Steel-in-Steel Direct-Buried Insulated Pipeline

Published:2025-07-31 | Last Updated: 2025-08-30    Views: 133

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

Steel-in-Steel Direct-Buried Insulated Pipeline is an underground pipeline system specifically designed for transporting high-temperature steam and media. It ingeniously combines the “steel-in-steel” structure with “direct burial insulation” technology, finding extensive application in urban heating and industrial sectors.

(1) What is the “Steel-in-Steel” Structure?

Its core is a composite system formed by double-wall insulated steel pipe:

The inner layer is the working pipe for conveying high-temperature steam;

The outer layer is the protective outer casing pipe;

Filled with high-efficiency insulation material.

This structure ensures pipeline safety and stability under high-temperature, high-pressure conditions. A typical example is the steel-in-steel insulated steel pipe.

(2) What is “direct burial insulation”?

“Direct burial insulation” refers to pipelines buried directly underground without trench construction. This installation method significantly saves space and costs while reducing heat loss and corrosion risks. It is commonly used in direct-buried preinsulated steam pipeline projects.

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

Such pipelines (e.g steel jacketed preinsulated pipe) are particularly well-suited for high-temperature transportation due to their:

Excellent thermal insulation, significantly reducing heat loss;

High safety and corrosion resistance, with the dual-pipe structure enhancing reliability;

Extended service life, requiring minimal maintenance.

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

Steel-in-steel direct burial insulated pipes demonstrate significant advantages in high-temperature transportation applications, particularly for heating steam pipeline systems, thanks to their unique structural design and performance. Key benefits include:

(1) Superior Thermal Insulation

Minimizes heat loss to the greatest extent, significantly reducing energy consumption and operational costs.

(2) High Safety and Reliability

Steel-in-steel steam insulated pipe provides armor-like high-strength protection, drastically lowering leakage risks and ensuring long-term system safety.

(3) Exceptionally Long Service Life

Designed for 30-50 years of operation, substantially reducing replacement frequency and maintenance expenses.

(4) Convenient and Cost-Effective Direct Burial Installation

Eliminates trenching requirements, conserving land, shortening construction schedules, and minimizing environmental impact.

(5) Superior High-Temperature and High-Pressure Performance

Engineered specifically for high-temperature, high-pressure steam, ensuring stable and reliable operation under demanding conditions.

III. Selection Recommendations

1. Pipe Diameter and Insulation Thickness Selection

(1) Pipe diameter should be determined based on the design flow rate.

Larger diameters offer greater flow capacity but also incur higher costs.

Example: DN200 is suitable for medium flow rates, while DN600 is more appropriate for high-flow systems.

(2) Insulation thickness primarily depends on the medium temperature and allowable heat loss. Higher temperatures and stricter insulation requirements necessitate greater thickness.

High-temperature steam (e.g., 350°C): Recommended thickness 100–200mm

Medium-temperature hot water (e.g., 150°C): Recommended thickness 80–150mm

Actual design requires thermal calculations and economic analysis.

2. Selection Guidance Based on Medium and Temperature

(1) Steam Conveyance (High-Temperature/High-Pressure Conditions)

Working pipes should use materials like 20# seamless steel pipe Q345B welded steel pipe or 12Cr1MoV;

Insulation materials should be high-temperature resistant, such as calcium silicate, composite silicate, or rock wool;

Outer casings must employ reinforced anti-corrosion coatings like epoxy coal tar or fiberglass to ensure a 30–50-year service life;

Steel jacketed steam pipe construction is recommended, incorporating bellows expansion joints or natural expansion design.

(2) High-Temperature Hot Water (Medium-Temperature, Medium-Pressure Conditions)

Working pipes may use Q235B or Q345B carbon steel welded pipe;

Beyond high-temperature materials, polyurethane foam may be selected within permissible temperature ranges;

Prefabricated insulated steam pipes are recommended to enhance construction efficiency and system consistency.

3、Common Specifications Reference

IV. Construction Guide for Steel-in-Steel Direct-Buried Insulated Pipeline

1. Burial Depth and Foundation Treatment

(1) Burial Depth Requirements:

The thickness of the soil covering the top of the pipeline should not be less than 0.6–1.0 meters to avoid direct external loads.

In cold areas, it should be buried below the permafrost line to prevent frost heave damage.

(2) Foundation Treatment:

The bottom of the trench should be flat and dense, without sharp stones. If necessary, a 150 mm sand cushion should be laid.

The trench bottom should have a slope to facilitate drainage.

When using polyurethane insulated steel pipes, the foundation flatness should be strictly controlled to avoid local stress concentration.

2. Pipeline welding and anti-corrosion

(1) Welding requirements:

Working pipe welding must be performed by a certified welder according to the process and non-destructive testing must be carried out.

Outer pipe welding should ensure strength and sealing.

The groove must be clean and free of contamination.

(2) Anti-corrosion treatment:

Welds should be repaired with compatible high-grade anti-corrosion materials.

Ensure that all steel surfaces are completely covered with no exposed spots.

Tip: It is recommended to purchase pipes from reputable seamless steel pipe suppliers to ensure the quality of the parent material and welding reliability.

3. Setting of fixed and sliding ends

(1) Fixed supports:

Need to bear the axial thrust of the pipeline, and the foundation should be a reinforced concrete structure.

The location should be strictly constructed according to the drawings and usually located at branches, corners or compensators.

(2) Sliding supports:

Allow the pipeline to expand and contract axially, and low-friction materials should be used.

The spacing should be determined by calculation to avoid the pipeline hanging in the air or excessive friction.

4. Waterproofing and backfilling measures

(1) Joint sealing:

All insulation joints must be strictly sealed to prevent water vapor intrusion.

For polyurethane foam insulation pipes, the waterproofing treatment of the ends should be paid more attention.

(2) Backfill requirements:

Use dry, non-corrosive fine-grained materials to backfill in layers and compact them.

High water level areas should add drainage measures or use waterproof casing.

(3) Leakage monitoring:

If the detection line is pre-installed, it must be ensured to be intact and connected to the system for real-time monitoring.

V. Frequently Asked Questions (FAQ)

(1) How long is the service life of an thermal insulation steel pipe?

Under standard design, construction and normal maintenance, its service life can usually reach 30-50 years. Long-term effectiveness mainly depends on the strength of the steel pipe itself, the high-quality anti-corrosion layer and the effective control of thermal stress.

(2) Compared with polyurethane insulated pipe, which one is more suitable?

It needs to be selected according to the medium temperature and use conditions:

Steel-in-Steel Direct-Buried Insulated Pipeline:

It is more suitable for high-temperature and high-pressure (150°C-550°C) steam or hot water. It has high mechanical strength, good corrosion resistance and long service life, but the cost and construction requirements are also high.

Polyurethane thermal insulation steel pipe:

It is usually used for hot water or liquid transportation below 140°C. It has good insulation, light weight, low cost and easy installation. If polyurethane foam pre-insulated steel pipe is used, the on-site installation efficiency and overall consistency can be further improved. However, it is not suitable for high-temperature steam, and the outer sheath has weak impact resistance and aging resistance.

(3) How should the pipeline be inspected and maintained after it is buried?

The insulation layer status and leakage can be monitored in real time through pre-buried leak detection lines;

Regularly check the ground for abnormal settlement, local heating, or seepage;

Monitor the displacement of the compensator to ensure that it does not exceed the limit;

Continuously monitor the pressure and temperature changes during system operation;

Use infrared thermal imaging equipment and other equipment to assist in troubleshooting when necessary.