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The guide allows even very small flow meters, with diameters of 1mm, to be characterised
EMPIR project ‘Metrology infrastructure for high-pressure gas and liquified hydrogen flows’ (20IND11, MetHyInfra) is developing new standards and technologies for ensuring accurate dispensing at hydrogen pumps.
The development of renewable zero-carbon fuel sources is key to achieving the targets set out by the European Green Deal. This includes the installation of 40 GW of renewable hydrogen electrolysers in Europe by 2030 to supply the growing use of hydrogen for vehicles and industry. However, in order to maintain a high energy capacity, hydrogen must be stored at high pressures. Existing flow metrology and calibration services have been unsuitable for measuring hydrogen flow at these pressures, resulting in a need for new flow standards and calibrations.
Good practice guide
Critical Flow Venturi Nozzles (CFVN), also known as ‘sonic nozzles’, are commonly used as reference flow meters as they have high reproducibility and precision. As gas is fed through the nozzle, the pressure ratio between inlet and outlet leads to the gas reaching local speed of sound in the ‘throat’ (a narrower section in the middle of the nozzle). The flow rate is than calculated by measuring the pressure and temperature at the inlet. CFVN have the advantage over differential pressure methods, as disturbances in flow downstream does not affect pressure measurements. However, to use a sonic nozzle for flow measurements without requiring the use of a hydrogen primary flow standard, the nozzle must be accurately characterised. There was also need for the method to be validated as suitable for hydrogen over the whole range of pressure.
This project has produced a good practice guide, “Good Practice Guide on Dimensional Characterisation” for characterising CFVN, including the throat diameter, inlet curvature and roughness. Characterising CFVN this way ensures that they conform with the tolerances specified in existing standards, and so the calculations provided in those standards can be used. This is particularly true for smaller nozzles with throat diameters of around 1mm, such as those constructed and calibrated by the project.
The guide will extend the use of CFVN for the accurate calibration of hydrogen flow meters. This will increase reliability in areas where hydrogen is used and transported, including in industry, and increase confidence in its use.
Other project achievements
- Cryogenic measurement campaigns
The project performed cryogenic measurement campaigns, calibrating LH2 (liquid hydrogen) flow meters with liquid nitrogen and LNG to establish traceability for LH2 flows.
A Coriolis flow meter was used for LH2 flow, after calibration in an LNG facility. Flow rates reached up to 3000 kg/h, which are directly relevant to industry. In another approach, a small Coriolis flow meter was used in liquified nitrogen and helium, with liquid flow compared to gaseous flow.
The project developed a new real gas model for the thermophysical properties of hydrogen, across specified temperature and pressure ranges, which was implemented in OpenFOAM. The model has also been validated by numerically simulating a CFVN.
Project coordinator Hans-Benjamin Böckler (PTB) has said about the work of the project:
“The ongoing project has a holistic and challenging approach to bring traceability to the whole field of hydrogen application.
On the gaseous side, we are preparing the high-pressure calibration campaigns. In parallel the CFD-model for the CFVN is making great progress. Already available are the good practise guide for dimensional calibration and the equation of state. With the knowledge of the inner shape and curvature of the CFVNs, which are used in the high-pressure calibration campaigns, twins will be implemented in the CFD to make both (real calibration and CFVN) comparable. For the final comparison it is essential to use the same equation of state in the CFD and the calibration. Beside this main action there is the nearly finished campaign with alternative fluids with CFVNs and the construction of test infrastructure in progress.
For the liquid hydrogen the consortium published a good practice guide about conversion from para to normal hydrogen and a report on the uncertainty of U-shaped Coriolis flow meters in liquified hydrogen. Aside, the measuring campaigns with liquified gases in the stage of evaluation and will give a better understanding about how to bring traceability to liquified hydrogen flow applications.”
This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.
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