Abstract

Plastic pipes are playing an increasingly important role in industrial applications. This article explores the selection and design of commonly used plastic pipe materials, focusing on key factors such as operating temperature, corrosion resistance, industry standards, pressure and temperature ratings, valve material compatibility, and other critical design considerations. The goal is to provide a practical reference for the proper use of plastic piping systems to ensure the safe and reliable operation of industrial projects.

Plastic pipes have seen widespread adoption in construction, water supply and drainage, municipal infrastructure, industrial installations, and other sectors, owing to their significant advantages—lightweight construction, corrosion resistance, low fluid resistance, ease of installation, and long service life. Compared to metal pipes, plastic alternatives are more cost-effective, rely on readily available raw materials, and benefit from shorter production cycles—all of which have accelerated their adoption and application across various industries.

This article focuses on the material selection and design of thermoplastic pipes commonly used in industrial installations. These include chlorinated polyvinyl chloride (PVC-C), unplasticized polyvinyl chloride (PVC-U), high-density polyethylene (PE), steel-reinforced polyethylene composite pipes, fiberglass-reinforced polypropylene (FRP/PP), and fiberglass-reinforced polyvinyl chloride (FRP/PVC). The discussion covers key factors such as service temperature, chemical resistance, industry standards, pressure and temperature ratings, and valve material compatibility.

1. Applicable Temperature of Plastic Pipes

Plastic pipes are inherently limited by their molecular structure and composition, which prevents them from withstanding high temperatures as effectively as metal pipes. Different plastics have distinct molecular formulas, structures, and thermal tolerances. Thermoplastics can only sustain internal pressure effectively while remaining in a highly elastic state below a specific temperature threshold. As the temperature increases, this elastic state gradually transitions into a viscous flow state. The Vicat softening temperature indicates the point at which the material begins to soften and its mechanical properties start to deteriorate.

The long-term continuous use temperature of plastic pipes depends on both the selected material and the design parameters. The maximum design temperature of a pipeline should be lower than the material’s maximum allowable long-term service temperature and significantly below its Vicat softening point. If the operating temperature is too low, the plastic may enter a glassy state, undergo brittle transition, and experience a decline in mechanical performance. Therefore, the minimum design temperature should also exceed the material’s minimum allowable long-term service temperature. Recommended long-term service temperatures for various plastic pipe materials are listed in Table 1.

Table 1: Recommended Temperature for Long-Term Continuous Use of Plastic Pipes

Plastic pipe design should also take into account the impact of environmental conditions. Factors such as ambient temperature, temperature fluctuations, and duration of sunlight exposure can affect material selection and pipe service life. Special attention should be given to outdoor plastic piping in colder northern regions, where low temperatures may lead to freezing and brittle transition. Appropriate measures must be taken to prevent such issues under extreme ambient conditions.

2. Corrosion Resistance of Plastic Pipes

Plastics consist of high molecular compounds held together by molecular bonding forces and intermolecular interactions. The corrosion resistance of plastic pipes is based on the principle that the plastic molecules and the media they come into contact with cannot chemically react or become physically miscible. The molecular structure must feature gaps small enough to prevent the penetration of harmful substances. A chemical reaction would signify a change in the plastic’s molecular structure, such as alterations in functional groups, molecular chain breakage, or degradation.

Physical miscibility occurs when the plastic molecules and the media share similar functional groups and molecular structures. The key factor in the corrosion resistance of plastic pipes is their ability to resist chemical corrosion. This article explores the corrosion resistance of common plastic pipes when exposed to corrosive media, such as brine, hydrochloric acid, sodium hypochlorite, and sulfuric acid, at different temperatures and concentrations.

In engineering, plastics with a chemical corrosion resistance rating of S grade or higher are typically recommended for pipes used to convey specific media. These pipes should undergo a series of tests, such as hydraulic tests and system suitability evaluations, to verify system safety. Plastics with an L grade chemical corrosion resistance rating are not recommended for pressure-bearing pipes in the corresponding media environments.

PE (polyethylene) is a long-chain aliphatic compound, and its molecular structure provides chemical inertness to many substances. However, it is not resistant to highly oxidizing agents such as sodium hypochlorite and concentrated sulfuric acid. PVC-U (unplasticized polyvinyl chloride) offers higher strength than polyethylene, while PVC-C (chlorinated polyvinyl chloride) is a modified form of PVC-U. Chlorination of polyvinyl chloride introduces irregularities in its molecular structure, which enhances its chemical stability. PVC-C is chemically inert to most acids, alkalis, and salts.

