FLEXALEN pre-insulated pipelines. Basic principles for the design of pre-insulated pipelines

Pipes pre-insulated with polyurethane foam are used in the system central heating.

This “pipe-in-pipe” product is currently characterized as reliable and very effective. Its service life is more than 25 years. Withstands high thermal loads up to +140 degrees, which can be increased for a short period of time up to 150 degrees.

Allowed use of pre-insulated pipes for transporting other substances– gas, oil, etc.

The steel pipe and polyurethane foam layer are tightly protected by a polyethylene, or, in some cases, a spiral-wound sheath of pre-galvanized steel. The product is manufactured with a system for special monitoring of the humidity of the thermal insulation layer or breakage and leakage in the pipe.

When assembled, a pre-insulated pipe looks like a single structure consisting of a steel pipe, a layer of polyurethane foam insulation, and an external waterproof shell. Tight adhesion of the layers is achieved during the manufacture of products by complying with technological standards:

• Initial shot blasting, brushing or shot blasting of the top coating of a steel pipe. As a result, its surface is freed from rust and various contaminants and becomes rough. These qualities of the product contribute to a strong connection between the insulating layer and the pipe.
• Maintaining temperature conditions to ensure a high-quality polyurethane foam foaming process;
• The polyethylene shell is internally treated with a coronary discharge, which provides the best bond between the polyurethane insulation and the shell.

Materials for the manufacture of polyurethane foam pipes

For the manufacture of thermally waterproofed products, pipes and fittings made of corrosion-resistant steel are used, which comply with GOST. For thermal insulation, polyurethane systems produced by Elostokam, Izolan, Dau, and Huntsman can be used, which are more consistent with the conditions imposed on thermal insulation materials. This PPU system is designed for a long service life with high temperatures– up to 150 degrees.

Pre-isolated polymer products equipped with an operational system remote control. It monitors the condition of the pipes, signaling if there is a malfunction, and indicates the exact location of the defect.

Checking the quality of pre-insulated products

Ready pre insulated pipes and their parts undergo mandatory quality control. In addition, all materials used in production are subject to control. Before use, the polyurethane foam system must also be tested for compliance with the foaming standards specified in the technical specifications. In addition, before use, polyethylene insulating materials are tested for elongation at break, as well as changes in length finished product after warming up.

When monitoring the quality of pre-insulated pipes, the laboratory checks:

• density of polyurethane foam;
• stability during compression, shear strength and deformation within 10%;
• volume fraction of closed pores;
• thermal conductivity of polyurethane foam;

An additional and important condition for the quality of polyurethane foam pipes is the use of a high-quality waterproofing shell made of polyethylene. When the polyethylene shell breaks, the permissible elongation in percentage terms should be 350. After heating to a temperature of 110 degrees, the change in length should be no more than 3%. Durability at elevated temperature at 80 degrees and a constant pressure of 165 (with an initial stress in the shell wall of 4.6 MPa), and not less than 1000 (with an initial stress in the shell wall of 4.0 MPa). Stability under uniform tensile loads of 4.0 MPa at 80 degrees in an aqueous surfactant solution – no less than 2000.

Characteristics of pipe insulation:

• The density of polyurethane foam should be within 60 kg per cubic meter;
• Compressive stability of at least 0.3 MPa at 10% deformation in the radial direction;
• water absorption by volume is no more than 10% at 90-minute boiling.

The ends of the thermal insulation of polyurethane foam and parts can be covered with a waterproofing layer. Foam insulation in cross-section should be a homogeneous fine-mesh suspension. Voids in it larger than 1/3 of the thickness of the composition are not allowed.

Polyurethane foam, which is used for the manufacture of pre-insulated structures and shaped products made from liquid formulations, mixing and dosing of which is carried out using special filling equipment. These foams can be produced both at industrial-scale enterprises and directly where they are used. The process of foaming and hardening of polyurethane foam occurs quite quickly, so that after a few tens of minutes the material is ready for use. Rigid PU foam can have a density of approximately 30 to 80, and sometimes more than 1 kg per cubic meter and contain insulated cells measuring 0.2 - 1 mm in diameter.

Advantages of thermally insulated polyurethane foam pipes

1. Lowest thermal conductivity and due to this quality minimum thickness isolation. Such properties of polyurethane foam make it possible to achieve high energy and heat-saving characteristics in household and industrial systems during its use.
2. Durability: the operational life of polyurethane foam exceeds 30 years, while all its properties are preserved.
3. Waterproof.
4. High and long-lasting adhesion (adhesion) to the pipe and waterproof shell.
5. Increased mechanical strength of the product.
6. Polyurethane foam insulation is seamless, monolithic, without the formation of “cold bridges”.
7. The material is inert to acid and alkaline compounds, protects the pipe from corrosion and aggressive chemical environments, thereby extending the service life of the structure, is non-toxic and completely safe for humans.

