At present, compared with traditional non-plastic pipes, biaxially oriented PVC (PVC-O) pipes have many advantages such as high cost performance, corrosion resistance, long service life, convenient installation and maintenance, low friction resistance coefficient, light weight and high strength, fatigue resistance, impact resistance, earthquake resistance, settlement resistance, leakage prevention, scaling resistance, aging resistance, rigidity and flexibility, ecological protection, and hygiene etc. The pipes have been widely used in many places.
Generally, the pipe socket adopts R type with elastic sealing ring for flexible connection. This article focuses on the development history of PVC-O pipes and its relevant technical indicators and characteristics. As well as the prefabricated sealing ring connection technology when the orientation factor reaches class 400-500, that is, before the pipe leaves the factory, the sealing ring has been prefabricated inside socket of PVC-O pipe. This type of sealing ring contains a steel skeleton inside. The steel skeleton is fully covered by the rubber ring and isolated from water. The steel skeleton sealing ring can only be removed from the socket by destruction, so as to achieve high installation efficiency, and can improve the resistance to foundation settlement, deflection angle, and displacement. This type of sealing also has better and long-term sealing performance.
1 Development of PVC-O pipes
According to relevant reports, PVC-O pipes were first developed by the United Kingdom in 1970. In the early days, they all adopted the “off-line” processing technology (two-step processing method). With the accumulation of experience and the advancement of technology, the technology of orienting during the extrusion process was later developed to achieve continuous production of PVC-O pipes. The emergence of the “online” (one-step) biaxial orientation production process has greatly improved production efficiency, reduced manufacturing costs, and enhanced the competitiveness of PVC-O against other pipes. At present, PVC-O pipes have been used abroad for many years, and the product standards for PVC-O have been released. The International Organization for Standardization has also published the PVC-O standard ISO/DIS 16422. In China, the government has also issued the national standard GB/T 41422 for PVC-O.
2 Processing technology and performance of PVC-O pipes
2.1 Biaxial stretching process of PVC-U pipe
PVC pipes are prone to partial uniaxial stretching and orientation during the forming process. Although it increases the axial strength of the pipe through partial stretching and orientation, the radial strength of the pipe, i.e. the circumferential strength, does not increase. The ideal stretching and orientation should be biaxial stretching and orientation. Through biaxial stretching and orientation, both axial and radial (circuferential) strength of the pipe increase. Because of the big improvement in pipe material strength, we can reduce the wall thickness and improve the hydraulic bursting strength of the pipe to save materials, reduce product costs, achieve energy conservation and emission reduction. Therefore, PVC-O pipes are called the “king of cost performance”.
2.2 Mechanical properties of PVC-O
The processing technology of PVC-O is to stretch the PVC-U pipe in both directions, so that the tensile strength and toughness of the pipe in the axial and circumferential directions are substantially improved. The thin layered structure formed by molecular orientation can prevent the expansion of cracks and has excellent impact resistance and fatigue resistance. Compared with ordinary PVC-U pipes, the tensile strength can be increased from 40-48MPa to 49-80MPa, or even higher; and the hydraulic ring stress can be increased from 36-38MPa to 41-65MPa, or even higher; and the impact strength can be increased by 200-300%.
2.3 Material grade
| Material grade | 315 | 355 | 400 | 450 | 500 | |||||||||||
| MRSa/MPa | 31.5 | 35.5 | 40 | 45 | 50 | |||||||||||
| C | 1.6 | 2 | 1.6 | 2 | 1.6 | 2 | 1.4 | 1.6 | 2 | 1.4 | 1.6 | 2 | ||||
| σs/MPa | 20 | 16 | 22 | 18 | 25 | 20 | 32 | 28 | 23 | 36 | 32 | 25 | ||||
| aCan choose R20 or higher grade MRS as long as it meets the range specified in GB/T 321. | ||||||||||||||||
Table 1 Material Grade
The higher circumferential and axial orientation of PVC-O pipes, the higher grade of the material, and the higher tensile strength, hydraulic strength, and impact strength. Conversely, the lower orientation, the lower material grade and the lower performance.
