PE-RT pipe (type 2) is used in systems for drinking and drinking purposes, hot water supply, low-temperature water (up to 80 ° C) heating, water heated floors and walls, soil heating, as well as technological pipelines , transporting liquids that are not aggressive to pipe materials. Operating classes according to GOST 32415-2013 - 1, 2, 4, XB.

VALTEC PE-RT pipes are connected using press fittings (VTm.200, VTc.712), which are also used to connect metal-polymer pipes. Compression fittings VTc.4410, VTc.709 can be used for connections of "cone" and "Eurocone" standards. During the installation work, you should be guided by the instructions given in the technical data sheets for the specified fittings.

Estimated service life - 50 years. Delivery form - pieces 200 m long in bays. The cost of the pipe is indicated for 1 running meter.

Pipe VALTEC PE-RT

Technical characteristics of the Valtec polyethylene pipe:

Manufacturer: Valtec
Producing country: Italy
Pipe material type: Polyethylene
Scope of the pipe: 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 admissible temperature: 90.0(deg.)
Guarantee period: 10 years)

Logistics: Supplied in 200m coils.

Buy Valtec pe-rt pipe for underfloor heating in Moscow in the company Teplodoma-msk

The system of underfloor water heating, in recent 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 a private country house. The quality of pipes, the materials from which they are made, is thought not only by customers, but also by people who independently mount such a system.

Which pipes are better to choose for a water-heated floor - an overview 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 manufacturer of the pipe and the country of production. Since it does not matter what material the pipe is made of, if it is not produced according to 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 underfloor heating pipe cannot be cheap.

Main properties and parameters of pipes used in floor and panel heating

When choosing a pipe for mounting it in the warm floor of a private country house or an apartment in a high-rise building, they start not only from the quality of the pipe and its applicability in a particular case, but also from the ease of installation. For a person who will install a warm floor in his house for the first time, it will be more convenient and pleasant to work with a more flexible and shape-holding pipe than a rigid and not pliable one, and this should also be taken into account, because. this can affect the quality (uniformity) of underfloor heating in the future.

What material is more suitable for underfloor heating pipes

Metal-plastic pipes

Metal-plastic pipes - the first and most popular, until recently, polymer pipes for underfloor heating. When viewed in section, such a pipe consists of two polymer layers, between which there is a layer of aluminum foil with a thickness of 0.2 mm or more. The most famous pipe for underfloor heating is the Henco pipe. Recently, it has not been very popular, because. the cost of the pipe is quite high. Due to the use of cross-linked PEX polyethylene and high-quality glue for gluing the layers.

Unlike Henco, other European manufacturers have switched to the production of metal-plastic pipes from heat-resistant PE-RT polyethylene. The elongation of this material when heated is several times less than that of cross-linked polyethylene PEX, respectively, the reliability of such a pipe is higher with sharp temperature fluctuations. So many Chinese manufacturers use cross-linked polyethylene, and given the savings on other materials, the overall quality of the pipe turns out to be quite low, so there are a lot of bad reviews on the forums about exfoliating pipes, cracking the outer layer (afraid of ultraviolet).

The presence of aluminum foil in the composition of the metal-plastic pipe allows you to completely avoid the ingress of oxygen into the coolant and reduce the linear elongation up to 5 times.

If you decide to use a metal-plastic pipe, it is better to stop at European manufacturers

1. Uponor (PE-RT/AL/PE-RT) Germany
2. Germany
3. HENCO (PEXc/AL0.4vmm/PEXc) Belgium
4. APE, STOUT (PEXb/Al/PEXb) Italy
5. COMPIPE (PEXb/Al/PEXb) Russia(Application up to class 5 operation)
6. 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. information will be typed into a whole article, and we will tell you which options for pipes are better to stop at.

The highest percentage of crosslinking (from 75%) in the peroxide crosslinking method is PEXa pipes. The most expensive method used by European manufacturers. The PEXb silane crosslinking method 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 the PEXb pipe obtains its strength properties only during the operation of the pipe with a heat carrier.

In the process of 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 receives increased elasticity.

