Steel-in-Steel Direct-Buried Insulated Pipeline

Published:2025-07-31 | Last Updated: 2025-08-01    Views: 3

I. What is a steel-in-steel direct burial insulated pipeline?

A steel-in-steel direct burial insulated pipeline is an advanced underground pipeline system specifically designed for the efficient and safe transportation of high-temperature steam and high-temperature media. It ingeniously combines the structural advantages of the “steel-in-steel” design with the “direct burial insulation” installation method, making it an indispensable component of modern industrial and urban infrastructure.

(1) What is the “steel-in-steel” structure?

The “steel-in-steel structure” refers to a pipeline with two layers of steel pipes:  

Inner layer: the working steel pipe for conveying high-temperature steam.  

Outer layer: the outer steel pipe that protects the inner layer and insulation layer.  

Intermediate layer: filled with high-efficiency insulation material.  

This double-layer steel pipe structure ensures the stability and safety of the pipeline under high-temperature and high-pressure conditions.  

(2) What is “direct burial insulation”?  

“Direct burial insulation” refers to the pipeline being directly buried underground without the need for additional trenches or supports. This approach saves space, reduces costs, and further minimizes heat loss and corrosion.  

(3) Why is it suitable for conveying steam and high-temperature media?  

Steel-in-steel direct burial insulation pipes are highly suitable for conveying steam and high-temperature media because they:  

Provide excellent insulation: significantly reduce heat loss and conserve energy.  

Are safe and reliable: the double-layer steel pipe structure is robust, corrosion-resistant, and reduces the risk of leaks.  

Have a long service life: can operate stably over the long term with minimal maintenance requirements.

II. Product Advantages of Steel-in-Steel Direct Burial Insulated Pipes

Super Insulation: Minimizes heat loss, saving money and energy.

Highly Safe: Dual-layer steel pipes provide armor-like protection, minimizing leaks and ensuring durability.  

Long Service Life: Designed for a service life of 30–50 years, reducing the need for replacement and maintenance.  

Direct Burial: No need for trenches, saving space, costs, and time, with minimal environmental impact.  

High-Temperature Compatibility: Specifically designed for transporting high-temperature, high-pressure steam, ensuring stable performance.

III. Recommendations for Selecting Steel-in-Steel Direct Burial Insulated Pipes

(1) Pipe Diameter and Insulation Layer Thickness

Pipe Diameter: Determined based on flow requirements. A larger diameter allows for higher flow rates but also incurs higher costs. For example, a DN200 pipe is suitable for medium flow rates, while a DN600 pipe is suitable for high flow rates.

Insulation Layer Thickness: Primarily determined by the medium temperature and heat loss requirements. Higher temperatures and lower heat loss requirements necessitate thicker insulation layers.

High-temperature steam (e.g., 350°C): Insulation layer thickness is recommended to be 100-200mm.  

Medium-temperature hot water (e.g., 150°C): Insulation layer thickness can be 80-150mm.  

Comprehensive thermal economic calculations must be performed.  

(2) Selection Based on Different Media and Temperatures  

Steam Transportation (High-Temperature and High-Pressure):  

Working Pipe: Recommended materials include 20#, Q345B, 12Cr1MoV, etc.  

Insulation material: Preferably calcium silicate, rock wool, etc., which are high-temperature resistant materials.  

Outer casing corrosion protection: Must use enhanced or extra-enhanced corrosion protection (e.g., epoxy coal tar asphalt, fiberglass) to ensure an underground service life of 30-50 years.  

Compensation: Consider bellows compensators or natural compensation to address thermal expansion and contraction.

Transporting high-temperature hot water (medium temperature and medium pressure):

Working steel pipe: Carbon steels such as Q235B and Q345B can be selected.

Insulation material: In addition to calcium silicate and rock wool, polyurethane foam may also be considered if temperature permits.

(3) Common specifications and models for reference

The following are common parameter ranges for steel-clad steel insulated pipes. Specific models should be determined based on project-specific design requirements:

IV. Installation and Construction Guidelines

(1) Burial Depth and Foundation Preparation  

Burial Depth:  

Sufficient Depth: The top of the pipeline must be covered with at least 0.6–1.0 meters of soil to prevent external loads (such as vehicles or buildings) from directly compressing the pipeline.  

Below the Frost Line: In cold regions, the pipeline must be buried below the local frost line to prevent soil frost heave from damaging the pipeline.

Foundation Preparation:  

Flat and Compact: The bottom of the trench must be flat and compact, free of sharp stones. If necessary, a layer of sand and soil (minimum thickness of 150 mm) may be laid to provide uniform support for the pipeline.  

Slope Control: The trench bottom should have a certain slope to facilitate drainage during construction and water drainage after pipeline operation.

(2) Pipeline Welding and Corrosion Protection Treatment  

Pipeline Welding:  

High technical requirements: The welding of the working pipe (inner pipe) must be performed by qualified welders, strictly following the welding procedure specifications to ensure that the weld quality meets design requirements (e.g., non-destructive testing).  

Outer pipe welding: The welding of the outer pipe is equally important, ensuring strength and密封性.  

Bevel preparation: Before welding, the pipe bevel must be cleaned thoroughly, free of oil and rust.