2.4 Falling weight impact test
The falling weight impact test of the prefabricated sealing ring pipe socket is carried out at 0℃ according to ISO/DIS 16422 or GB/T 14152. The impact hammer head radius of the test is 12.5mm, the drop hammer mass is shown in Table 2, the drop hammer height is 2m. The true impact rate (TIR) should not exceed 10%.
| Nominal outer diameter (dn/mm) | Falling weight mass (kg) |
| 63 | 4 |
| 75 | 5 |
| 90 | 5 |
| 110 | 6.3 |
| 125 | 6.3 |
| 140 | 8 |
| 160 | 8 |
| 180 | 10 |
| 200 | 10 |
| ≥225 | 12.5 |
| Note: Drop weight is determined based on pipe MRS 450 and 500. | |
Table 2 Drop Hammer Mass
2.5 Hydraulic ring stress
| MRS | 315 | 355 | 400 | 450 | 500 |
| Test hydraulic ring stress (MPa) | 41 | 46 | 52 | 60 | 65 |
Table 3 Hydraulic Ring Stress
3 Common PVC-O pipe connection technology
3.1 Traditional PVC-O pipe interface
Since PVC-U, PVC-M, and PVC-UH water supply and drainage pipes have no orientation, there is no necking phenomenon but even expansion phenomenon when pipe temperature changes from glass transition to high elastic state, this type of pipe can be made into prefabricated sealing rings.
Picture 1 PVC-O Pipe R Type Socket
Normally, when the orientation factor of PVC-O pipe reaches class 400 grade or above, the deorientation begins at 80℃ in the Vicat softening point and glass transition zone, and the deorientation is completed when the temperature reaches 130-150℃. The higher the temperature, the faster the deorientation speed. Therefore, after the pipe is produced, when it is heated to make the R socket, the pipe temperature has reached or exceeded 150℃. In fact, PVC-O pipe will change to the original PVC-U pipe at high temperature, so PVC-O pipe belling process is different from traditional PVC-U belling process. Now PVC-O pipe R belling process is mature, but need to insert rubber ring manually under this process technology. And it is impossible to insert steel skeleton sealing ring manually. So it is extremely difficult to make PVC-O pipe socket with prefabricated sealing ring. It is necessary to change the PVC-O pipe processing technology or expansion technology to realize the PVC-O pipe with prefabricated sealing socket (with steel skeleton sealing ring inside socket).
3.2 New prefabricated sealing ring socket for PVC-O pipe
This topic shows when PVC-O pipe is formed, the prefabricated sealing socket is already with steel skeleton sealing ring inside, which is much different from normal PVC-O pipe socket needs to insert normal rubber ring manually.
Picture 2 PVC-O Prefabricated Sealing Ring
4 Verification of prefabricated sealing ring socket performance
4.1 Tensile strength changes at prefabricated sealing ring socket
Picture 3 Sample Preparation at PVC-O Socket
4.1.1 Circumferential tensile strength of prefabricated sealing ring socket before and after belling
| Name | Size | MRS | Tensile Strength Standard (MPa) | Circumferential tensile strength before belling (MPa) | Circumferential tensile strength after belling (MPa) (prefabricated sealing ring socket) |
| PVC-O pipe | Dn110×3.1mm | 400-450 | ≥48 | 68.9、67.34、68.5、68.8 | 75.62、75.61、77.05、77.03 |
Table 4 Comparison of Circumferential Tensile Strength Before and After Belling
After experiments, using special process technology to implement biaxially oriented PVC-O pipe prefabricated sealing ring technology, the tensile strength of the socket showed an upward trend. This indicates that there is secondary orientation occurred at the socket, which improved the circumferential compressive strength and tensile strength, proving that the circumferential orientation increased by about 12%.