With an increase in the degree of crosslinking, strength, heat resistance, resistance to aggressive media and ultraviolet rays increase. However, along with an increase in the degree of crosslinking, the brittleness increases and the flexibility of the resulting pipeline decreases. If you bring the degree of crosslinking of polyethylene to 100%, then in its properties it will be similar to glass.

The biggest problem in choosing a specific manufacturer and pipe is the poor quality of crosslinking in pipes made in China, as well as in some representatives of the Russian one. 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 protection from oxygen becomes saturated with oxygen after a certain time, which can lead to corrosion of the 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 glued together. It should be noted that the EVOH layer does not completely prevent oxygen emission, but only reduces oxygen permeability to a value of 0.05–0.1 g/m3 day, which is acceptable for heating systems. In the PEX-EVOH pipe, the anti-diffusion layer is made on the outside, i.e. the pipe has a three-layer construction: PEX-adhesive-EVOH. There are also five-layer (PEX-adhesive-EVOH-adhesive-PEX) pipes on the market, but tests have shown that the three-layer construction is more reliable. The notion that the outer layer of EVOH in a three-layer construction is subject to abrasion is erroneous.

Another disadvantage of PEX pipes is a large linear elongation, therefore, such pipes are practically not used for outdoor installation, but only in a hidden one.

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 in editing. If during the installation of the pipeline a fracture, squeezing or other deformation is formed, then it is easily eliminated by heating the pipeline to a temperature of 100-120 ° C. (However, in the passport for the Russian-Chinese pipe Valtec it is written: "In the event of a" crease ", the damaged section of the pipe must be removed.")

On pipelines coated with an anti-diffusion layer, folds form after restoration. In these places, the anti-diffusion layer peels off from the PEX layer. This defect practically does not affect the characteristics of the pipeline, since the main bearing capacity of the pipeline is determined by the PEX layer, which has completely recovered. Slight delamination of the anti-diffusion layer slightly increases the oxygen permeability of the pipeline.

Pipelines made of cross-linked polyethylene, and especially PEXa made in Europe, are better than other polymer pipes for use not only in underfloor heating, but also in radiator heating, by a hidden method.

What pipes can be found on sale:

1. Germany
2. UPONOR COMFORT PIPE PLUS PE-Xa EVOH Germany(application up to class 5, underfloor heating and radiators)

(application up to class 5) BEST CHOICE FOR PRICE-QUALITY

3. SANEXT "Warm floor" PE-Xa Russia-Europe(application up to class 4)
4. Russia-China(application up to class 4)

Heat-resistant polyethylene PE-RT

Very often, heat-resistant PE-RT polyethylene is called cross-linked polyethylene. But the technology for the production of such polyethylene is as follows. In a chemical reaction, "flat" butene is replaced by octylene (formula C8P16), which has a spatially branched structure. In the future, it forms side branches near the main chain, which are mutually intertwined monomer chains. They are interconnected due to the mechanical interweaving of branches, and not due to interatomic bonds.

PE-RT pipes are mainly used for floor heating, where the temperature and pressure are lower than in plumbing and heating systems. Although the manufacturers of PE-RT pipes, pursuing their marketing policy, claim that the properties of their pipes are the same as those made from PEX. However, this is questionable as PE-RT is a common thermoplastic with limited overall resistance to elevated temperatures and pressures in hot water systems, as confirmed by hydraulic testing and subsequent practice.

Comparison of regression curves obtained by the independent Institute of Polymers Bodycoat (Belgium) indicates that the durability of PE-X pipes is higher, and the regression curve showing the loss of the ability to perform working functions over time for heat-resistant PE-RT polyethylene has a characteristic fracture (loss of strength continuous operation) already at 70 °C.

