Everything you need to know about custody transfer in the hydrogen industry

Custody transfer is a term traditionally associated with the oil and gas industry, however as hydrogen becomes ever more important in the energy mix, custody transfer of hydrogen is an increasingly common practice. Whether on-site, exchanging hands or crossing borders, hydrogen custody transfer has its own unique challenges that must be addressed in ensuring the process is safe, effective and highly accurate. As a relatively nascent industry, albeit one with huge potential, hydrogen has the opportunity to embrace new technologies that can ensure best practices are baked into custody transfer procedures from the outset. 

Hydrogen is a highly promising option for future energy needs chiefly because it can be combusted without producing any pollutants. The emergence of clean or green hydrogen, which is produced through an electrolysis process using renewable electricity, and produces little or no carbon emissions, is also an area where technology is helping to bring down renewable energy costs and reduce carbon impact. The more that renewable energy sources are brought online, the more opportunities there are to not only reduce emissions, but to also create a useful byproduct in the form of green hydrogen. This in turn can help to further accelerate efforts to decarbonise society. 

The potential applications for hydrogen are varied. In industry, it can be used as a clean combustion fuel, replacing natural gas. It may also be used in primary metal production, manufacture of semiconductors and fuel cells, along with a wide range of other manufacturing processes. In transport, hydrogen is a viable fuel for road vehicles, and is increasingly being considered as an environmentally friendly alternative for shipping and even aerospace. Hydrogen can be blended with natural gas, using existing networks, for space-heating and water heating in buildings, while it can also be used as a means of storing renewable energy. 

As the rollout of hydrogen as an energy source becomes more widespread, the economy for transporting, storing and trading it as a resource will rapidly and necessarily grow. Accurate flow measurement is therefore crucial in custody transfer. Minor errors can potentially create high financial impact, but precision can be hard to maintain when operating in harsh field conditions. Even a minor misreading can accumulate into a major error over time and ultimately undermine confidence in the process. For a growing industry like hydrogen, it is crucial that this is avoided. 

Hydrogen is usable in two physical states — gas or liquid — and is often pressurized when stored. Its low energy density compared to natural gas means that metering must be highly accurate and be able to handle high flow rates. Furthermore, hydrogen is the smallest observed molecule in the universe, which can result in a higher likelihood of leakage. This can be difficult to detect since hydrogen is odorless. For these reasons, accurate measurement of hydrogen flow is a challenge using traditional technologies. 

In order to facilitate the increased adoption of hydrogen, the regulatory landscape must be robust to ensure standardization across countries and markets. Custody transfer across borders is a particular challenge, as differing regulations and standards may apply. Collaboration between countries, regions and organizations will be fundamental in navigating the challenges of the growing hydrogen industry. In response, the American Gas Association (AGA) standard sets out the calculations required for measurement of hydrogen in normal conditions, and calculations in the field are certified against this standard. 

The problem here is that ‘normal conditions’ rarely occur. The physics of pure hydrogen are well understood, however in custody transfer hydrogen is often mixed with other gases, for instance natural gas, which adds an additional layer of complexity. Operators therefore not only need to use the calculations as set out in the standard, but also need to gain an understanding of what is happening inside the pipe to ensure a ‘true’ reading of hydrogen quantities. 

Even a small error can potentially have a large financial impact. It is therefore important to ensure that measurements are as accurate as possible. Investing in advanced measurement equipment now will most likely pay off down the line through the reduced error rate. Customers must also have confidence in the measurement system, as false reports can potentially have a significant effect on the market, influencing stock and commodity prices. 

Measurement errors can be caused by a wide range of factors, including human error, incorrectly calibrated equipment, flawed or faulty measurement equipment, flow computers with incorrect algorithms, and drifting analog inputs. Inadequate maintenance of measurement systems can also lead to problems. It is important to remember that a single mistake can render an entire custody transfer worthless, so the stakes are extremely high to ensure that measurement is as accurate as it can be. 

This is where a flow computer can help. This, when combined with Coriolis mass flow measurement, provides a more accurate picture of the actual conditions, taking into account the presence of other gases, along with other key variables. Coriolis mass flow measurement has been used for many years in oil and gas custody transfer applications and is widely considered to be one of the most accurate and cost-effective measurement techniques. Its advantages also make it a logical choice for the measurement of hydrogen. Coriolis meters operate on the mass flow measurement principle and are favored for their ability to measure multiple attributes over sustained periods with high repeatability and little maintenance requirements. The latest generation of devices from manufacturers such as ABB can measure a wide range of medium characteristics including aggregate state, conductivity, and density with an accuracy to 0.1% in direct mass flow measurement. 

From the mass flow rate, fluid density and temperature measurements taken by a Coriolis meter, other measurement values can be inferred from the data, such as volumetric flow rate and percent concentration. A single Coriolis meter can therefore carry out the work of multiple instruments, saving money by reducing the need for separate devices, while also cutting down maintenance. 

A flow computer is an electronic device that takes inputs from the flowmeter along with pressure and temperature sensors to compute a correct volume flow. In this sense, the flow computer essentially acts as the cash register in a hydrogen custody transfer application. 

The software is used to validate continuously and intelligently, in real-time, field signals and raise an error condition so that appropriate actions are taken when the measurement fails. Depending on the actual root cause, in many cases it can not only detect an error and warn the operator, but it can also semi-automatically or even fully automatically correct for the mismeasurement incident and regenerate the flow calculation results, in near real-time, without adding any further uncertainty. This means that mismeasurement incidents are now resolved in seconds, rather than in weeks, saving vast amounts of time compared to resolving issues manually. This means that paperwork can be filled out swiftly and correctly, removing barriers and complexity from the custody transfer process. 

This article was firsts published in the magazine Decarbonisation Technology.