The chemical corrosion resistance of commonly used plastic pipes to fluid media is outlined in the ISO 10358 standard. For example, for a 31% mass fraction of hydrochloric acid at room temperature, the corrosion resistance data for 30% concentration can be referred to. PE pipes, steel-reinforced polyethylene pipes, PVC-U, and PVC-C pipes all have an S corrosion resistance rating at room temperature, making them suitable for use in hydrochloric acid environments. PVC-U and PVC-C pipes are more commonly used in engineering applications. FRP-lined PVC and PP (polypropylene) composite pipes combine the corrosion resistance of the inner layer with the high strength, fatigue resistance, and aging resistance of FRP (fiberglass reinforced plastic). The FRP resin outer sheath is wound around the PVC or PP pipe, with the FRP layer primarily providing mechanical support.

3. Industrial Plastic Pipe Standards

The development of domestic plastic pipes was relatively slow, initially being used only in the construction and municipal industries. Relevant industrial plastic pipe standards were not implemented until the 1980s. When plastic pipes are used for industrial fluids, it is important to consider the specific application and project type. The resin raw materials used for construction plastic pipes differ from those used for industrial plastic pipes, resulting in variations in chemical resistance and structural permeability. Therefore, it is crucial to select the appropriate pipe standards for industrial applications. Currently, commonly used standards for industrial plastic pipes such as PVC-C, PVC-U, PE, steel-reinforced polyethylene, FRP/PP, and FRP/PVC are detailed in Table 2.

Table 2 Common Standards for Industrial Plastic Pipes

4. Pressure and Temperature Ratings

In addition to the inherent properties of the material, the pressure-bearing capacity of plastic pipes is primarily determined by the wall thickness. The pipe wall thickness series is established based on factors such as the pipe’s nominal pressure and the intended service life. Due to their poor weather resistance and susceptibility to aging, plastic pipes require careful selection of wall thickness. When determining the expected service life, this factor must be fully considered, as a longer service life generally requires a thicker wall. Plastic pipes also have a high thermal expansion coefficient, meaning their performance is significantly affected by temperature fluctuations. Therefore, when assessing nominal pressure at different temperatures, it is important to consider the temperature-pressure reduction factor.

4.1 PVC-U Pipe Pressure and Temperature Ratings

The domestic standard for PVC-U pipes used in industrial applications is GB/T 4219.1. According to this standard, the nominal pressure (PN) refers to the maximum operating pressure for conveying water at 20°C. As the temperature increases, the allowable working pressure decreases accordingly. The corresponding reduction coefficients are provided in Table 3.

Table 3 PVC-U Pipe Temperature Reduction Coefficient for Pressure

According to manufacturer data, the allowable pressure of PVC-U flanges at room temperature is approximately 10 bar. As the temperature rises, the allowable pressure decreases accordingly. The temperature reduction coefficient for flange pressure is provided in Table 4. In engineering applications, it is recommended to refer to the reduction coefficients in Table 4 to determine the maximum allowable pressure for PVC-U flanges.

Table 4 PVC-U Flange Temperature Reduction Coefficient for Pressure

4.2 PVC-C Pipe Pressure and Temperature Ratings

The applicable standard for industrial PVC-C pipes is GB/T 18998. This standard does not specify temperature reduction coefficients for pressure. In engineering applications, the maximum allowable pressure of PVC-C pipes can be determined using the temperature-to-pressure reduction coefficients provided by the manufacturer (Table 5). The allowable working pressure of the pipe decreases progressively as the temperature increases.

The allowable pressure for PVC-C flanges at room temperature is approximately 10 bar. As the temperature rises, the allowable pressure also decreases. According to manufacturer data, the flange temperature-to-pressure reduction coefficients are listed in Table 6. It is recommended to refer to these coefficients when determining the maximum allowable pressure of PVC-C flanges in engineering applications. 

Table 5 Temperature-to-Pressure Reduction Coefficient of PVC-C Pipes

Table 6 PVC-C Temperature-to-Pressure Reduction Coefficient of Flanges

4.3 PE Pipe Pressure and Temperature Ratings

For PE pipes manufactured in accordance with GB/T 13663, the maximum operating temperature must not exceed 40°C, and the minimum working temperature should not be lower than 0°C. It is recommended to adjust the nominal pressure of PE pipes based on the temperature-to-pressure reduction coefficient. The temperature reduction coefficients for PE pipes are provided in Table 7. PE pipeline flanges are loose flanges composed of a steel backing ring and a PE stub end. The flange material can be selected by the user, and the maximum allowable pressure of the flange will vary depending on the material used.

