PE-RT is a modern and universal polymer material, characterized by a large margin of strength and high resistance to temperature influences. Pipes made of PE-RT type 2 can be used for heating, cold and hot water supply. They are also widely used to protect power supply and communication cables. Compared to other types of polyethylene pipes, products made from PE-RT type 2 have the following advantages:
Widest operating temperature range. Acceptable constant temperature coolant 95°C. At the same time, flexibility is maintained sub-zero temperatures, down to -50°C, which eliminates the formation of cracks during storage and transportation in winter time. When the water inside freezes, the pipes are also not damaged; after defrosting, they retain their original shape.
Resistant to corrosion and chemical attack. Pipes made of PE-RT are not affected by hard water and can withstand acidic and alkaline environments. Deposits do not form on the walls of polyethylene pipes, reducing throughput.
Flexibility and resilience. Pipes of small diameters are suitable for hidden wiring and can be concreted. PE-RT pipes with a diameter of up to 110 mm inclusive can be supplied in coils, which allows such pipes to be laid using the trenchless method, as well as inside old metal pipelines without dismantling them.
High operating pressure. For pipes with sufficient wall thickness, the nominal operating pressure is 16 atmospheres at a temperature of the transmitted medium up to 95°C. Thin-walled pipes are designed for constant pressure up to 10 atmospheres.
Fast installation. The length of the section in the bay allows you to do without intermediate connections. This also reduces the amount of waste.
Low thermal conductivity allows minimizing heat loss in heating networks.
Reliable connection. PE-RT pipes can be welded just like any other polyethylene pipes- end-to-end or using electric welded fittings.
Using PE-RT pipes for cable laying
Type II PE-RT pipes are often used for cable installation. In this case, both standard pipes and pipes specially designed for this purpose with an additional protective layer can be used.
Manufacturers offer several options for pipes designed for laying power, telecommunications and signal cables, as well as fiber optic lines. For example, the Tekhstroy company produces a series of TEHSTROY TR (temperature resistant) pipes made of polyethylene with increased heat resistance. High resistance to temperature fluctuations allows the pipe to be laid at any depth, including above the soil freezing depth. Operating temperature ranges from -20°C to +95°C.
The TEHSTROY TR series includes single-layer pipes with a nominal outer diameter Dn and pipes with an additional protective layer, sold under the brand Technical Construction TR-1 Prosafe. The protective shell, made of thermoplastic polymer, may have a red or green color. Both types of pipes are available in diameters from 16 to 630 mm, which allows you to choose the optimal size for both mains and single signal and communication lines.
The use of pipes with a protective layer allows pipelines to be laid directly into the ground, as well as along the bottom of reservoirs with or without burial into the ground. Using the HDD method often reduces installation costs, since it does not require earthworks and greatly reduces the number of joints.
Underfloor water heating system, for last years, has become a leader in comparison with radiator and other heating in private and suburban construction. Many water heated floors began to be used as the main and only heating in private country house. Not only customers, but also people who independently install such a system think about the quality of pipes and the materials from which they are made.
Which pipes are best to choose for water heated floors - review of materials and manufacturers
Basic information about pipes for both underfloor heating and other systems (heating and water supply) that you need to know - this is the pipe manufacturer and country of production. Since it doesn’t matter what material the pipe is made from, if it is not produced using technology, with savings on the quality of raw materials and quality control, such a pipe will not last long. And as with other products, a good pipe for heated floors cannot be cheap.
Basic properties and parameters of pipes used in floor and panel heating
When choosing a pipe for installation in a heated floor of a private country house or an apartment in a high-rise building, are based not only on the quality of the pipe and its possibility of application in a particular case, but also on the ease of installation. For a person who will be installing a heated floor in his home for the first time, it will be more convenient and pleasant to work with a pipe that is more flexible and holds its shape than a rigid and inflexible one, and this also needs to be taken into account, because In the future, this may affect the quality (uniformity) of the underfloor heating.
Which material is more suitable for underfloor heating pipes?
Metal-plastic pipes
Metal-plastic pipes are the first and most popular, until recently, polymer pipes for heated floors. If you look in section, such a pipe consists of two polymer layers, between which there is a layer of aluminum foil 0.2 millimeters thick or more. The most famous pipe for underfloor heating is the Henco pipe. It hasn't been very popular lately, because... the cost of the pipe is quite high. Through the use of cross-linked PEX polyethylene and high-quality adhesive for gluing the layers.
Unlike Henco, other European manufacturers have switched to the production of metal-plastic pipes made of heat-resistant polyethylene PE-RT. The elongation of this material when heated is several times less than that of PEX cross-linked polyethylene; therefore, the reliability of such a pipe during sudden temperature fluctuations is higher. So, many Chinese manufacturers use cross-linked polyethylene, and taking into account the savings on other materials, the overall quality of the pipe turns out to be quite low, which is why there are a lot of bad reviews on the forums about delaminating pipes and a cracking outer layer (they are afraid of ultraviolet radiation).
The presence of aluminum foil in the composition of a metal-plastic pipe allows you to completely avoid the ingress of oxygen into the coolant and reduce the linear elongation by up to 5 times.
If you decide to use a metal-plastic pipe, it is better to focus on European manufacturers
- Uponor (PE-RT/AL/PE-RT) Germany
- Germany
- HENCO (PEXc/AL0.4vmm/PEXc) Belgium
- APE, STOUT (PEXb/Al/PEXb) Italy
- COMPIPE (PEXb/Al/PEXb) Russia(Use up to operating class 5)
- Valtec, Altstream, etc. Russia-China
XLPE pipes
Cross-linked polyethylene is the most popular material for underfloor heating pipes at present. We will not dwell on the description of this material, because... There’s enough information to fill an entire article, but we’ll tell you which pipe options are best to choose.
The highest percentage of crosslinking (from 75%) in the peroxide crosslinking method is PEXa pipes. The most expensive method used by European manufacturers. The silane crosslinking method of PEXb is the most common, the level of crosslinking is quite high, but for example in the USA such pipes are prohibited for use due to the presence of harmful chemical compounds. It is also believed that a PEXb pipe gains its strength properties only during operation of the pipe with coolant.