BioPipe (PERT) Russia

The most affordable option with the highest quality

Stainless steel and copper pipes

These types of pipes are practically not used in the installation of underfloor heating, and the main reasons are the high price. Due to the fact that polyethylene pipelines of the best German manufacturers are 2 times cheaper than metal pipes, and the service life is more than 50 years (in a warm floor), there is no need for such pipes. Installing a copper pipe floor is more expensive and the installer of such floors must have extensive experience and qualifications.

findings

As for other types of equipment and materials, when choosing a specific manufacturer, we recommend choosing European manufacturers. The fact that the European manufacturer must be determined by the barcode and the inscription "Made in ...". Many sellers offer Italian trumpet, but cannot confirm that it is made in Italy, because. the pipe is actually made in China, and the real home of the brand is Russia. And of course, if the pipe is produced in Europe, then the price of such a pipe will not be the lowest, because. quality cannot be cheap. If you compare an inexpensive German pipe and an expensive Chinese pipe, decide for yourself how confident you are in the real characteristics and quality of the Chinese pipe, for example, in the level of "crosslinking" of cross-linked polyethylene.

If we draw conclusions on the materials for underfloor heating pipes, then our experts arrange the materials in the following sequence, starting with the best:

1. Cross-linked PEXa polyethylene with an anti-diffusion layer
2. Metal-plastic with PE-RT inner layer
3. Cross-linked polyethylene PEXb,c
4. Heat-resistant polyethylene PE-RT

GOST 32415-2013

Available sizes:

The COMPIPE TM pressure pipe made of polyethylene of increased temperature 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, hot water supply and radiator heating of buildings, including underfloor 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.

The PERT/EVOH COMPIPE TM pipe is made from DOWLEX 2388 new generation PE-RT type II heat stabilized polyethylene 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 side bonds in polymer macromolecules by copolymerization of butene and octene (Fig. 1). In the process of synthesis, an area of ​​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 high temperature resistant polyethylene - 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 with an oxygen permeability index of not more than 0.1 g/m 3 per day in heating systems (the requirements are also GOST 32415-2013, DIN 4726).

Pipe specifications are shown in Table 1.

Table 1

Table 2. Table of characteristics of operation classes according to GOST R 32415-2013

Table 3. Package characteristics of COMPIPE TM PERT/EVOH pipes

The pipe has a certificate of conformity in the Rostest system in accordance with GOST 32415-2013, a certificate of state registration.

Plastic pipes for water supply - withstand temperatures from -70°C to 110°C (min and max temperature conditions are indicated), have a blue coating. For gas supply - has a yellow or orange color.

Production of PERT pipes

The HDPE pressure pipe is made of polyethylene and has a protective layer. Manufacturers use different coatings (protection) from modified additives, minerals, light and heat stabilized compositions based on propylene and polypropylene.

Connection methods

To connect pipes made of polyethylene with increased heat resistance, the same methods are used as for cross-linked PE:

• Sliding fittings. Installation is available to a specialist of any qualification. Mainly used for small diameter pipes.
• Crimp fittings. Polyethylene fills the entire docking space, so the connection is reliable and installation is quick.
• Electrofittings. The most reliable connection method.
• Flanges. It is used for pipes of large diameter and connection of sections of critical pipelines.

Connection of pe-rt pipes with pipes made of other material (PP, PA, PB, metal) transitional removable and non-removable couplings are used.

PERT pipe installation

Installation is carried out in several ways:

• Trench method, without the use of sand backfill.
• For laying are used incl. and gentle methods.
• Different types of drilling, for example, horizontal directional.
• Trenchless laying by the methods of punctures with a pneumatic punch.
• It is possible to lay in unstable soils using the plow-rotary laying method, as well as in other soils (rocky terrain, coarse-grained, excluding boulder, crushed stone, pebble-gravel).
• Rotary excavation - backfilling.
• Restoration in a trenchless way without dismantling the old one - relining.
• During the current repair of external systems - renovation.

Today, unfortunately, marketing moves and advertising tricks are increasingly affecting various technical solutions and the choice of a particular material and equipment for a project. More and more often, instead of a full-fledged technical passport or catalog for equipment, designers have advertising booklets and brochures on their desks, according to which they select. What is unacceptable to write in serious technical literature migrates to the pages of such booklets. Often, marketers assign overestimated or completely non-existent indicators to their product, misleading engineers. As a rule, outstanding technical features of the equipment in booklets are presented as undeniable advantages. Conversely, any technical information about competitive products is presented as significant and irreparable flaws.