4.1.2 Longitudinal tensile strength of prefabricated sealing ring socket before and after expansion
| Name | Size (mm) | MRS | Tensile Strength Standard (MPa) | Longitudinal tensile strength before belling (MPa) | Longitudinal tensile strength after belling (MPa) (prefabricated sealing ring socket) |
| PVC-O pipe | Dn110、160、200、315、400、500、630 | 400-450 | ≥48 | 52.44、51.13、52.23、52.18 | 49.07、49.94、49.2、49.3 |
Table 5 Comparison of Longitudinal Tensile Strength Before and After Belling
After experiments, for the biaxially oriented prefabricated sealing ring PVC-O pipe, the tensile strength at the socket showed a downward trend, but met the standard requirements, with a minimum value of 49.2MPa (standard ≥48MPa), indicating that the axial deorientation at the socket of the pipe after heating reached about 5%, which had no substantial effect on the use of the pipe.
4.2 Analysis of falling weight impact on prefabricated sealing ring socket before and after expansion
Picture 4 PVC-O Pipe and Socket are Cryogenically Frozen
| Name | Size (mm) | PVC-O pipe falling weight impact before belling | prefabricated sealing ring socket falling weight impact |
| PVC-O Pipe | Dn110 | 6.3kg, 0℃, 2m, no cracking after impact | 6.3kg,0℃，2m, no cracking after impact |
| Dn160 | 8.0kg, 0℃, 2m, no cracking after impact | 8.0kg,0℃，2m, no cracking after impact | |
| Dn200 | 10kg, 0℃, 2m, no cracking after impact | 10kg,0℃，2m, no cracking after impact | |
| Dn315-630 | 12.5kg, 0℃, 2m, no cracking after impact | 12.5kg,0℃，2m, no cracking after impact |
Table 6 Falling Weight Impact Test of Socket After Low Temperature Freezing
After testing, the biaxially oriented PVC-O water supply pipe produced by special process technology has reached the orientation grade of 400-450, and the falling weight impact performance of the prefabricated sealing ring socket at low temperature remains basically unchanged and remains qualified.
4.3 Explosion test of prefabricated sealing ring socket
Ultimate Damage of Prefabricated Sealing Ring Socket
Picture 9 Prefabricated Sealing Ring Socket Limit Burst Test
Conclusion: The connection between PVC-O pipe and prefabricated sealing ring socket passed the hydraulic test, and then the ultimate destructive test was performed. The pipe exploded but the pipe socket was not damaged. It was verified that the prefabricated sealing ring socket processed by special process technology have a compressive strength that exceeds the strength of the pipe itself.
5 Temperature sensitivity of PVC-O pipe prefabricated sealing ring socket
5.1 Temperature sensitivity of the prefabricated sealing ring socket outer diameter – change during deorientation
| Size (mm) | OD at 0℃ (mm) | OD at 23℃ (mm) | OD at 40℃ (mm) | OD at 50℃ (mm) | OD at 60℃ (mm) | OD at 70℃ (mm) |
| Dn110 | 117.5 | 117.5 | 117.5 | 117.5 | 117.5 | 117.5 |
Table 7 Temperature Sensitivity of PVC-O Pipe Socket
The purpose of this test is to verify whether second time orientation when doing the prefabricated sealing ring sealing socket will affect pipe usage in different seasons or under different temperatures. After the test, the outer diameter of the prefabricated sealing ring socket of PVC-O pipe did not change under the conditions of 0℃, 23℃, 40℃, 50℃, 60℃, and 70℃, indicating that the use of the prefabricated sealing ring pipe socket connection technology can be satisfied under the ambient temperature conditions required by the relevant standards.
