When it comes to maintaining the safety of LNG facilities, what are the gas and fire detection requirements?
The world interest for energy and cleaner fuels is growing and liquefied natural gas (LNG) plays an important role in meeting these needs. Scale and expansion of LNG demand raise fundamental questions concerning safe handling and use. Some LNG infrastructures, especially vehicle fuelling stations, are located within public areas and many countries now require installation of both gas and flame detectors to address public safety concerns. In Europe, several LNG facility safety standards have been adopted. EN 13645 Standard for the design and construction of onshore stationary LNG installations, with a storage capacity between 5t and 200t, stipulates in clause 6, “Consideration shall be given to the installation of fixed leak detection systems with executive action to stop the leak source, to isolate relevant sections of plant and shutdown sources of ignition in the vicinity.” The newest ISO 16924 specifies the design, construction, operation, maintenance and inspection of stations for fuelling LNG to vehicles, including equipment, safety and control devices, indicating that offloading area shall be equipped with leak detection systems. EN 1473 Standard for larger installation and equipment for liquefied natural gas specifies in clause 5.3, “A detection system shall be provided to give warning of any leakage of LNG or natural gas and also to give warning in the event of fire.”
Considering these requirements, practical deployment of fire and gas detectors that maximise detection efficiency is vital. This approach is based upon the notion that any single detection technique cannot respond to all hazardous events and consequently, risk of detection system failure is reduced by deploying detection instruments that deliver complementary strengths while minimising traditional technology limitations.
Improved safety through gas detection system diversity
Hazards associated with LNG include cryogenic temperature of constantly boiling liquid, vapour expansion and dispersion characteristics making it highly flammable. Although a combination of hazards occurs in most instances, the primary one is leakage of a flammable mixture that can lead to fire or explosion. As LNG’s minimum ignition energy at atmospheric pressure is around 0.28mJ, it can readily be ignited.
The first step in fire escalation and detonation is loss of gas containment. When released at temperatures below -110°C LNG vapour is heavier than air and the vapour blankets the ground, with clouds travelling in the wind direction. As soon as the temperature is above -110°C, the LNG vapour becomes lighter than air and will rise when sufficiently warmed by ambient air.
To address hazards posed by changing LNG vapour behaviour, MSA’s fire and gas detection systems work within the construct of layers of protection to reduce hazard propagation. Using such a model, each layer acts as a safeguard, preventing the hazard from becoming more severe. Detection technology layers encompass different hazard detection techniques that either improve scenario coverage or increase the likelihood that a specific hazard type is detected. Such fire and gas detection layers can consist of point and open path detectors as well as ultrasonic gas leak monitors and flame detectors.
Compared to just 20-30 years ago, when catalytic bead and electrochemical point sensors were the standard in fixed gas detection, there has been a steady development into more advanced sensing technologies such as enhanced laser diode spectroscopy (ELDS) open path gas detectors, which can respond much quicker to gas releases. In turn, continuous gas monitors such as infrared point detectors can contribute to detection of small leaks. To further protect an LNG plant against fires, flame detectors can supervise entire process areas.
Why choose a laser-based gas detection system?
The technology behind Senscient ELDS open path gas detectors relies on ELDS to detect specific flammable and toxic gases (including methane or H2S). In the event of a gas leak, the sensor’s laser technology recognises and analyses a gas’s specific harmonic fingerprint. During normal operation some of the detector’s laser light is reflected continuously through a sample of the target gas contained by a hermetically sealed reference cell. This design ensures the laser remains locked on the selected gas wavelength for the specific target gas.
The detector’s harmonic fingerprint technology helps ensure precise gas recognition, eliminating the potential for false alarms, even during adverse environmental conditions.
False alarms are a serious problem with many gas detection technologies. They can result in excessive plant downtime, which often requires complex investigations and regulatory reporting. From a safety perspective, frequent false alarms lead to a lack of confidence by employees in the gas detection technology and a culture of apathy that can cause employees to fail to act promptly during an actual emergency event.
Class 1 eye safe lasers designed into ELDS detectors are used to penetrate thick fog, heavy rain and snow beyond the capability of traditional open path infrared (OPIR) detectors. With its automated SimuGas safety integrity self-check, there is no need for the typical OPIR sensor gas checks and recalibrations requiring field technician time to address. Unlike electrochemical cells, ELDS sensors are also immune to sensor poisoning and interferent gases, thanks to their gas specific harmonic fingerprint detection method.
Although the cost of an open path detector may be higher than traditional point gas detectors, the total installed cost can be similar or less expensive than installing multiple fixed-point devices to achieve an equivalent coverage area.
