Preliminary study on the causes of mechanical damage of PVC-U water supply pipes and its prevention

Processing and application of PVCU water supply pipe mechanical damage causes and prevention Wang Xinghai (Huaya (Dongying) Plastic Co., Ltd., Dongying 257091, Shandong) The whole process of the analysis of the reasons for the mechanical damage of PVC U pipe, and put forward effective preventive measures.

Compared with traditional metal pipes, cement pipes and other polyolefin pipes, rigid polyvinyl chloride (PVC U) pipes produced with polyvinyl chloride (PVC) resin as the main raw material have corrosion resistance, light weight and low water flow resistance , Good mechanical strength, environmental protection and hygiene, convenient construction, high cost performance, long service life and other advantages, has been in urban water supply and drainage, building water supply and drainage, electrical, telecommunications, chemical, water conservancy, agricultural irrigation, aquaculture, greening, golf course engineering, etc. It is widely used in many fields. Taking the statistics in 2004 as an example, the domestic production capacity of PVC U pipes has reached 1.2 million t / a (excluding Taiwan, China), accounting for about 67 of the total production capacity of the entire plastic pipe that year, and PVC U pipes are used in the overall application of plastic pipes 'S share also accounted for more than 60. In particular, in March 2004, the Ministry of Construction Technical Announcement "Construction, Promotion, Application and Restriction of Prohibited Use Technology" re-clarified that PVC U pipes (production of non-lead salt heat stabilizers) are suitable for the 90th building water supply plastic piping system, which will Further enhance the first advantage of PVC U pipe in the overall application of plastic pipes. But in the process of more and more extensive application, even qualified products will encounter a small amount of mechanical damage caused by improper application. In addition to ensuring that the product quality complies with GB / T 10002.1 1996 national standards, this aspect also needs to enable the majority of users to understand the performance characteristics of PVC U pipes, and fully grasp their transportation, handling, design, construction, acceptance and use. The correct operation norms and essentials, take systematic and active preventive measures. This can effectively overcome the mechanical damage of PVC U pipes caused by improper application, and can clarify the doubts of the reliability of PVC U water supply pipes by a few users. Therefore, on the basis of summarizing a large number of production and application engineering cases, the author conducts a more comprehensive and in-depth analysis of the mechanical properties of PVC U water supply pipes and the causes of mechanical damage, and puts forward corresponding preventive measures. This will undoubtedly help the correct application of PVC U water supply pipes.

Polyvinyl chloride [Brief introduction] Wang Xinghai (1946―), male, Shandong native, senior engineer, graduated from Tianjin University in 1969, majoring in automation instrumentation, and currently serves as executive director and deputy general manager of Huaya (Dongying) Plastic Co., Ltd.

1 The performance difference between PVC U pipe and traditional pipe and its relationship with mechanical damage. The performance comparison between PVC U pipe and traditional pipe is shown in Table 1. Compared with metal pipes and cement pipes, PVC U pipes have many advantages as mentioned in the introduction, but their mechanical properties, such as tensile yield strength and elastic coefficient, are much lower than that of metal pipes, while the coefficient of linear expansion is higher than that of metal pipes. And cement pipes, and these coefficients of performance all change with changes in temperature, and high attention must be paid to installation and application at different temperatures to avoid unnecessary mechanical damage.

Project galvanized steel pipe nodular cast iron pipe stainless steel pipe copper pipe cement pipe roughness coefficient tensile yield strength / MPa elastic modulus / MPa linear expansion coefficient / ℃ corrosion resistance is good or bad can be good or bad water quality affects no impact easily red water slightly red Water does not affect, slightly affects workability, easy not easy, easy not easy to leak, use temperature / ℃ service life, more than 50 years, more than 5 years, more than 30 years, more than 50 years, more than 10 years, low price, medium high, low 1.1 tensile yield strength, PVC U pipe The tensile yield strength is the ratio of the maximum load at the time of breaking the test piece divided by the minimum cross-sectional area of ​​the test piece, which is determined by the tensile speed of 5 mm / min. This value is closely related to the temperature: when the temperature increases, the tensile force The tensile and yield strength and elastic modulus will be reduced accordingly, otherwise, it will be increased. But when the temperature is lower than 0 ℃, the brittleness of PVC U pipe will gradually occur, so the temperature conditions should be paid attention to in the pipeline design. Figure 1 is a graph of the relationship between the tensile yield strength and elastic modulus of PVC U pipes and temperature. Compared with Table 1, the tensile yield strength of PVC U pipes is only for metal pipes. According to the stress of plastic pipes under the action of hydrostatic pressure p The analysis shows that the relationship between the hoop stress (tensile strength) and the outer diameter and wall thickness of the wall thickness is: where: p is the breaking pressure, M Pa σ is the tensile stress (tensile strength), M Pa d is the average outer diameter of the pipe, mm e is the minimum wall thickness of the pipe, mm.

