How to Mitigate the Harm Caused by Water Hammer?

Water hammer is a common issue in water distribution systems, and there are various protective measures available to mitigate its effects. However, these measures must be tailored to the specific causes of water hammer. Below are some commonly used methods:

Lowering the flow rate in the pipeline can reduce water hammer pressure to some extent. However, this may require increasing the pipe diameter, which adds to the project costs. When laying out the pipeline, it is essential to avoid situations where there are sudden changes in slope or the formation of humps (high points) in the line.

Additionally, reducing the length of the pipeline can help, as longer pipelines generally result in greater water hammer during pump shutdowns. One approach is to split a single pump station into two and use a suction well to connect the two stations.

The magnitude of water hammer during pump shutdown is mainly related to the geometric head of the pump station. The higher the geometric head, the larger the potential for water hammer. Therefore, it is important to select an appropriate pump head based on local conditions.

After a pump shutdown, the system should wait for the pipe downstream of the check valve to fill with water before restarting the pump. During pump startup, it is crucial not to open the pump outlet valve fully, as this could cause significant water hammer. Many major water hammer incidents in pump stations occur under these conditions.

(1) Adopting Constant Pressure Control Technology:

A PLC (Programmable Logic Controller) automated control system can be employed to adjust the speed of pumps through variable frequency control. As the pressure in the water distribution network fluctuates with changing operating conditions, pressure surges or drops are common, leading to the risk of water hammer and damage to pipes and equipment. By monitoring the pressure and controlling the pumps' operation—turning them on or off, or adjusting their speed—the system maintains a constant pressure. This helps prevent large pressure fluctuations and reduces the likelihood of water hammer.

(2) Installing Water Hammer Arrestors:

These devices primarily prevent water hammer caused by pump shutdowns and are typically installed near the pump outlet. They use the pressure within the pipeline to activate a pressure-relief valve when the pressure drops below a set threshold, allowing water to be discharged to relieve pressure. This helps balance local pipeline pressures and prevent damage from water hammer. Water hammer arrestors are generally available in mechanical and hydraulic types. Mechanical arrestors require manual reset after activation, while hydraulic ones reset automatically.

(3) Installing Slow-Closing Check Valves on Large-Diameter Pipes:

Slow-closing check valves can effectively mitigate water hammer caused by pump shutdowns. However, because the valve's action allows some water to flow back, it requires an overflow pipe in the suction well. Slow-closing check valves come in two types: weight-based and energy-storing types. These valves can be adjusted to close within a specific time frame. Typically, the valve closes 70%-80% within 3 to 7 seconds after power failure, with the remaining 20%-30% of the closure taking 10 to 30 seconds, depending on the pump and pipeline conditions. It is important to note that when there are high points (humps) in the pipeline, water hammer caused by column separation may occur, in which case the slow-closing check valve is less effective.

(4) Installing a One-Way Pressure Regulating Tower:

A one-way pressure regulating tower can be built near the pump station or at a suitable point in the pipeline. The tower’s water level should be lower than the pipeline pressure at that point. When the pipeline pressure drops below the tower's water level, water is supplemented from the tower to the pipeline to prevent the water column from separating and to avoid water hammer. However, this measure is not very effective for preventing water hammer caused by valve closures. Additionally, the one-way valve used in the tower must be reliable, as a failure could lead to significant water hammer.

(5) Installing Bypass Pipes (Valves) in Pump Stations:

Under normal conditions, the pressure on the discharge side of the pump is higher than on the suction side, causing the check valve to close. When a sudden power failure occurs, the pressure on the pump discharge side drops sharply, while the suction side pressure increases dramatically. The pressure difference forces transient high-pressure water in the suction pipeline to push open the check valve, sending water to the low-pressure discharge side. This process helps equalize pressure on both sides of the pump, reducing the likelihood of water hammer.

(6) Installing Multiple Check Valves:

For long pipelines, installing multiple check valves can divide the pipeline into sections. Each section would have its own check valve. In the event of water hammer, the water flow is divided into smaller sections as each check valve closes in sequence. The smaller pressure head in each section reduces the magnitude of water hammer. This method is esp ecially useful for systems with a large vertical head difference. However, it cannot eliminate the risk of water column separation. A major drawback is that during normal operation, it increases pump energy consumption and operational costs.

By implementing these strategies, it is possible to effectively reduce the impact of water hammer on the water supply system, ensuring both safety and efficiency in operation.

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