Cause analysis of leakage of the hottest nano anti

2022-08-22
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Analysis of leakage causes of nano antibacterial stainless steel plastic composite pipe

nano antibacterial stainless steel plastic composite pipe is a new type of pipe. The pipe has innovations in product design, technical application of nano antibacterial materials and pipe connection. It is made of nano antibacterial agent and relevant antibacterial carrier mixed with thermoplastic to make antibacterial masterbatch as the inner antibacterial layer, and the thermoplastic is extruded into the inner plastic pipe, It is coated with hot melt adhesive, then coated with stainless steel on the outer layer, and made after welding, grinding and polishing. It is commonly used for indoor water supply, heating and gas supply Because the outer layer of this kind of pipe is covered with stainless steel, it is neither rusty nor corrosion-resistant. The inner layer of plastic pipe slows down the aging rate because it is not in direct contact with the sun and the atmosphere

in 2005, a large number of leaks occurred in the nano antibacterial stainless steel plastic composite pipes for water supply in six 32 story residential buildings in a residential area during the pressure test, and it developed to a serious accident of leakage as long as there is water in the pipes in the absence of pressure. In order to find out the cause of the leakage of the new water supply pipeline, the author carried out a series of inspections to determine the cause of the leakage

1 overview of pipes

the outermost layer of this batch of composite pipes is 0.8~1 mm stainless steel strip; The second layer is non uniformly distributed hot melt adhesive between the stainless steel strip and the third layer; The third layer is thermoplastic high-density polyethylene with a thickness range of 5~10mm; The fourth layer is a nano antibacterial layer with a thickness of about 0.5 mm. The pipe fittings are injection molded parts, and the connection between the pipe fittings and the straight pipe section adopts the inner plastic hot-melt method. The steel sample with a certain hardness should be tested for several times, and the outer layer is stainless steel compression connection. The pipeline is a water supply pipeline for high-rise buildings. The design pressure of the pipeline is 1.6 MPa and the hydrostatic test pressure is 2 MPa

2 macro morphological analysis

samples were taken randomly, that is, samples of dn160, dn90 pipes, elbows, flange connection accessories, dn110, dn75 pipes were taken respectively according to diameter and connection mode. It was found that the main part of leakage was at the flange connection. Before all samples are intercepted, it is important to check whether there are abnormal geometric dimensions and signs of permanent plastic deformation. Through measurement, no obvious abnormal deformation and plastic deformation are found

in order to find out the cause of leakage, the 150~200 mm long pipe near the flange is cut off together with the flange for inspection and analysis. Randomly select 13 pipe segments with flanges from 12 pairs of 24 short joints with flanges, including 8 for dn160 and 5 for dn90. Slowly cut the pipe segments with flanges with a hand saw, and plane the pipe segments with flanges along the direction of the pipe axis into 2~8 segments: the small diameter pipe has at least 2 segments, most of which are up to 4 segments, and the large diameter pipe has at least 4 segments, most of which are up to 8 segments, for further observation

throughout the samples taken, it is found that there are mainly the following problems:

(1) micro cracks if the tested material or parts are relatively coarse and penetrating cracks (see Figure LA). This phenomenon exists in 10 of the 13 pipe section samples, and the large-diameter pipe is more serious. Cracks or penetrating cracks are found in the planing parts of 8 dn160 pipe sections, accounting for 100% of the planing large-diameter pipe. Small diameter tubes account for a small proportion, and microcracks are found in 2 of the 5 samples, accounting for 40%

(2) injection molding or hot melt bonding defects (see Figure 1b). It mainly occurs between the injection flange ring or the flange inserted into the inner section of the pipe and the adhesive layer on the inner surface of the pipe, which is manifested as annular holes or interlayer gaps

(3) poor adhesion between stainless steel strip and plastic pipe (see Figure LC). The main performance is that the distribution of hot melt adhesive is seriously uneven, resulting in a large area of glue shortage, and there is a large gap between stainless steel and the outer wall of plastic pipe

