Classification of plastic pipes for floor radiant heating
With the development of construction industry and the improvement of residents' requirements for living comfort, the application of low-temperature hot water floor radiant heating system is becoming more and more popular. At present, there are the following kinds of plastic pipes commonly used in floor radiant heating system in China.
PE-X: crosslinked polyethylene 1
PP-B: impact-resistant polypropylene (once called PP-C in Korea).
PP-R: random polypropylene
Lead: polybutene
PE-RT: high temperature resistant polyethylene
Note 1: Crosslinked polyethylene can be divided into four types according to production methods: peroxide crosslinking (PE-Xa), silane crosslinking (PE-Xb), electron beam crosslinking (PE-XC) and azo crosslinking (PE-Xd). Among them, peroxide crosslinking and silane crosslinking are two commonly used crosslinked polyethylene pipe products in China.
Requirements of raw materials for plastic pipes and their respective characteristics
2. 1 pipeline raw material requirements
According to the requirements of national and international product standards, the raw materials for producing plastic pipes and plastic fittings for cold and hot water must be special pipeline materials that have passed the test and passed the qualification judgment according to GB/T 18252—2000 "Determination of long-term hydrostatic pressure of thermoplastic pipes by extrapolation method for plastic pipeline systems". In other words, each raw material manufacturer must make a creep failure curve that conforms to the predicted strength reference curve according to the pipe product standard to prove that the raw materials sold are qualified. Only under the hydrostatic pressure of 1 10℃, a little test of thermal stability for 8760 hours can not prove whether the raw materials of pipes are qualified. So far, the raw materials of hot water pipes sold in the domestic market have qualified creep failure curves, and only a few internationally renowned manufacturers such as Nordic Chemical, Basel and DSM have PP-R. PE-RT is only available in Dow Chemical in the United States; PB can only be obtained from Shell. So far, the raw materials of other manufacturers and the varieties of PE-X and PP-B have not reached the product standards. If you don't know whether the raw material products are qualified or not, it means that the cement has no definite label and the use strength cannot be determined. No matter what kind of pipes are used to lay floor heating, only by using raw materials that meet the requirements of product standards and qualified pipes produced by reasonable production equipment and technology can the service life of plastic pipes for floor heating be reliably guaranteed for 50 years through correct design, installation and use.
2.2 Characteristics of different pipeline varieties:
PE-X: In domestic production, silane or peroxide is generally used to cross-link medium density polyethylene or high density polyethylene. It connects the linear long molecular chains of polyethylene through chemical bonds to form a three-dimensional network molecular chain structure. Compared with ordinary polyethylene, the tensile strength, heat resistance, aging resistance, stress cracking resistance and dimensional stability are improved. The whole production process is a chemical reaction process. There are some difficulties in this variety, such as the crosslinking agent is not easy to disperse evenly, the degree of crosslinking is difficult to control evenly, and the screw needs to be cleaned regularly to prevent gel particles from producing. Product quality is difficult to control, which is difficult for small-scale production enterprises to do well. Qualified PE-X pipes have the advantages of good mechanical properties, high temperature resistance and low temperature resistance. However, PE-X pipes are not thermoplastic, so they cannot be connected and repaired by hot melt welding.
PP-B: Good low temperature resistance, high bending modulus, excellent connection performance and low raw material cost. However, under the design pressure of floor radiant heating system of 0.6MPa, PE-X, PP-R, PB and PE-RT need S3.2 series pipes (S5. ) should be selected. The wall thickness of de20 pipe is 2.8mm, and its inner diameter is 1.6mm less than that of PE-X, PP-R, PB and PE-RT, so it has poor heat resistance compared with other products. Raw material manufacturers have not yet made a creep failure curve.
PP-R: Good high temperature resistance, good mechanical properties and excellent connection performance. There are several companies in Europe whose raw materials have qualified creep failure curves. However, the impact resistance at low temperature is poor.
PB: Excellent creep resistance and mechanical properties, the softest of several pipes and the thinnest under the same design pressure. Under the same use conditions, the same series of pipes with the same wall thickness have the highest safety. However, the price of raw materials is the highest, which is more than double that of other varieties. At present, the domestic application area is less.
PE-RT: The raw material is a kind of medium density polyethylene with very stable mechanical properties, which is polymerized by ethylene and octene monomers under the catalysis of metallocene. Its unique ethylene backbone and octene short-branched chain structure make it have excellent ethylene toughness, stress cracking resistance, low-temperature impact resistance, outstanding long-term water pressure resistance and thermal creep resistance of octene. Can be hot-melt connection, careless damage can be repaired with hot-melt connection of pipe fittings. The raw material has a creep failure curve that meets the product standard, and it is the only polyethylene variety that can be used for hot water pipes without crosslinking at present. However, its processing range is narrow and its quality control is difficult.
Performance comparison and selection of products
3. 1 Because the heating system is not used at constant temperature for a long time, the water temperature in the system will rise and fall with the climate change, so in ISO 10508, the use conditions of plastic pipes are divided into five levels, and floor radiant heating belongs to four levels.
In all heating equipment, only water or treated water can be transported as a heat carrier.
The number of years in the table is "plus", not "or".
3.2 Because the design stress σD of each variety is different under the four-level service conditions, the S value of the piping system to be selected under the same design pressure is also different, and the S value of the piping system to be selected for different varieties of plastic pipes is also different under a given design pressure.
At present, in actual production and application, the S value of general piping system is the highest, which is 5.