By exposing the material to charged particles, 60% cross-linked PEXc polyethylene is obtained. The product is irradiated in the solid state. The main disadvantages of the method are the heterogeneity of the material as a result, but there are also advantages - cross-linked polyethylene gains increased elasticity.
As the degree of crosslinking increases, strength, heat resistance, and resistance to aggressive environments and ultraviolet rays increase. However, along with an increase in the degree of cross-linking, the fragility of the resulting pipeline increases and the flexibility decreases. If you increase the degree of crosslinking of polyethylene to 100%, then its properties will be similar to glass.
The biggest problem in choosing a specific manufacturer and pipe is the low quality of stitching in Chinese-made pipes, as well as in some Russian representatives. Another disadvantage of such pipes is the rigidity of the pipe; it does not hold its shape well and after bending it tries to take its previous shape and therefore it is more difficult to work with it than with a metal-plastic pipe, especially for an inexperienced installer.
The disadvantage of PEX material is that it is oxygen permeable. Water in pipelines without oxygen protection becomes saturated with oxygen after a certain time, which can lead to corrosion of system elements. To reduce the oxygen permeability of PEX, a thin layer of polyvinylethylene (EVOH) is used. The PEX base layer and the EVOH layer are joined together with glue. It is worth noting that the EVOH layer does not completely prevent oxygen emission, but only reduces oxygen permeability to 0.05–0.1 g/m3 day, which is acceptable for heating systems. In a PEX-EVOH pipe, the anti-diffusion layer is made on the outside, i.e. The pipe has a three-layer construction: PEX-adhesive-EVOH. Five-layer (PEX-adhesive-EVOH-adhesive-PEX) pipes are also available on the market, but tests have shown that the three-layer design is more reliable. The belief that the outer layer of EVOH in a three-layer construction is susceptible to abrasion is erroneous.
Another disadvantage of PEX pipes is their large linear elongation, so such pipes are practically not used for external installation, but only for hidden ones.
One of the advantages of pipelines made of cross-linked polyethylene is the presence of a memory effect. The shape memory effect is very useful during installation. If a kink, compression or other deformation occurs during pipeline installation, it can be easily eliminated by heating the pipeline to a temperature of 100–120 °C. (However, in the passport for the Russian-Chinese Valtec pipe it is written: “If there is a “kink”, the damaged section of the pipe must be removed.”)
Wrinkles form on pipelines coated with an anti-diffusion layer after restoration. In these areas, the anti-diffusion layer peels away from the PEX layer. This defect has virtually no effect on the characteristics of the pipeline, since the main load-bearing capacity of the pipeline is determined by the PEX layer, which has been completely restored. A slight peeling of the anti-diffusion layer does not significantly increase the oxygen permeability of the pipeline.
Pipelines made of cross-linked polyethylene, and especially PEXa produced in Europe, are better suited than other polymer pipes for use not only in underfloor heating, but also in radiator heating, using the hidden method.
What pipes can be found on sale:
- Germany
- UPONOR COMFORT PIPE PLUS PE-Xa EVOH Germany(use up to class 5, heated floors and radiators)
- SANEXT "Warm floor" PE-Xa Russia-Europe(use up to operating class 4)
- Russia-China(use up to operating class 4)
(use up to operating class 5) BEST CHOICE for PRICE-QUALITY
Heat-resistant polyethylene PE-RT
Very often, heat-resistant polyethylene PE-RT is called cross-linked polyethylene. But the production technology of such polyethylene is as follows. IN chemical reaction“flat” butene is replaced by octylene (formula C8P16), which has a spatially branched structure. Subsequently, it forms side branches near the main chain, which are mutually intertwined monomer chains. They are connected to each other due to the mechanical interlacing of branches, and not due to interatomic bonds.
PE-RT pipes are mainly used for underfloor heating, where the temperature and pressure are lower than in water supply and heating systems. Although the manufacturers of PE-RT pipes, when pursuing their marketing policy, claim that the properties of their pipes are the same as those made from PEX cross-linked polyethylene. However, this is questionable because PE-RT is a conventional thermoplastic with limited overall resistance to elevated temperatures and pressures in hot water systems, as evidenced by hydraulic testing and subsequent practice.
A comparison of regression curves obtained by the independent Bodycoat Polymer Institute (Belgium) suggests that durability PE-X pipes above, and the regression curve, showing the loss of ability to perform work functions over time, for heat-resistant polyethylene PE-RT has a characteristic break (loss of strength during long-term operation) already at 70 ° C.
BioPipe (PERT) Russia
Most affordable option with high quality
Stainless steel and copper pipes
These types of pipes are practically not used in the installation of heated floors, and the main reason is the high price. Due to polyethylene pipelines the best German manufacturers are 2 times cheaper, metal pipes, and the service life is more than 50 years (in a warm floor), there is no need for such pipes. Floor installation from copper pipe more expensive and the installer of such floors must have extensive experience and qualifications.
conclusions
As with other types of equipment and materials, when choosing a specific manufacturer, we recommend choosing European manufacturers. The fact that the manufacturer is European must be determined by the barcode and the inscription “Made in...”. Many sellers offer an Italian pipe, but cannot confirm that it was made in Italy, because... The pipe is actually produced in China, and the real homeland of the brand is Russia. And of course, if the pipe is produced in Europe, then the price for such a pipe will not be the lowest, because... quality cannot be cheap. If you compare an inexpensive German pipe and an expensive Chinese one, decide for yourself how confident you are in the real characteristics and quality of the Chinese pipe, for example, in the level of cross-linking of cross-linked polyethylene.