The use of polyurethane foam allows you to:

1. increase the service life of the pipeline to 40 years, compared to the old ones (their life is only up to 10 years);
2. reduce heat losses to 2% (old types of pipelines had losses of up to 40%);
3. reduce capital costs by 20%, operating costs by nine times, and repair costs by three times;
4. an adapted remote control system (RMS) with great accuracy allows you to identify and promptly eliminate any breakdowns that have occurred (for example, moistening of polyurethane foam), and prevent any accidents;
5. does not require protection from stray currents and the construction of a drainage system.

Significant capabilities of pre-insulated pipes

What are the possibilities for using foam insulation when transporting heat over long distances, when compared with mineral wool insulation? According to SNiP standards, mineral wool is considered a good heat insulator. But during operation, after two years it loses its technical properties under the influence of atmospheric factors and it requires replacement.

For example, when testing heat retention from pipes insulated mineral wool, in one of the villages it turned out that heat losses on the pipeline (with a diameter of 200 mm and the ability to receive hot water at 75 degrees, at atmospheric temperature 13 degrees), which was isolated by this material, was 104 kcal/m.h. But when installing polyurethane foam insulation – only 18 kcal/m.h. As a result, the difference was large - 122 kcal/m.h., naturally, in favor of pre-insulated pipes.

The use of pipes coated with polyurethane foam reduces heat loss by up to minimum sizes, rehabilitate centralized system heating, as well as transfer heat through pipelines over relatively long distances without much loss. And the use of pre-insulated pipes together with heat pumps makes it possible to transmit to the population located on long distance secondary, recycled industrial enterprises, the heat that is currently being discarded by these objects. Thus, pre-insulated pipes are good way reduce heat losses to a minimum in water supply networks.

Polyurethane foam products require careful handling

When storing pre-insulated pipes, mechanical damage, longitudinal deflection, contamination, and deformation are not allowed. When loading or unloading material, lifting mechanisms must be used that do not cause any damage to the insulated pipes. They are transported by water, rail and by car. Careful delivery of polyurethane foam pipelines with all components guarantees high-quality functioning of the future heating network.

Russia has the highest level of centralized heat supply (about 80%). The total length of heating networks in two-pipe terms with pipe diameters from 57 to 1400 mm is about 260 thousand km. The predominant method of laying heating networks is in non-passable channels with mineral wool thermal insulation.

Channelless installation, carried out from factory-made structures using reinforced foam concrete insulation and bitumen-containing masses (bitumen perlite, bitumen-overmiculite, bitumen-ceramsite), makes up 10% of the total length of heating networks. About 90% of fuel savings achieved through combined heat generation methods are lost in heating networks.

The service life of heating networks is one and a half to two times lower than abroad, and does not exceed 12-15 years. Most effective solution problem is the widespread introduction into practice of construction of thermal networks of pipelines with foam polyurethane thermal insulation"pipe-in-pipe" type. The idea is not new. Back in the 1960s, the USSR carried out experimental work on the use of polyethylene pipes and foamed polymer materials for insulation of underground heating networks. But then this direction was not widespread due to the limited production and high cost of the polymer materials used.

Technical requirements to thermal insulation

The materials used must have high thermal insulation properties(the thermal conductivity coefficient of the material should not exceed 0.06 W/(m⋅°C), durability (resistance to water, chemical and biological aggression), frost resistance and mechanical strength, fire and environmental safety. Polyurethane foam most fully meets these requirements.

Polyurethane foam thermal insulation is usually applied to pipes at the factory, and the joints are thermally insulated at the construction site after welding and testing of the pipeline. IN Western Europe Such designs have been used since the mid-1960s and meet European standards EN 253:1994, as well as EN 448, EN 488 and EN 489.

They provide the following advantages over existing structures: increasing the durability (resource) of pipelines by two to three times; reduction of heat losses by two to three times; decline operating costs twice (the specific damage rate is reduced by 10 times); reduction of capital costs in construction by two to three times; Availability of a system for operational remote monitoring of thermal insulation moisture.

Pre-insulated pipes are made from various materials depending on operating conditions. Steel pipes are most widely used for the construction of heating mains.

Compliance of pre-insulated pipes with state standards

For the manufacture of insulated pipes, steel pipes with outer diameters of 57-1020 mm, lengths up to 12 m are used, corresponding to GOST 550, 8731, 8733, 10705, 20295, requirements current standards active documents on heating network and Device Rules and safe operation steam pipelines and hot water. Steel bends, tees, transitions and other parts must comply with the requirements of GOST 17375, 17376 and 17378.

The main reason for the widespread use of steel pipes is due to their relatively low cost, ease of processing combined with high strength and the ability to use traditional welding as a pipe joining method. To avoid pipe corrosion, it is necessary to use treated water. Water treatment depends on local conditions, but it is generally recommended that the following requirements:

• pH = 9.5-10;
• lack of free oxygen;
• total salt content 3000 mg/l.

