grow to become large ones, resulting in cascading explosions or BLEVE’s that can wipe out an entire facility and cause substantial loss of life. Failure to detect these events quickly, has been identified as a common safety issue during the investigation of many past major accidents.
One of the most important factors to ensuring effectiveness of a F&G system is to ensure that the sensor is properly placed to see the fire or gas event. While process sensors require very little thought when placed, F&G sensor placement on the other hand is a highly specialized activity and is easy to get wrong. Poor detector placement is the greatest factor influencing why fire and gas systems fail.
Often, detectors are treated as an afterthought during the engineering phase. Very few details other than approximate location are typically provided to installers by engineers. It is typically the electrician that decides the final location of devices and aims the fire detectors. This is done with no specialty training, understanding of what area the detectors are meant to cover, or what site conditions may impair sensors.
This article will introduce the concept Fire and Gas detector coverage mapping to help decision makers understand that there is a scientific method to support these critical design decisions, and to maximize the return on investment of F&G systems. It will also highlight why these safety critical systems require specialized training and experience to design and support installers.
Fit for Purpose Design
Before any effort is spent to assess what detector coverage is achieved by a proposed design, one of the first things to consider is “what is the design target”. Detection systems can be expensive to purchase and maintain, therefore it is important to understand which areas actually need coverage. Low risk areas may not benefit from a high detector coverage, while high risk areas would benefit from the greatest detector density practical to maximize coverage. Taking a risk-based approach to detector coverage ensures a “Fit for Purpose” design.
The ISA TR84.00.07 standard describes a risk assessment process called Hazard Grading that can be used to set detector coverage targets. Factoring in many variables such as leak probabilities, area occupancy, operating pressures, fluid properties, level of congestion, ignition probabilities, toxicity and more, the user can use a calibrated risk table to determine a Hazard Grade with a corresponding coverage target for each risk zone. Hazard grades can also be used to determine the design basis coverage scenarios using Relative Heat Output (RHO) as a design input (how big of a fire do I wish to detect). This in turn allows the designer to consider the size of fire a specific model can “see” using the design basis fire sizing associated with the selected Hazard Grade. The hazard grading and coverage assessment process will need to be established in a F&G Philosophy before any risk assessment or coverage assessments begins. The standards do not dictate what the process must be, but rather provides guidance on what it could look like. The process is up to the end user to determine based on their corporate risk policy.
3D Detector Coverage Mapping (In Depth Analysis)
Once performance targets have been established, software tools are available to import or setup a 3D model of the hazard area. Hazard grading information can be calibrated into the software. Specific models of sensors can be selected allowing the software to take into consideration sensor capabilities and limitations. Detectors can be placed within the model and configuration and orientation data entered for analysis. Some major advantages of these software tools that make them worth the time, money and effort to perform mapping include:
• Accounting for obstructions which limits the field of view of the flame sensors
• Calibrated to account for flame cone size based on specific model, sensitivity settings and RHO in 3-dimensions.
• Some tools can simulate a gas release and calculate the probability of detection. This allows the user an option to analyze coverage using geographically or using scenario modelling.
• These tools are particularly useful when configuring voting sensors. The tool can calculate coverage for single or multiple detectors making it clear which percentage of an area or scenario has single versus dual detector coverage. This is not easy to do without software tools and often leads to either overdesign to compensate for lack of information, or the more dangerous alternative of insufficient coverage to work properly during an event.
This overview is just scratching the surface on coverage mapping and the associated tools. There are multiple tools available, each with their own strengths and weaknesses. The standard offers good guidance on processes that can be followed to work with which ever software tool seems to be the best fit for the user’s application. It should also be considered that 3D mapping may be overkill for some applications. An experience analyst should be able to provide guidance on the best approach.
Qualitative Detector Coverage Assessment (Simplified Analysis)
In some cases, a qualitative approach to detector coverage assessment may be the preferred method to determine if a proposed design achieves a tolerable level of coverage. After reading about the benefits of using software tools, hopefully it is apparent how complicated coverage assessments can be and how many variables need to be considered when doing this work. Qualitative coverage assessment therefore places a significant demand on the analyst to take these factors into consideration in their assessment without the support of software tools to simplify this process for them. Often this will result in a recommendation for more detection since the analyst may need to take a more conservative approach in their estimations. Qualitative assessments for this reason are most effectively performed by specialists with experience using the semi-quantitative approach and working with software tools since this experience helps the analyst apply their learnings from previous mapping work.
A visual representation of detector coverage should be provided to show what coverage has been allocated to detectors and which obstructions have been considered. Coverage targets should still be assigned using a Hazard Grade, however achieved coverage is an estimation and not quantified by software tools with this method. Designers should provide similar details to installers that are available from software tools including: location, elevation, orientation, declination and sensitivity settings depending on the sensor type.
Experience and Independence Should not be Overlooked
While the process described may seem straight forward, it is important that designers have specialized fire and gas systems training to do this properly. Decisions around where to place detectors can be affected by a wide range of factors that must be considered for the system to work when needed.
An independent eye focused on coverage and correct application of technology can be very beneficial. Avoid some common pitfalls such as: a) avoid using generalist engineering for this specialist activity; b) avoid limiting designers to technologies available from a single manufacturer and their sales rep; b) seek unbiased engineering advice on detector quantities and placement (i.e. companies that do not sell the hardware being recommended).
Hopefully this article will help readers recognize the importance of proper F&G detector placement, understand some of the issues contributing to detector effectiveness and what it takes to ensure fit for purpose coverage has been achieved. These activities can now be factored into new project plans. For existing installations, it is not too late for a detector coverage study. Quite often significant improvements can be made simply by re-orienting existing detectors. Many facilities have aging equipment due for replacement. This is a good time to ensure that coverage is optimized, detectors are accessible for maintenance and the quantities are appropriate.
Assuming the topic of detector coverage is now better understood and readers are on the path to ensuring good coverage is achieved, consider other factors that influence the overall F&G system effectiveness. In future articles, we will discuss other factors such as effectiveness of annunciation systems, and automatic actions that should be considered upon a fire or gas alarm.
Feel free to reach out to the author at firstname.lastname@example.org to discuss the information in this article or for support with F&G detector coverage assessments.
> ISA TR84.00.07: Guidance on the Evaluation of Fire, Combustible Gas, and Toxic Gas System Effectiveness
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