All these factors ultimately lead to the wrong choice of materials and equipment, which can eventually lead to an emergency. The fault 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 data sheet carefully hidden from the human eye with real data. Most often, advertising brochures provide information that does not contradict the passport data, but is presented in such a way that people have a false idea about the real technical features of the product. For example, the phrases “a pipe can withstand a temperature of 95 ºС and a pressure of 10 bar” and “a pipe can withstand a coolant temperature of 95 ºС at a pressure of 10 bar for 50 years” are fundamentally different from each other. In the first case, a riddle is posed: is the pipe capable of withstanding 95 ºС coolant temperature and 10 bar at the same time, or are these two critical points for the application of this pipe? And most importantly, there is no time indicator, that is, it is not known how long the pipeline can withstand these parameters - five minutes, an hour or 50 years?

This article lists the main marketing gimmicks and myths propagated by the PEX pipe manufacturers.

1st group of myths - about the superiority of one stitching method over another

Almost every manufacturer of PEX pipes claims that it is the method of sewing their pipes that is the best, while others are no good. Only polyethylene cross-linked according to their method will have increased strength characteristics and reliability indicators.

To begin with, I would like to recall some information about the crosslinking of polyethylene. By cross-linking is meant the creation of a spatial lattice in high-density polyethylene due to the formation of volumetric cross-links between polymer macromolecules. The relative amount of cross-links formed per unit volume of polyethylene is determined by the “degree of cross-linking”. 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, four industrial methods for crosslinking polyethylene are known, depending on which the crosslinked polyethylene is indexed by the corresponding letter.

Table 1. Types of crosslinking of polyethylene

Peroxide crosslinking (method "a")

Method "a" is a chemical crosslinking 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 highly hazardous substances. Their production is a technologically complex and expensive process.

To obtain PEX according to 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 a free bond), rice. 1.2. Peroxide radicals are taken from the polyethylene atoms by one hydrogen atom, which leads to the formation of a free bond at the carbon atom ( rice. 1.3). In neighboring polyethylene macromolecules, carbon atoms that have free bonds are combined ( rice. 1.4). The number of intermolecular bonds is 2–3 per 1000 carbon atoms. The process requires tight temperature control during the extrusion process, when pre-crosslinking occurs, and during further heating of the pipe.

Method "a" is the most expensive. It guarantees complete volumetric coverage of the mass of material by the action of peroxides, as they are added to the initial melt. However, this method requires that the crosslinking be at least 75% (according to Russian standards - not less than 70%), which makes pipes made of this material more rigid than other methods of crosslinking.

Silane crosslinking (method "b»)

Method "b" is a chemical cross-linking of polyethylene using organosilanes. Organosilanides are silicon compounds with organic radicals. Silanides are poisonous substances.

Currently, for the production of PEX pipes according to the “b” method, vinyltrimethoxyloxane (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 polyethylene is treated with water or water vapor, while organic radicals attach a hydrogen molecule from 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). Water displacement from PEX is accelerated by a tin catalyst. The process of final cross-linking takes place already in the solid stage of the product.

Radiation crosslinking (method "c")

The "c" method consists in exposing the C-H group to a stream of charged particles ( rice. 3.1). It can be a stream of electrons or gamma rays. With this exposure, some of the C-H bonds are destroyed. The carbon atoms of neighboring macromolecules, from which a hydrogen atom was knocked out, combine with each other ( rice. 3.3). Irradiation of polyethylene by a stream of particles occurs already after its molding, that is, in a solid state. The disadvantages of this method include the inevitable uneven crosslinking.

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 crosslinking along the length and thickness.

A cyclic electron accelerator (betatron) is most often used as an irradiation source, which is relatively safe both in production and in the use of a finished pipe.

Despite this, in many European countries the production of pipes sewn by 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 very doubtful.

Misconception #1 : “The cross-linking method (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 greater the degree of crosslinking of PEX, the stronger the material.”

