The recording and documentation of methane emissions are becoming increasingly important – not just in the future, but right now – when it comes to quantifying leaks. Whilst leak detection and localisation have been established processes in network operations for many years, LDAR (Leak Detection and Repair) programmes are increasingly focusing on the question of what the actual emission rate from a leak is.
From methane leak detection to emission quantification
If a methane leak is detected in an underground natural gas pipeline during a pipeline network inspection, the scale of the emissions should be investigated in more detail and documented once the leak has been successfully localised. The bagging method is currently the most commonly used method for this purpose; whilst it is typically employed for quantification in above-ground installations, it can also be used to assess methane emissions from underground pipelines.
Challenge in quantification: Locating leaks
Methane from underground pipelines is not released directly at the point of the leak; it first spreads through the ground and then reaches the surface via the path of least resistance. This often makes it difficult to estimate the actual volume of emissions.
Consequently, the challenge is not only to pinpoint the location of the leak, but also to obtain reliable information on the scale of the methane emissions. This information forms the basis for prioritising repair work and maintaining documentation as part of LDAR processes.
Why methane emission quantification matters for pipeline operators
Modern quantification methods offer the opportunity to assess methane emissions more effectively and to utilise available resources in a more targeted manner. This provides network operators, utility companies and service providers with additional – and legally significant – information on which to base their decisions, enabling them to prioritise leak detection on a risk-based basis and to plan measures efficiently.
How the bagging (surface enclosure) method works
The bagging method enables the rapid measurement of methane emissions directly at the surface. To do this, the area where methane is being released is covered with a tarpaulin and the escaping gas-air mixture is extracted in a controlled manner.
The DVGW worksheet G 425-1 refers to the extraction of soil air using a tarpaulin spread out on the ground as a method for quantifying underground releases. This means the method can also be used for underground pipes.
In combination with modern measurement technology, the method enables a practical assessment of the emission levels and provides important information for the further investigation of leak points.
Equipment requirements for methane emission quantification
To determine the emission rate, a metrologically determined or known volume flow rate of the extracted soil air is required. The gas measuring device used to determine the methane concentration in the extracted soil air must have a detection limit of 10 ppm, i.e. at least 10 ppm must be detectable. This must be checked before each operation.
This applies to both methods mentioned in this article: bagging using a dedicated bagging probe and the extraction method using our Vakumobil.
How to quantify methane emissions step by step
Step 1: Locating the leak
The starting point for the measurement is a previously detected and localised methane leak. Based on the gas concentrations determined, the measurement area is defined and prepared for the measurement.
Step 2: Extraction and measurement
The gas-air mixture collected beneath the tarpaulin is extracted using the bagging probe. The Laser HUNTER continuously measures the methane concentration, whilst the flow rate is monitored via the TONI FlowTest.
Step 3: Assessment of emissions
Conclusions regarding the order of magnitude of methane emissions can be drawn from the recorded measurement values. The data obtained assists network operators in assessing, documenting and prioritising leak locations.
Extraction method using the Laser HUNTER and Vakumobil
The extraction (suction) method for underground installations is carried out using ground probes inserted into the soil following leak detection or localisation, and is described in detail in worksheet DVGW G 425-2. Other suction methods are permitted to a limited extent in accordance with DVGW G 425-1.
To carry out emission measurements of a leak in underground pipework, we recommend using the Vakumobil with borehole probes in combination with the Laser HUNTER.
Step-by-step guide to measuring emissions in an underground pipeline
Step 1: Define the measurement area
The measurement area is determined on the basis of a recorded gas concentration above ground and prevailing environmental conditions, such as the nature of the subsoil, and, where necessary, cleared and secured. The ground probes should be placed symmetrically around the suspected leak. Furthermore, care should be taken to ensure that the probes are ventilated evenly.
Step 2: Install ground probes
Depending on the nature of the subsoil, boreholes may need to be drilled for the ground probes. These should be as deep as possible, whilst ensuring that the underground pipe is not further damaged. In most cases, a depth of approx. 35 cm has proved effective. Furthermore, the free outflow from the leak should not be affected, and the maximum hose length specified by the manufacturer should not be exceeded.
Step 3: Extract and analyse soil gas
Once the measuring equipment has been set up and connected, the soil air enriched with methane/natural gas is extracted via a vacuum pump connected to the soil probes and analysed by a concentration analyser linked to the pump. The combination of concentration measurement and flow rate measurement of the extracted soil air enables the methane emission rate from the leak to be determined.
Step 4: Calculate the methane emission rate
The extraction flow rate must be sufficiently high so that no further methane leakage is detectable at the surface. In most cases, this condition can only be achieved using a suitably sized vacuum pump. Once the measured gas concentration and the flow rate have remained constant for approximately 10 minutes, the enriched soil air has been extracted and the emission rate of the leak itself is recorded. The measurement can then be terminated. Depending on the soil conditions, environmental conditions and extraction flow rate, it may take several hours to reach these constant values.
Simply request a free consultation with no further obligations
Are you a network operator, energy supplier or service provider who needs to implement measures to reduce methane emissions? We would be delighted to help you!