The calculation formula of the instantaneous breaking pressure of the pipe under the action of hoop stress is: where: p is the instantaneous breaking pressure, M Pa is the instantaneous tensile stress (tensile strength), M Pa.

It can be seen from equation (2) that under the condition that the outer diameter and wall thickness of the pipe are constant, the instantaneous breaking pressure is proportional to the instantaneous tensile strength, that is, the higher the tensile strength, the stronger the water pressure resistance. Inversely proportional to temperature. Therefore, under the condition that the outer diameter and wall thickness of the pipe are constant, the instantaneous breaking pressure also decreases with increasing temperature. In other words, as the temperature increases, the ability of PVC U pipes to withstand internal pressure also decreases.

At different temperatures, the temperature reduction factor f should be used to modify its actual working pressure (), that is, the nominal pressure is multiplied by f to obtain the maximum working pressure that can withstand at this temperature. Therefore, in order to prevent the instantaneous pressure from damaging the pipe, it is necessary to prevent the brittle damage below 0 ° C, and also consider that the maximum service temperature of the PVC U pipe should not exceed 45 ° C.

Temperature / ° C reduction factor f 1.2 The modulus of elasticity The ratio of the stress on the PVC U pipe within its proportional limit to the corresponding strain generated is called the modulus of elasticity. There are two transfer points on the curve of the elastic modulus and temperature shown in Figure 1. The first transfer point shows that the PVC U pipe begins to soften. The corresponding temperature is the Vicat softening temperature of 80 ℃. For the second transfer point, Due to its relatively low ultimate elongation, the corresponding tensile strength increases but the gradual flexibility and processing of PVC disappears in 2005, the corresponding impact strength becomes weak, and brittleness characteristics begin to appear.

The elastic modulus of PVC U pipe is only about 2 of that of metal pipe.

In view of the fact that the tensile yield strength and elastic modulus of PVC U pipes are greatly reduced as the temperature decreases, especially at 0 ℃, brittleness occurs, so during winter construction, be sure to take it lightly to avoid damage from external forces.

1.3 Impact strength The impact strength of PVC U pipe is its maximum ability to withstand the impact load, that is, the ratio of the work consumed when the material is destroyed to the cross-sectional area of ​​the sample under the impact load is according to the drop hammer impact method or simply supported beam impact Method. Figure 2 is a graph of the relationship between the impact strength and temperature of PVC U pipes. The impact strength is proportional to the temperature. The higher the temperature, the better the impact performance, and the sharp decline. The impact strength at 20 ° C is nearly 200 at 0 ° C. The impact performance of PVC U pipes under low temperature conditions is significantly reduced, especially below 0 ° C. Press test (0 ℃) TIR≤5 (TIR is the true destruction rate). Huaya (Dongying) Plastic Co., Ltd. shows the drop weight impact test curve of 1 M Pa and D under low temperature impact as shown in Figure 3. The test conditions are: weight 0.25 kg, height 2 m, total impact 25 times. The qualified condition is: the number of impact failures ≤ 1 times.

63 The low-temperature impact characteristic curve of the water supply pipe needs to be emphasized that the scoring impact shown in Figure 2 refers to the scoring made with a sharp tool on the test piece. Up to 10 times. When the test piece has a score, the test piece must be broken at the score after the impact test. This characteristic is called the score effect.

