Reasons for brittle failure of pvc pipes in use


1. The influence of air hammer: When using PVC-U pipes, […]

1. The influence of air hammer: When using PVC-U pipes, the air in the pipeline cannot be exhausted or the exhaust valve is set improperly. When there is gas in the pipeline, it is easy to cause airbags in the pipeline. It cannot directly cause water hammer, but it can be used to cause harm. Under normal circumstances, the water flow in the pipeline can be approximately regarded as a constant flow (pressure, flow rate, and temperature remain unchanged). In this state of the pipe production line, the gas in the water will gradually separate out, forming bubbles of different sizes that rise to the pipe wall, and the bubbles move forward according to the flow rate of the water. In the uphill section, due to buoyancy, the flow rate of bubbles may be greater than that of water. Due to the roughness of the tube wall, the bubbles move in the same direction, which makes it difficult to gather large bubbles.

Small bubbles flow forward along a certain width of the pipe wall. When passing through the high-point exhaust valve, the bubbles in the diameter of the exhaust pipe are conditionally discharged, while other bubbles flow downstream by the thrust of the water flow. Due to the turbulence, flow velocity and tangential characteristics of the pipe wall, some small bubbles passing through the exhaust pipe will flow downstream through the exhaust hole. The bubble that crosses the exhaust valve travels along the slope, and the direction of movement is opposite to the direction of the component of the buoyancy of the bubble. The resistance generated by the resultant force of buoyancy will inevitably slow down the movement of the bubble. The subsequent bubble is likely to hit the previous bubble and become complete. Big bubbles. The air bag can cause the tube to burst due to the rapid opening and closing of the valve or the start and stop of the water pump, which causes a large pressure increase in the pipeline. The compressible characteristics of the gas causes the stress to concentrate on the airbag to generate high pressure and burst the tube.

2. The impact of scratches: during installation and transportation, scratches, bumps and other damage are caused to the surface of the PVC-U pipe. According to the crack theory of brittle failure, these scratches will concentrate the stress at the tip of the scratch, which is much higher than the average stress of the pipe. When this stress reaches and exceeds a certain critical value, the crack will expand, due to PVC- U is a brittle material, the crack propagation speed is very fast, causing brittle damage.

3. The influence of water hammer: Water hammer refers to the pressure transient process, which is a special and important phenomenon caused by the unstable flow in the pipeline. When the flow rate in the pipeline changes suddenly due to some reason, such as switching the valve too fast, sudden power failure and stopping the pump, it will cause the pressure in the pipeline to change suddenly and cause water hammer. Water hammer is also called water hammer. When the water hammer pressure is large enough, it will cause brittle failure of the PVC-U pipe. Even if the pipe is not damaged, when the water hammer occurs repeatedly, especially when the frequency is high, a large internal pressure will be generated in the pipe. Fatigue loads are also applied to the pipe.