Anti-Corrosion Structural Design of Pressure Vessels

Pressure vessels are essential and commonly used equipment in industrial production, widely applied in industries such as petrochemical, power generation, shipbuilding, and metallurgy. Due to the high temperatures, high pressures, or corrosive media they contain, pressure vessels face numerous potential corrosion risks during operation. To ensure the safety, reliability, and long service life of these pressure vessels, corrosion prevention design is crucial. A well-designed anti-corrosion structure can not only effectively slow down the corrosion process, preventing equipment failure due to corrosion, but also reduce maintenance costs and improve the overall operational efficiency of the equipment.

In the anti-corrosion structural design of pressure vessels, several important design factors must be considered, including stress distribution, liquid accumulation control, flow optimization, and overheat protection. Designers must approach the task from multiple angles, such as material selection, structural optimization, and process improvements, while considering the working environment and operating conditions of the equipment to ensure the anti-corrosion design can address various potential corrosion risks. Below are key design requirements and considerations that provide theoretical foundations and practical guidance to enhance the corrosion resistance of pressure vessels.

Avoiding Stress Concentration

Stress concentration is one of the critical factors affecting the performance of pressure vessels. Uneven stress distribution increases the likelihood of forming corrosion cells, and areas of concentrated stress often become the anode of corrosion, accelerating the process and potentially leading to more severe stress corrosion cracking (under fixed environmental conditions) or corrosion fatigue (under cyclic pressure conditions). To avoid stress concentration, the design should aim to simplify the structure, keep surfaces even, smooth, and clean, and ensure a streamlined component shape with as large a radius of curvature as possible. Sharp edges, notches, and sudden changes in section should be avoided. If unavoidable, these areas should be placed in low-stress regions and treated with necessary measures such as rounded transitions or filling internal corners. In some cases, gaps are inevitable, but they can be managed by welding closures or appropriately enlarging the gaps to prevent clogging and thereby avoid crevice corrosion.

Avoiding Liquid Accumulation

Liquids are generally more corrosive than gases, so areas prone to liquid accumulation should be minimized. The design should ensure that the vessel's shape facilitates the discharge of water, especially in regions prone to water accumulation, where effective drainage holes should be provided. The internal shape of storage tanks and vessels should be designed to aid liquid discharge, and the pipe systems should have streamlined designs, sloping downward where appropriate, to ensure smooth flow. Additionally, the design of pipes and equipment should avoid the formation of turbulence, vortices, and fluid impacts, as these flow instabilities increase corrosion risks. Flow rates should be maintained within appropriate limits, and changes in flow direction and cross-sectional area should avoid abrupt transitions to reduce flow resistance. Air and liquid flows should not directly impact the equipment wall to avoid localized wear and exacerbate corrosion.

Reducing Abrasive Corrosion

Bubbles and solid particles suspended in liquids, or droplets carried by gases, can intensify abrasive corrosion. Therefore, during design, measures should be taken to separate and remove these impurities. For areas where turbulence and impact are unavoidable, the design can be optimized by increasing the thickness of the impacted parts, upgrading local materials, installing baffles, or increasing flow areas. These measures can effectively slow down or prevent abrasive corrosion from damaging the equipment.

Preventing Local Overheating

Overheating is a key factor influencing corrosion rates. Increased temperatures typically accelerate material corrosion, so the design of equipment should avoid local overheating. In special cases, excessive temperatures may lead to rapid corrosion damage. For equipment handling high-temperature gases, certain parts must be protected from "cold spots", which are critical for insulation design. Furthermore, liquid flow along walls or splashing may increase local concentration, intensifying local corrosion conditions. Therefore, designs should ensure smooth liquid flow and avoid liquid stagnation on walls or spillage.

Reasonable Piping and Nozzle Design

The design of inlet pipes should ensure sufficient distance from the vessel shell to prevent liquid splashing or the formation of liquid pools around the pipes. The liquid level distance for nozzles should be minimized or directly inserted into the solution to avoid issues such as harmful component concentration or uneven temperature distribution at the gas-liquid interface. For shell-and-tube heat exchangers, the gas-liquid interface should be avoided, as these areas are prone to temperature unevenness, worsening corrosion. In coolers, the liquid level should completely submerge the tube bundle, while in reboilers, the gas-phase volume should be appropriately increased to reduce gas flow velocity and alleviate corrosion issues.

Gap Issues in Shell-and-Tube Heat Exchangers

The connection points between the tubes and tube sheets in shell-and-tube heat exchangers often have gap issues. While the widely used expansion method creates gaps between the tubes and tube sheets, the expanded-welded connection can eliminate gaps on the tube side, but gaps still exist on the shell side. Deep hole sealing welding can effectively solve this problem but comes with higher processing difficulty and cost. Therefore, the design should choose an appropriate connection method based on actual needs and optimize anti-corrosion measures for gaps to ensure the long-term stable operation of the heat exchanger.

Stress Corrosion and Fatigue Protection

Corrosion fatigue and stress corrosion are common issues during equipment use. To reduce the actual stress levels and ensure the equipment operates under low-stress conditions, the design should incorporate analysis results and rational structural optimization. Increasing the thickness of stressed components can effectively reduce stress levels, preventing corrosion problems caused by stress concentration. Additionally, appropriate material selection and localized material upgrades are key methods for preventing stress corrosion and fatigue.

Conclusion

The anti-corrosion structural design of pressure vessels is a complex task that requires comprehensive consideration from multiple aspects. Through reasonable stress distribution, liquid discharge design, flow system optimization, overheating protection, proper layout of piping and nozzles, and gap control for heat exchangers and connections, corrosion risks can be effectively reduced, thereby extending equipment service life. Furthermore, addressing issues of corrosion fatigue and stress corrosion, and adopting measures to lower stress levels and optimize structures, ensures the stability and safety of pressure vessels during long-term use.