Indeed, GOST R 52134 regulates a different minimum allowable degree of crosslinking of PEX pipes for different manufacturing methods ( tab. one), 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. The bond energy of the C-C type that is formed in the polyethylene cross-linked by the "a" and "c" methods is about 630 J/mol, while the bond energy of the Si-C type that is formed in the polyethylene cross-linked by the "b" method is 780 J/mol. The physicochemical and technical properties are also influenced by 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 bonds themselves. This is primarily characteristic of a silanol-crosslinked polymer, where there are a large number of silanol groups capable of forming additional engagement sites in amorphous regions, increasing the density of the structural network (which is 30% higher than with peroxide and 2.5 times higher than with irradiation). crosslinking) and reduce deformability at high temperatures.

Bench tests of cross-linked polyethylene pipes show some strength advantage of silane cross-linking. So, 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 (B.C Osipchik, E.D. Lebedeva, “Comparative analysis of the performance properties of polyolefins crosslinked by various methods and improvement of the physicochemical characteristics of silanol-crosslinked polyethylene”, May 24, 2011).

Thus, claims that PEX-a is the strongest material due to the greater degree of cross-linking are not true. This factor is rather a disadvantage than an advantage of this crosslinking 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 really cross-linked. Some manufacturers do not sew or do not sew the pipe at all, while indicating the same characteristics on it as on high-quality PEX pipes.

For example, in May 2013, GROSS pipes were withdrawn from circulation in Ukraine. Under this brand, pipes made of cross-linked polyethylene were distributed, on the pipes themselves there was a PEX marking ( rice. 4), but in fact these pipes consisted of ordinary non-crosslinked polyethylene, is it worth talking about their performance? There is an easy way to determine what is in front of you - cross-linked polyethylene or a fake made from ordinary 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, and cross-linked due to intermolecular bonds retains its shape even at such high temperatures ( rice. 5).

Rice. 4. Marking on pipe Gross

Rice. 5. Pipes Gross (sample 7) and VALTEC PEX-EVOH (sample 6) after heating in an oven for 30 minutes at a temperature of 180 ºС

Misconception No. 2: “Only polyethylene crosslinked according to the “a” method has the properties of temperature memory, polyethylene crosslinked by other methods does not have this property.

What is meant by the "temperature memory effect" in this case? The essence of this effect is that the pre-deformed pipe, after heating, restores its original shape, which it had before deformation. This property manifests itself due to the fact that during bending and deformation, the molecularly bonded areas are compressed or stretched, while accumulating internal stress. After heating in places of deformation, the elasticity of the material decreases. Internal stresses accumulated during the deformation process create forces in the thickness of the “softened” material directed towards the original shape of the pipe. Under the influence of these efforts, the pipe tends to recover.

Rice. 6.1. pipe fractureVALTEC PEX- EVOH(crosslinking method - PEX-b) and its recovery after heating to 100 °C

Rice. 6.2. Fracture of a PEX-a pipe with an anti-diffusion layer and its recovery after heating to 100 °C

Rice. 6.3. Fracture of a pipePEX- c without an anti-diffusion layer and its recovery 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 various stitching methods after a break. With all methods of stitching, the pipes restored their original shape. Wrinkles formed on pipes coated with an anti-diffusion layer after restoration. In these places, the anti-diffusion layer has delaminated from the PEX layer. This does not affect the characteristics of the pipe, since the working layer is a PEX layer that has been completely regenerated.

The memory effect is inherent in any cross-linked polyethylene. The only difference between PEX-a in the recovery technique is that PEX-a crosslinks during extrusion, and the original shape that the pipeline seeks to return to is straight. PEX-b and PEX-c, as a rule, are sewn together after being formed 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: “B-linking does not provide the required hygiene of pipes, since the silanes used in the production of these pipes are toxic.”

Indeed, silanes (SiH 4 - Si 8 H 18), used to obtain PEX-b, are extremely toxic. However, silicic acid for polyethylene crosslinking is used only in the cable industry. For the production of pipes, organosilanides are used, which are also poisonous, but their distinctive feature is that when cross-linked, they either completely go into a chemically bound state, or turn into chemically neutral organic alcohol, which is washed out during hydration of pipelines. To date, the most common reagent for cross-linking polyethylene using the “b” method is vinyltrimethoxylane (simplified formula: C 2 H 4 Si (OR) 3).

