Selection and Supply Assurance of Large Diameter Steel Pipes in Refining Units

Literature Overview

This paper by Wang Honghai, Sang Wei, and Chen Dong from Dexin Steel Pipe (China) Co., Ltd., published in Chemical Equipment and Piping in 2021, addresses a critical practical challenge in the design and construction of large-scale refining units: the selection and supply assurance of large diameter steel pipes. The paper is particularly relevant to refineries processing high-sulfur and high-acid crude oils, where the corrosion environment is severe and the consequences of material selection errors are catastrophic.

Corrosion Environments and Material Selection

Refining units operate in a variety of corrosive environments, each requiring specific material selection strategies. The paper identifies and categorizes these environments:

Corrosion Environment	Typical Location in Refinery	Primary Corrosive Species	Recommended Material Grades
Wet H2S corrosion	Crude distillation, hydrogenation units	H2S dissolved in water	Carbon steel with HIC-resistant grade; low alloy steel
High-temperature H2S corrosion	Hydrocrackers, hydrodesulfurizers	H2S at elevated temperature	Cr-Mo low alloy steel (e.g., 1.25Cr-0.5Mo)
Naphthenic acid corrosion	Distillation units processing naphthenic crude	Naphthenic acids	Stainless steel (e.g., 321, 347); high-alloy materials
Wet H2S + CO2 corrosion	Gas processing sections	Mixed acid environment	Duplex stainless steel; CRA alloys
Sulfuric acid dew point corrosion	Furnace exit, waste heat boiler	H2SO4 condensation	Alloy 800H; Inconel 600; ceramic lined steel
High-temperature sulfur attack	Catalytic cracking units	Elemental sulfur	Nickel-based alloys; high-alloy castings

The paper emphasizes that material selection alone is insufficient; the manufacturing process of the steel pipe must also be considered. This is a critical insight that distinguishes this paper from simpler material selection guides.

Seamless vs. Welded Pipe: A Critical Comparison

The central argument of the paper is that for large diameter steel pipes in refining applications, seamless pipes should be preferred over welded pipes within the production size range of seamless pipe mills. The reasoning is based on several technical and logistical factors:

Comparison Factor	Seamless Pipe	Welded Pipe	Advantage
Corrosion resistance	Homogeneous material throughout wall thickness	Weld and HAZ have different corrosion resistance than base metal	Seamless
Material supply flexibility	Billet stock can be pre-stored for emergency supply	Requires specific coil or plate stock for each order	Seamless
Customization capability	Medium frequency hot expansion allows flexible diameter customization	Limited to mill production sizes; custom sizes require additional processing	Seamless (with hot expansion)
Production lead time	Billet storage enables rapid production mobilization	May require coil procurement and production scheduling	Seamless
Cost	Higher unit cost	Lower unit cost	Welded
Size range	Limited by mill capacity	Available in very large diameters	Welded
Weld quality risk	No weld defects possible	Weld defects and HAZ degradation possible	Seamless

The paper specifically addresses the medium frequency hot expansion process, which allows seamless pipes to be produced in diameters that exceed the direct rolling capacity of the seamless pipe mill. In this process, a smaller-diameter seamless pipe is heated by medium frequency induction heating and then expanded over a mandrel to the desired final diameter. This process preserves the metallurgical integrity of the material while achieving the required dimensions. The key advantage is that the resulting pipe is still seamless — there is no weld or heat-affected zone — and therefore maintains the full corrosion resistance of the base material.

Damage Modes and Failure Analysis

The paper discusses common damage modes for large diameter steel pipes in refining service, which can be analyzed using an FMEA (Failure Mode and Effects Analysis) approach:

External corrosion: Caused by atmospheric or soil corrosion on the pipe exterior. Mitigated through proper coating and cathodic protection systems.
Internal corrosion: The dominant damage mode in refining service. Caused by corrosive species in the process fluid. Mitigated through appropriate material selection, corrosion inhibitors, and monitoring systems.
Stress corrosion cracking (SCC): Occurs when a susceptible material is exposed to a specific corrosive environment under tensile stress. Particularly relevant for stainless steel in chloride environments and carbon steel in wet H2S environments.
Weld degradation: For welded pipes, the weld metal and HAZ may have different microstructures and mechanical properties than the base metal, making them more susceptible to corrosion and mechanical failure. This is a fundamental disadvantage of welded pipes in corrosive service.
Hydrogen damage: Including hydrogen-induced cracking (HIC), sulfide stress cracking (SSC), and hydrogen blistering. Particularly relevant for carbon steel in wet H2S environments. The resistance to hydrogen damage is primarily a function of the steel's cleanliness (inclusion content) and microstructure.

Supply Assurance Strategy

The supply assurance challenge for large diameter steel pipes in refining projects is unique due to the combination of diverse specifications, small batch sizes, and long lead times. The paper proposes a supply strategy that leverages the advantages of seamless pipe production:

Billet stock pre-positioning: Maintaining strategic inventory of seamless pipe billets in the required grades allows rapid production mobilization when orders are received, reducing lead times significantly compared to the time required to procure and process coils or plates for welded pipe production.
Medium frequency hot expansion capability: The ability to expand seamless pipes to custom diameters provides flexibility in meeting project-specific requirements without the need for special mill setups or dedicated production runs.
Quality consistency: Seamless pipes offer more consistent quality because the material is homogeneous throughout the wall thickness. There is no weld line, no HAZ, and no risk of weld-related defects. This consistency reduces the risk of in-service failures and simplifies quality assurance procedures.
Multi-variety production capability: Seamless pipe mills can produce a wide range of grades from a single billet stock, making them well-suited for projects that require multiple material grades in small quantities.

Engineering Practice Implications

For engineers involved in the design and procurement of large diameter steel pipes for refining units, this paper offers several actionable recommendations:

Always consider the manufacturing process when selecting materials. A welded pipe of the same material grade as a seamless pipe may have significantly lower corrosion resistance due to the presence of a weld and HAZ.
For critical service applications involving high-sulfur or high-acid crude oils, seamless pipes should be the default choice unless the required diameter exceeds the seamless pipe production capability.
When seamless pipes are not available in the required size, consider the medium frequency hot expansion route as an alternative to welded pipe fabrication.
Establish strategic supply relationships with seamless pipe manufacturers that maintain billet inventories in the required grades, to ensure supply assurance for time-critical projects.
Implement rigorous quality assurance procedures that include chemical analysis, mechanical testing, and non-destructive examination for all large diameter steel pipes procured for refining service.

Summary

This paper provides a comprehensive and practical guide for the selection and supply assurance of large diameter steel pipes in refining applications. The central message is clear: for critical service in high-sulfur and high-acid crude oil processing, seamless pipes should be preferred over welded pipes within the available size range, due to their superior corrosion resistance, supply flexibility, and quality consistency. The medium frequency hot expansion technology extends the applicability of seamless pipes to larger diameters, further strengthening the case for seamless pipe preference. For engineers and procurement specialists, this paper serves as a valuable reference for making informed material selection decisions and developing robust supply strategies for large-scale refining projects.