PVC (Polyvinyl Chloride) pipes, valves, and fittings are widely used in liquid piping systems due to their durability, cost-effectiveness, and versatility. However, to ensure optimal performance and longevity, it is important to understand their chemical resistance, pressure ratings, temperature ratings, and the impact of temperature on these ratings (derating).
Please note that this information is provided for pressurized PVC liquid piping systems constructed from what is commonly "grey PVC," Type 1, Grade 1, Cell Class 12454 per ASTM D1784. Most of this does not apply directly to unpressurized "white" PVC such as a household drain system.
Chemical Resistance
One of the primary advantages of PVC piping is its excellent chemical resistance. PVC is capable of handling a wide range of chemicals, including acids, bases, salts, and some organic solvents. This makes it suitable for use in diverse applications such as water treatment, chemical processing, and wastewater management where metal pipe/valves/fittings would be subject to chemical attack.
- Acids and Bases: PVC can handle most inorganic acids and bases, making it ideal for applications involving these chemicals. For example, it is resistant to hydrochloric acid, sulfuric acid, and sodium hydroxide.
- Salts: PVC piping can effectively transport saline solutions without degradation, making it suitable for seawater and brine applications.
- Organic Solvents: While PVC is generally resistant to many organic solvents, it can be affected by certain types, such as aromatic hydrocarbons (e.g., benzene) and chlorinated solvents. Compatibility should be checked for specific solvents.
Manufacturers often provide chemical resistance charts that detail the compatibility of PVC with various chemicals. It is recommended to refer to these charts to ensure that PVC is suitable for your application.
Pressure Ratings
Pressure ratings for PVC pipes, valves, and fittings are determined by the pipe's diameter, wall thickness, and the material grade.
- Standard Sizes: PVC pipes are commonly available in schedules (e.g., Schedule 40, Schedule 80), which indicate the wall thickness. Schedule 80 PVC has a thicker wall and, consequently, a higher pressure rating than Schedule 40 PVC.
- Pressure Classes: Pressure ratings are typically expressed in PSI (pounds per square inch). For example, Schedule 40 PVC pipes are generally rated for pressures up to 280 PSI, while Schedule 80 pipes can handle up to 400 PSI at room temperature.
Temperature Ratings
PVC pipes, valves, and fittings have a maximum service temperature, beyond which their structural integrity and pressure handling capability may be compromised.
- Standard Temperature Range: PVC is typically rated for use up to 140°F (60°C). Exceeding this temperature can cause the material to soften, leading to deformation and potential failure.
- Temperature Derating: As temperature increases, the pressure rating of PVC decreases. This phenomenon, known as derating, is crucial to consider in high-temperature applications.
Derating for Temperature
Temperature derating is a critical factor in determining the safe operating pressure of PVC piping systems at elevated temperatures.
- Derating Factors: Manufacturers provide derating factors that specify how the pressure rating decreases with temperature. For example, a PVC pipe with a pressure rating of 280 PSI at 73°F (23°C) might be derated to 62 PSI at 140°F (60°C).
- Calculation: To determine the safe pressure rating at a higher temperature, multiply the original pressure rating by the derating factor. For instance, if the derating factor at 140°F is 0.22, a pipe rated at 280 PSI at 73°F would be rated at 61.6 PSI at 140°F (280 PSI * 0.22).
It should also be noted that in most cases, minimum recommended service temperature of pvc piping systems under pressure is 40ºF.
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Understanding the chemical resistance, pressure ratings, temperature ratings, and deratings of PVC pipes, valves, and fittings is essential for designing and maintaining efficient and reliable liquid piping systems. By considering these factors, engineers and system designers can ensure that their piping systems operate safely and effectively, even under varying chemical and temperature conditions. Always refer to manufacturer specifications and guidelines to make informed decisions about the suitability of PVC for specific applications.