If we draw conclusions about the materials for underfloor heating pipes, then our experts arrange the materials in the following order, starting with the best:
- Cross-linked PEXa polyethylene with anti-diffusion layer
- Metal-plastic with an inner layer of PE-RT
- Cross-linked polyethylene PEXb,c
- Heat-resistant polyethylene PE-RT
Today, unfortunately, marketing moves and advertising gimmicks increasingly influence various technical solutions and the choice of one or another material and equipment for the project. Increasingly, instead of a full-fledged technical passport or catalog for equipment, designers end up with advertising booklets and brochures on the table, from which they make the selection. What is unacceptable to write in serious technical literature migrates to the pages of such booklets. Often, marketers assign inflated or completely non-existent indicators to their products, misleading engineers. As a rule, extraordinary technical features of equipment in booklets are presented as undeniable advantages. Conversely, any technical information about competitive products is presented in the form of significant and irreparable defects.
All these factors ultimately lead to the wrong choice of materials and equipment, which can ultimately lead to an emergency. The blame in this case falls on the shoulders of the design engineer, since any manufacturer, along with colorful advertising that triumphantly describes all the delights of the product, has either footnotes in small print or a technical passport with real data carefully hidden from the human eye. Most often, advertising brochures contain information that does not contradict passport data, but is presented in such a way that people have a false impression of the real technical features goods. For example, the phrases “the pipe can withstand a temperature of 95 ºС and a pressure of 10 bar” and “the pipe can withstand a coolant temperature of 95 ºС at a pressure of 10 bar for 50 years” are radically different from each other. In the first case, there is a riddle: is the pipe capable of withstanding a coolant temperature of 95 ºC and 10 bar at the same time, or are these two critical points for the use of this pipe? And most importantly, there is no time indicator, that is, it is not known how long the pipeline maintains these parameters - five minutes, an hour or 50 years?
This article outlines the top marketing gimmicks and myths propagated by polyethylene (PEX) pipe manufacturers.
1st group of myths – about the superiority of one stitching method over another
Almost every manufacturer of PEX pipes claims that the method of stitching their pipes is the best, and that others are no good. Only polyethylene cross-linked using their method will have increased strength characteristics and reliability indicators.
To begin with, I would like to recall some information about cross-linking polyethylene. Cross-linking refers to the creation of a spatial lattice in high-density polyethylene due to the formation of volumetric cross-links between polymer macromolecules. The relative number of cross-links formed per unit volume of polyethylene is determined by the “degree of cross-linking” indicator. The degree of crosslinking is the ratio of the mass of polyethylene covered by three-dimensional bonds to the total mass of polyethylene. In total, there are four known industrial methods for cross-linking polyethylene, depending on which the cross-linked polyethylene is indexed with the corresponding letter.
Table 1. Types of cross-linking of polyethylene
Peroxide cross-linking (method “a”)
Method "a" is chemically cross-linking of polyethylene using organic peroxides and hydroperoxides.
Organic peroxides are derivatives of hydrogen peroxide (HOOH) in which one or two hydrogen atoms are replaced by organic radicals (HOOR or ROOR). The most popular peroxide used in pipe production is dimethyl-2.5-di-(bytylperoxy)hexane. Peroxides are particularly hazardous substances. Their production is a technologically complex and expensive process.
To obtain PEX using method "a", polyethylene is melted together with antioxidants and peroxides before extrusion (Thomas Engel process), rice. 1.1. With an increase in temperature to 180–220 ºС, peroxide decomposes, forming free radicals (molecules with free bonds), rice. 1.2. Peroxide radicals take away one hydrogen atom from polyethylene atoms, which leads to the formation of a free bond at the carbon atom ( rice. 1.3). In neighboring polyethylene macromolecules, carbon atoms having free bonds are combined ( rice. 1.4). The number of intermolecular bonds is 2–3 per 1000 carbon atoms. The process requires strict temperature control during the extrusion process, when preliminary cross-linking occurs, and during further heating of the pipe.
Method "a" is the most expensive. It guarantees full volumetric coverage of the material mass under the influence of peroxides, since they are added to the original melt. However, this method requires that the crosslinking be at least 75% (according to Russian standards - not lower than 70%), which makes pipes made from this material more rigid compared to other crosslinking methods.
Silane cross-linking (method "b»)
Method “b” is a chemical method of cross-linking polyethylene using organosilananides. Organosilanides are compounds of silicon with organic radicals. Silanides are toxic substances.
Currently, vinyl trimethaxyloxane (H 2 C=CH)Si(OR) 3 ( rice. 2.1). When heated, the bonds of the vinyl group are destroyed, turning its molecules into active radicals ( rice. 2.2). These radicals replace the hydrogen atom in polyethylene macromolecules ( rice. 2.3). Then the polyethylene is treated with water or steam; organic radicals add a hydrogen molecule from the water and form a stable hydroxide (organic alcohol). Neighboring polymer radicals are closed through the Si-O bond, forming a spatial lattice ( rice. 2.4). The displacement of water from PEX is accelerated by the use of a tin catalyst. The final cross-linking process occurs already in the solid stage of the product.
Radiation cross-linking (method “c”)
Method "c" is to influence the group C-H flow charged particles ( rice. 3.1). This may be a stream of electrons or gamma rays. With this effect, some of the C-H bonds are destroyed. The carbon atoms of neighboring macromolecules, from which a hydrogen atom has been knocked out, combine with each other ( rice. 3.3). Irradiation of polyethylene by a flow of particles occurs after its molding, that is, in solid state. To the disadvantages this method can be attributed to the inevitable unevenness of cross-linking.
It is impossible to position the electrode so that it is equidistant from all areas of the irradiated product. Therefore, the resulting pipe will have uneven stitching along its length and thickness.
The irradiation source most often used is a cyclic electron accelerator (betatron), which is relatively safe both in the production and in the use of the finished pipe.
Despite this, in many European countries the production of pipes stitched using the “c” method is prohibited.
To reduce the cost of the crosslinking process, radioactive cobalt (Co 60) is sometimes used as a radiation source. This method is certainly cheaper, since the pipe is simply placed in a chamber with cobalt, but the safety of using such pipes is highly questionable.
Misconception #1 : “Flip cross-linking (PEX-a) is better than others in terms of the strength of the resulting material, because the regulated minimum degree of cross-linking for this method is greater than for other methods. And the higher the degree of cross-linking of PEX, the stronger the material.”
Indeed, GOST R 52134 regulates different minimum permissible degrees of cross-linking of PEX pipes for different manufacturing methods ( table 1), and it is true that as the degree of crosslinking increases, the strength of the pipes increases.
