Why is it specified that the exhaust temperature of hydrogen reciprocating compressors must be below 135℃

My understanding is that, as stipulated by the limitations regarding the use of Nelson curves, the corrosive effect of hydrogen on steel materials increases sharply in high-temperature environments. I’m not sure if this is correct; everyone’s thoughts are welcome

Please refer to the fact that the exhaust temperature of hydrogen reciprocating compressors is limited to below 135°C based on the following safety and technical considerations: 1. The risk of spontaneous ignition and explosion of hydrogen. The spontaneous ignition temperature of hydrogen is around 500°C (in air), but high temperatures significantly increase its chemical reactivity. If there is localized overheating inside the compressor (such as due to mechanical friction or static sparks from leakage points), the high temperature can trigger combustion or even an explosion. Material tolerance: High temperatures may accelerate the aging of seals (such as polytetrafluoroethylene), leading to hydrogen leakage and the formation of an explosive mixture (the explosion limit for hydrogen is 4%–75%). 2. Material performance limitations: Decreased metal strength: Components such as compressor cylinders and pistons may suffer from metal fatigue or creep at high temperatures, especially with lightweight materials like aluminum alloys (which are commonly used in hydrogen-related equipment). Lubrication failure: Lubricating oil tends to carbonize and coking at high temperatures, blocking the oil passages or contaminating hydrogen (a critical problem for certain processes such as the electronics industry). When oil-free lubrication is used, the temperature tolerance limit of sealing materials such as PTFE is usually below 150°C. 3. Preventing hydrogen embrittlement and corrosion: Risk of hydrogen embrittlement: Under high temperature and pressure, hydrogen can penetrate the metal lattice, causing the material to become brittle. Rising temperatures will accelerate this process and shorten the equipment's lifespan. Oxidation reaction: High temperatures may cause hydrogen to react with trace amounts of oxygen inside the compressor to produce water vapor, thereby accelerating corrosion (such as rusting on the inner walls of the cylinders). 4. Design criteria for process safety margin: API 618 (Standard for Reciprocating Compressors Used in the Petroleum, Chemical, and Natural Gas Industries) recommends that the exhaust temperature of hydrogen compressors should not exceed 135–150°C, with 135°C being a conservative value that provides a safety margin. Monitoring redundancy: In actual operation, temperature sensors may experience errors or delays; setting a lower limit can trigger alarms in advance to prevent sudden overheating. 5. Balance between energy consumption and efficiency: Heat management in compression: The temperature rises significantly during hydrogen compression (the theoretical temperature increase under adiabatic compression can exceed 100°C per stage). Limiting the exhaust temperature is achieved through multi-stage compression and inter-stage cooling, but excessive cooling increases energy consumption. 135°C is the balance point between efficiency and safety. Measures in practical applications: Multi-stage compression: Typically 2 to 4 stages of compression are used, with the exhaust gas from each stage being cooled to below 40°C via a cooler. Emergency interlock: Automatic shutdown in case of overheating to prevent the expansion of accidents. Material selection: Such as stainless steel cylinders and special coatings to withstand high-temperature hydrogen environments. Through the above measures, the 135°C limit can effectively mitigate the main risks during hydrogen compression, while also taking into account the economic efficiency of the equipment.

The specification that the exhaust temperature should not exceed 135°C when compressing hydrogen is based on the following factors: 1. **Risk of spontaneous combustion of hydrogen**: Hydrogen has a wide ignition range, and when it leaks and the temperature is high, it is easier to reach the temperature at which spontaneous combustion occurs (around 400–560°C). Although 135°C is far below the auto-ignition temperature, reducing the exhaust temperature can effectively decrease the likelihood of accidents caused by local hot spots or transient high temperatures. 2. **Stability of seals and lubricating oil**: The lubricating oil and sealing materials inside reciprocating compressors are sensitive to temperature. At high temperatures, aging, deterioration, and coking occur easily, increasing the risks of wear and leakage and affecting the equipment’s lifespan and safe operation. 3. **Preventing the exacerbation of hydrogen embrittlement**: Hydrogen, in high-temperature and high-pressure conditions, intensifies its penetration into the metal components of compressors as well as the hydrogen embrittlement effect, leading to a decrease in the material’s strength. Controlling the temperature within a certain range can alleviate the hydrogen embrittlement effect. 4. **Overall operational safety of the equipment**: Operating at high temperatures increases the risk of mechanical fatigue in the compressor components; it may also lead to difficulties in process control, thereby increasing the complexity of managing the operation of the equipment and systems. In summary, keeping the exhaust temperature below 135°C is a decision made after considering various factors such as hydrogen leakage, seal aging, the risk of hydrogen embrittlement, material corrosion, equipment lifespan, and overall safe operation. .