The standard length of pipes is 6-12 m, but the technology makes it possible to apply thermal insulation to pipes of any length and made of other materials. Technical requirements for insulated pipes and pipeline parts are set out in GOST 30732-2001 “Steel pipes and fittings with thermal insulation made of polyurethane foam in a polyethylene shell”, put into effect on 07/01/01.

The standard applies to steel pipes and fittings with thermal insulation made of polyurethane foam in a polyethylene shell intended for underground channelless installation heating networks with the following design parameters of the coolant: operating pressure up to 1.6 MPa and temperature up to 130 °C (short-term temperature increases up to 150 °C are allowed). GOST 30732-2001 is compiled taking into account European standards:

• EN 253-1994 “Welded pipelines, pre-insulated for underground systems hot water supply. A pipeline system consisting of a steel main pipeline with polyurethane thermal insulation and an outer sheath of polyethylene";
• EN 448-1994 “Welded pipelines, pre-insulated, for underground hot water supply systems. Prefabricated fittings made of steel distribution pipes with polyurethane thermal insulation and an outer shell of polyethylene.”

Type and size

To provide maximum efficiency(insulation cost/heat loss) certain diameters are established external insulation pipelines made of polyurethane foam for various climatic zones. Pipes and fittings can have two types of insulation thickness: type 1 - standard, type 2 - reinforced. Protective covers are manufactured in the form thin-walled pipes made of polyethylene high density.

They are designed for pipelines located directly in the ground, ensuring their waterproofness and mechanical protection (Table 2). For pipelines located above the ground, a protective shell made of galvanized steel with a zinc coating thickness of at least 70 microns is used. Dimensions of shaped products (except for the diameters of steel pipes and polyethylene pipe shells) are recommended and determined by the design decision.

Design solutions usually based on manufacturers' recommendations. For example, some companies accompany their products with a design and construction manual, “Factory Insulated Steel Pipelines.” The wall thickness of the pipe and fittings is determined by calculation and rounded to the recommended thicknesses, which are given in the appendix to the standard.

For the manufacture of waterproofing pipe shells, high-density polyethylene of grades 273-79, 273-80 and 273-81, classified as PE63, is used. European companies also use PE80 polyethylene, which has higher minimum long-term strength and resistance to crack propagation. Rigid polyurethane foam used for thermal insulation is made from high molecular weight alcohols - polyol and isocyanate.

Polystyrene foam is a homogeneous mass with an average pore size of 0.5 mm. The service life of thermal insulation of pipes and fittings must be at least 25 years. Polyurethane foam does not have a harmful effect on environment and provides high-quality insulation at temperatures up to 130 °C.

Installation practice

Insulation of pipe sections with welded joints or repair of insulation can be carried out according to one of the following schemes:

1. Installation of insulating linings made of rigid polyurethane foam with further application of waterproofing material.
2. Installation of polyethylene couplings with polyurethane foam poured into the cavity of the coupling.

For waterproofing joints wide application received heat-shrinkable polyethylene shells, characterized by low cost and ease of installation. To insulate the joints of heat-insulated pipes with a protective shell made of galvanized steel, special steel couplings are used. They are used on straight sections of pipelines, on bends and branches for pipes with outer shell diameters of 63-450 mm, as well as during hot tapping, when a branch is installed without shutting off the supply.

The technology for installing couplings is simple and requires a minimum of tools. The joint consists of two parts, fastened using special cones or screws. The sealant located between the outer shell of the pipe and the coupling makes the joint waterproof. Thermal insulation is carried out using foam packages; they are easy to handle and provide precise dosage and uniformity of polyurethane foam when pouring.

To insulate and repair joints of pipes with diameters of 90-1300 mm, bandage couplings made of polyethylene with embedded electric spiral are used. Bandage couplings are available in three types and differ in the method of fixation on the outer shell during the welding process. Small bandage couplings are used for pipes with outer shell diameters of 90-200 mm. Medium-sized bandage couplings are used for diameters of 225-800 mm.

For outer shells with diameters of 800-1200 mm, bandage couplings consisting of two parts are used. All couplings are supplied with all necessary components. During welding, small couplings are pressed against the polyethylene shell of the pipe using mechanical clamps, and medium and large size couplings - using pneumatic ones. In all cases, the welding process is performed automatically and controlled using a special welding computer.

To ensure optimal adhesion between steel pipe and foam insulation, all steel pipes are pre-treated sandblasting. The outer shell is made of high-density polyethylene, and its inner surface is treated with corona discharge to obtain optimal adhesion between polyethylene and foam insulation.