The main indicator of the safety of the pipeline and fittings is the hygienic certificate. Only pipes and fittings that carry this certificate are approved for installation in potable water systems.

Misconception No. 4: “Only PEX-a pipes have a uniform degree of cross-linking throughout the cross section, while other pipes have uneven cross-linking.”

The main advantage of "a" crosslinking is that peroxides are added to the molten polyethylene before it is extruded into the pipe, and pipe crosslinking, with due attention to temperatures and peroxide dosages, will be uniform.

When cross-linked polyethylene pipelines were not widely used, cross-links using the “b” and “c” methods did have a drawback, which consisted in 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 of improving the quality and automation of these types of stitching. Using the silane method, it is possible to evenly sew the pipeline by choosing the right dosage of reagents, accurately maintaining the temperature and time parameters of pipe processing, and also using catalysts (tin).

In addition, the modern method of introducing silane differs from the original one, if earlier 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 (method B-MONOSIL).

Plants producing large volumes of pipes, by trial and error, have long reached the ideal crosslinking technology, and production automation has made it possible to obtain pipes with stable characteristics. Thus, the problem of uneven sewing of the pipeline remains only in small, non-automated industries.

Misconception #5: "PERT is a type of cross-linked polyethylene, and is not inferior to it in terms of performance."

Heat-resistant polyethylene PERT is a relatively new material used for the production of pipes. Unlike conventional polyethylene, which uses butene as a copolymer, PERT uses octene (octylene C 8 H 16) as a copolymer. The octene molecule has an extended and branched spatial structure. Forming side branches of the main polymer, the copolymer creates an area 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 pressures, and is also less acid-resistant than PEX. On the rice. 7 graphs of the long-term strength of cross-linked polyethylene PEX and high-temperature polyethylene PERT, taken from GOST R 52134-2003 with change No. 1, are presented. 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 drop in strength is straight and easily predictable. For PERT, the graph has a kink, and at high temperatures, this kink occurs after two years of operation. The break 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. Reference curves of long-term strength of pipes made of PEX (left) and PERT (right)

In addition, due to the lack of bonds between macromolecules, PERT does not have the properties of temperature memory.

Misconception No. 6: "PEX pipes can be unconditionally used for radiator heating systems."

The conditions for the applicability of plastic and metal-plastic 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 of exposure to a coolant with a certain temperature, they have operating classes ( tab. 2), which reflect the nature of the impact of certain temperatures on the pipe during the entire service life.

Table 2. Operation classes of polymer pipelines

At the same time, the use of pipelines in heating and water supply systems is limited by 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 working pressure P max 0.4; 0.6; 0.8 and 1.0 MPa and temperature conditions indicated in Table 26. The following classes of operation of pipes and fittings are established ... "

"5.2.4 Other operating classes may be established, but temperatures shall not exceed those specified for class 5."

In other words, the manufacturer can set any ratio of the time of influence of various temperatures. However, the maximum operating temperature must not be set above 90 °C. In most heating systems, the design temperature of the coolant is 95 °C. From here the data follows the conclusion: in old systems, PEX pipes are unacceptable to use. 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 ° C.

But why do most advertising products of PEX pipe manufacturers indicate a maximum operating temperature of 95 ° 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 the pipe has three main parameters: temperature, pressure and service life, and if you increase one of the parameters, then 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 allowable service life of pipelines according to SNiP 41-01-2003 is 25 years, and if the pipelines are laid hidden in the building structure, the service life must be at least 40 years. With an increase in the operating temperature to 95 ° C, the service life of the pipeline is reduced to 35-40 years, depending on the wall thickness, hence it can be concluded that pipes with such application parameters cannot be laid covertly.

Below are examples of the use of supplier omissions when specifying technical specifications:

The operating temperature of 95 ºС 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.

The maximum operating temperature of 95 ºС and the service life of 50 years are indicated, but it is silent that this temperature can act on the pipe for a maximum of 1 year out of these 50 years.

Misconception No. 7: “The oxygen-protective layer of the pipeline is a marketing ploy and does not have any effect on performance ...”