Table 7 PE Pipe Temperature-to-Pressure Reduction Coefficient

4.4 Pressure and Temperature Rating of Steel-Skeleton Polyethylene Pipe

Steel-skeleton polyethylene pipes offer higher strength and greater pressure-bearing capacity compared to PE pipes. According to HG/T3690, the maximum allowable temperature for these pipes should not exceed 70°C, and the minimum working temperature should not be lower than 0°C. The nominal pressure (PN) represents the maximum working pressure for conveying water at or below 20°C. As the temperature increases, the allowable working pressure decreases accordingly. The nominal pressure of the pipe is adjusted and reduced based on the temperature reduction coefficient. Details are provided in Table 8. Steel-skeleton polyethylene flanges are loose flanges composed of a steel flange and a steel-skeleton flanged stud. The material for the flange can be selected by the user, and the maximum allowable pressure of the flange will vary depending on the flange material.

Table 8: Temperature to Pressure Reduction Coefficient for Steel-Skeleton Polyethylene Pipe

4.5 Pressure and Temperature Ratings of FRP-Lined PVC and PP-Lined Composite Pipes

The allowable working pressure of FRP-lined PVC composite pipes at different temperatures is dependent on the nominal diameter of the pipe. For further details, refer to Table 9.

Table 9: Allowable Working Pressure of FRP-Lined PVC Composite Pipes

Similarly, the allowable working pressure of FRP-lined PP composite pipes at different temperatures also depends on the nominal diameter of the pipe. Please refer to Table 10 for more information.

Table 10: Permissible Working Pressure of Glass Fiber Reinforced Plastic-Lined PP Composite Pipes

The HG/T21579 and HG/T21636 standards specify the permissible working pressure for glass-lined PP and PVC pipes. In addition to temperature, the permissible working pressure is closely related to the nominal diameter of the pipe. This is a common characteristic of many plastic pipes. However, other plastic pipe standards do not differentiate the permissible working pressure based on pipe specifications when formulated.

5. Valve Material Selection

When industrial plastic pipes are used to convey corrosive media, the primary materials for valve bodies include corrosion-resistant metals, plastics, and steel-lined plastics. In general, corrosion-resistant metals are more expensive, and their use is limited due to cost considerations. The commonly used materials for plastic valves are mainly PVC-C, PVC-U, and PVDF.

Plastic valves offer several advantages over metal valves, such as excellent corrosion resistance, lightweight construction, and lower cost. However, the quality of products in the plastic valve industry varies significantly. Plastic valves also have certain limitations, including small nominal diameters, poor resistance to temperature variations, potential leakage from valve components, serious internal leakage, and short service life. The nominal pressure of plastic valves generally does not exceed PN10. As temperature increases, the allowable pressure decreases. The reduction factor can be referenced from the flange reduction factor of the corresponding material.

Steel-lined plastic valves combine the high pressure-bearing capacity of a steel valve body with the excellent corrosion resistance of a plastic lining. Currently, the most commonly used valve lining material is F46 (polyperfluoroethylene propylene). This fluoroplastic offers outstanding corrosion resistance and is widely applied in plastic pipeline systems.

6. Other Design Considerations

Plastic pipes generally have high electrical resistivity and are excellent insulators against electrostatic discharge. However, this also means that electrostatic charges generated by the flowing fluid are difficult to dissipate. Over time, static electricity can accumulate to a level that may cause dielectric breakdown of the pipe wall. If the transported medium is flammable or explosive, or if the surrounding environment contains leaked flammable substances, this discharge could ignite the medium and lead to a serious accident. Therefore, plastic pipes are typically not used for conveying flammable or explosive media and are unsuitable for use in hazardous environments—unless they are specially treated for static dissipation and shielding.

In addition, due to their high permeability, plastic pipes are not suitable for transporting highly permeable fluids, as this could result in leakage and pose a risk of personal injury.

7. Conclusion

Plastic pipes are widely used in modern industry due to their corrosion resistance, cost-effectiveness, and long service life. In engineering applications, designers should carefully assess the suitability of plastic pipes for specific operating conditions. This includes thoroughly understanding the performance characteristics and applicable standards of the selected plastic materials, making informed choices during material selection and system design, and evaluating potential application risks. Appropriate risk mitigation measures should also be implemented to ensure safe and reliable operation.