The service life of pre-insulated pipes in district heating systems depends on the aging process of the pipe itself, including possible corrosion of the steel pipe, the temperature resistance of the polyurethane foam insulating material, as well as a polyethylene shell. Other critical factors include changes in the strength properties of the above materials over time. long period, the influence of temperature and pressure, as well as deformation conditions in the pipeline system. Corrosion of a steel pipe depends on how tightly the system is sealed against the penetration of water from the outside, since internal corrosion working steel pipe can hardly be observed in systems operated with treated water.

Therefore, an indispensable condition is to maintain the tightness of the pipe-shell joints. Stresses and deformations depend on operating conditions, temperature conditions and pressure, as well as from pipe laying technology and the condition of the surrounding soil. Due to the fact that it is the properties of the material (polyurethane foam insulation and polyethylene sheath) that have a decisive influence on the service life of pre-insulated pipes in district heating systems, the characteristics of two properties of polyurethane foam were considered, namely: temperature resistance and compressive strength.

Temperature resistance

In accordance with the requirements of the European standard EN 253, the service life of pre-insulated pipes must be at least 30 years, provided that the system is continuously operated with a coolant temperature of 120 °C. In a system where the temperature is less than 95 °C, the service life can be virtually unlimited. Throughout the tests, the supply water temperature varied between 100-115 °C, and the temperature was maintained at 115 °C throughout the three coldest periods. winter months.

Assuming that Maximum temperature supplied water will be 110 °C for the remainder of the year, the system will have total term service life is 75 years, and this corresponds to the EN 253 standard. A service life of 75 years does not mean that the pipes do not need repairs at all. This means that the polyurethane foam insulation material is expected to maintain its strength characteristics over the specified period.

When designing a central heating system, a certain number of load cycles are calculated - temperature fluctuations from operating temperatures to soil temperatures and back to operating temperatures over a period of 30 years, which is used in calculating fatigue characteristics. In Russia, the service life of thermal insulation made of polyurethane foam is determined according to GOST R 30732, Appendix D - methodology integral assessment service life of polyurethane foam insulation of heating networks at variable temperature chart coolant.

The number of load cycles remains the same, although the polyurethane foam insulating material retains its properties further.

Compressive strength

The compressive strength of polyurethane foam insulating material is limited and determines the conditions for maximum depth of pipes being laid and the technology for laying pipes for district heating systems. It has been found that when exposed to a temperature of 140 °C over a long period, the compressive strength of polyurethane foam with a density of 75 kg/m3 drops to zero within 15 months.

At temperatures above 125°C, the compressive strength will remain the same as new polyurethane foam after approximately two years of service. The limited compressive strength of the insulating material dictates restrictions on the maximum depth of laid pipes in central heating systems, especially in cases where a change in the direction of the pipeline route is required. To reduce earth pressure when moving pipes horizontally, other precautions should be used as an alternative.

Economic justification

Data in table. 5 and 6 give an idea of ​​the economic efficiency of using various types of thermal insulation. The advantages of PPU insulation are visible, which are confirmed by many years of experience in operating heating networks in Russia and foreign countries. The design of heating networks is carried out on the basis of current standards using “ Standard solutions laying pipelines in PPU insulation", " Technological maps for builders”, developed at VNIPI-Energoprom, and methodological recommendations manufacturing plants.

Design and calculation methods do not differ from traditional channelless installation. Maximum use of existing standard building construction. It is possible to refuse drainage or switch to its lighter types.

Route and method of laying the heating network. Development wiring diagram

The technology of ductless installation of industrially insulated heat supply pipelines is a progressive way to save energy resources. Pipelines pre-insulated with polyurethane foam (PPU) in a waterproof pipe-sheath are a rigid “pipe-in-pipe” structure consisting of a steel pipe, an insulating layer of rigid polyurethane foam and an external protective pipe-shell made of polyethylene (PE) low pressure For underground laying or a spirally wound shell pipe made of thin-sheet galvanized steel - for above-ground installation.

PPU-insulated pipelines are intended for laying heating networks with a constant temperature of up to 393 o K (120 o C), as well as for laying heating networks operating according to a quality control schedule with a coolant temperature of up to 423 o K (150 o C).

The minimum depth for channelless installation is assumed to be 0.5 ÷ 0.7 m from the ground surface. The maximum depth of the pipeline is taken from the strength of its structure. As a rule, the depth of the pipeline should not exceed 3 m. When laying heating networks using a ductless method, the pipes are laid on a sandy base with a thickness of at least 0.1 m with a sand backfill of at least 0.1 m. After backfilling, the sand must be compacted to ensure uniform friction between the pipeline shell and the soil.

Prev insulated pipelines can be laid traditional way(in canals, above ground). When reconstructing heating networks, it is possible to lay insulated pipelines in an existing impassable channel and backfill the latter with sand.

