Introduction

Ordinary high-density polyethylene (HDPE and MDPE) pipes have linear macromolecules, and their biggest disadvantage is poor heat resistance and creep resistance. Therefore, ordinary high-density polyethylene pipes are not suitable for conveying media with a temperature greater than 45¡ãC. "Cross-linking" is an important method for polyethylene modification. After cross-linking, the linear macromolecular structure of polyethylene becomes a three-dimensional network structure of PEX, which greatly improves the heat resistance and creep resistance of polyethylene. At the same time, its aging resistance, mechanical properties and transparency are significantly improved. The higher the degree of cross-linking, the more obvious the improvement of these properties. At the same time, it inherits the inherent chemical corrosion resistance and flexibility of polyethylene pipes. There are three types of commercialized PEX pipes: PEXa pipe, PEXb pipe, and PEXC pipe.

Features

Excellent heat and cold resistance, high thermal strength at high temperature;

Excellent low temperature toughness;

No melting when heated;

Extraordinary creep resistance: Creep data is an important basis for product design and engineering material selection. Compared with traditional materials such as metals, the strain behavior of plastics has a significant dependence on loading time and temperature. The creep characteristics of PEX pipes are almost one of the most ideal pipes among common plastic pipes;

Semi-permanent service life: After the PEX pipe has passed the test of temperature 110¡æ, hoop stress 2.5MPa, and time 8760h, it can be deduced that its continuous service life at 70¡æ is 50 years.

The common PE-Xa pipes on the market are Boya, Rifeng, etc.

Abstract: Based on the data and its actual engineering situation, the excellent performance, application field, manufacturing process, and design and construction process of PEX pipes are introduced.

Keywords: Water supply pipe PEX pipe application

Introduction

Recently, with the announcement by the construction departments of more than a dozen large and medium-sized cities and regions such as Shanghai, Dalian, Guangdong, Fujian, Hubei, and Hunan that the use of galvanized pipes and cast iron pipes for water supply and drainage pipes is prohibited, the use of several new pipe materials has spread rapidly across the country. The focus of competition is mainly on three types of pipes, namely UPVC pipes, aluminum-plastic composite pipes and PEX pipes. These three types of pipes have been widely used in the Chinese market and have a high market share.

The application of PEX pipes began in the early 1990s and has been widely promoted and applied in developed Western countries. It is currently the best product to replace traditional galvanized pipes and is known as a green and environmentally friendly pipe.

Manufacturing process

1.1 Main raw materials for the production of PEX pipes

The main raw materials for the production of PEX pipes are HDPE, as well as additives such as initiators, cross-linking agents, and catalysts. If there are special requirements, other modifiers can also be added.

PEX pipes are manufactured using the world's most advanced one-step method (MONSOIL method). Ordinary polyethylene raw materials are added with silane grafting materials to form chemical covalent bonds between polymer macromolecular chains to replace the original van der Waals forces, thereby forming a three-dimensional cross-linked network structure of cross-linked polyethylene, the degree of cross-linking can reach 60% to 89%, making it have excellent physical and chemical properties.

1.2 Production process

Raw material pretreatment + auxiliary agent preparation ¡ú mixing ¡ú melt grafting extrusion ¡ú pipe molding ¡ú cross-linking ¡ú inspection and packaging ¡ú finished product.

1.3 The role and purpose of PEX pipe additives

The cross-linking agent is to crack its original linear molecular structure to form a grafting material, and then form a three-dimensional network structure. The role of antioxidants and color particles is to improve the corrosion resistance of PEX pipes, prevent physical changes caused by long-term exposure to sunlight, and resist long-term erosion by ultraviolet rays, thereby extending their service life.

Performance and characteristics

2.1 Performance comparison between PEX pipe and galvanized steel pipe (see Table 1)

Table 1 Performance comparison between PEX pipe and galvanized steel pipe

2.2 Similarities and differences between PEX pipe, UPVC pipe and aluminum-plastic composite pipe

In terms of performance, the three pipes have some common characteristics such as no corrosion, no scaling, no bacteria breeding and easy installation, but also some differences:

2.2.1 Excellent temperature resistance

The operating temperature of PEX pipe is -70¡«110¡æ, the operating temperature of UPVC pipe is 5¡«45¡æ, and the operating temperature of aluminum-plastic composite pipe is -40¡«85¡æ.