1.4 Linear expansion coefficient The percentage of change in the length of the material for every 1 ° C change in temperature is the linear expansion coefficient. Generally, the average expansion coefficient is used to express the linear expansion characteristics of a material in a certain temperature range.

Like other materials, PVC U pipe has the characteristics of thermal expansion and contraction. The coefficient of linear expansion also changes with temperature. When the temperature increases, the coefficient of linear expansion also increases. At room temperature, the coefficient of linear expansion of PVC U pipes is much larger than that of metal pipes, and is generally nearly 6 times larger. Therefore, when piping, especially when using the adhesive connection method, the expansion and contraction of PVCU must not be ignored.

It can be known from the performance comparison of the above PVC U water supply pipe and metal pipe that in order to ensure that the mechanical properties of the PVC U water supply pipe are in the best or good condition, the lower limit of the use temperature of the PVC U water supply pipe should be limited to 0 ° C.

2 The reasons for the mechanical damage of PVCU water supply pipes and the prevention of the mechanical damage of general plastic pipes are related to the size of the load, the temperature and the length of the load. The correct use of plastic pressure pipes, including PVC U water supply pipes, must take into account the relationship between the temperature and the static pressure of the medium in the pipe and the failure time of the pipe. The destruction process of PVC U water supply pipe, that is, the creep and destruction behavior of PVC U water supply pipe under time under the action of stress and temperature. The pressure-bearing process of PVC U water supply pipe, that is, the polymer material mainly made of PVC U resin, under the pressure, continuously produces the process of creep and relaxation, that is, its mechanical properties continue to decline. Therefore, the reasons for the mechanical damage of PVC U water supply pipes are various. According to the process of PVC U water supply pipe quality control chain, in addition to raw material quality and product manufacturing reasons, there are many factors such as whether the pipeline design is reasonable, the construction and installation are correct, the transportation and storage meet the national standards, and whether the product application meets the specifications. The decision should be based on a comprehensive analysis and determination of many specific conditions of production and application.

2.1 The quality of the PVCU resin compound has a decisive effect on the pressure resistance of PVCU water supply pipes. In recent years, the International Technical Committee for Standardization of Plastic Products ISO / TC138 sub-technical committee and the National Technical Committee for Standardization of Plastic Products TC48 / SC3 technical committee have formulated or revised There is an obvious change in the standard of plastic pipe products, which is to increase the chapters on material requirements. For example, ISO 4422.2: 1996 PVC U pipe standard for water supply, the minimum required strength (MRS) of the pipe material is not less than 25 MPa in the section of raw materials, that is, the grade of the material grade of the special material for PVCU water supply pipe should be able to reach PVCU 250 ( The definition of MRS is: the stress value obtained by rounding the value of the lower limit of σ at 20 ° C and 50 years, and the lower limit of σ refers to the long-term hydrostatic pressure when the pipe made of the material is at 20 ℃ and 50 years of water pressure with a confidence of 97.5 Strength, it is an evaluation of the stress value of the material). The M RS and the lower confidence limit σ of plastic material processing and application materials are the design basis for the design stress, design pressure, pressure level and wall thickness of plastic pipe products. The requirements of the PVCU water supply pipe standards of various countries on the mechanical properties of raw materials are shown in Table 3.

Project (draft) long-term strength or design basis / MPa no design stress / MPa As can be seen from Table 3, PVC U pipe standards in developed countries have requirements for the long-term strength of the material, and the K value of PVC resin (characterization of polymer molecules) The index of quality or degree of polymerization, K value or polymerization degree is large, the material's long-term strength is large, but the processing performance is also difficult to control.) When it is greater than 64, its long-term strength can guarantee the product's mechanical properties. PVC resin and its pipe products have been used for 50 years. It is not necessary to use extrapolation method to determine its long-term static pressure strength under certain temperature and stress conditions according to ISO / DIS 9080: 1997 standards like PP and other new materials. Based on this, in the section on repairing materials, China's TC48 / SC3 added new requirements for PVC U resin K value greater than 64 in comparison with the PVC U pipe standards of developed countries, but has not yet adopted its M RS value requirements. At the same time, compared with the ISO 4422.2: 1996 standard, the requirements for PVC U compound ingredients are added, that is, the compound resin should be mainly based on PVC resin, and the additives necessary for the production of pipes in this part should be added. The addition of any additives should not damage the product. The processing and bonding properties and other properties that affect this part. In the long-term production and application, it is found that the change of any additive in the compounding material, such as the test of a certain component substitute, or the change of the quality ratio of a certain component, and the use of the same specification but different manufacturers of PVC resin, are all The processing performance and mechanical properties of the pipe are affected, and a type inspection should be carried out to verify whether the finished product meets the product standard. Therefore, the quality of any additive of PVC resin and its compounding ingredients is the decisive factor affecting the mechanical properties of PVCU water supply pipes. There is no doubt that the quality control of PVC U pipes starts with the quality control of raw materials, and the focus is on the quality level of raw materials. Without qualified PVC resin and compounding additives, it is impossible to produce qualified PVCU water supply pipes that meet the standards. The insufficient long-term strength of the raw materials is the main safety hazard that causes rapid cracking or creep cracking of PVC U pipes.