However, it is unacceptable to compare the degrees of crosslinking of PEX-a, PEX-b and PEX-c, since the molecular bonds of these materials formed as a result of crosslinking have different strengths, and therefore even these types of polyethylene crosslinked to the same degree will have different strengths. Communication energy type S-S, which is formed in polyethylene cross-linked by method “a” and “c” is about 630 J/mol, while the bond energy of the Si-C type, which is formed in polyethylene cross-linked by method “b” is 780 J/mol. For physico-chemical and technical properties The interaction of macromolecules due to hydrogen bonds that arise in the polymer due to the presence of polar groups and active atoms, as well as the formation of associates as a result of the interaction of the cross-links themselves, also influences. This is primarily characteristic of a silanol cross-linked polymer, where there is a large number of silanol groups capable of forming additional adhesion nodes in amorphous regions, increasing the density of the structural network (which is 30% greater than with peroxide, and 2.5 times greater than with radiation crosslinking) and reducing deformability at high temperatures.
Bench tests of cross-linked polyethylene pipes show some strength advantage of silane cross-linking. Thus, at a test temperature of 90 °C for pipes with a diameter of 25 mm and a length of 400 mm, the fracture pressure of pipes made of PEX-a, PEX-b and PEX-c was 1.72, 2.28 and 1.55 MPa, respectively (V.C Osipchik, E.D. Lebedeva, " Comparative analysis operational properties cross-linked polyolefins by various methods and improving the physico-chemical characteristics of silanol cross-linked polyethylene”, May 24, 2011).
Thus, claims that PEX-a is the strongest material due to its higher degree of cross-linking are not true. This factor is more a disadvantage than an advantage of this stitching method.
The stitching method is not the most important indicator of a pipe when choosing it. First of all, you should make sure that the polyethylene from which the pipe is made is actually cross-linked. Some manufacturers under-sew or do not sew the pipe at all, but indicate on it the same characteristics as high-quality PEX pipes.
For example, in May 2013, GROSS pipes were taken out of circulation in Ukraine. Pipes made of cross-linked polyethylene were distributed under this brand; the pipes themselves were marked PEX ( rice. 4), but in fact these pipes consisted of ordinary non-crosslinked polyethylene, is it worth talking about their performance characteristics? There is an easy way to determine whether you are looking at cross-linked polyethylene or a fake made from regular polyethylene. To do this, a piece of pipe must be heated to a temperature of 150–180 ºС; ordinary polyethylene loses its shape at this temperature, but cross-linked polyethylene due to intermolecular bonds retains its shape even at such high temperatures ( rice. 5).
Rice. 4. Marking on pipe Gross
Rice. 5. Gross pipes (sample 7) and VALTEC PEX-EVOH (sample 6) heated in an oven for 30 minutes at a temperature of 180 ºС
Misconception No. 2: “Only polyethylene cross-linked using method “a” has temperature memory properties; polyethylene cross-linked by other methods does not have this property.”
What is meant by “temperature memory effect” in this case? The essence this effect lies in the fact that a pre-deformed pipe, after heating, restores its original shape that it had before deformation. This property manifests itself due to the fact that during bending and deformation, molecularly bonded areas are compressed or stretched, accumulating internal stress. After heating in places of deformation, the elasticity of the material decreases. Internal stresses accumulated during deformation create forces in the thickness of the “softened” material directed towards the original shape of the pipe. Under the influence of these forces, the pipe tends to recover.
Rice. 6.1. Pipe breakVALTEC PEX- EVOH(crosslinking method - PEX-b) and its restoration after heating to 100 °C
Rice. 6.2. Fracture of a PEX pipe with an anti-diffusion layer and its restoration after heating to 100 °C
Rice. 6.3. Pipe break fromPEX- c without an anti-diffusion layer and its restoration after heating to 100 ° C (uncolored cross-linked polyethylene becomes transparent at high temperatures)
In Figures 6.1 – 6.3 shows the restoration of pipes with different ways stitching after creasing. With all stitching methods, the pipes regained their original shape. Wrinkles formed on pipes coated with an anti-diffusion layer after restoration. In these areas, the anti-diffusion layer has peeled away from the PEX layer. This does not affect the performance of the pipe, since the working layer is a PEX layer that has been fully restored.
The memory effect is inherent in any cross-linked polyethylene. The only difference between PEX-a in the restoration technique is that PEX-a is cross-linked during extrusion, and the original shape that the pipeline strives to return is straight. PEX-b and PEX-c, as a rule, are stitched together after forming into coils, and, accordingly, the shape to which the pipelines will tend is a circle with a radius equal to the radius of the coil.
Misconception No. 3: “Crosslinking using method “b” does not provide the required hygiene of pipes, since the silanides used in the production of these pipes are toxic.”
Indeed, silicas (SiH 4 – Si 8 H 18), used to produce PEX-b, are extremely toxic. However, hydrogen silica for cross-linking polyethylene is used only in the cable industry. For the production of pipes, organosilananides are used, which are also poisonous, but their distinctive feature is that when crosslinked, they either completely transform into a chemically bound state or turn into a chemically neutral organic alcohol, which is washed out when the pipelines are hydrated. Today, the most common reagent for cross-linking polyethylene using method “b” is vinyl trimethoxylane (simplified formula: C 2 H 4 Si (OR) 3).
The main indicator of the safety of pipelines and fittings is a hygienic certificate. Only pipes and fittings that have this certificate are allowed for installation in drinking water supply systems.
Misconception No. 4: “Only PEX-a pipes have a uniform degree of cross-linking throughout the entire cross-section, while other pipes have uneven cross-linking.”
The main advantage of crosslinking using method "a" is that peroxides are added to the molten polyethylene before it is extruded into the pipe, and the crosslinking of the pipe, with due attention to temperatures and dosages of peroxides, will be uniform.