Picture 10 The Effect of Temperature on Socket Outer Diameter
5.2 Temperature sensitivity of prefabricated sealing ring socket length
| Size (mm) | Length at 0℃ (mm) | Length at 23℃ (mm) | Length at 40℃ (mm) | Length at 50℃ (mm) | Length at 60℃ (mm) | Length at 70℃ (mm) |
| Dn110 | 200 | 200 | 200 | 200 | 200 | 200 |
| Dn160 | 200 | 200 | 200 | 200 | 200 | 200 |
| Dn200 | 200 | 200 | 200 | 200 | 200 | 200 |
| Dn315 | 200 | 200 | 200 | 200 | 200 | 200 |
| Dn400 | 200 | 200 | 200 | 200 | 200 | 200 |
Table 8 Temperature Sensitivity of Socket Length
After testing, the length of PVC-O prefabricated sealing ring socket did not change under the temperature of 0℃, 23℃, 40℃, 50℃, 60℃ and 70℃, indicating this type of PVC-O socket can be used under normal ambient temperature conditions required by relevant standards.
Picture 11 Prefabricated Sealing Ring Socket Before Deorientation
Picture 12 Prefabricated Sealing Ring Socket After Deorientation
Picture 13-1 Shape Before Deorientation
Picture 13-2 Shape of Incomplete Deorientation
Picture 13-2 Shape After Complete Deorientation
The above pictures mainly show the complete deorientation of PVC-O pipe under temperature 130-150℃. The outer diameter of the pipe changed from dn116.5mm to about 63mm. Since there is a steel skeleton in the prefabricated sealing ring and it support the socket, the outer diameter of the socket part is basically unchanged, see Picture 12.
Picture 14 Deorientation Under Different Temperatures (PVC-O Straight Pipe, No Socket)
6 General Characteristics of PVC-O Prefabricated Sealing Ring Pipes
- Installation efficiency is improved, reducing leakage caused by improper placement of the rubber ring during installation.
- Reduce the leakage caused by sealing ring being overturned or displaced during pipe insert.
- After long-term use, due to the effect of the steel skeleton structure inside prefabricated sealing ring, the pipe interface has better anti-settlement and anti-displacement capabilities.
- Due to the effect of the steel skeleton, when the pipe interface achieves a larger deflection angle, it is helpful to prevent leakage.
7 Installation and application characteristics and engineering cases of PVC-O pipe with prefabricated sealing ring socket
7.1 Installation characteristics
(1) The installation tools are simple and convenient, and are not affected by weather. Construction can be carried out in windy and rainy weather. The installation progress is 2-3 times that of PE pipes. Water can be supplied immediately after installation, which improves efficiency and safety.
(2) Flexible connection method, each interface is equivalent to an expansion joint, which solves the leakage problem caused by thermal expansion and cold contraction of pipes.
7.2 Project cases
Picture 15 Installation of PVC-O pipe with prefabricated sealing ring
- Case study of a large-scale water supply project in Yibin, PVC-O water supply pipe with prefabricated sealing ring, MRS 450, specifications dn110, 160, 200mm, nominal pressure 1.6MPa, 2.0MPa.
Picture 16 Yibin Large-scale Water Supply Project
- Xichang large-scale water supply project case, biaxially oriented PVC-O water supply pipe with prefabricated sealing ring, specifications dn315 and dn500mm, nominal pressure 1.0MPa, 1.6MPa, MRS 450.
Picture 17 Xichang Large-scale Water Supply Project
- Chengdu Water Group water supply project case, PVC-O pipe with prefabricated sealing ring, specification dn400mm, nominal pressure 2.0MPa, MRS 450.
Picture 18 Chengdu Water Group Project
8 Conclusion
The biaxially oriented PVC-O water supply pipe has changed from the traditional R tyep socket (the elastic sealing ring is manually installed), to an prefabricated elastic steel skeleton sealing ring in the pipe socket during production. The steel skeleton is fully covered by the elastic sealing ring and has no contact with water during long-term use. The sealing ring has been prefabricated in the pipe socket before it leaves the factory. The PVC-O pipe with prefabricated sealing ring has been subjected to hydraulic test, ultimate blasting test, low-temperature falling weight impact test, tensile strength, deorientation temperature test, engineering use and other verifications, which shows that the product has good safety, reliability and applicability.