A FRESH APPROACH TO NATURAL GAS NETWORK SURVEYS
Against the current backdrop of a general review of energy issues, gas operators are playing a key role in national and local energy policies. Even though their status varies by country - some are public, others semi-private or private - their mission is the same: transporting gas to the end user in a manner that offers the highest quality, the greatest safety and the least cost. While quality and cost issues are covered by the policies specifi c to each company, the risks inherent to gas make safety a concern that goes well beyond the operator: an issue of national or even supranational importance.
For example, European laws make it necessary for natural gas distributors to keep natural gas systems safe through the systematic survey of the totality of their network at least once every four years. In recent years, they have also been required to provide computerised traceability of surveys, with precise dating and location of the leak indices that may have been detected.
All of which makes gas network survey and leak detection a science that must be as exact as possible.
What Does Network Survey Mean?
A gas network survey is carried out within the regulatory framework set by each country, at frequencies that vary widely from one country to another depending on the characteristics of the system: age, type of pipes, service pressure, etc. Its purpose is to heighten the safety of the distribution network by giving priority to the detection of leaks with a major safety risk (explosion) and identifying leaks that need to be treated in a programme of preventive system maintenance.
Survey operations are generally carried out from a vehicle and/or on foot, with an appropriate detector. The survey of transmission pipelines involves the use of helicopters or drones with thermal cameras or a remote laser measurement system.
In all cases, the principle is the same: the aim is to measure the methane (or propane) in an atmospheric sample taken on the ground level, as close to pipes as possible.
Survey on foot or from a vehicle makes it possible to identify leak indices, which then need to be confi rmed by precise location (sampling) or precise measurement above the ground. The risk generated by the leak will depend on the concentration level, the closeness of public buildings and/or dwellings or the risk of the gas fl owing along sewers and underground electrical or telephone cables.
The Limits of Each Method
Historically, network survey was done on foot. Even today, it is the best method for detecting the smallest leak indices, for two reasons. The sample is taken directly on the ground, just above the pipe. The drawback is that an operator on foot can only cover 6 to 8 km on average every day. It is therefore easy to understand why the extension of distribution systems and economic constraints have contributed greatly to the development of vehicle-based network surveying. Depending on traffi c density, a vehicle can cover 30 to 50 km every day, or 5,000 to 6,000 km every year.
However, vehicle surveying has its own limits: its on-board detectors must be more sensitive, and it does not allow access to all the zones requiring monitoring. Piping segments with connections and meters that can only be inspected on foot account some 10 to 15% of the totality of networks in many European Union countries.
IET May / June 2017
www.envirotech-online.com
The Many Constraints of Gas Network Surveying
Network surveying is subject to a number of constraints in the fi eld, which are critical in defi ning the qualities of an effective detection system.
• Locating leaks outdoors using fast-moving mobile equipment demands ppm-level sensitivity. Low gas pressure in the pipe, the type of road surface, the presence of gases that are heavier than air, for example in butane-air or propane-air networks, also make inspection more complicated.
• Road traffi c produces carbon dioxide and petrol fumes that can lead to false alerts. As a result, the measuring system must be perfectly methane selective.
• Increased inspection speed requires the shortest possible response time to locate the leakage point accurately.
• Changes in temperature and working conditions in the fi eld make high demands of equipment, which is required to offer measurement stability over time.
• Lastly, the obligation to date and geolocate survey operations makes it necessary to use maps, digital if possible, which are precise and up-to-date. That requires device intelligence and communication.
Use of New Technologies
Measurement techniques were initially based on fl ame ionisation (FID) or semiconductors, but these have gradually given way to measurement systems using optical technology that are more reliable and more selective.
At GAZOMAT™, a French instrument manufacturer working in close collaboration with a number of gas operators like GRDF (formerly known as Gaz de France) in France, we aim to offer a comprehensive solution for network surveying. The range includes a laser spectroscopy methane analyser (INSPECTRA®
LASER
portable) and the NGS survey software that controls the analyser, the maps and the GPS system.
The NGS survey software installed on a PC or PC tablet provides real-time visualisation of the survey circuit with continuous display and recording of gas leak detected and survey events along with associated GPS co-ordinates.
The patented measurement principle of the portable INSPECTRA® LASER analyser from GAZOMAT is based on the emission of a laser beam in a wavelength specifi c to methane and a multipass cell to provide 1ppm sensitivity over a measurement scale up to 100% gas volume.
The principle of measuring methane absorption by photodetectors means that only methane is detected. The device does not respond to other hydrocarbon gases, and so provides accurate measurements. Selectivity reduces false alarms and unnecessary travelling by gas emergency intervention teams. The immunity of the device to humidity and temperature keeps the measurements stable. The INSPECTRA®
response time, close to a second, with the benefi t of less dilution of the detected leaks in the air, and a shorter operating time.
Lightweight and compact, the INSPECTRA® in a portable version with the INSPECTRA®
system is available LASER detector
for detecting leaks on foot, and a vehicle version named VSR INSPECTRA®
, with an inspection speed up to 40 km/h.
For greater optimisation and survey simplifi cation, we integrated the computerised recording of survey data, along with traceability and geolocation using an associated GPS system. The NGS system survey software developed in house thus takes care of interaction with measurement and geolocation equipment. It can be used on foot or from vehicles, with the same guarantee of traceable leak detection.
While the INSPECTRA® vehicle travels along pipes, atmospheric INSPECTRA®
LASER portable analyzer with advanced Laser spectroscopy technonology for superior measuring performances
samples are taken directly from the surface of the ground through the intake manifold fi xed to the front bumper of the vehicle. The sample is taken close - 4 to 5 cm maximum - to the surface of the ground, avoiding excessive dilution in the air. With standard vehicles, air is taken in up to 4 m on each side of the vehicle. A fi ltration device protects the instrumentation from water and dust.
system was developed to offer a very short
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