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This article provides insight into the planning, design, installation, operation and maintenance of fixed gas detectors in the oil and gas industry.
The analysis of past accidents in the oil and gas sector leads us to believe that the loss of primary containment could trigger a major accident, which would pose potential risk to people, assets and the environment – let alone the reputation of the operating company involved.
While oil and gas plants are designed to contain, one should recognise that they are asset intensive and each section has a number of flanges to connect valves, gauges and pumps. In addition to wear, corrosion and human error, these flange joints are the weak links for a potential leak. Any such leak must be detected early to prevent and mitigate the major consequences. The main objective of installing gas detectors is to identify any dangerous conditions, including flammable or explosive atmospheres, oxygen deficient environments and high levels of toxic airborne contaminants.
These environmental conditions translate into the categories of sensors required to detect:
• Flammable gases
• Toxic gases
• Oxygen
In terms of fixtures, gas detectors can be divided into fixed and portable types. As indicated by their name, fixed detectors are permanently installed in a chosen location to provide continuous detection of gas leaks. These detectors provide early warning of leaks, which can then be actioned appropriately to prevent and mitigate the consequences. Portable detectors are small, handheld devices that can be used for testing an environment prior to entry or carrying out a task, for example, testing a confined space for the presence of flammable or toxic gas or vapour before entering.
Plan and design
Gas detector arrangement has to be optimised based on the hazards associated with specific scenarios, as well as to cover the entire facility, reduce the potential for false alarms and enable access during inspection and maintenance. It is important to understand that installing high quality detectors at inappropriate locations may not serve the gas detection objectives. As outlined in the following sections, there are four approaches for planning and designing a gas detector arrangement for any given condition: experience based, prescriptive standards, risk based, and a combination of all the above.
Experience based In this approach a facility relies on the experience of staff and in particular their previous experience of installing gas detectors in similar facilities. This is neither based on any scientific justifications nor on the standards.
Generally a walk through is conducted on the facility to ascertain the gas detector requirements and arrangements. This approach may identify locations, but may overlook the gas accumulation zones and may not fulfil the requirements of the local standards and regulations.
Prescriptive standards In this method the detectors are designed based on standards and regulations. The most widely adopted standards are NFPA 72, fire alarm and signalling code and EN 54, fire detection and fire alarm standards. These standards are encyclopaedic and cover entire aspects including selection, installation and maintenance. Standards also specify requirements for signalling after detection.
• Generally the following factors are specified as guiding principles in locating gas detectors in prescriptive standards:
• Vapour density – below potential leak source for products heavier than air and above potential leak sources for products lighter than air
• Leak pattern and environmental conditions – close to potential leak sources along the predicted leak trajectory
• Location of potential ignition sources
• Impediments
• Access for inspection and maintenance
Due to the innovations, technological changes, evolving compliance requirements and changing market conditions, over time gas detector placement techniques have transformed. Companies have started providing complete and optimal solutions rather than just supplying equipment. These factors drive the oil and gas community towards adopting more scientific approaches. Further to this, it is to be recognised that prescriptive standards have shortcomings, as they are oriented towards occupied buildings and not oil and gas processing facilities.
The oil and gas sector is evolving to meet the changing market conditions. Changes in environmental legislation, for example, led to changes in gasoline and diesel fuel specifications. These changes encourage the oil and gas facilities to use hydrogen for improving the product specification. Further, the ever increasing crude oil prices have compelled processing of heavier crudes, requiring more extensive conversion and product treatment processes, which eventually require more scientific and risk based approaches in gas detection regimes.
Risk based In the risk based approach, a quantitative risk of individual hazards is assessed and risks are quantified to ensure they are as low as is reasonably practicable (ALARP). Use of this technique enables the facility to be more responsive and specific to the gas detection needs. The entire facility is classified into different zones to determine gas detector coverage.
The following factors can be used to determine this zoning:
• The nature of the material handled and its inventory
• Process parameters such as pressure and temperature
• Layout considerations and ventilation patterns such as open area, congested or confined
• Asset protection value
• Occupancy rate
Once the zones are identified, a computer simulation model is used to map the potential gas dispersion in all the zones. A preliminary gas detector arrangement can then be designed by considering the potential gas dispersion and the gas detection targets to be set by the designer. The detection target is driven by the minimum gas cloud size that could result in a significant consequence. Some empirical values can also be used to set detection targets.
A study by the UK’s Health and Safety Executive (HSE), for example, determined that in an offshore platform the energy contained in a six metre propane or methane gas cloud could create damaging pressure waves after ignition. The HSE determined that gas detectors placed at five metre intervals could potentially minimise the damage levels. Readers should also note that toxic gas mapping is more challenging, as there is no general rule for toxic gas cloud size. In this case, concentrations that are considered as immediately dangerous to life or health (IDLH) could be a guiding factor. If the toxic gas concentration can reach levels IDLH, then there is a need to detect its release.
Selection of appropriate performance targets plays a critical role in effective gas detection. The general rule of thumb is the smaller the gas cloud size, the smaller the hazard size and the more effective the gas detection system. Such an approach, however, will require a plethora of detectors all over the facility, which will incur huge cost. This emphasises the need for optimisation, which can be achieved by adopting the ALARP principle. In simple terms, ALARP attempts to strike a balance between cost and benefits.
Based on the above principle, a preliminary gas detector arrangement should be designed. This arrangement must then be verified with the manufacturer’s specifications for suitability and facility visits will be made to check for impediments and access for inspection and maintenance. If it is a facility to be built then this could be verified during the model review process.
