HDPE (High-Density Polyethylene) pipes require restrained type couplings in some applications, while other types of pipes can use non-restrained type couplings. The need for restrained couplings in HDPE pipes, compared to other pipe materials, is mainly due to the specific physical properties and behavior of HDPE pipes under various conditions. Here’s why HDPE pipes require restrained couplings and why other materials can use non-restrained couplings:

1, Flexibility and Thermal Expansion of HDPE Pipes

Thermal Expansion: HDPE pipes are highly sensitive to temperature changes. Due to their relatively high coefficient of thermal expansion, HDPE pipes can expand or contract significantly with temperature fluctuations. Without a restrained coupling, the pipe ends may shift or slide, causing loose connections or leaks.

Flexibility: HDPE pipes are flexible and can adjust to complex ground conditions like soil settlement or movement. However, excessive movement due to pressure fluctuations or external forces can destabilize the connection points or cause joint failures. A restrained coupling helps prevent this unnecessary movement, ensuring a secure connection.

2, Preventing Pipe Disconnection

Connection Stability: HDPE pipes, due to their flexibility and lower rigidity, are more prone to disconnections at the joints, especially under external forces such as pressure changes, vibrations, or ground shifts. A restrained coupling locks the pipe ends in place, preventing them from loosening and ensuring connection stability.

Pressure Surge Impact: During liquid transport, pipelines can experience sudden pressure surges. Without a restrained coupling, these surges could cause the pipe ends to shift, resulting in loose joints or leaks. A restrained coupling provides additional holding power to prevent movement during pressure fluctuations.

3, Preventing Axial Movement

Slippage Issue: HDPE pipes are often connected using heat fusion or electrofusion, which do not provide the same rigidity as traditional metal pipe connections. Without a restrained coupling, axial movement can occur due to external forces or temperature changes, leading to disconnections or leaks.

Role of Restrained Couplings: Restrained couplings secure the pipe ends and limit the longitudinal movement, ensuring the pipes do not slip or break under force.

4, Lower Tensile Strength of HDPE

Tensile Strength: HDPE pipes have lower tensile strength compared to metal pipes, meaning they are less able to withstand axial forces. Without a restrained coupling, the pipes could experience excessive tensile forces or pressure, leading to leaks or breaks at the connection points. Restrained couplings offer added tensile strength, reducing the risk of failure.

5, Different Properties of Other Pipe Materials

Other pipe materials, such as steel pipes, cast iron pipes, or PVC pipes, are generally more rigid and less sensitive to temperature changes and ground movement. These materials can typically handle pressure fluctuations and external forces without the need for a restrained coupling.

Steel and cast iron pipes are stiffer and can withstand higher internal and external pressure fluctuations, making them less prone to slippage or loosening under normal conditions.

PVC pipes are rigid and structurally stronger, allowing them to maintain secure connections even with non-restrained couplings. The rigidity of PVC makes it less susceptible to movement or expansion, reducing the need for restrained connections.

Conclusion
In summary, HDPE pipes require restrained couplings because of their flexibility, sensitivity to thermal expansion, and lower tensile strength, which make them more prone to slippage, disconnection, or joint failure under external forces or temperature changes. On the other hand, other rigid materials like steel, cast iron, and PVC can typically handle external pressures, temperature fluctuations, and movement without the need for restrained couplings. Therefore, selecting the appropriate coupling type is crucial for maintaining the stability and leak-proof integrity of the pipeline system.