Aboveground and ductless installation of heating networks on the territory of preschool, school and medical institutions is not allowed. In the case of laying pre-insulated pipelines in places subject to dynamic loads, at a height of at least 30 cm above the surface of the pipeline, it is necessary to lay reinforced concrete slab or lay the pipeline in protective pipes or reinforced concrete channels. At a distance of 30 cm above the network pipeline, it is necessary to lay a warning (signal) tape.

When laying pipelines without ducts, the horizontal distance from the outer surface of the insulated pipeline to the foundations of buildings and structures should be taken according to Table 3. 1". If it is impossible to maintain these distances, pipelines must be laid in channels or steel cases at a distance of at least 2 m from the foundations of buildings.

When a straight section of a channelless pipeline is heated, covered with soil, the ends of which end in a compensator, a stationary point appears that has no movement, from which the pipe expands into different sides. This point is called a conditional fixed support. There is no need to install an actual fixed support in this location.

Maximum length Lm (Table 5.1) is the maximum possible distance between the conditionally fixed support and the compensator, at which the axial stress in the steel pipe does not exceed the permissible one (σ permissible).

Maximum length Lm, m, straight section pipeline is determined by the formula

where S – area cross section steel pipe walls, mm 2;

σ add – permissible axial stress, MPa;

F – friction force between the soil and the shell pipe, N/m.

Permissible axial stress σ permissible, MPa, pipelines:

– from low alloy steels 17G1S, 17G1SU (GOST 19281) – 170;

– from steels St10, St20 (GOST 1050) – 150;

– from steel VSt 3sp4-5 (GOST 380) – 130.

The friction force F, N/m, (Table 5.1) between the soil and the shell pipe arises due to soil pressure on the outer surface of the pipes. Due to friction forces, the temperature elongations that occur in ductless pipelines when the coolant temperature increases are partially compensated.

The maximum length of straight sections of heating network pipelines (where compensation for temperature expansion is not required, Fig. 5.1).

To ensure the strength of the pipeline, the length of straight sections should not exceed 2L m, and in the center of the straight section the elongation Δl=0, and here the conditional arises fixed support, where the pipeline is fixed and an extension appears at its free ends Δl.

Fig.5.1 – Maximum length of straight sections of heating network pipelines

If the length of the straight section is more than 2L m, then additional compensation should be provided in this area due to natural angles turn.

Table 5.1

End of table 5.1

Pre-insulated pipelines for district heating systems

Ph.D. V.E. Bukhin, senior researcher,

NPO "Stroypolymer"

Russia is a country with high level centralized heating (up to 80%). The country is penetrated by about 280 thousand km of heating networks (in two-pipe calculation) with pipe diameters from 57 to 1400 mm, a tenth of which are main lines, the rest are distribution heating networks.

The predominant method of laying heating networks in Russian Federation is laying in non-passable channels with mineral wool thermal insulation (80%). Channelless installation, carried out from factory-made structures using reinforced foam concrete insulation and bitumen-containing masses (bitumen-perlite, bitumen-overmiculite, bitumen-ceramsite), accounts for 10% of the total length of heating networks.

Due to the moistening of the materials used during operation, the heat-protective properties of thermal insulation structures are sharply reduced, which leads to heat losses that are 2-3 times higher than the standard ones.

Total heat losses in district heating systems amount to about 20% of the supplied heat (78 million tons standard fuel per year), which is 2 times higher than the same figure in advanced countries of Western Europe.

District heating systems in the Russian Federation currently provide heat consumption of 2171 million Gcal per year, which approximately corresponds to the annual heat consumption of all Western European countries and is almost 10 times higher than the heat consumption provided by district heating systems in these countries. Being a pioneer in the field of centralized heating and possessing the world's largest system of heating networks, Russia is significantly behind advanced foreign countries in the technical level - in use modern materials and technologies for laying heat pipelines.

About 90% of fuel savings achieved through combined heat generation methods are “lost” in heating networks. The durability of heating networks is 1.5-2 times lower than abroad and does not exceed 12-15 years. The situation in the hot water supply system is no better.

The volume of planned repairs and reconstruction of heating networks in the Russian Federation currently amounts to 10-15% of the total demand, but due to economic problems, no more than 4-6% is actually carried out.

The most effective solution to the problems posed above is the widespread introduction into practice of constructing heating networks of pipelines with polyurethane foam thermal insulation of the “pipe-in-pipe” type.

This idea is not new. The newspaper "Evening Moscow" dated December 10, 1963 reported that the Mosinzhproekt Institute carried out experimental work on the use of polyethylene pipes and foamed polymer materials for insulating underground heating networks. However, in those years this direction was not widespread.

Taking into account the expanding use in Russia of pre-insulated pipes in district heating systems and the great interest shown in this problem by specialists in design, construction and operational organizations, this article discusses the main provisions of the new technology.

