Basics of hydrogen flow measurement

Hilko den Hollander

Industry Manager Oil & Gas and Sustainable Energy

Introduction

Green hydrogen produced from renewable energy in combination with water electrolysis is foreseen to play an important role in the energy transition. Next to its traditional role as feedstock for refineries and the chemical industry, hydrogen will be applied as an energy carrier. As such it replaces fossil fuels in for example steel production and will help to decarbonize the industry. Other sectors where hydrogen can drive decarbonization are long-range and heavy-duty mobility applications and seasonal storage of renewable energy. To be able to serve these applications, investment in infrastructure for the production, transportation and storage of hydrogen is being made. With the growing size of the hydrogen economy, the need for process and custody transfer measurement of hydrogen is increasing. In this article, we give further background about the technical challenges that operators face when measuring hydrogen.

Physics

Hydrogen is the lightest element in the periodic system with a molecular weight of 2g/mol and consequently has a very low density, which is approximately 8 times lower than natural gas, but has a very high speed of sound, which is approximately a factor 3 higher than for natural gas. Concerning the energy content of hydrogen this results in an interesting situation where the energy density per unit mass is very high, but the energy density per unit volume is approximately a factor 3 lower than for natural gas. Therefore, for mobility applications hydrogen is applied at high pressures of 350 to 700barg ( 5075 to 10150 psig). For many other applications such as industrial processes, electrolysers and pipeline transportation hydrogen is applied at moderate pressures of 30 to 60barg (435 to 870psig). Hydrogen only becomes liquid at very low temperatures; at ambient pressures, it liquifies at -253°C (-423°F), which is only 20°C (36°F) above absolute zero.

How to measure Hydrogen flow

Hydrogen flow is usually measured in volume or in mass. The three most commonly used flow measurement technologies for this task are Ultrasonic, Coriolis and Variable Area. Each technology presents individual benefits and challenges for hydrogen flow measurement. For custody transfer measurement of hydrogen usually Ultrasonic or Coriolis flowmeters are used.

Ultrasonic

Ultrasonic flowmeters offer negligible pressure drop and are available full-bore (without constrictions) up to very large line sizes. The meters are generally not affected by pulsating flow and do not have rotating or moving parts that wear. For hydrogen applications, the low density and high speed of sound must be taken into account.

The low density of hydrogen makes it relatively difficult for the ultrasonic signal to travel between the emitting and the receiving transducer, as there is less efficient acoustic coupling between the transducer and the hydrogen gas. This results in a reduced signal-to-noise ratio compared to natural gas. At ambient pressures it is still possible to perform ultrasonic flow measurement on hydrogen gas; with increasing pressure, the signal-to-noise ratio increases.

The high speed of sound in hydrogen results in very short transit times of the ultrasonic signal between the transducers and in a large opening angle of the ultrasonic beam. Due to the short transit time, the receiving transducer must be ready in time to receive the signal from the sending transducer. This is made possible by adjusting the electronics to move the time window for the reception forward. The large opening angle can cause crosstalk, especially in small-diameter pipes. By selecting higher frequency transducers the crosstalk can be removed.

As there are no large-scale ISO 17025 accredited hydrogen flow calibration facilities available yet, flowmeters are typically calibrated on water, air or natural gas. For ultrasonic flowmeters, that rely on the flow profile in the meter, a Reynolds-number based calibration can be done. In this way, the flow profiles during calibration will be similar to that in the field. For example, the Reynolds numbers that are seen on a hydrogen application can be reached by calibrating a flowmeter on natural gas at an 8 times lower flow rate or lower pressure. By taking into account some additional effects, the natural gas calibration can be transferred to hydrogen applications.

Coriolis

Coriolis meters provide high accuracy measurement of mass, density and volume flow in line sizes DN02…400 / ?…16". Offering a direct mass measurement, the meters are not affected by flow profiles and will continue to measure on multiphase flow. In the case of gas measurement, especially with low-density hydrogen, care should be taken to meet the meter minimum density requirement to ensure the performance of the flowmeter. In practice this means that a minimum pressure is required for custody transfer applications. Since Coriolis meters measure mass by vibrating one or more tubes, Coriolis meters also have a maximum operating pressure based on these relatively thin-walled tubes. Higher pressures (typically over 200 bar) require thicker tubes that are more difficult to vibrate, which might make the Coriolis meter lose some of its accuracy.

Coriolis meters are not affected by flow profiles. To guarantee highest meter performance and accuracy, they are usually calibrated with water against a high-precision weighing scale based calibration facility. It is not necessary to perform calibrations using a fluid similar to that being used in the final application. At KROHNE, this method is used throughout the manufacture of all Coriolis mass flowmeters and is widely accepted by users and certification bodies. To calibrate a flowmeter to ±0.05% accredited measurement uncertainty, the large mass flow calibration rigs at the KROHNE Coriolis production in Wellingborough, UK, are accredited by UKAS (calibration laboratory 0812), the United Kingdom’s metrology agency, to an uncertainty of less than 0.017%.

Variable Area

Variable Area flowmeters provide a simple and cost-effective low-flow measurement of hydrogen in process applications such as hydrotreating or oxide reduction or auxiliary applications like purging or sample flow measurement in analyzers or sampling systems. ?Diameters go from 4 mm/ 1/8¨?up to DN100 / 4¨. The majority of variable area flowmeters are used as mechanical local flow indicators without the need for auxiliary power and in combination with a valve for flow adjustment. However, optional limit switches, analogue signal outputs or digital fieldbus interfaces allow the integration into a monitoring or control loop. Variable Area flowmeters can measure very low flow rates of low-density hydrogen down to atmospheric conditions as well as high-pressure hydrogen. With constant process conditions (pressure, temperature) it is possible to achieve accuracies in the range of 1-2.5% of the reading.

Summary

With the shifts related to the energy transition, hydrogen becomes important as an energy carrier. For hydrogen flow measurement, Ultrasonic, Coriolis or Variable Area are the most common technologies. For each technology, individual benefits were pointed out, including the possibilities of use for Custody Transfer measurement, as well as parameters influencing calibration.

Additional information on products & measurement technologies

Ultrasonic flowmeter products & free eLearning

Coriolis flowmeter products & free eLearning

Variable area flowmeters products & free eLearning