H2Tech - Q1 2021 - 32

INFRASTRUCTURE AND DISTRIBUTION
the lower 10% of the flowrate range.
Ensuring that the meters achieve a similar level of accuracy with H2 compared to
what is presently achieved with natural gas
is important. Given the number of transactions that occur between energy suppliers and consumers using domestic gas meters, even small errors compared to those
presently observed with natural gas would
likely result in a large financial exposure. If
the performance of the flowmeters in H2
is not verified, this could ultimately lead
to a lack of market confidence and hamper
the incorporation of H2 into the gas grid.
When H2 is used as opposed to natural gas, the accuracy of the diaphragm
flowmeters may be affected. The energy
content of natural gas is approximately
three times greater than H2; so, to deliver
the same number of kW to consumers, the
flowrate of H2 must be three times larger
than natural gas. As the meters were designed only for operation in flowrates that
are typical for natural gas, they may show
increased mechanical wear if the flowrate is
increased by this amount. The higher flowrate may also increase the potential for internal leakage, due to the greater tendency
of H2 to leak compared with natural gas.
For blends where only a proportion
of natural gas is replaced with H2 , the total volumetric flowrate will not be three
times larger. Provided that the increase
in flowrate is not excessive, diaphragm
gas meters may still be used for blends
while maintaining conformance to the
accuracy requirements of the MID. However, for pure H2, this is unlikely to be the
case. For mechanical-based meters such
as the diaphragm type, larger flowrates
of gas are accommodated by increasing
the physical dimensions of the flowmeters. Existing domestic gas meter boxes
are based on the existing meter size, so
larger diaphragm meters may need a new
or retrofitted meter box, which would be
expensive and time-consuming.
Aside from diaphragm meters, ultrasonic meters are the second-most-common domestic gas meter type used in the
UK. The principle of ultrasonic meters is
based on measuring the time it takes for
an ultrasonic signal to pass upstream and
downstream through the flowing fluid.
The difference in time taken between the
upstream and downstream signals can
be used to determine the velocity of the
flowing fluid and the volumetric flowrate.
Like diaphragm meters, ultrasonic me32 Q1 2021 | H2-Tech.com

ters demonstrate conformity to the MID
through the harmonized standard for
ultrasonic domestic gas meters: BS EN
1359:2017. This standard requires that
ultrasonic meters conform to either the
Class 1.0 or 1.5 requirements in the MID.
These classes have an MPE of ±1% and
±1.5% upon initial testing, respectively,
with allowances up to ±2% and ±3% for
the lower end of the flowrates tested.

One aspect that is
commonly overlooked is
the ability to effectively
measure and, therefore,
trade hydrogen.
The performance of an ultrasonic
meter is known to be more affected than
that of a diaphragm-type meter by physical properties such as the speed of sound,
attenuation, viscosity and density of the
gas. The speed of sound of H2 is approximately three times greater than methane
at standard conditions, meaning that the
time taken for an ultrasonic pulse to travel
to the detector would be three times less.
At the very least, this would increase the
uncertainty in the ultrasonic pulse transit
time and, ultimately, the overall uncertainty in the volumetric flow. Also, the
correction factor used for ultrasonic meters is generally dependent on the Reynolds number. The change in viscosity,
density and velocity of H2 compared to
natural gas will result in a change in Reynolds number; therefore, it is vital that the
correction factor accounts for this.
Despite the issues posed for the use of
existing ultrasonic meters in H2, they are
not insurmountable, and it is possible that
existing meters could be adjusted for H2
service. In fact, ultrasonic meters provide
a distinct advantage over diaphragm meters, in that they should be able to cope
with the increased flowrates without requiring larger physical dimensions.
As with diaphragm-type meters, while
conformance to the MID is essential for
domestic gas meters to be used in the EU,
it is important that this is proven for the
use of H2 to ensure market confidence.
As there has been no requirement for dia-

phragm and ultrasonic meters for domestic application to be tested using H2, there
is a lack of traceable testing facilities that
have this capability. This means that there
is no way to unequivocally confirm that
the accuracy of the flowmeters used in H2
will be the same as for natural gas.
To address this, we have built a facility
to accommodate testing in H2. As previously discussed, the switchover from natural gas to H2 may take place in a staged approach involving H2 and natural gas blends
or direct switch to pure H2. To accommodate this, the facility allows for a complete
assessment of meter performance across a
range of H2-to-natural gas compositions
from 0%-100% H2. The facility makes use
of high-purity bottled gases fed through a
manifold to supply gas-and-gas mixtures
to a test section at precisely controlled
pressures and flowrates. The flowrates
are measured, using precision reference
instruments calibrated to national standards. The facility can operate over a range
of flows, pressures and temperatures to reflect those experienced in service.
This new facility provides a platform
not only to investigate the performance
of existing meter stock, but also to allow
the development of new meters designed
specifically for H2 service. The findings
from the research and testing carried out
at this facility will help the switch to H2
take place safely and ensure that the end
users are billed accurately and suppliers
have economic certainty. It will also provide a facility capable of rendering the ongoing type-testing and meter verification
services required by the industry.
Flowmeter testing at this facility is due
to start imminently. Initially, the facility
will be used to test various domestic gas
meter types such as diaphragm, ultrasonic
and thermal mass with H2 and methane
blends. It will also be used in the EMPIR
NEWGASMET project as part of an inter-comparison study with standards developed by other flow laboratories around
Europe. This study will help validate the
performance of the facility and its uncertainty budget.
DALE ANDERSON is a Clean Fuels Engineer at
TÜV SÜD National Engineering Laboratory (NEL)
in Glasgow, Scotland, where his primary focus is
understanding the flow measurement challenges
for H2, CO2 and LNG. Since joining NEL, he has been
involved in various projects related to the design
and uncertainty assessment of physical testing
facilities. NEL is the UK's Designated Institute
for Flow Measurement.

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