2.2 The control of the processing process of PVCU water supply pipes is a link that cannot be ignored. The quality of PVCU water supply pipes is also closely related to the rationality of the processing process and the advantages and disadvantages of the plasticization process. Extrusion temperature and vacuum control, the performance of the pipe extrusion equipment and whether it is in good condition, etc., all affect the mechanical properties of the pipe, such as the hydraulic test and the drop weight impact test.

The large-scale PVC U pipe enterprise has not only established a perfect production management system, but also passed the ISO 9001: 2000 quality management system certification, starting from raw material procurement, inspection and supplier evaluation, to production process control, equipment management, monitoring, In the whole process of measurement and inspection, all links have traceable system management, and since it was put into production, PVC resin with K value greater than 64 and qualified additives have been used. It is safe to say that these companies are fully confident that their products comply with national standards.

2.3 Reasonable pipeline engineering design is the premise to ensure its long-term service life. CECS 17: 2000 "Technical Specifications for Buried Hard Polyvinyl Chloride Water Supply Pipeline Engineering" organized by the Ministry of Construction aims to regulate the rational use of PVCU pipes in buried water supply pipes. It is suitable for the design, construction and acceptance of PVCU pipe engineering for buried water supply under general geological conditions.

The design of the water supply pipeline system should be carried out corresponding hydraulic calculations, structural calculations, and the use of supporting accessories and structures and other relevant design content, etc., to determine the reasonable service life and temperature, pipe laying method and connection form.

2.3.1 Design the relationship curve between the reasonable service life and the change of service temperature and time.

The long-term hydrostatic strength decreases with the increase of load time, but the rate of decrease decreases with the increase of time, and it will hardly decrease after about 1 000 h. The long-term hydrostatic strength value at room temperature is about 50% of the instantaneous tensile strength value shown in Figure 1, which is the basis for the general design of PVC U piping systems. The design life of PVC U pipe is set according to the relationship curve of Figure 4 for processing and application of polyvinyl chloride in 2005 for 50 years. The relationship curve in Fig. 4 is similar to the results of the long-term hydrostatic strength prediction of PVC U pipe using the ISO 9080 extrapolation method.

GB / T 10002.1 1996 water supply pipe standard stipulates: the water temperature does not exceed 45 ℃, and the minimum temperature lower limit of the temperature-pressure reduction factor is set to 0 ℃. The general rules of CECS 17: 2000 technical regulations stipulate that it is applicable to PVC U pipes for water distribution and distribution with water temperature not lower than 0 ℃ and not higher than 45 ℃. Therefore, all aspects of PVC U pipe production and application, such as production, transportation, handling, construction, installation and use, should be kept above 0 ℃. If you need to construct below 0 ℃, you must be careful to prevent cracking caused by pressure damage .

2.3.2 Calculate the axial thrust and design the concrete to consolidate the protective measures. The horizontal or vertical change of the pipeline, the diameter reduction, the tee joint, the valve and the fire hydrant should calculate the axial thrust according to the pressure in the pipe and design the corresponding Concrete protection measures to prevent the mechanical damage caused by the pipe. Figure 5 is the concrete consolidation method of PVC U pipe bend pipe plane piping, and Figure 6 is the axial thrust calculation diagram.