The use of an oxygen-protective layer is primarily due to the fulfillment of the requirements of SNiP 41-01-2003 "Heating, ventilation and air conditioning" paragraph 6.4.1

“... Polymer pipes used in heating systems together with metal pipes (including in external heat supply systems) or with instruments and equipment that have restrictions on the content of dissolved oxygen in the coolant must have an oxygen permeability of not more than 0.1 g / m day ... »

The oxygen permeability of a pipe made of cross-linked polyethylene with a wall thickness of 2 mm and a diameter of 16 mm at an air temperature of 20 ºС is 670 g/m³·day. Obviously, a conventional XLPE pipe does not meet the requirements of this SNiP. The requirements of SNiP did not appear by chance, the fact is that a specially prepared coolant is used in heating and heat supply systems. Water in boiler houses or in heating points is deaerated using special installations. All this is done in order to prevent corrosion of steel and aluminum elements of the system, 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 course of 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 (Fe 2+ + 2e -) accumulates on the surface of the steel. In water, due to the presence of impurities, cations and anions H + and OH - are formed. Iron ions with a negative charge, which have gone into solution, combine with anions of the hydrogen group, forming an iron hydrate that is poorly soluble in water (it is this substance that gives the brown, rusty color to the coolant): Fe 2+ + 2OH - → Fe (OH) 2.

Hydrogen cations (H +), having 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), which reduces the corrosion rate.

As you can see, the corrosion of steel in the absence of oxygen is temporary, until the entire inner surface of the pipe is covered with a protective film, and the reaction slows down.

In the case 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 iron surface (oxygen depolarization). And ferrous iron is oxidized to ferric:

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 in this case 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 oxide hydrate, and the "swelling" of the iron layer subject 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 diffusion of gases, a process in which a gaseous substance can penetrate through the thickness of an amorphous material due to the difference in partial pressures of this gas on both sides of the substance. The energy that allows the gas to pass through the thickness of the plastic arises as a result of the difference in the partial pressures of oxygen in air and oxygen in water. The partial pressure of oxygen in air under 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 is not difficult to quantify the harm that a pipe without an oxygen barrier can cause.

For example, let's take a heating system with pipes made of cross-linked polyethylene without an oxygen barrier. The total length of 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\u003d 650 (0.16 - 2 0.002) 2 100 365 \u003d 3416 g of oxygen.

In the above 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 650 g / m 3 · day; d n and s- the 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 ferrous iron (2Fe + O 2 → 2FeO) and subsequent oxidation to ferric iron (4FeO + O 2 → 2Fe 2 O 3).

In the oxidation reaction of ferrous iron, its mass will be equal to:

m Fe = m o2· n Fe· M Fe /(nO 2 · M O2) = 3416 2 56 / (1 32) = 11 956

In this calculation m Fe is the mass of ferrous iron that has reacted, g, m o 2 is the mass of oxygen that entered into the reaction, g, n Fe and nO2- the amount of the substance that entered into the reaction: (iron, Fe, - 2 mol, oxygen, \u003d 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· M Fe /(nO 2 · M O2) = 3416 4 56 / (3 32) = 7970

Here, the amount of the substance that reacted with iron ( n Fe) is 4 mol oxygen ( nO2) - 3 mol.

It follows that when 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 steel walls, and 11,956 - 7,970 = 3,986 (3.98 kg) of iron will remain in a divalent state and enter the coolant, polluting it. For comparison: if we take the oxygen permeability of the pipeline as the maximum allowable according to the norms (0.1 g / m 3 day), then 0.52 g of oxygen per year will dissolve in water, 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 of the oxygen that enters the pipe interacts with iron, some of the oxygen will interact with impurities in the coolant, and some can 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 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. Release of dissolved gases occurs only when the flow rate or pressure is abruptly reduced, which is rare in conventional systems. To remove oxygen, special flow-through deaerators are installed, in which there is a sharp decrease in speed and the removal of released gases. On the rice. 9.1 and 9.2 shows the usual version of the installation of the air vent and the version with a deaeration chamber. 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: “The temperature elongation of PEX pipes is many times greater than the temperature elongation of other materials, due to such a large temperature elongation, the embedded pipe breaks the screed and plaster ...”