(4) the flange and the pipe are cured after hot-melt bonding, and the non smooth connection formed between the flange and the inner wall of the pipe protrudes seriously (see Figure 1D), resulting in the discontinuity of geometry, which not only increases the fluid resistance in the pipe, but also causes the sudden change of the pipe wall thickness, which is one of the important factors causing stress concentration, but also the origin of all observed cracks

Figure 1 macro morphology analysis

3 finite element stress analysis of composite plastic pipe

the plastic layer material of composite plastic pipe is HDPE, and its elastic modulus E is 413.7~1034 MPa, when 20 ℃, e ≥ 800 MPa. In finite element analysis, e is 800 MPa and Poisson's ratio is 0.3. The tensile strength of the material ranges from 21.4 to 37.9 MPa

use the finite element analysis software ANSYS to conduct stress analysis, and ignore the influence of the edge effect at both ends of the straight pipe when establishing the model. Generally, the stress of the straight pipe lining is mainly composed of two parts: one is the stress caused by the internal pressure of the pipe fitting during operation; The second is the bonding force between the lining and the steel pipe wall. However, due to the serious poor connection between stainless steel and plastic layer, in order to better simulate the actual situation, only the effect of internal pressure on pipe fittings is considered. At the same time, compared with the pipe diameter, the pipe length can be regarded as infinite without considering the friction between stainless steel and plastic layers, and the quality of composite pipe is ignored. Solid 45 element in ANSYS software is selected to simulate the plastic deformation element (see Figure 2)

Figure 2 Structural schematic passing division

the model is manually divided into grids, loaded and solved, and the stress and displacement results are as follows: the maximum radial stress is at the plastic pipe wall thickness discontinuity a (see Figure 2), and its value is 11.623 MPa; The maximum axial stress is at the plastic pipe wall thickness discontinuity a, and its value is 7.247mpa; The maximum circumferential stress is 13.505 MPa, which occurs at the thin wall of the plastic pipe, and the circumferential stress at the discontinuous wall thickness is greater than 10.898mpa; The maximum overall equivalent stress is 14.51 MPa at the thin wall of the plastic pipe, and the maximum equivalent stress at the discontinuous wall thickness of the plastic pipe is greater than 10.767 MPa; The maximum deformation is 1.565 mm at the thin wall of the plastic pipe, the maximum radial deformation is 1.062 mm, and the maximum axial deformation is 1.555mm

from the above analysis results, it can be seen that without the strengthening effect of the stainless steel layer, the radial expansion deformation of the pipe is relatively obvious, and the maximum radial deformation can reach 1.062 mm, up to 20% of the total thickness of the plastic layer; At the same time, the stress concentration in the discontinuous area of plastic layer wall thickness is very prominent, and the maximum radial stress is 11.623 MPa

4 cause analysis of pipeline leakage

through the macro analysis of the planed samples and the ANSYS analysis of the pipeline, the author believes that the bonding part between the end of the flange inserted into the pipe and the inner wall of the pipe is the weakest part of the whole flange connection area, where cracks occur under the hydrostatic test pressure, and some cracks gradually expand and penetrate the pipe wall. After the crack penetrates the pipe wall, due to the serious disconnection between the outer stainless steel strip and the plastic pipe layer, the water in the pipe can easily penetrate the inner wall of the outer stainless steel strip along the inner wall after the tensile strength and elongation of the main test data penetrate through the plastic pipe, and then penetrate the flange ring along the inner wall, leaking out of the ring gap of the pressure ring. The leakage channel formed above can cause leakage as long as there is a little pressure, which is the reason for leakage as soon as there is water

the main causes of cracks are:

(1) sudden changes in geometric dimensions make the stress concentration phenomenon prominent under hydrostatic test and working pressure. The size of the pipe wall is more than twice that of this part, and the geometric shape is seriously discontinuous. From the ANSYS analysis, we can also know that the stress is particularly large here, reaching the allowable stress

(2) defects such as injection holes exist in the material, resulting in weak links