Applicable thermal insulation materials must have high thermal insulation properties (the thermal conductivity coefficient of the material should not exceed 0.06 W/(m°C)), durability (resistance to water, chemical and biological aggression), frost resistance, mechanical strength and environmental safety, i.e. be safe for the life and health of people and the environment natural environment. Polyurethane foam most fully meets these requirements.

Polyurethane foam thermal insulation is usually applied to pipes at the factory, and the joints are thermally insulated at the construction site, after welding and testing the pipeline. A diagram of a pipe with thermal insulation made of polyurethane foam and a protective shell made of polyethylene pipe is shown in Fig. 1.

For example, in Western Europe, such designs have been successfully used since the mid-60s and are standardized by the European standard EN 253:1994, as well as EN 448, EN 488 and EN 489. They provide the following advantages over existing designs:

• · increased durability (pipeline service life) by 2-3 times;
• · reduction of heat losses by 2-3 times;
• · reduction in operating costs by 9 times (specific damage rate is reduced by 10 times);
• · reduction of capital costs in construction by 1.3 times;
• · availability of a system for operational remote monitoring of thermal insulation moisture.

Pre-insulated pipes have been successfully used for construction:

• · heating networks;
• · hot water supply systems;
• · process pipelines;
• · oil pipelines.

The pipes themselves are made of various materials depending on operating conditions. Currently, steel pipes are most widely used for the construction of heating mains, the main physical and chemical indicators of which are given in Table 1.

Table 1. Basic physical and mechanical parameters steel pipelines

For the manufacture of insulated pipes, steel pipes with outer diameters of 57 - 1020 mm, up to 12 m long, complying with GOST 550, GOST 8731, GOST 8733, GOST 10705, GOST 20295, the requirements of current regulatory documents for heating networks and the "Rules for the design and safe operation of pipelines" are used steam and hot water."

Steel bends, tees, transitions and other parts must comply with the requirements of GOST 17375, GOST 17376 and GOST 17378.

To avoid pipe corrosion, it is necessary to use treated water. Water treatment depends on local conditions, but the following requirements are recommended:

• · pH=9.5-10;
• · lack of free oxygen;
• · total salt content not more than 3000 mg/l.

The standard length of pipes is 6.0-12.0 m, but the technology makes it possible to apply thermal insulation to pipes of any length and made from other materials (see, for example, the magazine "Pipelines and Ecology" 1997, No. 1, p. 5 about polypropylene pipes PPR with thermal insulation for hot water supply).

In Russia, pre-insulated steel pipes with thermal insulation made of polyurethane foam and a polyethylene waterproofing shell have been used since 1993. Their production is organized at several enterprises (JSC MosFlowline, Moscow; JSC TVEL Corporation, St. Petersburg; JSC NPO Stroypolymer, JSC Moscow; CJSC "Teploizolstroy", Mytishchi; 000 Plant of thermally insulated pipes "Alexandra", Nizhny Novgorod; CJSC "Sibpromkomplekt", Tyumen, etc.), united in the Association of Manufacturers and Consumers of Pipelines with Industrial Polymer Insulation.

Technical requirements for insulated pipes and pipeline parts are standardized in GOST 30732-2001 “Steel pipes and fittings with thermal insulation made of polyurethane foam in a polyethylene shell”, which came into force on July 1, 2001 by Decree of the State Construction Committee of Russia dated March 12, 2001 No. 19.

The standard for steel pipes and fittings with thermal insulation made of polyurethane foam in a polyethylene shell is compiled taking into account the following European standards developed by the European Committee for Standardization (CEN):

EN 253-1994. Welded pipelines, pre-insulated for underground hot water supply systems - A pipeline system consisting of a steel main pipeline with polyurethane thermal insulation and an outer sheath of polyethylene;

EN 448-1994. Welded pipelines, pre-insulated for underground hot water supply systems - Prefabricated fittings made of steel distribution pipes with polyurethane thermal insulation and an outer sheath of polyethylene.

In the new standard, which united technical specifications Russian manufacturers, the values ​​of indicators relating to apparent density, compressive strength at 10% deformation, thermal conductivity, water absorption, volume fraction of closed pores correspond to those specified in European standards. In addition, the requirements for polyurethane foam in terms of safety and environmental protection requirements also comply with the requirements of European standards: hazard class, explosive™ production category, flammability group of polyurethane foam, requirements for the disposal of waste generated during the production of pipes, their removal and disposal.

The standard applies to steel pipes and shaped products with thermal insulation made of polyurethane foam in a polyethylene shell (hereinafter referred to as insulated pipes and products) intended for underground ductless installation of heating networks with design coolant parameters: operating pressure up to 1.6 MPa and temperature up to 130 °C (a short-term increase in temperature up to 150 °C is allowed).

In order to ensure maximum efficiency (insulation cost/heat loss), a certain thickness of polyurethane foam thermal insulation is established for different climatic zones. Therefore, pipes and fittings can be of two types in terms of insulation thickness: type 1 - standard, type 2 - reinforced. The dimensions of insulated pipes are shown in table. 2, design - in Fig. 1.

