Factors to Consider for Material Selection of Pyrolysis Steam Drum

Factors to Consider for Material Selection of Pyrolysis Steam Drum

Corrosion Characteristics of Medium

The pyrolysis oil and gas contain sulfides (H₂S), acidic components and trace chloride ions, which are prone to cause corrosion on the inner wall of the equipment. Corrosion-resistant materials should be preferred: Q345R boiler and pressure vessel plate is suitable for normal working conditions; for high-sulfur feedstock working conditions, 304/316 stainless steel or carbon steel with anti-corrosion coatings (such as epoxy resin, polytetrafluoroethylene coating) on the inner wall is recommended; the welding parts shall be subject to anti-corrosion passivation treatment to avoid intergranular corrosion.

Temperature and Pressure Resistance Capacity

The temperature of oil and gas in the steam drum is usually 250–350℃, and the operating pressure is slight positive pressure (0–5kPa absolute pressure). The batch process also has frequent temperature fluctuations. The material shall meet the requirements of temperature resistance and thermal fatigue resistance: Q345R can withstand high temperatures below 475℃ and has good toughness, which is suitable for the temperature fluctuations of batch working conditions; for continuous processes operating at long-term high temperatures, Q245R or heat-resistant stainless steel can be selected. Meanwhile, the thickness of the material shall be determined through strength calculation to meet the requirements of pressure resistance and load-bearing capacity.

Adaptability to Process Conditions

Batch process: The batch operations of feeding, pyrolysis and slag discharge will cause repeated heating and cooling of the equipment. The material shall have good fatigue resistance to avoid weld cracking caused by thermal expansion and contraction.

Impact of medium scouring: There is airflow impact at the oil-gas inlet. Wear-resistant liners can be installed in this area or wear-resistant alloy materials can be selected to prevent local scouring thinning.

Safety and Standard Compliance

The steam drum is classified as a pressure vessel, so its material must comply with the national standard GB150 Pressure Vessels, and be provided with material qualification certificates and third-party inspection reports. Non-standard steel shall not be used to avoid safety accidents such as leakage and explosion caused by substandard materials.

Cost and Maintenance Economy

Stainless steel has strong corrosion resistance but high cost; carbon steel has low cost but needs supporting anti-corrosion measures. A comprehensive trade-off shall be made based on project budget, feedstock characteristics (sulfur content) and operation cycle: carbon steel with anti-corrosion coating is applicable for short-term projects or low-sulfur feedstock; stainless steel is recommended for long-term high-sulfur working conditions to reduce later maintenance costs.