(3) the heating and cooling process of the material is complex, the residual stress level is high, and the cracks preferentially occur and expand at the bonding part between the end of the flange inserted pipe and the inner wall of the pipe. When the flange pipe joint is inserted into the inner wall of the pipe, due to the heating effect, the melt in the inner wall of the pipe migrates to the top of the flange short pipe under the action of surface tension, weakening the wall thickness of the main pipe at this position, and the wall thickness of the pipe at this transition position is the smallest. In the cooling stage, due to the slow cooling of the flange short pipe, the high cooling speed of the pipe away from the flange short pipe and the serious surface shrinkage, a high level of residual stress is generated on the inner surface of the pipe in this area. The residual stress is in the state of tensile stress along the axial direction of the pipe, which is superimposed with the working stress or the water pressure test stress, resulting in the maximum stress of the pipe at the stress concentration part

the main reason why the crack preferentially originates from the bonding part is that the pipe needs to be replaced in case of damage. The wall thickness of the cable at the transition part is the smallest, and the overall stress level after the superposition of working stress and residual stress exceeds the yield limit of the material. It is found that all pipe cracks with cracks originate from this part, regardless of the finite element calculation or the investigation of the planing pipe

5 suggestions for improvement

in view of the causes of the above pipe leakage, the author suggests the following improvement measures:

5.1 improve the design of the flange connection structure to avoid the sudden change of wall thickness structure

although the method of using hot-melt flange connection is simple and convenient, and because the pipes and fittings are the same homogeneous material, the connection quality is more reliable, but according to the above analysis, If this connection method is not handled well, it will cause structural mutation, and because the material needs to be hot-melt for many times, it will cause weak links at the flange connection. Therefore, it is suggested to improve the flange connection method, It can be improved in the following two ways:

(1) the hot-melt flange connection method is changed to the stainless steel sleeve flange connection method (as shown in Figure 3) provided in the engineering technical specification for ultra thin wall stainless steel plastic composite pipe for building water supply (cecs135:2002), so as to avoid the structure of sudden change in wall thickness.

Figure 3 stainless steel sleeve flange connection

(2) for small pipe fittings (DN ≤ 32) , the flange connection is changed to the ferrule connection. This connection method has good tightness, easy installation and disassembly. The snap ring is equivalent to adding a movable joint on the pipeline. It is different from the flange sealing by pressing, but by its own back pressure, which prolongs the service life of the product, and its quality can be reduced by 40%~50% compared with the flange connection quality

5.2 reasonable selection of pipe materials and welding parameters

reasonable selection of pipe materials and welding parameters to improve the ability of materials to resist the influence of repeated heating process, especially the flange joints need multiple hot melts and cold cuts, and the materials with better thermal stability in high density polyethylene (HPDE) should be selected. These mainly depend on the glass transition temperature Tg, viscous flow temperature TF, crystal melting temperature Tm and thermal decomposition temperature TD. When the crystal melting temperature of crystalline plastics is above TM or the viscosity flow temperature of non crystalline plastics is above TF, the solid material melts into viscous fluid. Therefore, the temperature zone between TM or TF and TD is the temperature window for thermoplastic processing. The welding of plastics should be carried out in this temperature window. The temperature and the width of the window directly affect the quality of hot melt connection of plastics. The thermal stability of the material can be judged from whether there is material accumulation at the pipe connection after hot melting, whether the accumulation is uniform, and whether a uniform flange is formed. If it is uneven, or even there is a gap on one side, it means that the welding is not ideal, and it is difficult to ensure that the joint is watertight, so it should be reconnected in time

5.3 optimize the flange heating and cooling process

optimize the flange heating and cooling process, reduce the residual stress level, control the heating temperature and stabilize the product quality, which can be compensated through mold design and processing process adjustment

6 conclusion

nano stainless steel plastic composite pipe combines the advantages of stainless steel and HPDE in water supply pipeline, and has been favored by more and more industries. If we can give better play to the advantages of the composite pipe, improve its raw material selection, production process quality control, structural design and construction technology, this new type of pipe will be better applied. (end)

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