The thrust generated by the water pressure at the elbow is calculated by equation (3): where: F is the thrust generated at the elbow, M Pa d is the inner diameter of the pipe, and cm θ is the bending angle of the elbow, °.

The concrete consolidation protection area refers to the area outside the pressure direction in Fig. 5, and its area size varies according to the water pressure level and the soil quality, and is calculated according to formula (4). Taking into account the impact of the residual pressure of the water hammer, 1.5 times the water pressure is used as the design internal water pressure.

In the formula: S is the concrete protection area, cm F is the thrust generated at the elbow, M Pa K is the safety support coefficient of the backfill (see Table 4).

Soil horizontal and vertical direction soft silt wet silt soft clay dense clay wet sand and mud mixed wet sand coarse sand pebbles mixed with coarse stone clay pebbles or coarse sand solidified layer 2.3.3 calculation of longitudinal deformation, design of expansion joints Reasonable configuration of PVC U pipe straight piping needs to consider expansion and contraction. The socket type live bushing is adopted because the live bushing socket is a type of expansion joint, and the longitudinal temperature deformation of the pipeline may not be calculated. For pipes with adhesive joints, the longitudinal deformation caused by the temperature difference between the internal and external media should be calculated according to the temperature changes during laying and use.

The longitudinal deformation of the pipeline caused by the temperature difference can be calculated according to formula (5): where: ΔL is the longitudinal deformation caused by the temperature difference, mm Δt is the temperature difference between the internal and external media during laying and use, and ℃ is not set in the application. Examples of expansion joints and thermal expansion and contraction that produce stress damage to pipes or fittings occur from time to time. Generally speaking, when the length of the straight line of the pipe with adhesive joint exceeds 50 m, an expansion joint should be installed. The number of expansion joints should be designed according to the calculation result of formula (5).

2.3.4 Calculate the water hammer pressure to prevent the mechanical damage of the pipe caused by pulses and cyclic loads. The reason for the water hammer is that water is incompressible and air is compressible. When the pump is turned on or off, the flow state of water and gas changes suddenly when it is processed and applied. , Causing the water pressure in the pipe to fluctuate and rise rapidly, up to several times or even dozens of times the normal water pressure. Water hammer pressure caused by improper design or operation can cause huge mechanical damage to PVCU pipes.

The water hammer pressure can be calculated according to formula (6) and formula (7): where: ΔF is the water hammer pressure, MPaΔv is the average flow rate of water in the pipe, m / s α is the pressure wave return velocity, and m / sr is the water density , Kg / mk is the bulk modulus of water, 2 000 M Pa d is the inner diameter of the pipe, m E is the elastic modulus of the pipe wall, M Pa e is the thickness of the pipe wall, m.

Commonly used measures to eliminate water hammer are: in the pipeline design, set the exhaust valve at the highest point according to the terrain elevation, install the water hammer eliminator on the terrain or the complex use conditions, and switch the pump and valve according to the correct operating specifications.

For the design of water supply pipes for long-distance water delivery, large flow rates, agricultural irrigation, mining areas and golf courses supplied by pumps, especially in the case of undulating hillsides, complex terrain and large drop, it is necessary to carry out head loss and The water hammer pressure calculation ensures that the pump head and pipe wall thickness are accurately selected to effectively prevent the mechanical damage caused by the pulse, cyclic load and water hammer during operation. In general, water hammer elimination measures are provided on the water supply pipeline. Therefore, the general water supply pipeline design only needs to consider the possible residual pressure of the water hammer.

The design requirement is that the design pressure of the water supply pipeline is the sum of the working pressure of the water in the pipe and the residual pressure of the water hammer, and is considered based on the working pressure of 1.5 times.

It can be seen from equation (7) that the pressure of the water hammer is mainly determined by the flow rate and the ratio of the tube inner diameter to the wall thickness d / e (or SDR standard size ratio), that is, the greater the flow rate, the smaller the d / e (or SDR) ratio , The greater the water hammer pressure is, conversely, the greater the d / e (or SDR) ratio, the smaller the water hammer pressure. Check the relevant graphs of CECS 17: 2000 to know: the nominal pressure p series, when the water flow rate is 0.8 m / s, the calculated water hammer pressure increase value is 0.31 MPa, which is about 31 of the static pressure in the pipe. This shows that the small-diameter water supply pipe It is often more susceptible to mechanical damage than medium and large diameter water supply pipes, so special attention should be paid when using small diameter water supply pipes.

