What Are Water Hammers?
A water hammer is a kind of hydro blast produced by the inertia of pressure water flow, just like a hammer beating the water at the moment of a sudden power cut or fast closing of valves. The force generated by the back-and-forth shock wave of water flow can sometimes be very large, thus damaging valves and pumps.
When the open valve suddenly closes, the water flow will generate pressure on the valve and pipe wall. Due to the smooth pipe wall, the subsequent water flow quickly reaches the maximum speed under the action of inertia, which will cause damage. This is the "water hammer effect" in fluid mechanics, that is, the positive water hammer. This factor should be considered in the construction of a water supply pipeline.
On the contrary, when the closed valve is suddenly opened, it will also produce a water hammer, called negative water hammer, which also has a certain destructive power, but it is not as big as the former. When the electric water pump unit suddenly loses power or starts, it will also cause pressure shock and water hammer effect. The shock wave of this pressure transmits along the pipeline, which easily leads to a local overpressure of the pipeline, resulting in pipeline rupture and equipment damage, etc. Therefore, water hammer effect protection has become one of the key technologies in water supply engineering.
Conditions in which water hammers generates
1. The valve suddenly opens or closes.
2. The water pump unit suddenly stops or starts.
3. Deliver water through the monotube to a higher place (the terrain elevation difference of water supply is more than 20 meters).
4. The total head of delivery (or working pressure) of the water pump is large.
5. The speed of water flow in the water pipeline is too high.
6. The water pipeline is too long and the terrain changes greatly.
Harms of water hammers effect
The pressure increase caused by the water hammer can reach several times or even dozens of times the normal working pressure of the pipeline. This kind of large-scale pressure fluctuation can cause several harms to the pipeline system mainly including:
1. It can cause strong vibration of the pipe, leading to pipe joint disconnection.
2. The valve can be damaged, and the serious pressure is too high, which causes the pipe to burst and the pressure of the water supply network to decrease.
3. On the contrary, if the pressure is too low, the pipe will collapse, and the valves and fixtures will be damaged.
4. It causes the pump to reverse, damaging the equipment or pipelines in the pump room, which seriously causes the pump room to be flooded, and other major accidents such as personal injury and death, affecting the production and daily life.
The protective measures to eliminate or reduce water hammer
There are many protective measures for water hammers, but different measures should be taken according to the possible causes of water hammers.
1. Reducing the flow rate of the water delivery pipeline can reduce the surge pressure to a certain extent, but it will increase the diameter of the water delivery pipeline and increase the project investment. The layout of water pipelines should be considered to avoid hump or abrupt slopes whose terrain change should be as little as possible. The size of the water hammer for stopping the pump is mainly related to the geometric lift of the pump house. The higher the geometric lift, the larger the value of the water hammer for stopping pumps. Therefore, a reasonable pump head should be selected according to the local actual situation. After stopping the pump in an accident, the pump should be started after the check valve is filled with water. Do not fully open the pump outlet valve when starting the pump, otherwise, it will cause great water impact. Many major water hammer accidents in pumping stations often occur in this situation.
2. Set the water hammer elimination device.
2.1 Adopt the constant pressure control technology.
As the pressure of the water supply network changes constantly with the change of working conditions, the phenomenon of low pressure or overpressure often occurs during the operation of the system, which easily leads to water hammers, resulting in the destruction of pipelines and equipment. By using the automatic control system and detecting the pressure of the network, through the feedback of controlling the start, stop and speed adjustment of the water pump and controlling the flow rate, it can keep the pressure at a certain level. By controlling the microcomputer, the water supply pressure of the pump can be set, and the water supply can be maintained at constant pressure, so as to avoid excessive pressure fluctuation and reduce the probability of water hammer.
2.2 Install the water hammer eliminator
This device mainly prevents the water hammer from stopping the pump, which is generally installed near the outlet pipeline of the water pump. It uses the pressure of the pipeline itself as the power to realize the low-pressure automatic action, that is, when the pressure in the pipeline is lower than the set protection value, the water outlet will automatically open to release water and pressure to balance the pressure in the local pipeline and prevent the impact of the water hammer on the device and pipeline. The eliminators can generally be divided into mechanical and hydraulic types. The mechanical eliminators are recovered manually after action, and the hydraulic eliminators can automatically reset.