When pipelines made of cross-linked polyethylene were not widely used, cross-linking using methods “b” and “c” did have the disadvantage of uneven cross-linking along the length and width of the pipeline. However, when the volume of pipe production reached several kilometers per week, the question arose about improving the quality and automation of these types of stitching. Using the silane method, you can evenly cross-link a pipeline by selecting the correct dosage of reagents, accurately maintaining the temperature and time parameters of pipe processing, and also using catalysts (tin).
Besides modern method The introduction of silane differs from the original, if previously silane was added to the polyethylene melt during extrusion (B-SIOPLAST method), now, as a rule, silane is pre-mixed with peroxide and a certain amount of polyethylene and only then added to the extruder (B-MONOSIL method).
Factories that produce large volumes of pipes have long ago, through trial and error, arrived at the ideal crosslinking technology, and production automation has made it possible to produce pipes with stable characteristics. Thus, the problem of uneven pipeline stitching remains only for small, non-automated industries.
Misconception No. 5: “PERT is a type of cross-linked polyethylene, and is not inferior to it in terms of characteristics.”
Heat-resistant polyethylene PERT is a relatively new material used for the production of pipes. Unlike conventional polyethylene, which uses butene as a copolymer, the copolymer in PERT is octene (octylene C 8 H 16). The octene molecule has an extended and branched spatial structure. By forming side branches of the main polymer, the copolymer creates a region of intertwined copolymer chains around the main chain. These branches of neighboring macromolecules form spatial cohesion not due to the formation of interatomic bonds as in PEX, but due to the cohesion and interweaving of their “branches”
Heat-resistant polyethylene has a number of properties of cross-linked polyethylene: resistance to high temperatures and ultraviolet rays. However, this material does not have long-term resistance to high temperatures and pressure, and is also less acid resistant than PEX. On rice. 7 graphs of the long-term strength of cross-linked polyethylene PEX and high-temperature polyethylene PERT are presented, taken from GOST R 52134-2003 with change No. 1. As can be seen from the graphs, cross-linked polyethylene loses little in its strength over time, even at high temperatures. At the same time, the graph of the decrease in strength is straight and easy to predict. The PERT graph has a break, and at high temperatures this break occurs after two years of operation. The breaking point is called critical; when this point is reached, the material begins to actively accelerate the loss of strength. All this leads to the fact that the pipe, which has reached a critical point, very quickly fails.
Rice. 7. Long-term strength reference curves for PEX (left) and PERT (right) pipes
In addition, due to the lack of connections between macromolecules, PERT does not have temperature memory properties.
Misconception #6: “PEX pipes can absolutely be used for radiator heating systems.”
Conditions of applicability of plastic and metal plastic pipes Pipelines on the territory of the Russian Federation are regulated by GOST 52134-2003. Since the strength of plastic pipelines is quite significantly affected by the time they are exposed to a coolant at a certain temperature, service classes have been established for them ( table 2), which reflect the nature of the influence of certain temperatures on the pipe during its entire service life.
Table 2. Classes of operation of polymer pipelines
| Service class | Application area | T slave, °C | Time at T slave; years | T max, °C | Time at T max, years | T avar, °C | Time at T emergency, h |
| Hot water supply (60 °C) | |||||||
| Hot water supply (70 °C) | |||||||
| Low temperature underfloor heating High temperature underfloor heating | |||||||
| Low temperature heating heating devices | |||||||
| High temperature heating heating devices | |||||||
| Cold water supply |
In this case, the use of pipelines in heating and water supply systems is limited to paragraphs 5.2.1 and 5.2.4:
“5.2.1 Pipes and fittings made of thermoplastics should be used in water supply and heating systems with a maximum operating pressure P max 0.4; 0.6; 0.8 and 1.0 MPa and temperature conditions specified in table 26. The following classes of operation of pipes and fittings have been established...”
“5.2.4 Other operating classes may be established, but the temperature values should not exceed those specified for class 5.”
In other words, the manufacturer can set any ratio of the time of exposure to different temperatures. But the maximum operating temperature cannot be set above 90 °C. In most heating systems, the design coolant temperature is 95 °C. From this data the conclusion follows: it is unacceptable to use PEX pipes in old systems. And if these pipes are used for high-temperature radiator heating, then only in a system that is designed for a maximum operating temperature of 90 o C.
But why do most promotional products of PEX pipe manufacturers indicate the maximum working temperature 95 o C? The fact is that in clause 5.2.1 GOST establishes standards only for the use of plastic pipes, in other words, it regulates the types of systems in which pipes can be used, but not the pipelines themselves, which gives manufacturers the right to write almost any operating temperature in the technical characteristics of pipes .
“The difference is only 5°C does not significantly affect the long-term strength of the pipe" - can be heard as a justification for the use of a pipe. But a pipe has three main parameters: temperature, pressure and service life, and if you increase one of the parameters, the other two will inevitably decrease. Thus, it is possible to use the pipe at higher temperatures, but one must take into account the fact that this will inevitably cause a reduction in service life. The minimum permissible service life of pipelines according to SNiP 41-01-2003 is 25 years, and if pipelines are laid secretly in building structure, service life must be at least 40 years. When the operating temperature increases to 95 o C, the service life of the pipeline is reduced to 35–40 years, depending on the wall thickness, from which we can conclude that pipes with such application parameters cannot be laid hidden.
Below are examples of the use of omissions from suppliers when specifying technical characteristics:
An operating temperature of 95 ºC at a pressure of 0.8 MPa cannot correspond to a service life of 50 years. From the chart on rice. 5 it can be seen that the maximum service life of the pipeline at a temperature of 95 ºС is 8 years.
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The maximum operating temperature is 95 ºС and the service life is 50 years, but it is silent that the pipe can be affected by this temperature for a maximum of 1 year out of these 50 years.
Misconception #7: “ Oxygen protective layer pipeline is a marketing ploy and has no impact on performance characteristics does not provide..."
The use of an oxygen-protective layer is primarily due to compliance with the requirements of SNiP 41-01-2003 “Heating, ventilation and air conditioning” paragraph 6.4.1
“...Polymer pipes used in heating systems in conjunction with metal pipes(including in external heat supply systems) or with devices and equipment that have restrictions on the content of dissolved oxygen in the coolant, must have an oxygen permeability of no more than 0.1 g/m2 day..."