2.4 Perform correct installation and construction in strict accordance with the technical regulations. The technical requirements of the GB / T 10002.1 1996 water supply pipe standard are as follows: the inner and outer surfaces of the pipe should be smooth, without obvious scratches, dents, visible impurities and other effects that meet the requirements of this part. Surface defects. This is not only a requirement for product quality, but also a requirement for installation, construction, transportation, and storage quality. Obvious scratches, depressions and surface defects caused by external forces due to improper construction, transportation and storage can all cause stress concentration and cause pipe rupture (that is, due to the scoring effect described in Section 1.3 of this article).

2.4.1 Pipe trench excavation should be straight and the bottom should be provided with suitable construction space to facilitate the connection and laying of pipes and fittings, to prevent the pipes from bending due to irregular pipe trenches, and to prevent the pipes and joints from being broken due to bending stress. The cross-section and dimensions of the trench excavation should meet the requirements shown in Figure 7 and Table 5.

Excavation of pipe trenches with a nominal outer diameter of D / mm below 140 and above 355 must be carried out in accordance with the design of the pipeline. The construction must be straight and smooth, without any deflections or twists. If the trench is dug to a specified depth, if a gravel layer or a hard object is found, a depth of 10 cm needs to be added to facilitate sand filling before piping, and then place the PVC U pipe. It is necessary to ensure that the bottom of the pipe trench is on the undisturbed undisturbed soil, or after back excavation and backfilling treatment, the compaction coefficient reaches the design requirements of the backfill soil layer. For high and soft soils, the bottom of the pipe trench needs to be backfilled and tamped. If necessary, proper foundation treatment should be carried out to prevent the pipe trench from falling and causing the pipe to sink, causing the pipe to bend and the joint to be stretched, resulting in additional The stress causes the pipe to rupture.

2.4.2 The backfill soil is divided into layers. The purpose of backfill soil compaction is twofold: â‘  The friction between the backfill soil around the buried pipeline and the pipe wall exerts a resistance to the expansion and contraction of the pipeline. This resistance can balance part of the temperature difference. The generated longitudinal tension â‘¡ ensures the foundation is level, and prevents the vibration and movement caused by the change of flow velocity during the pipeline water conveyance operation. In both cases, it is possible to process and apply PVC. In 2005, due to improper filling and compaction, stress concentration occurred, which caused the pipe to rupture.

According to the CECS 17: 2000 technical regulations, when backfilling the soil, you must first backfill the sand with a support angle of 90 ° below the pipe, and then use sand or qualified raw soil to backfill both sides of the pipeline at the same time. The thickness is 10 ~ 20 cm. After tamping this layer, fill it with another layer until it is backfilled to the minimum height of the backfill above the top of the pipe before the pressure test. This height is generally not lower than 0.3 m. During the backfill process, When pouring or ramming soil, the pipe and its accessories must not be damaged, and there should be no pipe displacement and rotation, and no large stones or hard objects should be placed under the pipe (this is easy to form the so-called point load, which is the pipe The common cause of rupture due to stress concentration). See Figure 7 for the requirements of the backfill soil quality and compaction coefficient of each part of the pipe trench.

2.4.3 Pipeline laying should avoid excessive external pressure load and excessive bending. The external pressure load acting on the pipeline should include vertical earth pressure, ground vehicle load and accumulated load. In order to prevent the external deformation caused by the vertical deformation greater than 5 of the pipe diameter and causing the pipe to break, it is stipulated that the buried depth of the pipe top under the road should not be less than 1 m, and the buried depth of the pipe top should not be less than 0.75 m when it is greater than 63 under the sidewalk. At D 63, the buried depth of the pipe top should not be less than 0.5 m. In seasonal and permanent frozen soil layers, the buried depth of the pipe top should be below the freezing line. When crossing roads and rivers, metal or concrete sheath pipes must be added. In addition, when the pipeline laying needs to be bent, the bending radius should not be less than 300 times the outer diameter of the pipe, and the bending angle should not be greater than 2 °. When the construction environment temperature is less than 5 ℃, elastic bending is not allowed. The above regulations are all measures to prevent creep cracking caused by long-term stress concentration at the bend of the pipe.