2.3 Install a slow closing check valve on the outlet pipe of the large-diameter water pump.
It can effectively eliminate the water hammer when the pump stops. However, as there is a certain amount of water flowing backward when the valve moves, the suction well must be equipped with an overflow pipe. There are two kinds of slow closing check valves: heavy hammer type and energy storage type. This kind of valve can adjust the closing time of the valve within a certain range as required. Generally, 70% to 80% of the valves are closed within 3 to 7 seconds after a power failure, and the remaining 20% to 30% of the closing time is adjusted according to the situation of the water pump and pipeline, generally in the range of 10 to 30 seconds. It is worth noting that the slow closing check valve is very effective when there is a hump in the pipeline to bridge the water hammer.
2.4 Set up one-way surge tower
Built near the pumping station or at the appropriate position of the pipeline, the height of the one-way surge tower is lower than the pipeline pressure there. When the pressure in the pipeline is lower than the water level in the tower, the surge tower supplies water to the pipeline to prevent the water column from breaking and avoid bridging the water hammer. However, it has a limited depressurization effect on water hammers other than pump stop water hammers, such as shut-off valve water hammers. In addition, the performance of the one-way valve used in the one-way pressure regulating tower should be absolutely reliable, and once the valve fails, it may lead to a larger occurrence.
2.5 Set the bypass pipe (valve) in the pump station.
During the normal operation of the pump system, the check valve is closed because the water pressure on the water pressure side of the pump is higher than that on the water suction side. When the pump suddenly stops after the accident power failure, the pressure at the outlet of the pump station drops sharply, while the pressure at the suction side rises sharply. Under this differential pressure, the transient high-pressure water in the suction main pipe is the transient low-pressure water that pushes the check valve plate away and flows to the main pressure pipe, and the low water pressure there is increased. On the other hand, the pressure rise of the water hammer on the suction side of the water pump is also reduced. In this way, the rise and fall of the water hammer on both sides of the pump station are controlled, thus effectively reducing and preventing the harm of the water hammer.
2.6 Set the multi-stage check valve
One or more check valves are added in the long water delivery pipeline to divide the water delivery pipeline into several sections. Each section is provided with a check valve. When the water in the water pipe flows backward in the process of water hammer, the check valves are closed one after another to divide the backwash water into several sections. Because the hydrostatic head in each section of water pipes (or backwash water section) is quite small, the water hammer pressure surge is reduced. This protective measure can be effectively used in the case of large geometric water supply height differences. But the possibility of water column separation cannot be eliminated. Its biggest disadvantages are the power consumption of water pump increases and the cost of water supply increases during normal operation.
2.7 Automatic exhaust and air supply devices are set at the high point of the pipeline to reduce the influence of water hammers on the pipeline.
When the open valve suddenly closes, the water flow will generate pressure on the valve and pipe wall. Due to the smooth pipe wall, the subsequent water flow quickly reaches the maximum speed under the action of inertia, which will cause damage. This is the "water hammer effect" in fluid mechanics, that is, the positive water hammer. This factor should be considered in the construction of a water supply pipeline.
On the contrary, when the closed valve is suddenly opened, it will also produce a water hammer, called negative water hammer, which also has a certain destructive power, but it is not as big as the former. When the electric water pump unit suddenly loses power or starts, it will also cause pressure shock and water hammer effect. The shock wave of this pressure transmits along the pipeline, which easily leads to a local overpressure of the pipeline, resulting in pipeline rupture and equipment damage, etc. Therefore, water hammer effect protection has become one of the key technologies in water supply engineering.
Conditions in which water hammers generates
1. The valve suddenly opens or closes.
2. The water pump unit suddenly stops or starts.
3. Deliver water through the monotube to a higher place (the terrain elevation difference of water supply is more than 20 meters).
4. The total head of delivery (or working pressure) of the water pump is large.
5. The speed of water flow in the water pipeline is too high.
6. The water pipeline is too long and the terrain changes greatly.
Harms of water hammers effect
The pressure increase caused by the water hammer can reach several times or even dozens of times the normal working pressure of the pipeline. This kind of large-scale pressure fluctuation can cause several harms to the pipeline system mainly including:
1. It can cause strong vibration of the pipe, leading to pipe joint disconnection.
2. The valve can be damaged, and the serious pressure is too high, which causes the pipe to burst and the pressure of the water supply network to decrease.
3. On the contrary, if the pressure is too low, the pipe will collapse, and the valves and fixtures will be damaged.
4. It causes the pump to reverse, damaging the equipment or pipelines in the pump room, which seriously causes the pump room to be flooded, and other major accidents such as personal injury and death, affecting the production and daily life.
The protective measures to eliminate or reduce water hammer
There are many protective measures for water hammers, but different measures should be taken according to the possible causes of water hammers.
