Gas Separation Unit (GSU) for electrolyzers

A pressure difference between the H₂ and O₂ gas storage units (GSU) of more than approximately 50 mbar is considered a safety hazard because it can promote gas crossover through the stack membrane. Our design solutions:

• The pressure loss characteristics of both GSUs are reflected in the design (matched pair)

• Hydraulic lye balancing system between the two vessels (standard with AEL)

• Differential pressure measurement with redundant sensors and a SIL-2/SIL-3-compliant interface

• Level control loop with tolerances < ± 10 mm

• Upon request: integrated H₂-to-O₂ and O₂-to-H₂ analysis via sampling ports

Standard operating pressures range from 6 to 30 bar for conventional AEL and PEM systems. Upon request, we manufacture GSUs with a design pressure of up to 80 bar for pressure electrolysers and special systems. For pressures above 50 bar, we typically use 1.4571 or Duplex 1.4462 with wall thicknesses up to 100 mm and perform a complete fatigue analysis in accordance with EN 13445-3, Annex B – cyclic loading is a key design criterion in volatile renewable energy operations.

The residence time is the key design parameter for a GSU. We use industry-standard values of 30–120 seconds as a guideline, depending on the foaming behavior of the lye, the gas concentration (vol/vol), and the desired droplet separation. For PEM electrolysers, we assume shorter residence times (15–60 s); for alkaline electrolysers with a higher risk of KOH foaming, we assume correspondingly longer residence times (60–180 s). We derive the tank dimensions from these specifications using established design rules and coordinate them closely with the plant manufacturer’s process engineering design team.

The choice depends on the KOH concentration, the operating temperature, and the service life targets. Our proven recommendations:

• 20% KOH, up to 80 °C, 6 bar: 1.4404 (316L) – cost-effective standard

• 30% KOH, up to 90 °C, 30 bar: 1.4571 (316Ti) – Best Practice

• 30% KOH, up to 100 °C, > 30 bar: 1.4539 (904L) or Alloy 600

• Corrosive conditions, trace amounts of chloride: Alloy 625

If necessary, we conduct targeted KOH corrosion tests prior to material approval.

Both options are possible. For alkaline electrolysers, we generally recommend delivery as a matched pair, because the pressure loss characteristics of both vessels must be matched to ensure hydraulically balanced operation (equal liquid levels on the H₂ and O₂ sides). For PEM electrolyzers, often only the anode-side O₂ GSU is required—water separation on the cathode side is significantly simpler here and can be integrated into a compact condensate separator.

We therefore recommend:

• AEL: matched pair (H₂ and O₂ GSU units of identical construction or mirror-image design)

• PEM: anode-side O₂ GSU + compact cathode-side condensate separator

• AEM: matched pair, often with an internal coating

A conventional hydrogen pressure storage cylinder stores only gas under pressure over a long period of time. A GSU in an electrolyzer, on the other hand, is an active process component: It receives the fresh gas-liquid mixture directly from the stack, separates the phases, serves as a level buffer for load spikes, and actively returns the liquid phase to the process. To do this, a GSU always contains a demister, a level control system, and usually several sensor ports—a conventional pressure storage tank does not require any of these components.

Development time varies depending on the complexity of the project. For simple vessels, the design can be completed in a few weeks, while more complex projects, such as custom-designed reactor vessels, can take several months. However, we always provide transparent project planning and keep all stakeholders informed of progress.

Yes, our team of 6 engineers and designers specializes in tailor-made solutions. We perform detailed calculations and simulations based on the specific requirements of each project, be it for pressures, temperatures, material behavior or complex loads.

We are certified according to international standards to ensure the highest quality and safety:

• DIN ISO 9001 (Quality Management)
• PED 2014/68/EU (Pressure Equipment Directive)
• EN 13445 (Pressure vessel construction)
• AD 2000 (German standard for pressure vessels)
• ASME U1, U2, and S stamps (for pressure vessel construction according to U.S. standards / pressure vessels and piping)
• EN 1090-1/-2 (Construction of load-bearing steel structures)
• DIN EN ISO 3834 (Welding Technology)

Yes, we offer customized material solutions for critical applications, especially in the chemical and energy industries. We select the most suitable material based on your requirements, taking into account factors such as pressure, temperature, corrosion resistance and mechanical properties.