Every industrial plant has extreme safety requirements, particularly if combustible gases are present. But selecting gas detection technology isn’t a straightforward issue. There are many different types of gas detectors, each with their own advantages and disadvantages that make them ideal for certain types of gas, but inappropriate for others that exist in the same plant. Anyone concerned with gas safety needs to know the basics of how to select and combine gas detectors for their specific installation. I recently had an opportunity to write an article for Chemical Processing that does just that – walks you through the selection process for gas detection systems. Here is a segment that introduces the challenge:
As shown in the selection decision tree (shown to the right), catalytic and infrared (IR) technologies can detect combustible gases. Of the two techniques, only the optical method lends itself well for point and area monitoring. Choose IR devices for detecting a single type of hydrocarbon or simple mixtures. Because they are unaffected by oxygen levels, such detectors are ideal for anaerobic processes, operations with high concentrations of corrosive agents, and areas with constant background of combustible gases. On the other hand, catalytic gas detectors are a better choice for monitoring hydrogen or hydrocarbon mixtures. Because catalytic detectors require oxygen gas concentrations in excess of 10% by volume to operate, they only suit situations where oxygen always is present. Moreover, catalytic gas detectors perform best in applications where the target gas normally is absent.
For monitoring toxic gas leaks in congested spaces, electrochemical and solid-state gas detectors are the instruments of choice. Use electrochemical detectors in applications where the target gas is not normally present and in environments with relative humidity (RH) above 15%. By contrast, solid-state detectors should be used for extreme high temperatures or low humidity applications or where ambient conditions are stable. Oxygen deficiency is best monitored using electrochemical gas detectors sited near the breathing zone. As for open or uncongested spaces, open-path near infrared or ultraviolet detectors offer the widest coverage per device.
Hazardous material releases can take many forms. For pressurized gas releases, choose ultrasonic gas leak detectors to provide the fastest response to a leak. They excel at detection in open, well-ventilated areas where other detection methods may not be wholly effective. For pressurized liquid releases, select air particle monitors.
Integration of appropriate technologies is essential to meeting each plant’s unique requirements. Which gas detection types should you use in your plant?
For a more in-depth analysis of the pros and cons of gas detection technologies, click HERE.
By Chris Duncan, Business Manager, Emerson Automation Solutions
Detecting flammable gas that has the potential to threaten people and property in areas where gas clouds easily disperse is a problem in many applications. This recent case history is a classic example.
A UKCS (United Kingdom Continental Shelf) operator was issued an improvement notice from the Health and Safety Executive after a large volume gas leak went undetected on one of their North Sea platforms. The release had not been picked up by the platform’s existing flammable gas detection system because the gas had been dispersing as soon as it escaped.
The company contacted Emerson for help. The solution was to provide the Incus, an ultrasonic gas leak detector that, while working in conjunction with the existing gas detection system, added another line of defense and provided an early warning when gas releases occurred. Most importantly, it increased the safety of platform personnel.
Ultrasonic gas leak detection uses acoustic sensors to identify fluctuations in noise that are imperceptible to human hearing within a process environment. The sensor and electronics are able to detect these ultrasound frequencies (25 to 100KHz), while excluding audible frequencies (0 to 25KHz).
Unlike traditional gas detectors that measure the accumulated gas, ultrasonic gas detectors “hear” the leak, triggering an early warning system. The sensors respond to sound generated by escaping gas at ultrasonic frequencies. Leak rate is mainly dependent on the size of the leak and the gas pressure. In most facilities, the majority of process noise is in the audible range, while limited ultrasonic noise is generated in normal operation. Highly pressurized gas releases produce ultrasound (25-100 kHz) which the sensors are able to pick up despite the presence of audible noise. Ultrasonic (acoustic) gas leak detection technology has several advantages over conventional gas sensor technologies: it does not have to wait until a gas concentration has accumulated to potentially dangerous concentrations; it does not require a gas cloud to eventually make physical contact with a sensor; and the response is instantaneous for all gas types.
