What is water hammer
What is water hammer?
In the pressure pipeline, due to some external reasons (such as the valve suddenly closed, the pump unit suddenly stopped), the water velocity suddenly changes, resulting in water hammer. This kind of hydraulic phenomenon is called water hammer.
Water hammer accident in steam pipeline
Explanation of water hammer phenomenon
Water hammer is the piping system in the process of conveying liquid, due to the sudden closure of the valve, the subsequent flow under the action of inertia, produce water shock wave, just like the hammer, so it is called water hammer. The water hammer caused by valve closing becomes normal water hammer. When the valve suddenly opens, it will produce water hammer, which becomes negative water hammer.
Water hammer is in a sudden power failure or when the valve is closed too fast, due to the inertia of the pressure water flow, produce water shock wave, just like a hammer, so it is called water hammer. The back and forth force of the water shock wave is sometimes very large, which damages the valve and pump.
When the electric water pump starts at full voltage, it can accelerate from static state to rated speed in less than 1s, and the flow in the pipeline increases from zero to rated flow. Because of the momentum and compressibility of the fluid, the rapid change of the flow rate will cause the impact of too high or too low pressure in the pipeline, as well as the phenomenon of “cavitation”. The impact of pressure will force the pipe wall to produce noise, just like a hammer hitting the pipe, which is called “water hammer effect”.
The water hammer effect is only related to the inertia of water itself, not to the pump.
The destructive effect of water hammer is very large. For example, the positive water hammer may cause the rupture of the pipe, while the negative water hammer may cause the collapse of the pipe, which is very unfavorable to the liquid transportation pipeline.
The conditions of water hammer
- 1. The valve suddenly opens or closes;
- 2. The water pump unit stops or starts suddenly;
- 3. Water delivery from single pipe to high place (the elevation difference of water supply terrain is more than 20m);
- 4. The total head (or working pressure) of the pump is large;
- 5. The water flow velocity in the pipeline is too high;
- 6. The water pipeline is too long and the terrain changes greatly.
- 7. If there is liquid in the gas-liquid two-phase pipeline, it is more dangerous for the valve to open rapidly before running the pipeline.
Harm of water hammer effect
The pressure rise caused by water hammer can reach several times or even dozens of times of the normal working pressure of the pipeline. This kind of large amplitude pressure fluctuation mainly causes harm to the pipeline system.
- 1. Strong vibration of the pipeline and disconnection of the pipeline joint;
- 2. The valve was damaged, and the serious pressure was too high, which caused the pipe burst and the pressure of water supply network decreased;
- 3. Too low pressure will lead to collapse of the pipe and damage the valve and fixing parts;
- 4. Cause the pump to reverse, damage the equipment or pipeline in the pump room, seriously cause the pump room to submerge, cause casualties and other major accidents, affect production and life.
Protective measures to eliminate or reduce water hammer
1. Reducing the flow velocity of water pipeline can reduce the water hammer pressure to a certain extent, but it will increase the pipe diameter and increase the project investment. Hump or abrupt change of slope should be avoided as far as possible in the layout of water pipeline. The longer the pipeline is, the greater the water hammer is. From one pumping station to two pumping stations, the two pumping stations are connected by suction wells. The size of the pump stop water hammer is mainly related to the geometric head of the pump house. The higher the geometric head is, the greater the pump stop water hammer is. Therefore, the reasonable pump head should be selected according to the local actual situation. After the pump is stopped in case of an accident, the pump shall be started after the pipe behind the check valve is filled with water. When starting the pump, do not fully open the outlet valve of the pump, otherwise it will cause great water impact. Many major water hammer accidents in pumping stations occur in this case.