Table 2. Dimensions of thermally insulated pipes, mm.

Protective housings are usually made in the form of thin-walled pipes (shells) made of high-density polyethylene. They are intended for pipelines directly located in the ground, ensuring their waterproofness and mechanical protection (Table 3). For pipelines located above the ground, a protective shell made of galvanized steel with a zinc coating thickness of at least 70 microns is used.

Table 3. Dimensions of polyethylene shell pipes, mm.

The dimensions of shaped products (except for the diameters of steel pipes and polyethylene shell pipes) are recommended and are determined by the design solution. Design decisions are usually based on manufacturer recommendations. For example, NPO "Stroypolymer" accompanies its products with a guide to the design and construction of "Steel pipelines with factory thermal insulation."

The wall thickness of the pipe and fittings is determined by calculation and rounded to the recommended thicknesses, which are given in the appendix to the standard.

Insulation connecting parts pipeline (bends, tees) is made by cutting the polyethylene shell, followed by contact or extrusion welding.

For the manufacture of waterproofing shell pipes, high-density polyethylene of grades 273-79, 273-80 and 273-81, classified as PE 63, is used. European companies also use PE 80 polyethylene, which has higher minimum long-term strength and resistance to crack propagation. The main characteristics of polyethylene shell pipes are given in table. 4.

Table 4. Main characteristics of waterproofing polyethylene shell pipes

Rigid polyurethane foam used for thermal insulation is made from high molecular weight alcohol - polyol and isocyanate. The foam is a homogeneous mass with an average pore size of 0.5 mm and has the physical and mechanical characteristics given in table. 5.

Table 5. Properties of rigid polyurethane foam in thermal insulation construction

Thermal insulation is applied along the entire length of steel pipes and fittings, with the exception of the end sections, equal to 150 mm for pipe diameters up to 219 mm, and 210 mm for pipes with a diameter of 273 mm or more.

The service life of thermal insulation of pipes and fittings must be at least 25 years. Polyurethane foam does not have a harmful effect on the environment and ensures high-quality insulation performance at temperatures up to 130 °C.

Insulation of pipe sections with welded joints or repair of insulation can be carried out according to one of the following schemes:

• 1. Installation of insulating linings (shells) made of rigid polyurethane foam with further application of waterproofing material.
• 2. Installation of polyethylene couplings with polyurethane foam poured into the cavity of the coupling.

For waterproofing joints, heat-shrinkable polyethylene shells are widely used, characterized by low cost and ease of installation.

To insulate the joints of heat-insulated pipes with a protective shell made of galvanized steel, special steel couplings are used. They are used on straight sections of pipelines, on bends and branches for pipes with outer shell diameters of 63-450 mm, as well as during hot tapping, when a branch is installed without shutting off the heat supply.

The technology for installing couplings is simple and requires a minimum of tools. The joint consists of two parts, which are fastened together using special cones or screws. The sealant located between the outer shell of the pipe and the coupling makes the joint waterproof. Thermal insulation is carried out using foam packages, which are easy to handle and, when pouring, provide an accurate dosage and uniformity of polyurethane foam throughout the entire volume.

To insulate and repair joints of pipes with diameters of 90-1300 mm, bandage couplings made of polyethylene with embedded electric spiral are used. Bandage couplings are available in three types and differ in the method of fixation on the outer shell during the welding process.

Small bandage couplings are used for pipes with outer shell diameters of 90-200 mm. Medium-sized bandage couplings are used for diameters of 225-800 mm. For outer shells with diameters of 800-1200 mm, bandage couplings consisting of two parts are used. All couplings are supplied with all necessary components. During welding, small couplings are pressed against the polyethylene shell of the pipe using mechanical clamps, and medium and large couplings are pressed using pneumatic clamps. In all cases, the welding process is performed automatically and controlled using a special welding computer.

Bandage couplings meet the highest requirements for strength and reliability. In 1993, a central heating pipe 2.5 m long and 200 mm in diameter was tested. The joint with the bandage coupling has successfully passed tests, including 1000 axial vibrations in a box of sand and 600 hours in a container of water at high blood pressure. This test corresponds to 30 years of operation. Currently, more than 350,000 bandage couplings are installed in world practice. Special tools and computer controlled welding guarantee fast and reliable installation insulation of joints. The equipment required for welding is mounted on vehicle trailers and includes a generator, compressor and computerized welding unit.

The described system of heating networks with polymer thermal insulation is intended for direct installation in the ground. The system is "connected", i.e. The steel pipe, thermal insulation and outer shell are firmly connected to each other. The joints are insulated using connecting parts that ensure 100% tightness.