The oxygen permeability of a cross-linked polyethylene pipe with a wall thickness of 2 mm, a diameter of 16 mm at an air temperature of 20 ºC is 670 g/m³·day. It is obvious that a conventional cross-linked polyethylene pipe does not meet the requirements of this SNiP. The SNiP requirements did not appear by chance; the fact is that heating and heat supply systems use specially prepared coolant. Water in boiler rooms or heating points is deaerated using special installations. All this is done in order to prevent corrosion of steel and aluminum system elements, which, one way or another, are present in any system.
To understand the detrimental effect that oxygen gives in the coolant, let us explain the process of steel corrosion itself. Steel corrodes both in water in which oxygen is dissolved and in deaerated water, but the process is somewhat different.
In water that does not contain oxygen, corrosion proceeds as follows: under the influence of water, some of the iron atoms go into solution, as a result of which a negative charge of iron atoms accumulates on the surface of the steel (Fe 2+ + 2e -). In water, due to the presence of impurities, cations and anions H + and OH - are formed. Iron ions with a negative charge, which have passed into solution, combine with anions of the hydrogen group, forming iron hydrate, which is poorly soluble in water (it is this substance that gives the coolant a brown, rusty color): Fe 2+ +2OH - → Fe(OH) 2.
Hydrogen cations (H+), which have a positive charge, are attracted to the inner surface of the pipe, which has a negative charge, forming atomic hydrogen, which forms a protective layer on the surface of the pipe (hydrogen depolarization), reducing the rate of corrosion.
As can be seen, corrosion of steel in the absence of oxygen is temporary until the entire inner surface of the pipe is covered protective film, and the reaction will not slow down.
When steel comes into contact with water containing oxygen, corrosion occurs differently: the oxygen contained in the water binds hydrogen, which forms a protective layer on the surface of the iron (oxygen depolarization). And divalent iron undergoes oxidation to trivalent:
4Fe(OH) 2 + H 2 O + O 2 → 4Fe(OH) 3,
nFe(OH) 3 + H 2 O + O 2 → xFeO yFe 2 O 3 zH 2 O.
Corrosion products do not form a protective layer tightly adjacent to the metal surface. This is due to the increase in volume that occurs during the transition of iron hydroxide to ferrous hydroxide, and the “swelling” of the iron layer susceptible to corrosion. Thus, the presence of oxygen in water significantly accelerates the corrosion of steel in water.
The elements that suffer from corrosion in the first place are boilers, pump impellers, steel pipelines, taps, etc.
How does oxygen penetrate through the thickness of polyethylene and dissolve in water? This process is called gas diffusion, a process in which any gaseous substance can penetrate through the thickness of an amorphous material due to the difference in the partial pressures of this gas on both sides of the substance. The energy that allows gas to pass through the thickness of the plastic arises as a result of the difference in the partial pressures of oxygen in the air and oxygen in the water. Partial pressure of oxygen in air at normal conditions is 0.147 bar. The partial pressure in absolutely deaerated water is 0 bar (regardless of the coolant pressure) and increases as the water is saturated with oxygen.
Rice. 8. EVOH layer of VALTEC PEX-EVOH pipe at x100 magnification
It's not hard to quantify how much damage a pipe without an oxygen barrier can cause.
For example, let's take a heating system with cross-linked polyethylene pipes without an oxygen barrier. Total length pipes with an outer diameter of 16 mm is 100 m. During the year of operation of this system, the following will enter the water:
Q = D O 2 ( d n – 2 · s) 2 · l · z= 650 · (0.16 – 2 · 0.002) 2 · 100 · 365 = 3,416 g of oxygen.
In the given formula D O 2 – oxygen permeability coefficient, for PEX pipes with an outer diameter of 16 mm and a wall thickness of 2 mm, it is equal to 650 g/m 3 · day; d n and s– outer diameter of the pipeline and its thickness, respectively, m, l– pipeline length, m, z– number of days of operation.
In the coolant, oxygen will be in the form of O 2 molecules.
The mass of iron that entered into the oxidation reaction can be calculated using the stoichiometric calculation of the equations for the oxidation of divalent iron (2Fe + O 2 → 2FeO) and subsequent oxidation to ferric iron (4FeO + O 2 → 2Fe 2 O 3).
In the oxidation reaction of divalent iron, its mass will be equal to:
m Fe = m o2· n Fe· MFe/(nABOUT 2 · M O2) = 3,416 2 56 / (1 32) = 11,956 g
In this calculation m Fe – mass of divalent iron that entered into the reaction, g, m o 2 – mass of oxygen that entered into the reaction, g, n Fe And nO2– amount of substance that reacted: (iron, Fe, – 2 mol, oxygen, = yes, O 2, – 1 mol), M Fe And M O 2 – molar mass (Fe – 56 g/mol; O 2 – 32 g/mol).
In the oxidation reaction of ferric iron, its mass will be equal to:
m Fe = m o2· n Fe· MFe/(nABOUT 2 · M O2) = 3,416 4 56 / (3 32) = 7,970 g
Here is the amount of reacted iron ( n Fe) is 4 mol, oxygen ( nO2) – 3 mol.
It follows that if 3416 g of oxygen enters the coolant, the total amount of iron subject to corrosion will be 11,956 g (11.9 kg), while 7,970 g (7.9 kg) of iron forms a rusty layer on the walls of the steel, and 11,956 – 7,970 = 3,986 (3.98 kg) of iron will remain in the divalent state and enter the coolant, polluting it. For comparison: if we take the oxygen permeability of the pipeline as the maximum permissible according to the standards (0.1 g/m 3 day), then 0.52 g of oxygen will dissolve in water per year, which will lead to corrosion of a maximum of 1.82 g of iron, that is, in 6,500 times less.
Of course, not all the oxygen that gets into the pipe interacts with iron; some of the oxygen will interact with impurities in the coolant, and some may reach the deaeration station, where it will be removed from the coolant again. However, the danger of the presence of oxygen in the system is very significant and is by no means exaggerated.
