Safety in Green Hydrogen Storage and Transportation: Compressor Integrity and Diaphragm Failure Mechanisms

Ensuring compressor safety is critical within green hydrogen infrastructure. Diaphragm compressors, widely used for their excellent static sealing in high-pressure hydrogen transport, face reliability challenges. The metallic diaphragm, a core component separating oil and gas chambers, is a primary failure point. This analysis examines diaphragm failure mechanisms, focusing on complex stress states during operation—including stress concentration from pressure imbalances—and material degradation due to hydrogen embrittlement under high-pressure, high-temperature, and cyclic loading. It highlights that failure results from combined mechanical overstress and hydrogen-induced weakening of material properties like ductility and fatigue strength. Recommendations for improvement include optimized diaphragm cavity design, strict operational controls, and the development of advanced materials with enhanced resistance to hydrogen-assisted damage.

1. Safety of Hydrogen Compressors

The compressor is a critical unit operation in hydrogen storage and transportation, responsible for pressurization. It is widely used in green hydrogen export, supply stations, and refueling stations. Given hydrogen's small molecular size, high leakage potential, and stringent purity requirements, sealing integrity in the compression stage is paramount. Two common compressor types in use are diaphragm compressors and liquid-driven piston compressors. The former, relying on effective static seals, is extensively applied in high-pressure hydrogen transfer. However, diaphragm compressors face complex operational issues during service, including high temperatures from gas compression, pressure imbalance across the diaphragm due to hydraulic oil leakage, and mechanical wear from high-speed crankshaft rotation. These issues manifest as component failures, with the compressor diaphragm being one of the most severely affected and frequently failing parts. As the physical barrier between hydraulic oil and hydrogen, the diaphragm not only provides a static seal but also transmits rapidly changing oil-side pressure to the gas chamber to drive compression. Diaphragm rupture has been a primary factor in multiple refueling station incidents. Recent research increasingly focuses on the diaphragm failure mechanism, often centered on how complex service conditions affect its mechanical performance. Diaphragm service life is influenced by a combination of material properties: strength, hardness, wear resistance, ductility, fatigue performance, and resistance to hydrogen embrittlement. A slight change in external conditions—such as oil pressure, temperature, or the presence of particulate contaminants—can cause any load to exceed the material's threshold, leading to failure.

2. Analysis of Stress State in Compressor Diaphragms

Diaphragm materials for hydrogen compressors have strict performance requirements, needing sufficient hardness and strength to resist stress concentration at specific locations. During reciprocating motion, the diaphragm's outer annular region is clamped, placing the central and outer annular areas under combined tensile and bending stresses, where stress peaks typically occur. For an ultra-high-pressure diaphragm compressor with a discharge pressure of 200 MPa, stresses at the diaphragm center can reach 249.91 MPa (radial) and 149.79 MPa (hoop). Considering real service conditions where oil deficiency may create a pressure differential across the diaphragm, causing it to conform to the specially shaped oil channels in the distribution plate and creating stress concentration, studies applying large and small deflection plate theory have calculated a maximum total radial stress of 275.7 MPa under such conditions—far exceeding a typical allowable stress of 170 MPa.

Furthermore, to enhance isolation and create a leak detection channel, diaphragms are typically used in a three-layer stack. This means impact and micro-friction occur between the layers during operation, constituting another life-limiting factor. Research indicates that friction velocity, stress load, and surface roughness all affect wear rate. Applying a PTFE coating on stainless steel surfaces can reduce the friction coefficient. The diaphragm undergoes cyclic deformation; for a thickness of 0.3–0.5 mm, center deflection can reach 5–10 mm. The industry often targets over 3,000 hours of continuous operation to minimize downtime. However, real-world cases show diaphragm lives of only 205 to 900 hours for 45 MPa compressors, falling short of requirements. This demands high material ductility, toughness, and fatigue resistance. Studies show that the diaphragm over gas ports, oil distribution grooves, and the clamping start ring area are all at risk of fatigue failure due to insufficient fatigue strength. Therefore, diaphragm service life is a combined function of strength, hardness, ductility, and fatigue performance, making comprehensive material improvement key to reliable compressor operation.

3. Mechanism of Hydrogen-Induced Degradation in Diaphragm Materials

Diaphragms operate under complex stress states, demanding stringent mechanical properties. However, much testing and simulation uses material data from non-hydrogen environments. In reality, performance degradation due to hydrogen atom ingress into the metal matrix is well-established. The diaphragm's service environment—involving high-pressure hydrogen, cyclic temperature excursions, and tensile stress—further accelerates failure. Local temperatures can exceed 200°C during compression. Experiments at this temperature show that hydrogen charging reduces the yield strength of stainless steel, attributed to hydrogen-enhanced localized plasticity (HELP). Hydrogen's non-uniform distribution can also cause internal stress.

Notably, while austenitic stainless steel may show less pronounced hydrogen embrittlement at high temperatures, another common diaphragm material, Inconel 718 nickel-based alloy, can exhibit significant embrittlement. Under tensile stress, dislocation movement generates vacancies and defects that act as high-energy trapping sites for hydrogen. Accumulated hydrogen at crack tips accelerates crack propagation. High-cycle fatigue tests on pre-charged austenitic stainless steel confirm hydrogen reduces fatigue life. For example, data shows the high-cycle fatigue life of 316L diaphragm material at 450 MPa and 500 MPa maximum stress decreased by 4.1% and 11.6%, respectively, in a 35 MPa hydrogen environment compared to nitrogen.

In summary, diaphragm rupture results from the combined effect of abnormal mechanical loads and hydrogen-induced material degradation. Abnormal loads arise from oil deficiency, pressure imbalance, or friction from gas impurities. Meanwhile, the high-pressure, high-temperature hydrogen environment weakens mechanical properties, especially ductility and fatigue strength. The superposition leads to operational stress exceeding the degraded material's capacity, initiating cracks that propagate rapidly under fatigue loading, resulting in failure.

4. Improvement Measures

To reduce diaphragm failure frequency and extend service life, a three-pronged approach is recommended:

    1.  Design Optimization: Optimize the diaphragm cavity profile and rationally design the oil channel geometry in the distribution plate and the dimensions of the discharge valve to minimize stress concentrations.

    2.  Operational Process Control: Implement strict controls over discharge temperature and hydrogen inlet purity. Employ measures like compensation pumps to ensure stable and balanced oil pressure.

    3.  Material Enhancement: Develop or select diaphragm materials with higher allowable stress and superior resistance to hydrogen embrittlement. Apply surface treatments or coatings to improve performance against hydrogen-assisted damage and fatigue.