The Incus is ideally suited for monitoring outdoor applications such as on an oil platform. The Incus has been engineered to withstand even the most extreme conditions. Performance is not affected by inclement weather, wind direction, leak direction or any potential gas dilution, with an instantaneous response to all gas types. It is an excellent addition to many safety systems adding another layer of detection to existing technologies.
The Health and Safety Executive subsequently inspected the platform and approved the solution provided, resulting in a very happy customer!
Do you have an application that might require the addition of ultrasonic detection? It might be worth a discussion. Contact us HERE today.
By Jim Cahill, Emerson
This post was originally published on Emerson Exchange 365 and we wanted to share it with our readers as well.
At the Emerson Exchange conference in Austin, Emerson’s Sean McLeskey presented Fixed Gas and Flame Detection Best Practices. His abstract:
Many industrial processes involve dangerous gases and vapors: flammable, toxic, or both. With the different sensing technologies available, and the wide range of industrial applications that exist, selecting the best sensor and locating them properly for the job at hand can be a challenge. This workshop will help you get a better understanding of application challenges, learn basic installation best practices, and understand the benefits of using flame and gas detection solutions.
Sean opened with a safety case study at a refinery where personnel heard a “pop” and saw what appeared to be steam. It was a gas release that led to people being injured and the refinery being shut down for 3 months with losses in the tens of millions.
Fixed flame and gas systems are for detecting releases of process hazards and provide time to alert personnel and put the process into a safe state. These systems protect people, property, provide regulatory compliance and maintain good relations in the surrounding community.
Ultrasonic gas leak detection listens for the ultrasonic sound caused by escaping gas. It is not impacted by wind direction that some other detector technology relies upon. It provides first detection but does not provide composition of the gas detected.
Another technology is point gas detection which requires the gas to pass by to be detected by the sensors. This technology is typically applied near leak sources where the escape points, such as gaskets are known. One example technology is catalytic bead combustible gas detection. This technology is used to monitor several targeted gasses across applications including hydrogen.
Infrared sensors are another type of point gas detectors. Their strengths compliment catalytic bead. It is unaffected by high concentrations of hydrocarbon and works in the absence of oxygen, unlike the catalytic bead technology.
Multi-spectrum infrared flame detectors are the highest performing detectors and have excellent immunity to false alarms.
After performing a risk assessment, installation considerations include pressure of gas source—is it high enough for ultrasonic listening. For the point gas family of technologies gas must be able to reach the sensor. Considerations include the properties of the gas, ambient conditions and obstructions between the gas source and detector, open path technology to cross beam path. Depending on the area where the detectors are located, beams, point gas and ultrasonic listening have advantages and disadvantages.
For flame detectors, the optical sensor is like an eye. Considerations include the size of the area to be monitored, detection technology, obstructions, nature of flame source, and potential blind spots.
No one detector is a silver bullet for use in all applications. Each has their advantages and disadvantages and you will need to consider your application. You can connect and interact with other gas and flame detector experts in the Analytical group in the Emerson Exchange 365 community.
Edward M. Marszal, PE, ISA84 Expert
The proper placement of gas detectors can be difficult and complex to determine. Our last blog post discussed how gas detectors were placed in the past and current technologies for detector placement, including geographic coverage mapping. This post will address where the technology is headed in the future. The next level of analysis, which is defined in the ISA 84.00.07 technical report, is scenario coverage. While geographic coverage is a big step over grid placement because it allows for multiple different critical cloud sizes for different hazards, it still suffers from the lack of a more sophisticated mechanism for considering where leaks are coming from. Also lacking is information on the dimensions of the leak created clouds as this will be considering factors such as wind direction. The mechanism that can better deal with these issues is scenario coverage.