Innovative gas detection sensing technologies for LNG facilities
It is important to understand that for optimum performance and protection, more than one single gas sensing technology is recommended for installation. This means that open path detectors should be used in combination with complimentary conventional gas detectors to achieve optimum protection at LNG facilities. Standard point gas detectors using catalytic sensing technologies have been available for many years and MSA XCell sensor features significant technology improvements worth mentioning. Catalytic bead sensing element has been designed and engineered with a larger pellistor and support infrastructure having more active gas reaction/sensing sites for better field stability and less drift.
Specifically, for LNG applications point infrared gas detectors of Ultima X5000 series with XIR Plus sensor often play a vital role providing the fastest response time, long term stability and significant reduction of calibration gas cylinder consumption and overall cost of ownership. The advantage of infrared sensing technology in detection of natural gas leaks comes from immunity to poisons such as silicones and lead compounds. Because no sensor combustion occurs, IR sensors do not experience corrosion due to by-product exposure.
Although the safety industry standard has always been one sensor per transmitter, the Ultima X5000 Gas Monitor doubles the coverage available with the ability to connect two sensor inputs into one transmitter. This economical dual sensor design dramatically reduces the cost of wiring, conduit and technician installation time to make safety even more affordable. It allows now for example to have supplementary oxygen or toxic gas detection in areas where, there is a risk of asphyxiation from LNG release or exposure to toxic gases.
Additionally, H2S detectors are also equipped with XCell digital sensors featuring TruCal technology, validating the ability of response to a gas leak at certain intervals and compensating for sensitivity drift due to changing environmental conditions. This is especially important for the beginning of LNG processes where H2S from extracted sour natural gas has to be removed and detected. Such distant location can now benefit from digitalised sensing technology with extended calibration intervals as well as user warnings for upcoming sensor replacements.
Choosing the right detector platform for complex LNG facilities would not only provide a better response to the hazard but also contribute to a lower total cost of ownership.
Expecting more from gas detection systems
Whenever there is a need for providing a containment system for pressurised gas fails, the leak can be quickly detected using ultrasonic gas leak detectors (UGLD) that detect airborne ultrasound produced by turbulent flow above a pre-defined level. Using ultrasound as a proxy for gas concentration is a major advantage since ultrasonic gas leak detectors do not require transport of gas to the sensor element and are unaffected by leak orientation, gas plume concentration gradient and wind direction. Due to the nature of LNG and low storage pressure, UGLD are recommended for installation at the point of regasification, within gas turbine skids and enclosures, and also during the LNG liquefaction process where pressurised refrigerants are used.
Optical flame detectors provide another important layer of protection against LNG fire hazards. Delayed ignition of an LNG vapour cloud in open air will typically burn steadily along the flame front back towards the release source. Depending on the source of the release, either a pool fire, jet fire or two-phase release fire could be the subsequent result. However, in case of an LNG release within a confined space contacting an ignition source, the LNG release could result in an explosion. In all cases, optical fire detection systems play a significant role in protecting LNG facilities.
Optical flame detectors sense radiant energy from an open flame by employing infrared, visible, and/or ultraviolet sensors usually in combination within a single detection instrument. However, for LNG applications, the technology with multiple IR sensors is to one enhancing nuisance alarm rejection and optimising detection performance of a true fire. The design intent of the optical flame detection system is to provide early and fast detection of an LNG ignition event and subsequently execute automatic mitigating actions and alarm notification to all people and systems potentially affected by the fire.
Multi-spectrum gas detection system
Multi-spectrum IR flame detectors use multiple infrared sensors, each monitoring slightly different spectral regions, to further improve discrimination of nuisance non-flame energy from true fire hazards as well as enhance detection range and overall flame sensitivity compared to other technologies.
They exhibit relatively high immunity to infrared radiation produced by arc welding, sunlight and other hot objects and are optimal for jobsites where optical contaminants may be present.
The presence of airborne optical contaminants and potential nuisance non-fire energy sources within monitored/protected zones onsite can have a significant impact on the performance of certain sensing technologies and proposed installation locations. Recent technical innovations using neural network technology (NNT) has proven very useful in LNG flame detection applications.
MSA’s multi-spectrum FL4000 flame detectors offering NNT signal processing provide better application coverage with fewer instruments, faster speed of response, and enhanced nuisance/false alarm immunity than legacy multiple IR technologies. The NNT processor converts the various spectral sensor signal levels into a digital format then extracts time and frequency information. The information is then processed by the neural network algorithm, which classifies it as being emitted from flame or non-flame sources and drives output signals accordingly.
LNG production, transfer, storage and distribution processes continue to expand and grow globally. However, rising demand for production and distribution must be matched by a comprehensive approach to safety. Informed and sophisticated users and consultants will ensure adequate LNG facility safeguards are specified to protect people and equipment from potential life safety hazards, at the same time the design of old facilities should also be revisited to ensure that sufficient layers of protection are available to minimise accidents and control failure. Safety systems that deploy diverse, complementary early hazard detection technologies can counteract possible effects of leaks, fire and explosions, preventing equipment or property damage, personal injury and loss of life.
