Boil-Off Gas

How do proportional relief valves improve hydrogen boil-off gas management?

February 14, 2025

With 2030 now approaching fast, EU and UK clean energy initiatives are driving a rapid increase in demand for liquid hydrogen (LH2) across multiple sectors, from aerospace to heavy industry. 

The demand for LH2 is also expected to grow significantly in the long term, too. Hydrogen’s potential as a low-carbon alternative to fossil fuels is realisable partly thanks to developments in the ability to store and distribute it in liquid form. 

However, while liquid hydrogen's higher volumetric density makes it more cost-effective for large-scale transportation, mitigating boil-off gas (BOG) remains a major challenge. 

With LH2 requiring storage temperatures of -253°C (20 Kelvin), even advanced insulation materials cannot completely prevent heat transfer. Any heat ingress results in vaporisation, which is difficult to halt once it begins (Fig. 1).

As governments and industries continue to invest in the transition to clean energy, proportional relief valves can play a key role in addressing the complexities of BOG that are critical to ensuring the safe, economical, and sustainable operation of LH2 systems.

Improving cryogenic system safety 

Because liquid hydrogen vaporisation can lead to a significant pressure increase within storage tanks, boil-off gas management is critical to the safe design and operation of cryogenic vessels. Failure to control BOG effectively could jeopardise safety across the entire storage and transportation system, which is why they need to incorporate a robust pressure management solution tailored to the unique demands of cryogenic hydrogen.

For tank builders, this is reflected in the standards they must adhere to during design and construction:

• ASME Boiler and Pressure Vessel Code (BPVC): This code includes requirements for the design and construction of pressure vessels used for storing LH2. It emphasises the need for pressure relief systems to safely manage boil-off vapour and prevent overpressure situations.
• ASME B31.12: Hydrogen Piping and Piping Systems: This standard provides guidelines for hydrogen piping systems, including considerations for managing BOV from LH2 storage. It outlines requirements for venting and pressure relief to ensure safety.
• 49 CFR Part 178: Details specifications for containers used to transport LH2, including provisions for pressure relief valves to handle boil-off vapour safely.
• ATEX Directive (2014/34/EU): Regulates equipment and protective systems for use in potentially explosive atmospheres. This is relevant for facilities handling LH2 and managing boil-off vapour, as it requires measures to mitigate risks of explosion.
• Pressure Equipment Directive (PED) (2014/68/EU): This applies to the design, manufacture and conformity assessment of stationary pressure equipment in Europe. 

Parker’s proportional relief valves (Fig. 2) safeguard hydrogen storage tanks by gradually reducing pressure and managing boil-off gas and are the first cryogenic valves with a proportional relief mechanism. 

Unlike traditional bellows safety relief valves, which operate in a binary ‘open’ or ‘closed’ fashion, Parker's proportional relief valves allow for finer control over pressure levels, providing more accurate and continuous pressure regulation. They are smaller and lighter than their traditional counterparts and are unique in their ability to be connected to pressurised recirculation systems.

When the upstream pressure exceeds the closing force exerted by the valve’s spring, the lower stem opens, permitting flow through the valve’s outlet port, which can be ducted to a safe location for recirculation or released into the atmosphere. 

The flow rate increases proportionately to the upstream pressure increase. The unique balanced bellows design of Parker's proportional relief valves employs a unique mechanism to balance the forces within the valve, maintaining a consistent set pressure even under fluctuating pressure conditions. 

The proportional relief valve is instrumental in addressing another safety challenge specific to the transportation of liquid hydrogen, as it also helps eliminate the risk of gas accumulation in roofed parking areas during boil-off and gas escape from the vessel. 

The innovative bellows seal design allows for the ducting of excess boil-off gas to a safe location, such as secure storage vessels, enhancing safety and mitigating potential gas buildup hazards.

Cutting boil-off loss to improve profitability

With liquid hydrogen storage commonly suffering boil-off loss ranging from 0.1% to 0.3% per day, reducing or eliminating the waste associated with BOG can make a significant impact on operators’ return on investment.

Because Parker's proportional relief valve outlets can be connected to the vessel’s pressurised recirculation system, this provides you with the ability to return gas to the process or use it as fuel in fuel cell trucks. 

The valves can also help drive down costs during their installation, with integrated A-LOK® connections that make them easier to install with lower labour and material costs when compared to NPT threads. And because integral connections allow you to dictate the orientation of the valve, you can position it and then tighten it, which is not possible with NPT threads.

Reducing the impact of hydrogen on the atmosphere

While hydrogen is rightly regarded as having the potential to revolutionise the energy sector, its environmental benefits are significantly undermined by the improper management of boil-off gas. 

Traditional safety relief valves can only release hydrogen into the atmosphere. Because hydrogen can enhance the lifetime of methane (CH4) by affecting its breakdown, this can increase the concentration of greenhouse gases to the detriment of the climate.

In this context, the bellows seal design of Parker's proportional relief valves plays a crucial role by ensuring that excess boil-off gas is effectively managed and ducted to safe storage locations, thereby preventing unnecessary venting into the atmosphere.

The valves’ integrated A-LOK® connections help reduce fugitive emissions even further by providing high-quality, leak-free connections to the pressurised system.

Advancing cryogenic technology for a sustainable hydrogen future

Hydrogen producers and engineers are facing major challenges in their quest to improve operational efficiency, reduce waste, and enhance the safety of liquid hydrogen systems. Our cryogenic proportional relief valves represent just one aspect of our ongoing efforts to support the development of sustainable, efficient, and safe hydrogen infrastructure. By investing in cutting-edge technologies designed to optimise resource utilisation and minimise environmental impact, Parker is helping drive the transition to a cleaner, more sustainable energy future. 

The complete range of our products for the whole hydrogen economy can be found on our Hydrogen solutions page.

Learn more about Parker's cryogenic solutions