As usual, these myths are based on reliable facts (the temperature elongation of a pipe made of cross-linked polyethylene is almost 8 times greater than that of a metal-plastic pipe), but the conclusion is made incorrect.

In order to find out whether the destruction of the floor screed will occur or not, it is necessary to understand the processes taking place in a monolithic pipe.

The pipeline laid in the open, when heated to a certain temperature, will begin to lengthen. The relative elongation of the pipeline is easy to calculate by the formula:

Δ L = k t · Δ t · L,

where k t- coefficient of thermal elongation of the pipe material, Δ t- the difference between the temperature of the coolant and the air temperature during pipe installation; L- pipeline length.

Rice. ten

But in the floor screed, the pipe cannot lengthen, since the cement-sand screed prevents its thermal expansion. In this case, for each unit of pipeline extension, 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. The elongated section of the pipe simply turns into stress, which the pipe exerts on the floor screed. And the answer to the question of whether the screed will withstand the thermal 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, the elastic deformation of materials. The voltage that the pipe will give will be equal to:

N = Δ L · s · e / L,

where s is the cross-sectional area of ​​the pipeline walls, e is the elasticity modulus of the pipeline material, L- pipeline length.

But even if a certain voltage value is obtained for a specific pipe, then there will be little practical benefit from this, since this value must be compared with the maximum allowable floor screed stress, and based on this comparison, a conclusion can be drawn about the use of this pipe. But it is quite difficult to calculate the maximum allowable stress in the screed, and the resulting value, as a rule, will not be accurate, since there are bumps and stress concentrators, etc. in the screed.

But using this formula, you can compare the pipelines with each other in terms of the voltage that they exert on the screed. If we substitute in the stress formula, the formula for thermal elongation, we 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 it is heated by 50 ° C, the stress in the screed is:

N= 0.26 10–4 50 8.7 10–5 8400 = 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 exerts 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 modulus of elasticity, which is relatively low for cross-linked polyethylene compared to other types of materials. Steel, due to the high modulus of elasticity, despite the lowest coefficient of thermal expansion, causes much more stress in the screed than pipes with high thermal expansion.

Misconception No. 9: “You cannot mount a PEX pipe using press fittings, since the property of temperature memory is not involved in the process of ensuring tightness.”

To date, two types of connections are used to connect PEX pipelines: press fittings and fittings with a compression sleeve.

First you need to understand the mechanism for connecting press fittings:

After pressing 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 greater than the modulus of elasticity of cross-linked polyethylene, it is not the sleeve that undergoes deformation, but polyethylene, which goes deeper into the grooves of the fitting and seals the joint. Rubber rings in this case serve two main purposes:

First ring (on rice. 12 left) is outside the crimping area of ​​the press tool. It is used to ensure tightness in case of small movements of the fitting during operation (such movements can be caused by temperature fluctuations). The modulus of elasticity of EPDM (the material from which the sealing gum is made) is many times less than the modulus of elasticity of PEX, so this material in such cases fills all the voids formed as a result of fitting displacement.

Rice. 12. Compression of the 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 under load from the steel sleeve. It serves to compensate for the difference in thermal expansion of polyethylene and brass. With sudden heating or sudden cooling of the fitting, 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 reduce the service life of the connection. This ring in this case will fill the resulting gap and ensure tightness.

Pipes made of cross-linked polyethylene using the “b” method are not mounted using compression sleeve fittings 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 compared to PEX-a is lower due to stronger silane bonds. Therefore, the expansion procedure for the PEX-b pipeline leads to the accumulation of microcracks, which shorten the service life of the connection.

The press fitting provides reliable and hermetic fixation of the pipeline during the entire working period.

Conclusion

On the one hand, the use of modern materials leads to cheaper production, faster installation, environmental friendliness and safety. All these factors lead to an improvement in the quality of human life. But at the same time, unhealthy competition between manufacturers of modern materials causes fear of consumers in the perception of everything new, and also significantly complicates the choice of one or another material.