1. Reducing the flow rate of the water delivery pipeline can reduce the surge pressure to a certain extent, but it will increase the diameter of the water delivery pipeline and increase the project investment. The layout of water pipelines should be considered to avoid hump or abrupt slopes whose terrain change should be as little as possible. The size of the water hammer for stopping the pump is mainly related to the geometric lift of the pump house. The higher the geometric lift, the larger the value of the water hammer for stopping pumps. Therefore, a reasonable pump head should be selected according to the local actual situation. After stopping the pump in an accident, the pump should be started after the check valve is filled with water. Do not fully open the pump outlet valve when starting the pump, otherwise, it will cause great water impact. Many major water hammer accidents in pumping stations often occur in this situation.
2. Set the water hammer elimination device.
2.1 Adopt the constant pressure control technology.
As the pressure of the water supply network changes constantly with the change of working conditions, the phenomenon of low pressure or overpressure often occurs during the operation of the system, which easily leads to water hammers, resulting in the destruction of pipelines and equipment. By using the automatic control system and detecting the pressure of the network, through the feedback of controlling the start, stop and speed adjustment of the water pump and controlling the flow rate, it can keep the pressure at a certain level. By controlling the microcomputer, the water supply pressure of the pump can be set, and the water supply can be maintained at constant pressure, so as to avoid excessive pressure fluctuation and reduce the probability of water hammer.
2.2 Install the water hammer eliminator
This device mainly prevents the water hammer from stopping the pump, which is generally installed near the outlet pipeline of the water pump. It uses the pressure of the pipeline itself as the power to realize the low-pressure automatic action, that is, when the pressure in the pipeline is lower than the set protection value, the water outlet will automatically open to release water and pressure to balance the pressure in the local pipeline and prevent the impact of the water hammer on the device and pipeline. The eliminators can generally be divided into mechanical and hydraulic types. The mechanical eliminators are recovered manually after action, and the hydraulic eliminators can automatically reset.
2.3 Install a slow closing check valve on the outlet pipe of the large-diameter water pump.
It can effectively eliminate the water hammer when the pump stops. However, as there is a certain amount of water flowing backward when the valve moves, the suction well must be equipped with an overflow pipe. There are two kinds of slow closing check valves: heavy hammer type and energy storage type. This kind of valve can adjust the closing time of the valve within a certain range as required. Generally, 70% to 80% of the valves are closed within 3 to 7 seconds after a power failure, and the remaining 20% to 30% of the closing time is adjusted according to the situation of the water pump and pipeline, generally in the range of 10 to 30 seconds. It is worth noting that the slow closing check valve is very effective when there is a hump in the pipeline to bridge the water hammer.
2.4 Set up one-way surge tower
Built near the pumping station or at the appropriate position of the pipeline, the height of the one-way surge tower is lower than the pipeline pressure there. When the pressure in the pipeline is lower than the water level in the tower, the surge tower supplies water to the pipeline to prevent the water column from breaking and avoid bridging the water hammer. However, it has a limited depressurization effect on water hammers other than pump stop water hammers, such as shut-off valve water hammers. In addition, the performance of the one-way valve used in the one-way pressure regulating tower should be absolutely reliable, and once the valve fails, it may lead to a larger occurrence.
2.5 Set the bypass pipe (valve) in the pump station.
During the normal operation of the pump system, the check valve is closed because the water pressure on the water pressure side of the pump is higher than that on the water suction side. When the pump suddenly stops after the accident power failure, the pressure at the outlet of the pump station drops sharply, while the pressure at the suction side rises sharply. Under this differential pressure, the transient high-pressure water in the suction main pipe is the transient low-pressure water that pushes the check valve plate away and flows to the main pressure pipe, and the low water pressure there is increased. On the other hand, the pressure rise of the water hammer on the suction side of the water pump is also reduced. In this way, the rise and fall of the water hammer on both sides of the pump station are controlled, thus effectively reducing and preventing the harm of the water hammer.
2.6 Set the multi-stage check valve
One or more check valves are added in the long water delivery pipeline to divide the water delivery pipeline into several sections. Each section is provided with a check valve. When the water in the water pipe flows backward in the process of water hammer, the check valves are closed one after another to divide the backwash water into several sections. Because the hydrostatic head in each section of water pipes (or backwash water section) is quite small, the water hammer pressure surge is reduced. This protective measure can be effectively used in the case of large geometric water supply height differences. But the possibility of water column separation cannot be eliminated. Its biggest disadvantages are the power consumption of water pump increases and the cost of water supply increases during normal operation.
2.7 Automatic exhaust and air supply devices are set at the high point of the pipeline to reduce the influence of water hammers on the pipeline.