3.3 Need to verify the quality control points and comparison of mechanical properties of products.
3.4 Physical and mechanical properties of various plastic pipes.
3.5 Design stress of different kinds of plastic pipes under the condition of Grade 4 service.
The calculated pipeline value Scalc of each pipeline variety under different design pressures can be calculated, and then the value can be rounded down to a similar pipeline S value, so that the pipeline S value under design pressure can be selected.
For example, under the condition of service level 4 and design pressure of 0.9MPa, the calculated pipe value of PE-X is 4.44, which should be s 4 after rounding.
3.6 From the above and comparison, it can be seen that as long as the raw materials and products meet the requirements of product standards at the same time, all five varieties can be reliably applied to the floor radiant heating system, but under the same design pressure, the pipe wall thickness is different, and the construction efficiency and difficulty are also different.
PE-X is the most widely used type of floor radiant heating system in Europe. Some European countries have good control methods and decades of management experience in the production and application of PE-X. The raw materials are the products of several internationally renowned chemical companies such as BASF and Nordic Chemical, and the equipment is designed according to the characteristics of the raw materials of the selected manufacturers. However, it is worth noting that at present, raw material manufacturers at home and abroad have not carried out product qualification verification, and raw material prices are quite different. Some domestic manufacturers choose raw materials at will, which makes it difficult to guarantee the quality of products, and there are certain risks in the application of this variety in choosing low-priced products.
PP-B is widely used in radiant floor heating system of wall-hung boiler in Korea. Because of its low pressure, when the pressure is 0.3MPa, it is only necessary to design the pipe of pipe system S6.3, and generally choose the pipe of pipe system S5, which is not easy to cause problems in the domestic use in Korea. At present, there is no qualified manufacturer of this variety of raw materials, and its design stress is the lowest among the five varieties, but its bending modulus is the highest, which is relatively difficult to lay. It is not suitable for hot water pipes such as high-pressure radiant floor heating pipes, but it is an ideal cold water pipe.
The creep failure curves of PP-R, PB and PE-RT from different manufacturers meet the product standards, which is the basic premise for the reliable use of hot water pipes for 50 years.
PP-R is an ideal product for domestic hot water pipes and radiator systems at present. Because of its poor low-temperature impact resistance and flexibility, it is rarely used in radiant floor heating pipes, and hot water needs to circulate in the pipes when laying. However, from the perspective of system reliability and service life, pipes with qualified product quality are very superior to supervisors and other hot water pipes used for floor radiant heating.
PB is one of the plastic hot water pipes with the greatest design stress under the same service conditions. For example, under the use condition of floor radiant heating, S8 should be selected as the series of pressure calculation tubes for 0.6MPa system, and the wall thickness of de20 is only 1.3mm, but S5 series is generally adopted in actual production due to the connection requirements of the system and the scratch damage in construction. It can be seen that the service life of PB pipe is much longer than that of other S series plastic pipes. This product is the most expensive and reliable variety among several plastic pipes for hot water at present.
When using PE-RT varieties under the design pressure of 0.6MPa, the pipe system S5 should be selected, which is the same as PE-X, PP-R and PB in practical application. It has the advantages of hot-melt connection, stable and reliable raw material performance, good flexibility and so on. The bending modulus of the product is less than 70% of that of PE-X. PE-RT plastic pipe adds a new variety to the pipe used in the field of floor radiant heating, and its comprehensive excellent characteristics make it competitive in the field of floor radiant heating.
Development trend of PE-RT
PE-RT raw material not only has qualified creep failure curve, but also has moderate pipe price, which is more convenient and faster than PE-X, PP-B and PP-R in the construction of floor radiant heating system. The connection mode belongs to the most reliable hot-melt connection mode at present. The pipes and fittings of the same material are welded with each other, and the diameter of the fittings is larger than the inner diameter of the pipes of the same specification. In the system, the fluid resistance is relatively small because there is no mechanical connection mode of local diameter reduction.
Due to the advantages of PE-RT pipes used as hot water pipes at present, the International Organization for Standardization (ISO) has formulated the expected strength requirements of raw materials as an international standard when only one company produces this product, and its standard number is ISO/AWI24033. The English name of this international standard is high temperature resistant polyethylene (PE-RT) pipe-the influence of time and temperature on the expected strength. From this point of view, it also fully shows the importance and development prospect of PE-RT as a pipeline product in the world.
System adaptability of plastic hot water pipe
The system adaptability test of hot-melt connection of plastic pipes only needs two tests: internal pressure resistance test and thermal cycle test, while the plastic pipes with mechanical connection need six tests: internal pressure resistance test, thermal cycle test, bending test, pull-out test, cyclic pressure impact test and vacuum test. It can be seen that the hot-melt connection of plastic pipes is superior to mechanical connection in meeting the requirements of pipeline system. On the other hand, the minimum inner diameter of the connecting part of S5 series de20 pipe is 20mm in hot melt connection and 90mm in mechanical connection. It can be seen that the local fluid resistance of mechanical connection is greater than that of hot melt connection, which is easy to cause blockage.
In the operation of plastic pipes laid underground, there will inevitably be leakage caused by construction damage and decoration damage. Repairing with hot-melt connection can ensure that the life of the repair point is synchronized with the life of the whole system, which is not easy to do with mechanical connection.
In the hot water plastic pipeline system, the service temperature continuously exceeds the design temperature 10℃, and the service life is shortened by 2.5 times under the same service pressure; On the contrary, when the service temperature is lower than the design temperature 10℃, the service life is prolonged by 2.5 times under the same service pressure. In addition, when the service pressure exceeds the design pressure, the damage to the plastic pipe is greater. Therefore, during the operation of the heating pipeline system, it is necessary to avoid running in a state exceeding the design temperature and pressure as much as possible.
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