Such systems meet all SNiP requirements for the design and construction of heating networks. To ensure optimal adhesion between the steel pipe and the foam insulation, all steel pipes are pre-sandblasted. The outer shell is made of high-density polyethylene, and its inner surface is treated with corona discharge to obtain optimal adhesion between polyethylene and foam insulation.

What is the expected service life of pre-insulated pipelines? This issue is significant for all district heating (DH) enterprises.

The article "Tests to determine the service life of pre-insulated pipes in district heating systems", published in the journal "Pipelines and Ecology", 2000, No. 1, examines the results of studies and observations carried out in Denmark on a network of main pipelines, including supply and return pipelines 100 km long with diameters 100-800 mm. Tests have been carried out since 1987.

The service life of pre-insulated pipes in DH systems depends on the aging process of the pre-insulated pipe, including possible corrosion of the steel pipe, the temperature resistance of the polyurethane foam insulation material, and the polyethylene sheath. Other critical factors include changes in the strength characteristics of the above materials over a long period, the effects of temperature and pressure, and deformation conditions in the piping system.

Corrosion of a steel pipe depends primarily on how hermetically the system is sealed against the penetration of water from the outside, since internal corrosion of a working steel pipe can hardly be observed in systems operated with treated water. Therefore, an indispensable condition is to maintain the tightness of the pipe-shell joints. pipe thermal insulation polyurethane foam polyethylene

The polymer materials used in pre-insulated pipes impose restrictions on the temperature of the supplied water and thus affect the service life of the pipes. Technical impacts on the system throughout its entire service life are increased requirements to the insulating material (polyurethane foam), its compressive strength and adhesion (cohesion) between the steel pipe and the waterproofing shell.

Stresses and deformations depend on operating conditions, temperature conditions and pressure, as well as on pipe laying technology and the condition of the surrounding soil. Due to the fact that it is the properties of the material (polyurethane foam insulation and polyethylene sheath) that have a decisive influence on the service life of pre-insulated pipes in district heating systems, the characteristics of two properties of polyurethane foam were considered, namely: temperature resistance and compressive strength.

Temperature resistance. In accordance with the requirements of the European standard EN 253, the service life of pre-insulated pipes must be at least 30 years, provided that the system is continuously operated at a temperature of 120 °C. In a system where the temperature is less than 95 °C, the service life can be practically unlimited. Throughout the tests, the supply water temperature varied between 100 and 115°C, and the temperature was maintained at 115°C throughout the three coldest winter months. Assuming a maximum supply water temperature of 110°C for the remainder of the year, the system will have a total service life of 75 years and this is in accordance with EN 253. A service life of 75 years does not mean that the pipes in a particular area are pre-insulated the pipelines do not need repairs at all. This only means that the polyurethane foam insulation material is expected to maintain its strength characteristics over the specified period. When designing a central heating system, a certain number of load cycles are calculated - temperature fluctuations from operating temperatures to soil temperatures and back to operating temperatures over 30 years, which should be used in calculating fatigue characteristics. (In Russia, the service life of thermal insulation made of polyurethane foam is determined according to GOST R 30732, Appendix D - Methodology for the integral assessment of the service life of polyurethane foam insulation of heating networks with a variable temperature schedule of the coolant.). The specified number of load cycles remains, although the polyurethane foam insulating material retains its properties over a longer period. Thus, it is very important to ensure that pipes for district heating systems, in constant daily use, are subjected to fewer load cycles than allowed in accordance with the calculations, so that more high term services of polyurethane foam insulation material.

The compressive strength of polyurethane foam insulating material is limited and determines the conditions for maximum depth of pipes being laid and the technology for laying pipes for district heating systems. It was found that when exposed to a temperature of 140 °C over a long period, the compressive strength of polyurethane foam with a density of 75 kg/m3 drops to zero over a period of approximately 15 months. At temperatures above 125°C, the compressive strength will remain the same as new polyurethane foam after approximately two years of service. The limited compressive strength of the insulating material dictates restrictions on the maximum depth of laid pipes in central heating systems, especially in cases where a change in the direction of the pipeline route is required. To reduce earth pressure when moving pipes horizontally, other precautions should be used as an alternative.

The tables below 6 and 7 give a clear idea of ​​the economic efficiency of using various types of thermal insulation.

Table 6. Cost of laying 1 km of a two-pipe heating main

Table 7. Estimation of economic efficiency of 1 km of two-pipe heating main in USD

From the tables above, you can see the advantages of polyurethane foam insulation, which are confirmed by many years of experience in operating heating networks in Russia and foreign countries.

The design of heating networks is carried out on the basis of current standards using “Standard solutions for laying pipelines in polyurethane foam insulation”, “Technological maps for builders” developed at the VNIPIENERGOPROM Institute, and methodological recommendations from manufacturers. Design and calculation methods are practically no different from traditional channelless installation. Existing standard building structures were used to the maximum extent possible. There is also the possibility of abandoning drainage or switching to lighter types.