Heat preservation pipe is a steel pipe treated with a heat preservation process to ensure the temperature of the transported medium meets requirements. It is widely used in liquid and gas pipeline networks, central heating, and municipal engineering.

It is mainly divided into two types: steel-in-steel composite insulated steel pipe and polyurethane insulated steel pipe. The steel-in-steel composite type uses a nested inner and outer steel pipe structure and includes a multi-layer design with air insulation and anti-corrosion layers, allowing for direct burial. The polyurethane type consists of a working steel pipe, a polyurethane insulation layer, and a polyethylene protective layer. Both types offer low heat loss, a long service life (30-50 years), and convenient construction.

This material reduces excavation and heat loss (75% lower than traditional pipes), reducing project costs by 10%-25%. Its waterproof and anti-corrosion properties support direct burial installation. Prefabricated direct-buried insulated pipe technology effectively solves the insulation and waterproofing issues of high-temperature heat transmission pipelines and is gradually being adopted in heating projects in China. However, there is still significant room for improvement in the use of polyurethane materials.

Classification

Steel-in-Steel Composite Insulated Steel Pipe

The insulation structure of steel-in-steel composite insulated steel pipe can be divided into two categories based on the sliding method:

1. Internal Sliding Type: The insulation structure consists of a working steel pipe, aluminum silicate, a drag reduction layer, microporous calcium silicate, an insulation layer, a stainless steel fastening belt, an aluminum foil reflective layer, a polyurethane insulation layer, an outer steel pipe, and an external anti-corrosion layer. Internal Sliding Type insulated steel pipes are constructed with a steel pipe for conveying the medium, a composite silicate or microporous calcium silicate, rigid polyurethane foam, an outer steel pipe, and a fiberglass shell for corrosion protection. The insulation treatment technology for various pipe joints is mature and reliable.

2. External Sliding Type: The insulation structure consists of a working steel pipe, a glass wool insulation layer, an aluminum foil reflective layer, a stainless steel fastening belt, a sliding guide bracket, an air insulation layer, an outer steel pipe, and an external anti-corrosion layer.

External Sliding Insulated Steel Pipe Structure

1. Inner Steel Pipe
2. Zinc-Rich Primer
3. Sliding Guide Bracket
4. High-Temperature Glass Wool
5. Aluminum Foil Reflective Layer
6. Air Insulation Layer
7. Outer Steel Pipe
8. Outer Steel Pipe Anti-Corrosion Layer

Steel-in-steel composite insulated steel pipe is a type of direct underground pipeline. It can be buried directly underground even without a concrete structure. Thermal expansion of the working steel pipe occurs within the outer pipe, reducing material costs, shortening construction time, and ensuring the safety of the heating pipe. It can be used safely and widely in diverse temperature environments, and is particularly suitable for high-temperature steam pipeline projects. The operating temperature can reach 150°C-450°C. Pipe ends are generally sealed with polyethylene film or three-layer PE cold wrap tape to prevent moisture and water ingress before and during installation. Multi-layer insulation wrapping with staggered seams effectively reduces heat loss. Control measures are implemented on the outer casing surface to prevent cold bridges, ensuring temperature control of the outer casing anti-corrosion layer. Multi-layer aluminum foil reflective layer wrapped with insulation effectively reduces heat loss, making steam pipelines more economical and cost-effective. The drainage system is fully enclosed, offering flexible layout, rational structure, and safety and reliability. The moisture drain pipe on the steel casing not only promptly removes humid gases but also serves as an alarm signal pipe for daily operation. The pipeline's thermal compensation utilizes high-quality bellows compensators installed within the casing. Direct burial eliminates the need for observation wells, facilitating easy installation and shortening the construction period. Suitable for transporting steam or other media at temperatures below 2.5 MPa and 350°C, this product utilizes a steel pipe outer protective layer, offering high strength, resistance to damage, ease of installation and maintenance, and a long service life.

Polyurethane Insulated Steel Pipe

To reduce heat loss from medium- and small-diameter crude oil or heating pipelines to the soil, an insulation composite layer is applied to the exterior. Rigid polyurethane foam is commonly used as the insulation material, with an operating temperature range of -185°C to 120°C. This soft material is reinforced by a high-density polyethylene layer applied over the insulation layer to create a composite structure that prevents groundwater from seeping into the insulation layer. High-pressure polyurethane foaming equipment is used to shot-blast the outer surface of the steel pipe for rust removal, and corona-treat the inner surface of the outer casing to improve the adhesion of the insulation pipe. The insulation layer is made of rigid polyurethane foam with a density of 60kg/m³ to 80kg/m³. It fully fills the gap between the steel pipe and the casing and provides a certain degree of bonding strength, forming a solid, integrated structure between the steel pipe, outer casing, and insulation layer. Polyurethane foam has excellent mechanical and thermal insulation properties, typically withstanding temperatures of 120°C. Modifications or combinations with other insulation materials can increase the temperature to 180°C.

