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HOW DO YOU SOLVE

A PROBLEM LIKE

SAMPLING? as it breaks down the waste product

used to produce the methane gas. It
is a colourless chalcogen hydride gas
and is commonly known for its foul
‘rotten egg’ odour, even at sub parts-
per-million (ppm) levels. H₂S is a
Sophia Rushton, Project Engineer at Orbital Gas Systems, safety critical measurement due to its
corrosive and poisonous nature.
explains the importance of a representative sample for
critical measurements in biomethane It all starts from the pipeline
The design of a measurement system
iomethane is a more sustainable Obtaining a representative sample is does not just include selecting the
alternative to natural gas an essential first step toward a reliable correct analyser for the application.
found in conventional wells, measurement that can be utilised to There are many elements to consider,
as it uses readily available regulate the process and ultimately from the conditioning of the sample
B organic material already what enters the network. to its transportation. Nevertheless,
present on the surface of the earth. the design should start as early in the
Any gas, biomethane included, that Critical measurements in biomethane system as the process tap i.e., where
enters the natural gas transmission In biomethane, there are many critical the sample is extracted from the source
and distribution networks must meet measurements that are taken prior to stream of interest. The sample system
certain requirements for quality and allowing the gas to enter the network. is typically perceived as a simple part
safety, set by the government’s Health A selection of the main required of the measurement system, required
and Safety Executive (HSE) and outlined measurements includes: only to deliver a sample to the analyser.
in the Gas Safety (Management) Calorific value (CV) However, it is often cited as the cause
Regulations (GS(M)R). Gas composition of measurement problems during
Understanding and analysing a Moisture (H₂O) operation and with the number of
representative gas sample in any system Oxygen (O₂) components, materials and surfaces
is a vital process to undertake. This Hydrogen sulphide (H₂S) incorporated, it can have a number of
is even more important when dealing This article focuses on H₂S, due detrimental effects on the measurement
with biomethane, ensuring critical to the chronic effects and severe that may not be evident to the operator.
contaminants are measured so they do consequences this gas can cause within Therefore, careful consideration of the
not find their way into the network. the gas network. design of the sample system should
H₂S is produced as a result of the begin with the extraction point as a
The purpose of obtaining microbial breakdown of organic necessity and continue through until
a representative sample materials in the absence of oxygen. delivery to the analyser inlet.
The ability to acquire a representative For biomethane, H₂S is generated The most commonly used sample
sample can often bring difficulties for within the anaerobic digestion (AD) extraction tool is a sample probe. When
sample system designers, due to the plant as a by-product of the bacteria exploring this specific component of
requirement of obtaining a ‘true’ value. the sample system, in relation to taking
The final measurement should be a a representative sample, there are two
reflection of the process fluid to a known FIGURE 1 VE TECHNOLOGY SAMPLE PROBE WITH ENHANCED ways to enhance the accuracy of the
performance criterion. However, if the MEASUREMENT PERFORMANCE AND SAFETY FEATURES measurement:
sample has been altered in any way
between extraction from the process The use of an enhanced probe tip to
stream and delivery to the analyser, eliminate or reduce flow disturbances,
then the instrument measurement is no therefore improving the accuracy of
longer a true reflection of the process the sample taken.
fluid. It is no longer representative, There are several international codes of
meaning incorrect and even dangerous practice for sample probe design (GPA
decisions can be made by the 2166, API 14.1, EEMUA 138 and ISO
surrounding systems or operators that 10715). However, these focus principally
rely on that data. on designing to avoid mechanical

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