Flow, level & control


Figure 4: Relative measurement error at the reference gas flow for five flow sensors in 100 per cent H2 (a), 98 per cent H2 + two per cent CO2 (b).


maximum allowed for class A hydrogen as defined by ISO 14687. Again, the error limits for class 1.5 meters of ± 3.5 per cent and ± 2.0 per cent are indicated in black (for a meter with Qmax of 20m3


/h).


One can see that the meters perform very well when working both with pure hydrogen


and with hydrogen with two per cent of CO2 contamination. It is important to mention that the size of the sensor (and consequently the meter too) can stay the same whether it operates with a 6m3 20m3


/h flow of natural gas or /h of hydrogen. OperatiOnal safety


Sensirion’s thermal-mass flow sensors do not have any safety-related limitations when operated with natural gas or hydrogen. Both the maximum temperature and the maximum thermal energy stored on the micro-sensor element are significantly below hydrogen/air mixtures’ ignition temperature or ignition energy, even if the flow sensor’s voltage regulator malfunctions. This is why Sensirion’s thermal- mass flow measurement technology has been successfully used for years in challenging gas analysis applications with pure hydrogen.


COnsistently COmpaCt size fOr any hydrOgen COntent


When substituting natural gas with hydrogen, it is important to bear in mind that hydrogen’s calorific value by volume is about three times lower than that of typical natural gas mixtures. In practice, this means that, if a gas appliance is operated with pure hydrogen instead of natural gas, a gas volume approximately three times greater must be supplied to achieve comparable heating power. In this case, gas meters originally designed for operation with natural gas need to be capable of measuring an increased gas volume due to the admixture of hydrogen. Consequently, it may be necessary to choose purely volumetric natural gas meters in a larger size and that also have an extended dynamic range (if they have to be compatible with both natural gas and pure hydrogen). Larger meter designs can be more costly and require more installation space. If larger gas volumes flow through a meter than it was originally designed for during operation


56


with hydrogen-free natural gas, this can increase the wear on the meter mechanics, thus shortening the service life. Similarly, ultrasonic gas maters face a


challenge due to hydrogen having an increased speed of sound compared to that in natural gas (approximately by a factor of three). This means that the sound flow path has to be physically lengthened and the electronics used for the measurement have to become significantly faster. Consequently, one should expect such a meter to be larger, more complex and more expensive. In contrast, thermal-mass flow measurement


technology is a static measurement principle that has no moving parts and directly measures mass flow. Consequently, increased volume flow does cause additional wear and has no influence on a thermal-mass gas meter’s service life. Unlike with volumetric or ultrasonic gas meters, thermal-mass gas meters can remain the same size and just as simple whether they are operated with natural gas or with any optional hydrogen content. In the thermal-mass measurement principle, the key parameter that ought to be considered is not the gas volume flowing through the meter; rather, it is the relevant gas mixture’s Reynolds number. This is a fluid dynamics parameter that tells us whether turbulent (high Reynolds number) or laminar (low Reynolds number) flow conditions are formed in a system. Comparing the Reynolds numbers for pure


methane ReCH4 (representing a natural gas mixture) and for pure hydrogen ReH2 shows that ReH2 is lower than ReCH4 by a factor of more than six for the same meter housing


geometry. Assuming that hydrogen flow increases by a factor of three (to compensate for hydrogen’s lower calorific value, which is


three times lower than natural gas), ReH2 is still about two times lower than ReCH4. Compared to methane, the lower Reynolds


number for hydrogen means that measuring conditions remain stable at all times with the same meter housing geometry, even if the volume flow increases by a factor of three. Similarly, the pressure drop across the meter


will not increase with the higher hydrogen flow required. The pressure drop across the meter is proportional to gas density×velocity2


. Since


the density of hydrogen is a factor of 14 lower than that of methane, the pressure drop when flowing hydrogen at three times a higher rate than methane will be actually lower. Consequently, the same meter size can be


readily used for both natural gas and for operation with up to 100 per cent hydrogen. A typical G4 thermal-mass meter can handle Qmax of 6m3


gas and over 20m3


/h during operation with natural /h when used with


hydrogen. Moreover, no recalibration or setting change is needed to switch between different gases and the same sensor can seamlessly adjust itself to the gas supplied.


COnClusiOn and OutlOOk


The measurement data presented here demonstrates that the thermal-mass measurement principle complies with the error limits for measurement accuracy and the air-gas relationship as stipulated by the MID for various natural gas/hydrogen mixtures and pure hydrogen. There are no limitations as regards operational safety, even during operation with 100 per cent hydrogen. The size of thermal-mass gas meters, which is already very compact, can be maintained regardless of a gas’ hydrogen content. This is a distinct advantage when compared to mechanical and ultrasonic meters. It eliminates the need for expensive, large meter designs and keeps the logistics and installation of thermal-mass gas meters simple and affordable. In recent years, technological advancements


in gas meters have mainly involved enabling them to communicate as smart meters. Hydrogen admixtures will drive further modernisation in the gas meter industry, seeing a move away from old, mechanical and volumetric measurement principles toward modern technologies that can offer significant advantages for operation with hydrogen. Over six million gas customers worldwide are already benefiting from reliable and fair billing thanks to their thermal-mass gas meters. Going forward, hydrogen admixtures will promote the ever-faster uptake of this compact, static metering technology.


Sensirion www.sensirion.com January 2022 Instrumentation Monthly


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