2. Set water hammer elimination device
(1) Constant voltage control technology is adopted
The PLC automatic control system is used to control the frequency conversion and speed regulation of the pump, and the operation of the whole water supply pump room system is controlled automatically. As the pressure of water supply network changes with the change of working conditions, the phenomenon of low pressure or overpressure often occurs in the process of system operation, which is easy to produce water hammer and lead to the damage of pipes and equipment. Through the detection of pipe network pressure, feedback control of pump start, stop and speed regulation, control of flow, and then make the pressure maintain a certain level, through the control of microcomputer set pump water supply pressure, maintain constant pressure water supply, avoid excessive pressure fluctuations, reduce the probability of water hammer.
(2) Installation of water hammer eliminator
The equipment is mainly used to prevent water hammer when the pump is stopped. It is generally installed near the outlet pipe of the pump. The pressure of the pipe itself is used as the power to realize the low-pressure automatic action. That is, when the pressure in the pipe is lower than the set protection value, the drainage outlet will automatically open to release water and pressure, so as to balance the pressure of the local pipe and prevent the impact of water hammer on the equipment and pipe, Generally, there are two types of eliminators: mechanical and hydraulic. The mechanical eliminator can be restored manually after operation, and the hydraulic eliminator can be reset automatically.
(3) Install slow closing check valve on the outlet pipe of large diameter water pump
It can effectively eliminate the water hammer when the pump stops, but because there is a certain amount of water backflow when the valve acts, the suction well must have an overflow pipe. There are two types of slow closing check valve: hammer type and energy storage type. This kind of valve can adjust the closing time of the valve within a certain range according to the needs. Generally, the valve is closed 70% – 80% within 3-7s after power failure, and the remaining 20% – 30% closing time is adjusted according to the situation of water pump and pipeline, generally within 10-30s. It is worth noting that when there is hump in the pipeline and water hammer occurs, the function of slow closing check valve is very limited.
(4) One way surge tank
The height of one-way surge tank is lower than the pressure of the pipeline near the pump station or at the proper position of the pipeline. When the pressure in the pipeline is lower than the water level in the tower, the surge tank will supply water to the pipeline to prevent the water column from breaking and to avoid bridging water hammer. But it has limited depressurization effect on water hammer other than pump stop water hammer, such as valve closing water hammer. In addition, the performance of the one-way valve used in the one-way surge tank should be absolutely reliable. Once the valve fails, it may lead to large water hammer.
(5) Setting bypass pipe (valve) in pump station
During the normal operation of the pump system, the check valve is closed because the water pressure on the pressure side of the pump is higher than that on the suction side. When the pump stops suddenly after the accident power failure, the pressure at the outlet of the pump station decreases sharply, while the pressure at the suction side rises sharply. Under this differential pressure, the transient high-pressure water in the main suction pipe is the transient low-pressure water flowing to the main pressure pipe by pushing the check valve plate open, and the low-pressure water pressure at this place is increased; On the other hand, the water hammer pressure rise on the suction side of the pump is also reduced. In this way, the rise and fall of water hammer on both sides of the pumping station are controlled, so as to effectively reduce and prevent the harm of water hammer.
(6) Set multi-stage check valve
One or more check valves are added in the long water pipeline to divide the water pipeline into several sections, and check valves are set on each section. In the process of water hammer, when the water in the water delivery pipe flows back, the check valves are closed one after another to divide the backwash flow into several sections. Because the hydrostatic head in each section of water delivery pipe (or backwash flow section) is quite small, the pressure rise of water hammer is reduced. This protective measure can be effectively used in the case of large height difference of geometric water supply, but can not eliminate the possibility of water column separation. Its biggest disadvantage is that the power consumption of water pump increases and the cost of water supply increases during normal operation.
3. In industrial pipeline, the pipe diameter can be increased and the flow velocity can be reduced; Open and close the valve slowly, and set the bypass line if necessary; For the steam pipeline, pay attention to warm the pipeline, increase the drainage, especially pay attention to the pressure test at the low point of the pipeline or the water storage after the termination of operation before warming the pipeline and operating the pipeline, and carry out the necessary displacement monitoring to avoid water hammer.
Source: China Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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