Prefabricated polyurethane insulated steel pipes are constructed from the inside out in three layers:

First Layer: Working Steel Pipe Layer

Seamless, spiral, or straight-seam steel pipes are typically used, depending on the design and customer requirements. After advanced shot blasting, the steel pipe surface achieves a rust removal grade of Sa2 as specified in the GB/T8923-1988 standard, and a surface roughness of R=12.5 microns as specified in the GB6060.5-88 standard.

Second Layer: Polyurethane Insulation:
This layer is created by injecting a rigid polyurethane foam concentrate into the cavity between the steel pipe and the outer sheath using a high-pressure foaming machine. This process is commonly known as "tube-in-tube foaming."

Third Layer: High-Density Polyethylene Protective Layer:
This layer is prefabricated from black or yellow polyethylene pipe of a specific wall thickness. Polyethylene is odorless, non-toxic, and has a waxy feel. It offers excellent low-temperature resistance, with a minimum operating temperature of -70°C to -100°C. It also exhibits excellent chemical stability and is resistant to most acids and bases (but not oxidizing acids). It is insoluble in common solvents at room temperature and has low water absorption. However, due to its linear molecular structure, it can be slowly dissolved in some organic solvents without swelling, and exhibits excellent electrical insulation properties. Its functions are to protect the polyurethane insulation layer from mechanical damage and to provide corrosion resistance and waterproofing. High-density polyethylene (HDPE) polyurethane foam insulated steel pipes comply with the SY/T114-2000 and SY/T115-2001 standards.

Advantages and Features of Polyurethane Insulated Steel Pipes

1. Reduced Project Costs
According to relevant department estimates, polyurethane insulated steel pipes can generally reduce project costs by approximately 25% (using fiberglass as a protective layer) and 10% (using HDPE as a protective layer).

2. Low Heat Loss, Saving Energy
Polyurethane has a thermal conductivity of λ = 0.013-0.03 kcal/m·h·oC, significantly lower than other commonly used pipe insulation materials, improving insulation effectiveness by 4-9 times. Furthermore, it has a very low water absorption rate of approximately 0.2 kg/m². This low water absorption is due to the polyurethane foam's high closed-cell ratio of approximately 92%. The low thermal conductivity and water absorption, combined with the insulation layer and waterproof high-density polyethylene (HDPE) or fiberglass outer shell, eliminate the "wet cotton jacket" problem of traditional trench-laid heating pipes. This significantly reduces overall heat loss in the heating network, resulting in a 2% heat loss, far below the international standard of 10%.

3. Excellent corrosion resistance and insulation properties, ensuring a long service life. Because the rigid polyurethane foam insulation layer is tightly bonded to the steel pipe outer skin, it isolates air and water from infiltration, providing excellent corrosion protection. Furthermore, its closed pores minimize water absorption. Both the HDPE and FRP outer shells offer excellent corrosion resistance, insulation, and mechanical properties. Therefore, the outer skin of the working steel pipe is less susceptible to erosion by air and water. As long as the water quality inside the pipe is properly treated, according to international data, the service life can reach over 50 years, three to four times longer than traditional trench or overhead installations.

Performance and Advantages

Prefabricated direct-buried insulated steel pipes offer significant advantages over traditional steel pipe insulation methods:

1. Prefabricated direct-buried insulated steel pipes do not require extensive trench construction; simply bury the insulated pipes underground. This significantly reduces project land occupation, contributing to environmental protection, and reduces earthwork excavation by over 50%, while reducing civil engineering and concrete construction by 90%. Furthermore, insulated pipe construction is carried out in parallel with on-site trenching, requiring only on-site jointing, shortening construction time by over 50%.

2. They offer excellent thermal insulation performance, with heat loss as low as 25% of that of traditional pipes. This can significantly reduce energy costs over long-term operation.

3. They offer strong waterproofing and corrosion resistance, eliminating the need for trenching and allowing for direct burial underground or underwater, resulting in simple, rapid construction and a low overall cost.