Sometimes in publications there is a phrase: “...automatic air vents will remove all oxygen that has entered through the walls of the pipeline" This statement is not entirely true, since an automatic air vent can release oxygen only if it is released from the coolant. The release of dissolved gases occurs only when there is a sharp decrease in flow speed or pressure, which is rare in conventional systems. To remove oxygen, special flow-through deaerators are installed, in which a sharp decrease in speed occurs and the released gases are removed. On rice. 9.1 And 9.2 The usual version of installing an air vent and an option with a deaeration chamber are shown. In the first case, the air vent removes only a small amount of gases accumulated in the pipeline, in the second - gases that are forcibly “extracted” from the flow due to a sharp increase in cross-section and a decrease in speed.
Misconception No. 8: “Thermal elongation of PEX pipes is many times higher than the thermal elongation of other materials; due to such a large thermal elongation, a monolithic pipe breaks the screed and plaster...”
As usual, these myths are based on reliable facts (the thermal elongation of a cross-linked polyethylene pipe is almost 8 times greater than that of a metal-plastic pipe), but the conclusion was drawn incorrectly.
In order to find out whether the destruction of the floor screed will occur or not, it is necessary to understand the processes occurring in the embedded pipe.
A pipeline laid open will begin to lengthen when heated to a certain temperature. The relative elongation of the pipeline can be easily calculated using the formula:
Δ L = k t · Δ t · L,
Where k t– coefficient of thermal expansion of the pipe material, Δ t– the difference between the coolant temperature and the air temperature during pipe installation; L– pipeline length.
Rice. 10
But in a floor screed, the pipe cannot lengthen, since its thermal expansion is prevented by the cement-sand screed. In this case, for every unit of extension of the pipeline, the tie will compress it by the same distance. Ultimately, the pipeline will be compressed by the floor screed to a distance equal to its thermal elongation ( rice. eleven), its length will not change. The question arises, where does the extra piece of pipe go? The fact is that a certain force is required to compress the pipe. An elongated section of pipe simply turns into the stress that the pipe exerts on the floor screed. And the answer to the question whether the screed will withstand the temperature stress of the pipe depends only on what stress the pipe will exert on the screed.
Rice. eleven
The stress that the pipeline exerts on the floor screed can be estimated using Hooke's Law of elastic deformation of materials. The voltage that the pipe will give will be equal to:
N = Δ L · s · e / L,
Where s- square cross section pipeline walls, e– modulus of elasticity of the pipeline material, L– pipeline length.
But even if you obtain a certain stress value for a particular pipe, there will be little practical benefit from this, since this value must be compared with the maximum permissible stress of the floor screed, and based on this comparison, a conclusion can be drawn about the use of this pipe. But calculating the maximum permissible stress in a screed is quite difficult, and the resulting value, as a rule, will not be accurate, since the screed contains irregularities and stress concentrators, etc.
But using this formula, you can compare pipelines with each other based on the stress they exert on the screed. If you substitute the temperature elongation formula into the voltage formula, you get:
N = k t · Δt · L · s · e / L = k t · t · s · e.
For a metal-plastic pipe with a diameter of 16 mm when heated to 50 °C, the tension in the screed is equal to:
N= 0.26 10 –4 50 8.7 10 –5 8 400 = 9.5 10–4 MPa.
N= 1.9 10 –4 50 8.7 10 –5 670 = 5.5 10 –4 MPa.
N= 0.116 · 10 –4 · 50 · 16.2 · 10 –5 · 200,000 = 187.9 · 10 –4 MPa.
Thus, it can be seen that PEX puts less stress on the screed than a similar metal-plastic pipe. The load from the pipeline on the screed depends not only on the thermal expansion of the pipeline, but also on the elastic modulus, which for cross-linked polyethylene is relatively low compared to other types of materials. Steel, due to its high modulus of elasticity, despite the lowest coefficient of thermal expansion, causes much greater stress in the screed than pipes with high thermal expansion.
Misconception #9: “You cannot install PEX pipe using press fittings because temperature memory is not involved in the sealing process.”
Today, two types of connections are used to connect PEX pipelines: press fittings and fittings with a sliding sleeve.
First you need to understand the mechanism for connecting press fittings:
After crimping the fitting with a press tool, the outer steel sleeve is deformed, squeezing the polyethylene wall. At the same time, polyethylene is also deformed, and due to the accumulated stress in the spatial bonds of molecules, polyethylene tends to return to its original shape (shape memory). Since the modulus of elasticity of steel is many times higher than the modulus of elasticity of cross-linked polyethylene, it is not the sleeve that is subject to deformation, but the polyethylene, which goes deeper into the grooves of the fitting and seals the connection. Rubber rings in this case serve two main purposes:
The first ring (on rice. 12 on the left) is outside the compression zone of the press tool. It serves to ensure tightness during small displacements of the fitting during operation (such displacements can be caused by temperature fluctuations). The modulus of elasticity of EPDM (the material from which the sealing rubber is made) is many times less than the modulus of elasticity of PEX, so this material in such cases fills all the voids created as a result of the fitting being displaced.
Rice. 12. Compression of VALTC PEX-EVOH pipe with a press fitting
The second ring is partially in the compression zone (on rice. 12 on right). This ring is constantly subject to the load from the steel sleeve. It serves to compensate for the difference in thermal expansion of polyethylene and brass. If the fitting is suddenly heated or cooled sharply, a situation may arise when a micron gap appears between the fitting and the pipe wall, which, although it will not lead to leakage, will significantly shorten the service life of the connection. In this case, this ring will fill the resulting gap and ensure a tight seal.
Pipes made of cross-linked polyethylene using the “b” method are not installed using fittings with a sliding sleeve due to the fact that during such installation the end of the pipe is expanded using an extractor. The elongation at break of PEX-b is lower compared to PEX-a due to stronger silane bonds. Therefore, the pipeline expansion procedure for PEX-b leads to the accumulation of microcracks, shortening the service life of the connection.
The press fitting ensures reliable and tight fixation of the pipeline during the entire working period.