2.2.2 Anti-vibration performance

UPVC pipe is slightly inferior to aluminum-plastic composite pipe and PEX pipe in anti-vibration performance due to glue bonding.

2.2.3 Service life

The service life of UPVC pipes and aluminum-plastic composite pipes is about 20 to 30 years, and that of PEX pipes is about 50 years.

2.2.4 Chemical corrosion resistance

Due to the molecular structure of PEX pipes, they can transport a variety of chemical substances without being corroded even at high temperatures.

2.2.5 Good memory performance

When the PEX pipe is heated to an appropriate temperature (less than 180¡ãC), it will become transparent, and when cooled, it will return to its original shape, that is, any wrong bends during use can be corrected by a hot air gun, making it easier to use.

2.2.6 No scaling

The presence of calcium bicarbonate and magnesium ions in water can easily form precipitates, which slowly clog the pipes, while PEX pipes have a lower surface tension so that water with high surface tension will not infiltrate the pipe wall, thereby preventing the formation of scale in the pipe.

2.2.7 Environmental performance

After many thorough demonstrations and tests, PEX pipes are green pipes that do not pollute the environment. They do not contain any toxins, do not release any harmful substances, and only produce water and carbon dioxide after incineration. However, UPVC pipes are added with heavy metal additives during the production process, and the remaining vinyl chloride in them enters the medium being transported during use, which can easily cause medium pollution; aluminum-plastic composite pipes are prone to cracking due to the different expansion coefficients of aluminum and plastic, and are non-degradable substances. Their waste can easily cause environmental pollution. Therefore, PEX pipes are more worthy of the name of green and environmentally friendly pipes.

Fields

(1) Indoor water supply pipes, hot water pipes, and pure water delivery pipes.

(2) Liquid food delivery pipes in food.

(3) Water heating systems, central air conditioning pipe systems, floor radiant heating systems, solar water heater systems, etc.

(4) Telecommunications and electrical piping.

(5) Electroplating, petroleum, and chemical plant transportation pipeline systems.

Design Guidelines

4.1 Pipeline Layout and Laying

(1) The location and direction of the piping should first be determined according to the structure and process layout requirements of the building.

(2) Pipes should be concealed in suspended ceilings, pipe holes, and pipe wells or embedded in walls.

(3) When pipes are laid in the suspended floor or in the whole pouring layer of the floor, hardened polyvinyl chloride corrugated pipes should be installed.

(4) When the pipe passes through the floor slab roof, a fixed support point should be set at the crossing point, and strict waterproofing measures should be taken. When passing through walls, beams, and columns, a casing should be installed.

(5) Pipes should not pass through settlement joints and expansion joints of buildings. When they have to pass through, anti-settlement and expansion measures should be set at the crossing point.

(6) When outdoor pipes pass through the concrete wall slabs of the foundation basement, anti-settlement measures should be taken.

(7) When the pipeline passes through a pool or water tank, the pipe section between the inlet and outlet pipes and the valve should be made of corrosion-resistant metal pipes.

4.2 Pipeline Insulation

(1) For outdoor exposed water supply pipes, insulation measures should be taken in accordance with the national water supply pipe antifreeze and insulation engineering technical regulations.

(2) For hot water pipes laid in suspended ceiling pipe wells, pipe holes and exposed pipe sections, insulation measures should be taken.

(3) The distance between the pipeline and the heat source is generally not less than 1m, and the distance between the indoor exposed pipe and the household stove shall not be less than 0.4m.

4.3 Residential Pipeline Manifold Installation

(1) For residential hot and cold water pipes, when the pipe diameter is less than ?De?25, it is advisable to centrally install manifolds and use the shortest distance to each water distribution point.

