Hydrogen Heat Exchanger Considerations for Design and Safety
Hydrogen is increasingly being used as an alternative fuel and chemical feedstock. Hydrogen service is also found in refineries, specifically in Feed Effluent Exchangers in hydrocracker units. Here are some important considerations for shell and tube and air-cooled heat exchangers in hydrogen service below.
Hydrogen Service and the Effects on Metals
Hydrogen causes embrittlement and cracking in many metals. At high pressures and temperatures, diffusion of hydrogen atoms into the metal lattice leads to loss of ductility and strength. When hydrogen diffuses into these areas, it can cause several microstructural changes and mechanisms that lead to embrittlement:
- Hydride Formation: Some metals, especially those containing titanium and zirconium, can form metal hydrides that are brittle and can lead to cracking.
- Hydrogen-enhanced Localized Plasticity (HELP): Hydrogen can enhance the local plastic deformation in some regions of the metal, leading to premature failure under tensile stress.
- Hydrogen-enhanced Decohesion (HEDE): Hydrogen can weaken the metallic bonds, particularly at grain boundaries, making the material more susceptible to crack initiation and propagation.
The impact of hydrogen embrittlement is more pronounced under high hydrogen pressures and at lower temperatures. Materials used in such environments must be carefully selected and often require special treatments or coatings to mitigate the effects of hydrogen embrittlement. Additionally, controlling the operating conditions, such as temperature and pressure, and using proper design standards can also help in reducing the risk of embrittlement.
Potential for Hydrogen Embrittlement and Cracking
The lightweight nature of hydrogen allows rapid diffusion even at moderate temperatures. Components such as header-boxes, shells, tubesheets, flanges, and heads are all vulnerable. Cracks are microscopic at first but can link up leading to eventual failure. Selecting more resistant materials and designing to minimize high-stress areas is very important and not doing so could result in exchanger failure.
Chrome-Molybdenum in Hydrogen Service
1-1/4Cr-1/2Mo materials for shell & tube heat exchangers in high-temperature and high-pressure hydrogen service typically adhere to API 934-C and API 941 standards. API 934-B provides specific guidelines on material selection, fabrication processes, and post-weld heat treatments to mitigate hydrogen embrittlement. It emphasizes the importance of controlled welding environments and preheating procedures to maintain material integrity.
2-1/4Cr-1Mo materials in high-temperature and high-pressure hydrogen service, compliance with API 934-A and API 941 is essential. API 934-A details specific fabrication and material requirements, including careful selection of welding procedures, pre-heat and post-weld heat treatments to prevent hydrogen embrittlement, and enhanced inspection protocols to ensure material integrity under service conditions.
API 941 provides critical guidance on hydrogen attack susceptibility, utilizing Nelson curves to define the safe operational limits for temperature and hydrogen pressure. Testing protocols include comprehensive non-destructive testing to verify weld quality and material suitability for prolonged exposure to harsh hydrogen environments.
Stainless Steel and Nickel Alloys in Hydrogen Service
Stainless steels and high-nickel alloys are often considered for hydrogen service, but their suitability depends on specific conditions such as pressure, temperature, and the presence of impurities like sulfur and phosphorus.
Stainless steels are generally resistant to hydrogen embrittlement due to their face-centered cubic (FCC) structure, which is more tolerant to hydrogen. However, the susceptibility varies with the type:
- Austenitic stainless steels (e.g., 304, 316) are commonly used in hydrogen service because they maintain good ductility and strength even when exposed to hydrogen. They are less susceptible to hydrogen embrittlement compared to other steel families.
- Martensitic (410, 420, etc) and ferritic (409, 430, etc) stainless steels are more prone to hydrogen embrittlement (due to their BCC structure) and are usually not recommended for high-pressure hydrogen service.
High-nickel alloys, such as Inconel, Hastelloy, and Monel, are excellent for use in hydrogen service, especially under high temperatures and pressures. These alloys offer exceptional resistance to hydrogen embrittlement:
- Inconel alloys (Alloy 625 / N06625 and 718 / N07718) are known for their strength and corrosion resistance in extreme environments, including high-temperature hydrogen service.
- Hastelloy (C276 / N10276) provides strong resistance to oxidizing and reducing environments and is effective in preventing hydrogen embrittlement.
- Monel alloys (M400 / N04400) also perform well in hydrogen environments, with good mechanical properties and corrosion resistance.
Material Selection Considerations in Hydrogen Service
When selecting materials for hydrogen service, consider the following:
- Hydrogen Purity: Impurities can dramatically affect material performance. High purity hydrogen reduces the risk of embrittlement and corrosion.
- Temperature and Pressure Conditions: Higher temperatures can mitigate hydrogen embrittlement effects but may introduce other mechanical challenges.
- Design and Fabrication: Material selection must be accompanied by appropriate design codes (such as ASME BPVC) and fabrication practices to avoid introducing stress concentrators where hydrogen can accumulate.
- For critical applications in hydrogen service, it’s crucial to conduct thorough material testing under simulated service conditions to ensure compatibility and safety.
Design Considerations for Heat Exchangers in Hydrogen Service
Butt-welded joints are preferred in hydrogen service due to their inherent structural continuity and smooth transitions, which significantly simplify the process of performing Non-Destructive Examination (NDE). These types of joints provide a uniform cross-section, which is beneficial for inspection techniques like ultrasonic testing, radiography, and magnetic particle inspection.
The smooth, straight configuration of butt-welds allows for more effective and thorough scanning, ensuring reliable detection of flaws or discontinuities that could lead to failures in high-pressure hydrogen environments. This is crucial for maintaining the integrity and safety of hydrogen service equipment.