In scenario coverage, the covered area is not the desired output, but instead the fraction of area that is covered, the intention is to calculate the fraction of release scenarios that are covered. In this method, cloud dimensions from a release of process equipment (as determined through dispersion modeling) are determined from each piece of equipment in a zone. These releases are then simulated in the process area and compared against the detector locations to determine if a release gas cloud overlaps the location of a detector. If so, that release scenario is determined to be “covered.” For each release source in a zone, the released gas cloud is distributed in many directions (usually around 720 directions), and the effect of wind direction is considered in this distribution process. As such, if a typical zone is considered, there could be hundreds of thousands of release scenarios that are considered. The software then determines what fraction of the release scenarios are detected and compares that against a quantitative target. In addition to the quantified coverage target, colorized maps can be drawn that show, by the overlaid color, the frequency at which an undetected gas cloud is expected to exist in a certain geographic location. This method provides a quantum leap in richness of knowledge about the degree of effectiveness of a gas detection array, but it comes at a price of increased analysis time. We have seen, though, that many end users find enough value in the additional insight provided by scenario coverage to go through the additional effort.
If this level of analysis is not enough, more can be done, but at this point, the actual commercial applications are limited due to the significantly increased analysis cost and effort. Loss prevention specialists know all too well the limitation of the dispersion modeling that is often employed in scenario coverage. Dispersion models are quite weak in their ability to analyze “near field” effects of obstructions, and they are all but useless for indoor releases where the size and shape of a released gas cloud is almost entirely driven by the action of the building’s HVAC system.
In cases where dispersion modeling is less than effective, a more rigorous method for determining the size and shape of a released gas cloud is through Computational Fluid Dynamics (CFD). Gas detector placement using CFD models is a type of scenario coverage modeling, but the results are much harder to develop and to interpret. Essentially, the analyst builds a three-dimensional model of a facility, places releases, and then determines the gas concentration at the various detector locations after a release has occurred to determine if the release is “covered.”
While the richness of data results from a CFD modeling run is orders of magnitude richer than for dispersion modeling-based scenario coverage, it comes at a high price. CFD modeling consumes large amounts of analyst time for the development and interpretation of the models, and also the sheer computational effort often makes it prohibitive. Whereas a dispersion modeling-based scenario coverage assesses hundreds of thousands or even millions of scenarios, CFD assessment of a zone will typically only include 6-10 scenarios. The reason for the paucity of scenarios in CFD assessment is mainly attributed to computational resources. In order to run millions of scenarios that would be used in scenario coverage, CFD would require literally dozens of years of computing time. This degree of effort is simply not feasible. As a result, analysts need to use judgment to select a small subset of scenarios that would be representative of the complete data set. While this type of judgment works well for indoor facilities, where release patterns can be more easily predicted, it is difficult to reasonably represent the full range of outdoor scenarios and weather conditions. In any case, CFD-based analysis is already being used successfully for indoor scenarios and, with advancements in methods and computing power, could soon be feasible for outdoor analysis too.
by Edward M. Marszal, PE, ISA84 Expert
Gas detection is ubiquitous in a wide range of process industry applications where leaks of process equipment can result in either toxic or flammable gas clouds that can hurt people or property. Even though the detectors are ubiquitous now, their application is relatively new. In fact, only a few dozen years ago in the offshore oil and gas industry, the state-of-the-art gas leak detection was to measure the pressure in vessels in piping. If it was found to be below the trip point, then there had to be a leak somewhere. You would probably be shocked to know how many existing platforms in the Gulf of Mexico are still operating under this philosophy. As a result of the relative novelty of gas leak detection systems, the methods utilized to determine how many detectors are required and where they should be installed are also still in their infancy. But with the help of organizations like ISA, companies are starting to get more technically rigorous in gas leak detection and adoption is growing. In this blog series, I’d like to present a brief history of how detectors were placed in the past, discuss gas detection best practices, and then talk about where the industry is headed.