2.4.4 To ensure the bonding quality at low temperature The main component of PVC U pipe adhesive is the solvent of PVC resin. After the volatile solvent volatilizes, the surface of the socket of the PVC U pipe socket forms a bond between molecules and becomes a solid one. According to the application experience, two points should be noted: â‘  The amount of adhesive should be appropriate. If the amount is small, the sealing and strength cannot be guaranteed. However, if the amount is large, it will overflow the socket and flow into the inner and outer walls of the pipe, corroding the pipe and causing the surface of the pipe wall Irregular recesses â‘¡ especially when bonding at low temperature, the volatility of the solvent is significantly reduced, and the overflowing adhesive of the socket can make the corrosion depth more serious. In both cases, the thickness will be significantly reduced or the strength of the depression will be reduced, and it will easily break from the place when the water is boosted.

2.4.5 The hydraulic test of the pipeline system should be strictly carried out. The hydraulic test is carried out after the pipeline system is installed, which is the most comprehensive and effective inspection of the quality of all links including pipes, fittings and design, installation, so this important step cannot be ignored And directly into the application. The hydraulic pressure test shall strictly prevent the test pressure from exceeding the nominal pressure of the pipes and fittings. Generally, two methods are used: pump water injection or valve water injection from the original system, but both need to be emptied of air and then pressure tested. Usually the water test pressure is 1 M Pa (or 1.5 times the water pressure used) and maintained for 1 h. For long-distance pipelines, the pressure test should be sectioned, and the length of the sectioned pressure test pipeline should be 500 m. In addition to the pipeline compressive strength test, a rigorous test is also conducted to determine whether it is qualified or not based on the allowable water leakage and pressure drop specified in the CECS 17: 2000 technical regulations.

2.5 Strictly transport and store in accordance with product standards GB / T 10002.1 1996 The water supply pipe product standard has strict regulations on transportation and storage: the pipe shall not be exposed, stained, stressed, thrown or damaged during transportation. Pipe storage must be done: the pipes should be stacked neatly, and the socket parts should be staggered to avoid extrusion deformation. The pipes shall not be exposed to light, not less than 1 m away from the heat source, and the stacking height shall not exceed 2 m. The CECS 17: 2000 technical regulations also stipulate that pipes and fittings should be handled lightly, arranged neatly, and avoided during transportation, loading and unloading, and promotion. Grease. Do not subject to violent impact and sharp objects, throw, drop, roll, drag. These hard rules are based on avoiding surface damage caused by external forces that can cause stress concentration or accelerate creep at high temperatures to reduce strength. Take Huaya (Dongying) Plastic Co., Ltd. as an example. When loading pipes, blanket pads are used as mats to move the pipes. To prevent the pipes from being damaged, a special transport vehicle is used. In order to prevent the vehicle bumps and vibrations from rubbing and damaging the pipes.

3 Conclusion Many factors of the mechanical destruction of PVC U pipes are sometimes not enough to be affected. However, if two or more factors are superimposed together, it is possible that the pipe will be broken at the weakest place first, and then it will affect the correlation. Rupture of scope. An example of winter construction is enough to illustrate this problem: the elbow of the highest part of the water supply pipeline is not well bonded, because of the low temperature, it is difficult to consolidate the thrust concrete, and there is no exhaust valve installed at the highest part. As soon as the valve is opened, the elbow ruptures due to water hammer pressure, and then the riser ruptures. Therefore, even a domestic first-class enterprise should highly strengthen the all-round and full-process application service to users, so that they can strictly abide by the standards and regulations in the design, handling, construction, installation and use of PVC U pipe In this way, the application of PVC U pipe, which is one of the new chemical building materials promotion products, can further promote the advantages and avoid the shortcomings, and obtain more healthy and substantial development.

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