Conclusion
On the one hand, the use modern materials leads to cheaper production, faster installation, environmental friendliness and safety. All these factors lead to an increase in the quality of human life. But at the same time, unhealthy competition between manufacturers of modern materials causes consumers to be wary of accepting everything new, and also significantly complicates the choice of one material or another.
Pipe made of polyethylene with increased heat resistance PE-RT (type 2) is used in drinking and drinking water systems, hot water supply, low-temperature water (up to 80 ° C) heating, water heated floors and walls, soil heating, and also as process pipelines transporting liquids that are not aggressive to pipe materials. Operating classes according to GOST 32415-2013 - 1, 2, 4, ХВ.
The connection of VALTEC PE-RT pipes is carried out using press fittings (VTm.200, VTc.712), which are also used for connecting metal-polymer pipes. For connections of the "cone" and "eurocone" standards can be used compression fittings VTc.4410, VTc.709. In progress installation work should follow the instructions given in technical passports to the specified fittings.
Estimated service life is 50 years. Delivery form: 200 m long sections in coils. The cost of the pipe is indicated for 1 linear meter.
VALTEC PE-RT pipe
Technical characteristics of Valtec polyethylene pipe:
Manufacturer: Valtec
Manufacturer country: Italy
Pipe material type: Polyethylene
Pipe application area: For hot, cold water supply and heating systems
Pipe section: Round
Outside diameter: 16.0(mm)
Inner diameter: 12.0(mm)
Pipe wall thickness: 2.0(mm)
Maximum operating temperature: 80.0(deg.)
Maximum short-term permissible temperature: 90.0(deg.)
Guarantee period: 10 years)
Logistics: Supplied in 200 m coils.
Buy Valtec pe-rt pipe for heated floors in Moscow in company Teplodoma-msk
GOST 32415-2013
Available sizes:
COMPIPE TM pressure pipe made of polyethylene with increased heat resistance (PERT) with a barrier (anti-diffusion) layer of ethylene vinyl alcohol (EVOH) is intended for the construction and repair of internal networks of cold and hot water supply and radiator heating of buildings, including floor heating (operation classes 1, 2 , 4, ХВ according to GOST 32415-2013).
PERT/EVOH COMPIPE TM pipes are ideal for low-temperature underfloor heating systems.
PERT/EVOH COMPIPE TM pipe is made from the new generation of thermostabilized polyethylene PE-RT type II DOWLEX 2388, manufactured by the Dow Chemical Company. DOWLEX 2388 - polyethylene with high temperature resistance and resistance to aging is produced by the method of directed spatial formation of lateral bonds in polymer macromolecules by copolymerization of butene and octene (Fig. 1). During the synthesis process, a region of mutually intertwined chains is formed around the main chain, due to which neighboring macromolecules are mutually intertwined, forming spatial cohesion. Thanks to this structure, PERT, like PEX, has increased long-term heat resistance and strength, but retains the flexibility inherent in conventional polyethylene.
Figure 1. Synthesis of polyethylene with increased heat resistance - copolymerization of butene and octene.
The PERT/EVOH COMPIPE TM pipe meets the requirements of SNiP 41-01-2003, which prescribes the use of polymer pipes in heating systems with an oxygen permeability index of no more than 0.1 g/m 3 per day (requirements also GOST 32415-2013, DIN 4726).
The technical characteristics of the pipes are given in Table 1.
Table 1
| Indicator name | COMPIPE TM PERT/EVOH | |||
| Outer diameter, mm | 16 | 20 | ||
| Inner diameter, mm | 12 | 16 | ||
| Wall thickness, mm | 2,0 | 2,0 | ||
| vendor code |
1620200-5 /1620100-5 |
2020100-5 | ||
| Coil length, m | 200/600 | 100 | ||
| Series S | 3,5 | 4,5 | ||
| Standard SDR Size Ratio | 8 | 10 | ||
| Weight 1 l.m. pipes, g | 82 | 131 | ||
| Volume of liquid in 1 l.m. pipes, l | 0,113 | 0,201 | ||
| Working temperature | (0÷80)ºС | |||
| Emergency temperature (no more than 100 hours) | 100ºС | |||
|
Maximum working pressure 1st, 2nd, 4th grades |
0.8 MPa |
0.6 MPa |
||
|
Maximum operating pressure at 20ºС |
1.0 MPa | |||
| Coefficient of thermal linear expansion | (1.95x10 -4) K -1 | |||
| Change in pipe length after heating at a temperature of 120ºC for 60 minutes | less than 2% | |||
| Equivalent uniform-grain roughness coefficient | 0,004 | |||
| Coefficient of thermal conductivity | 0.4 W/mK |
|||
| Oxygen diffusion | less than 0.1, g/m 3 per day | |||
| Warranty period, years | 10 | |||
| Service life subject to installation and operation rules, years | 50 | |||
Table 2. Table of characteristics of operating classes according to GOST R 32415-2013
| Service class | T slave, °C | Time at T work, year | Tmax, °C |
Time at Tmax, year |
T emergency, °C | Time at T emergency, h | Application area |
| 1 | 60 | 49 | 80 | 1 | 95 | 100 | Hot water supply (60 o C) |
| 2 | 70 | 49 | 80 | 1 | 95 | 100 | Hot water supply (70 o C) |
| 4 | 20 | 2,5 | 70 | 2,5 | 100 | 100 |
High temperature floor |
| 40 | 20 | ||||||
| 60 | 25 | ||||||
| 5 | 20 | 14 | 90 | 1 | 100 | 100 | High temperature heating heating devices |
| 60 | 25 | ||||||
| 80 | 10 | ||||||
| HV | 20 | 50 | - | - | - | - | Cold water supply |
|
The following notations are used in the table: T slave - operating temperature or combination of temperatures of transported water, determined by the area of application; T max - maximum operating temperature, the effect of which is limited in time; T emergency - emergency temperature that occurs in emergency situations in case of violation of regulatory systems. HOW TO USE THE TABLE |
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Table 3. Packaging characteristics of COMPIPE TM PERT/EVOH pipes
The pipe has a certificate of conformity in the Rostest system according to GOST 32415-2013, a certificate of state registration.