When gas detection instruments such as catalytic bead systems, point IR detectors, and electrochemical cells were first put into use in process industries, their placement was much more of an art than science. Industry employed veteran instrumentation and control engineers and safety engineers to use their “experience” to place detectors. These experts considered rules such as finding points where gases would accumulate, points where gases would be released, and measured the density of the released gas versus air to determine where detectors would be required. Unfortunately, these rules were often inconsistent among different experts, and led to widely different designs for similar facilities. Furthermore, studies, including ones performed by the UK Health and Safety Executive (HSE) found that the placement of fixed gas detection systems only identified less than 70% of the “major” gas releases that occurred in the process industries. This type of performance was deemed to be not acceptable, especially after a number of process industry accidents where detection systems failed to identify problems.
The poor performance of the “expert judgment” or heuristic placement techniques resulted in the first-wave application of more quantitative scientific analysis of detector placement. The next generation of detector placement is what we refer to as “the grid.” Loss prevention engineers, with a great deal of help from the UK HSE, made the decision that a good philosophy for the placement of gas detection equipment would be that if a gas cloud exists in a facility that is large enough to cause damage, it should be detectable by the gas detection array. HSE then undertook a program of study that determined that this objective could be achieved if gas cloud diameters could be limited less than 7 meters. If limited to this size, then the distance that a flame could travel through a gas cloud would be less than 7 meters which has been experimentally determined to not result in a vapor cloud explosion in most typical release scenarios.
Once the distance across the cloud was determined, the objective was to make sure that a cloud of that critical size could not hide in-between detectors without being identified.
The Dirty Bubble
Some of us practitioners affectionately call this cloud of critical size, “The Dirty Bubble” – a reference to a super-villain in the SpongeBob Square Pants cartoon that most of our children watch (and usually, us with them…). In order to ensure that the dirty bubble would always be identified by at least one detector, HSE determined that detectors placed on a five-meter grid would be required. Subsequently, a lot of designs – at least for offshore oil and gas production – were based on a five-meter grid.
A couple of problems were identified in the five-meter grid process. First off, the five-meter grid only worked for point detectors, and a lot of operating companies were starting to use open-path detectors. Second, the five-meter grid process did not differentiate areas of plants where leak sources did not exist, and thus leak detection would seem inappropriate. The grid system also was only applicable to combustible gases, and application of this approach to toxic gases was not effective, as there is no “safe” toxic gas cloud size. And finally, even if the hazard of concern was combustible gas, the efficacy five-meter grid is dependent on a number of specific parameters, such as the reactivity of the hydrocarbon gas and the amount of confinement of the gas cloud. For highly reactive hydrocarbons – such as ethylene oxide – five meters allows too big of a cloud, whereas methane in an open area would require a cloud much larger than five meters to be dangerous. The five-meter grid was a great starting point, but the limitations quickly became obvious, and solutions to the limitations were rapidly developed.
The next evolution of methodology for gas detector placement was the advent of what is now referred to as fire and gas mapping. This evolution focused on considering what fraction of an area is “covered” by a gas detector array. For gas detectors, this is essentially a function of the size of the “dirty bubble.” If the critical gas cloud size is five meters, then if a detector is within five meters of the center point of the cloud, it is “detectable.” Based on this theory, one could then plot the areas surrounding a detector that could be covered. The most primitive forms of this type of coverage analysis simply drew circles around point detectors on a plot plan drawing. As the technique developed, computer software was developed that would draw the coverage areas and calculate the fraction of covered area, along with distinguishing areas that were covered by a single detector from areas that were covered by two or more detectors. The advantages of this approach over simple grid placement were quickly apparent and quickly and widely adopted. The advantage of the coverage approach was that different critical cloud sizes could be defined for different hazard scenarios, and the total area that is desired to be covered could be limited to “graded areas” where hazards are known to exist – allowing non-hazardous areas to be ignored in the analysis. An example of a geographic coverage map created in the Kenexis Effigy™ FGS mapping software is shown in the figure above.
Gas Detection Coverage Map – Geographic Coverage – Kenexis Effigy™
At present, geographic coverage mapping for gas detector placement is the most commonly deployed sophisticated methodology for gas detector placement. But, other methodologies that are technically more robust are in rapid development and deployment. Our next blog post will address these new next-generation technologies.