Corrosion Protection:

Weld Seam Repair: Pipe connections (especially weld seams on outer pipes) are vulnerable points for corrosion protection. On-site repair must use corrosion-resistant materials of the same or higher grade as the outer pipe. The repair material must be compatible with the outer pipe's corrosion-resistant coating and bond securely.

Complete Coverage: Ensure that all exposed steel surfaces, particularly weld seam areas, are fully, uniformly, and completely covered with corrosion-resistant material.

(3) Key points for setting up sliding ends and fixed ends

Fixed end (fixed support):

Forced fixation: Fixed supports are used to restrict axial displacement of the pipeline, dividing it into several compensation sections. They must be able to withstand the enormous thrust generated by thermal expansion and contraction of the pipeline.

Solid foundation: The foundation of the fixed support must be extremely robust, securely connected to the ground or structure, typically requiring reinforced concrete piers.  

Accurate positioning: The position of the fixed support must strictly follow the design drawings, typically located at branch points, turning points, or on both sides of compensators.  

Sliding end (sliding support):

Allowable displacement: Sliding supports allow the working pipe to freely expand and contract within the outer pipe, accommodating thermal expansion and contraction.  

Reduce friction: Sliding supports typically use low-friction materials or rolling structures to minimize resistance during pipe expansion and contraction, thereby protecting the pipe.  

Reasonable spacing: The spacing of sliding supports should be determined based on design calculations, ensuring sufficient support strength while avoiding excessive pipe overhang.

(4) Moisture-proofing and waterproofing measures recommendations  

Pipe joint sealing: All pipe joints, especially those at insulation joints, must be strictly sealed to prevent groundwater or moisture from infiltrating the insulation layer, which could reduce insulation effectiveness or even cause corrosion of the steel pipes.  

Installation below the high water line: Avoid installing pipes in areas with high groundwater levels or prone to water accumulation whenever possible. If unavoidable, additional waterproofing measures should be taken, such as installing drainage ditches, gravel layers, or using higher-grade waterproof sleeves around the pipes.

Backfill Material: Backfill soil should be selected using dry, non-corrosive, uniformly graded materials. Layers should be compacted to avoid uneven stress on the pipeline or damage to the corrosion-resistant coating.

Leak Detection Wire: Some high-end buried pipelines are pre-installed with leak detection wires. During installation, ensure the detection wire is intact and properly connected to the monitoring system to enable real-time monitoring of pipeline conditions. Upon detecting an alarm, promptly locate and address the leak.

V. Frequently Asked Questions (FAQ)

(1) How long can steel-clad steel direct burial pipes be used?

Under normal design, strict construction, and good operation and maintenance conditions, the design service life of steel-clad steel direct burial insulated pipes can typically reach 30 to 50 years. Their long service life is mainly due to the strength of the inner and outer steel pipes, the protection of the highly efficient anti-corrosion layer, and the effective handling of thermal expansion and contraction.

(2) Which is better compared to polyurethane insulated pipes?  

This depends on the specific application scenario and the temperature of the transported medium. Each has its own advantages:  

Steel-clad steel direct burial insulated pipes:  

Advantages: Suitable for high-temperature, high-pressure steam or hot water (typically capable of withstanding temperatures from 150°C to 550°C or even higher). The outer steel pipe provides robust mechanical protection and corrosion resistance, ensuring a long service life and extremely high safety and reliability.

Disadvantages: Relatively high cost and stricter construction requirements.  

Polyurethane-insulated pipes (typically referring to pre-insulated polyurethane buried pipes):  

Advantages: Suitable for medium-to-low-temperature hot water or liquids (typically below 140°C). Excellent insulation performance, lightweight, relatively simple construction, and lower cost.  

Disadvantages: The temperature limit of polyurethane material is relatively low, making it unsuitable for high-temperature steam transportation; The outer protective pipe is typically made of HDPE (high-density polyethylene), which has lower mechanical strength and aging resistance compared to steel pipes.  

(3) How to inspect and maintain after burial?  

Leak detection line monitoring: Many modern steel-jacketed buried insulation pipes have pre-installed leak detection lines inside. These wires are connected to a monitoring system. Once the insulation layer becomes damp or the pipe leaks, the alarm system will immediately trigger an alert, indicating the leak location for timely repairs.

Regular inspections: Even for buried pipes, regular ground inspections are necessary to observe any abnormal ground subsidence, steam emission, or water accumulation, which may indicate issues with the underground pipes.

Compensator displacement monitoring: For pipe sections with compensators, the displacement of the compensators should be regularly checked to ensure it remains within the design range, indicating whether the pipes have abnormal thermal expansion/contraction or failed fixed points.

Pressure and temperature monitoring: Use pressure gauges and thermometers in the pipeline system to monitor changes in medium pressure and temperature in real time. Abnormal fluctuations may indicate internal pipeline issues.

Professional detection equipment: When suspecting a fault, use professional underground pipeline detection equipment (such as ground microphones, infrared thermal imagers, etc.) to assist in locating the problem area.

The key to maintaining buried pipelines is “prevention first.” Through high-quality installation and effective monitoring systems, the likelihood of faults occurring can be reduced.