Nozzle Design for Heat Exchangers in Hydrogen Service
Nozzle re-pads are not permitted in hydrogen service primarily due to concerns about the integrity and safety of the welded joints under high-pressure hydrogen conditions. Re-pads are additional pieces of metal welded around the nozzle area on a pressure vessel or piping to reinforce the structure. Here are the main reasons re-pads are avoided in hydrogen service:
- Hydrogen Embrittlement and Cracking: Hydrogen has a high propensity to diffuse into welds and the heat-affected zones, leading to embrittlement or hydrogen-induced cracking. The multiple welds involved in attaching a re-pad introduce more risk areas for such issues.
- Complexity in Inspection: Re-pads complicate the geometry at the nozzle junction, making it difficult to perform effective non-destructive testing (NDT). Regular NDT methods might not provide reliable results due to the intricate weld patterns and overlapping materials, increasing the risk of undetected defects.
- Stress Concentration: The addition of a re-pad can alter the stress distribution around the nozzle, potentially creating stress concentrations that are not favorable under the cyclic loading conditions typical in hydrogen service. These stress concentrations can be initiation points for cracks.
- Quality Control: Ensuring the quality and consistency of welds in re-pads is challenging, especially in the demanding environments of hydrogen service where perfect weld integrity is critical.
For these reasons, designs that minimize welds and potential failure points, such as integrally forged nozzles or improved nozzle and butt-welded shell joint designs like “Q-Lip” nozzles, are preferred in high-pressure hydrogen applications. These approaches enhance safety and reliability by reducing the likelihood of leaks or failures.
Exchanger Design to Mitigate Hydrogen Leakage at Bolted Joints
Hydrogen’s small molecule can easily leak through seals and gaskets. Bolted connections must account for hydrogen’s tendency to cause loss of preload stress. Joints may need more frequent re-torquing. One way to avoid hydrogen leaks at bolted joints is to eliminate the bolted joints entirely – nozzle flanges can be eliminated and butt-welded to piping instead. Body flanges on shell & tube heat exchangers can be eliminated (or minimized) by selecting BEU, CEU, or DEU TEMA types and welding the tubesheet to the shell and/or channel cylinders. Welded diaphragm plates in lieu of gaskets for channel covers reduces the likelihood of leaks and also helps reduce flange and bolt sizes.
Pre-Heat and Post-Weld Heat-Treatment for Heat Exchangers in Hydrogen Service
For heat exchangers in hydrogen service, managing thermal processes during welding is crucial for material integrity. Pre-heat temperatures are used to minimize temperature gradients and reduce the risk of hydrogen cracking, typically set based on material thickness and type.
Holding temperatures during welding ensure that the weld area remains uniformly heated to prevent stress concentrations and embrittlement.
Post-weld heat treatment (PWHT) is essential to relieve welding stresses and reduce residual hydrogen in the weld area, typically conducted at higher temperatures dictated by the material and weld properties to ensure optimal diffusion and tempering of the weldment.
Inspection and Testing Requirements
Non-Destructive Examination (NDE) and Non-Destructive Testing (NDT) are crucial processes for ensuring the structural integrity and safety of heat exchangers operating in hydrogen service. Due to the high pressures and corrosive nature of hydrogen, heat exchangers must undergo rigorous inspection to detect any potential flaws that could lead to failure.
Radiographic Testing (RT): X-rays or gamma rays are used to capture images of the internal structure of welds. This method is particularly effective for identifying internal defects such as cracks, voids, or inclusions in welded areas.
Ultrasonic Testing (UT): High-frequency sound waves are used to detect flaws and measure the thickness of materials. UT is highly sensitive to small defects and is capable of providing precise information about the location and size of flaws deep within the metal.
Phased-array ultrasonic testing (PAUT) uses multiple ultrasonic probes controlled independently to focus and steer sound waves, enabling detailed imaging of internal structures. It efficiently detects flaws and assesses material integrity by providing precise, real-time cross-sectional views.
Crack Tip Opening Displacement (CTOD) testing measures a material’s fracture toughness—its ability to resist crack extension under a loading state. In hydrogen service, CTOD testing is vital to assess how susceptible materials are to cracking when exposed to hydrogen, ensuring their reliability and safety in harsh environments.
Magnetic Particle Inspection (MPI): This technique is used for detecting surface and near-surface discontinuities in ferromagnetic materials. It involves magnetizing the part and applying ferrous particles that are attracted to areas of flux leakage, indicating surface flaws.
Dye Penetrant Inspection (DPI): Suitable for non-porous materials, DPI uses a visible or fluorescent dye solution to detect cracks open to the surface. The dye is applied to the clean surface and drawn into any flaws by capillary action.
Hardness testing measures a material’s resistance to indentation, providing valuable data on strength, wear resistance, and ductility. Common methods include Rockwell, Brinell, and Vickers tests.
Impact testing evaluates a material’s toughness—its ability to absorb energy and resist fracture under sudden loads. Common methods include Charpy and Izod tests.
Visual Inspection: Even simple visual checks are vital for assessing the general condition of heat exchangers, ensuring there are no obvious signs of wear, corrosion, or deformation.
Each method has its specific applications and limitations, and often, a combination of these techniques is employed to achieve a comprehensive assessment of the heat exchanger’s condition, particularly in high-stake environments like hydrogen service.
Contact Altex Industries for Services and Manufacturing
With long experience in heat exchanger services and manufacturing, Altex Industries also offers heat exchanger solutions tailored to hydrogen applications across petrochemical, refining, fuel cell and other sectors.
Contact our team to discuss your shell and tube or air-cooled heat exchanger project requirements.