(2) The manifold shall be equipped with a manifold box, the center of which shall be 0.3m above the ground for the cold water pipe and 0.45m above the ground for the hot water pipe.

(3) The manifold shall be made of corrosion-resistant materials, and copper valves shall be installed at the ends of the water inlet and outlet pipes.

Construction Guide

5.1 Preparation

(1) The design drawings and other technical documents shall be complete and have been reviewed.

(2) The construction organization design has been determined, and the technical disclosure has been made to understand the laying method.

(3) Before installing the pipeline, the appearance quality of the materials and the matching tolerance of the pipe fittings shall be carefully checked, and the dirt inside the contaminated pipes and pipe fittings shall be thoroughly cleaned.

(4) Construction personnel shall understand the building structure, ceiling height, number of pipes in the pipe well, determine the pipe position, and master the pipe fitting connection technology and other basic operation points.

5.2 Coordination with buildings

(1) When installing small-diameter pipes, the bendability of the pipes should be utilized, and pipe connectors should be avoided or minimized as much as possible. The minimum bending radius of the pipe without connectors is 8¡ã.

(2) When the pipe passes through the floor slab, roof concrete wall slab, or pool wall, the civil engineering should reserve holes, embed sleeves, or pipe fittings according to the design requirements. The reserved hole diameter should be 70mm larger than the outer diameter of the pipe, and the inner diameter of the embedded sleeve should not be larger than the outer diameter of the pipe 50mm.

(3) When the pipe passes through the roof slab, anti-leakage measures should be strictly implemented.

(4) When laying pipes embedded in the wall, the civil engineering should reserve or dig pipe grooves at the determined locations. The distance between the groove wall and the outer wall of the pipe should not be less than 10mm, and the groove depth should not be less than 5mm between the outer wall of the pipe and the rough wall surface. The groove mouth should be neat and flat.

5.3 Pipeline support

(1) General support and fixed support are set for pipeline risers and transverse pipes. The support spacing is shown in Table 2.

Table 2 Commonly used pipeline support spacing

(2) Fixed support parts should be made of steel and should be located near pipe fittings and pipelines; the main pipe part of the pipeline system diversion.

(3) Pipeline expansion and contraction should be compensated by pipe bending. The length of the cantilever end should not be greater than the length of the pipe section.

(4) For exposed straight pipelines, the expansion joint should be set between two fixed support points and meet the pressure requirements.

(5) When the pipeline passes through the wall, it is a general support point. Soft fillers should be used to fill the gaps in the casing or holes.

(6) After the pipeline is installed, it should be sealed with a pipe plug. The pressure resistance performance of the plug should meet the pipeline pressure test requirements.

5.4 Pipeline Insulation

The basic material for the insulation of hot and cold water pipes should be single-sided open high-foam polyethylene pipe shell, and the thickness should not be less than 15mm. After the insulation material is wrapped, the roof cold water pipe should be wrapped with two layers of hot water pipe insulation with a width of 100-120mm and a thickness of 0.22mm, and the construction should be carried out according to the design requirements. Black polyvinyl chloride film, the surface of the protective layer is tied with 1mm plastic-impregnated iron wire, and the spacing is 400-450mm.

5.5 Pipeline Pressure Test

After the pipeline is installed, a water pressure test should be carried out. The test pressure is 1.5 times the working pressure of the pipeline system, but not less than 0.60MPa. The system should be pressurized slowly. After reaching the specified pressure, the pressure should be stabilized for 1h, and then the joints should be observed for leakage. The pressure drop within 15min does not exceed 0.5MPa to be qualified.

Conclusion

With the continuous improvement of people's living standards, the demand for new pipes will inevitably increase. PEX pipes have advantages that metal pipes and other plastic pipes cannot match, but nothing can be perfect. For example, I have used PEX pipes to transport purified water in an industrial project, which has disadvantages such as small overhead span and many supporting points. Therefore, PEX pipes should be used correctly in the design process to better serve and benefit mankind. At the same time, we call on the country to issue national standards for the design, construction, acceptance, etc. of plastic pipes as soon as possible to accelerate the promotion of the application of plastic pipes in China.

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