4. They offer excellent corrosion and impact resistance even in low-temperature conditions and can be buried directly in frozen soil.

5. They offer a service life of 30-50 years. Proper installation and use can minimize pipe network maintenance costs. 6. An alarm system can be configured to automatically detect pipe network leaks, accurately indicate the fault location, and automatically generate an alarm. Application

Insulated Pipe Features

High-temperature prefabricated direct-buried insulated pipe offers excellent thermal insulation, increased safety and reliability, and low construction costs. It effectively addresses the insulation, sliding lubrication, and waterproofing issues faced by prefabricated direct-buried insulated pipes used for high-temperature heat transmission in urban centralized heating systems (130°C-600°C). High-temperature prefabricated direct-buried insulated pipes not only offer advanced technology and practical performance unmatched by traditional trench and overhead pipelines, but also offer significant social and economic benefits and are a powerful measure for energy conservation in heating systems. High-temperature prefabricated direct-buried insulated pipes utilize direct-buried heating pipeline technology, marking a new beginning in the development of heating pipeline technology in China.

The insulated pipes are constructed of a steel pipe, a fiberglass inner sheath, and a fiberglass outer shell. They feature a high-temperature-resistant insulation layer, a lubricating layer, and an elastic seal. The utility model effectively solves the problems of insulation, sliding lubrication and waterproofing of exposed pipe ends of prefabricated direct-buried insulated pipes for high-temperature heat transmission at 130℃-600℃ in urban centralized heating. The high-temperature prefabricated direct-buried insulated pipe mainly consists of four parts.

(1) Working steel pipe: According to the technical requirements of the transmission medium, seamed steel pipe, seamless steel pipe and double-sided submerged arc spiral welded steel pipe are respectively used.

(2) Insulation layer: Rigid polyurethane foam plastic is used.

(3) Protective shell: High-density polyethylene or fiberglass is used.

(4) Leakage alarm line: When manufacturing high-temperature prefabricated direct-buried insulated pipes, an alarm line is buried in the insulation layer near the steel pipe. Once a leak occurs somewhere in the pipeline, the alarm line conducts and the exact location and degree of the leak are displayed on the special detection instrument, so as to notify the maintenance personnel to quickly deal with the leaking pipe section and ensure the safe operation of the heating network.

High-temperature prefabricated direct-buried insulated pipe - advantages and characteristics

1 Reduce the project cost. According to relevant department estimates, dual-pipe heating pipes can generally reduce project costs by approximately 25% (using fiberglass as a protective layer) and 10% (using high-density polyethylene as a protective layer).

2. Low heat loss and energy conservation.

Current Development Status

Insulated steel pipes are a key factor in energy conservation, and their development and application are gaining increasing attention worldwide. Since the 1970s, international attention has been focused on the production and application of insulated steel pipes, striving to significantly reduce energy consumption, thereby reducing environmental pollution and the greenhouse effect. The international insulation industry has a long history, and new insulation materials are constantly emerging. Before 1980, the development of insulated steel pipes in my country was very slow, with a small number of insulation plants producing only small quantities of direct-buried insulated steel pipes. However, after over 30 years of hard work, and particularly the rapid development of the past decade, the Chinese insulation industry has developed numerous products from scratch, from a single product to a diverse range, and from low quality to high quality, with increasing widespread application. Polyurethane is the most commonly used insulation material internationally. Rigid polyurethane boasts many excellent properties and is widely used for thermal insulation in Europe and the United States. Approximately 49% of insulation materials in developed countries like Europe and the United States are polyurethane, while in China, this proportion is less than 20%. Therefore, there is significant room for growth in polyurethane-insulated steel pipes in China.

Structural Mechanism

1. Outer Sheath: Protects the insulation layer from groundwater erosion, supports the working pipe, and withstands certain external loads, ensuring proper operation.
2. Anti-corrosion Layer: Protects the outer steel pipe from corrosive agents, extending its service life.
3. Drag Reduction Layer: Ensures free movement of the working pipe due to thermal expansion and contraction.
4. Polyurethane Foam Layer: Maintains the medium temperature and maintains a constant temperature on the outer sheath surface.
5. Barrier and Reflective Layer: Prevents organic foam from entering the inorganic rigid, high-temperature-resistant layer and reflects some of the heat away from the high-temperature-resistant layer.
6. Working Pipe: Ensures the proper flow of the conveying medium.

7. Inorganic hard insulation layer: resistant to high temperatures, ensuring the interface temperature with the organic insulation layer and preventing the foam from carbonizing.