Combined approach It is ideal to use both prescriptive and risk based approaches for designing gas detector arrangements. It will be particularly beneficial to use a prescriptive approach for occupied buildings outside the process areas. The location of the gas detector should be determined by analysing the air movement in the building. In an oil and gas facility these approaches compliment each other to formulate an optimal arrangement.
Signalling requirements In determining the signalling requirements, the following factors should be considered:
• Standards and requirements to ensure compliance
• Time required to respond to the signal
• Actions to be taken following the signal
• Toxicity of the gas or vapour
• Any other locality specific factors
It is also necessary to account for ventilation of dead spots where vapours could accumulate, as well as the variability of natural ventilation. To overcome this, it is industry practice to set two levels of signals. One signal, at a lower level, is set as a warning of a potential problem requiring investigation. The higher level signal could trigger an emergency response, such as evacuation or shutdown or both. In general, the lower level signal is set at less than or equal to 10% of the lower explosion limit (LEL) and the higher level signal is set at less than or equal to 25% LEL.
There is a threat that gas detectors are prone to false detection, which can lead to emergency shutdowns. To avoid this a voting system is adopted in the design. Gas detector signalling response should be designed based on the voting system. As an example, a voting configuration of two out of three implies at least two out of the three detectors must detect hazardous gas levels before any signalling system is activated. One limitation of the voting system is that the gas cloud has to grow and be big enough to be detected by multiple detectors before the signalling system will activate.
On the contrary, a false signal in a system indicates that it is partly or completely not functional and in the case of a gas leak the system may not respond as intended.
Management of change Any changes in the materials or process should go through the change management process to ensure the detection and signalling systems are appropriate for the change. If not, measures should be taken to redesign the detector and signalling system.
Gas detection technology
Various gas detection technologies are available in the market place, which can broadly be grouped into:
1. Point gas detection such as catalytic bead sensors or infrared sensors, which work on the gas diffusion principle.
2. Open-path detectors, also called beam detectors, typically consist of a radiation source and a remote detector. The detector measures the average concentration of gas along the path of the beam. The unit of measurement is concentration multiplied by path length, % LEL x m or ppm x m. Systems can be designed with path lengths ranges from 40-140 m.
3. Ultrasonic Gas Leak Detectors (UGLD) have emerged to overcome certain limitations of the above traditional technologies. In traditional gas detection systems the gas has to form a cloud and it should be either in close proximity to detector or within a defined area. UGLD respond by sensing the sound emitted by high pressure gas leaks. The advantages of these detectors are they are not affected by the wind direction or gas diffusion, but they can be affected by background noise within the facility.
Installation
After planning and design the gas detectors have to be procured and installed in the facility. It is important to maintain integrity standards during manufacturing and installation. A well-designed detector arrangement will be effective only if it is installed as planned and designed. Location and orientation of the detectors are the primary factors to be strictly adhered to while installing the detectors. If for any reason the detectors cannot be placed at the specified locations and orientations, it is then essential to rerun the risk models to confirm the coverage of hazards.
Installation should then be followed by a loop test and Site Acceptance Test (SAT) to confirm that the detectors are functioning as per design intent.
Operation and maintenance
After successful completion of SAT, gas detectors should be put into normal operation. Throughout the operation phase regular functional tests have to be performed to ensure proper function of gas detectors. Successful function of a gas detector system not only depends on the function of the gas detectors, it also depends on the functions of the interfaces such as emergency shutdown, blow down system and ventilation, as well as utilities including an uninterrupted power supply. Signals activated by a gas detector will also provide insights into the facility’s maintenance schedule.
Similar to any other equipment, gas detector performance deteriorates over time. It may become aggravated further if gas detectors are installed in harsh environmental conditions. Gas detectors should be included in facility inspection and maintenance schedules, considering manufacture’s recommendations and the local environmental conditions. Depending upon the criticality some of the gas detectors may require replacements while others are taken out for maintenance.
As part of the maintenance activities, gas detectors must be calibrated at a definite frequency using the same gas that the gas detector is designed to detect. The calibration gas should contain the same mixture and concentration to those being monitored. Sometime this is not practical, in which case another gas mixture that gives a similar response to the target gas can be used. The calibration output then has to be corrected with a calibration or correction factor, generally provided by the manufacturer. After calibration, the calibration mixture gas has to be disposed of safely as per local regulatory requirements.
Conclusion
Unfortunately, many gas releases are not visible to the human eye. Further to this, our limited experience of gas behaviour and dispersion after release adds to the complication. In today’s world we rely on our experience with smoke clouds caused by fires to understand gas releases. It is worth note, however, that these are two different subjects, as thermal energy generated by fire dictates the behaviour of smoke clouds, which is not the case in gas releases. The gas release will have a high concentration close to the leak source and this reduces over distance, until the release reaches a point where harmful effects are unlikely.
Companies put a great deal of effort and resources into the planning and design of facilities and none are designed to leak. Unfortunately, some equipment still has the potential to leak, and a combination of minor leaks may lead to major accident. Having an effective gas detection system in place, therefore, could prevent and mitigate the major consequences.
It should be understood that the intention is to achieve excellence and optimise spending. Sound integration between safety and operations can lead to cost savings in production, replacement, insurance and lost time. It is necessary to develop solutions that are supported by an appropriate business strategy that includes the necessary policies, regulations and tools. Safety should no longer be perceived as a cost proposition. It adds value if it is understood, appreciated and applied consistently.
Published: 29th Oct 2014 in Health and Safety Middle East
