Water hammer is also called water hammer. During the transportation of water (or other liquids), due to sudden opening or closing of Api Butterfly Valve, gate valves, check vavles and ball valves. sudden stops of water pumps, sudden opening and closing of guide vanes, etc., the flow rate changes suddenly and the pressure fluctuates significantly. The water hammer effect is a vivid term. It refers to a severe water hammer caused by the impact of water flow on the pipeline when the water pump is started and stopped. Because inside the water pipe, the inner wall of the pipe is smooth and the water flows freely. When an open valve is suddenly closed or the water supply pump is stopped, the water flow will generate a pressure on the valve and pipe wall, mainly the valve or pump. Because the pipe wall is smooth, under the action of inertia of the subsequent water flow, the hydraulic force quickly reaches the maximum and produces destructive effects. This is the “water hammer effect” in hydraulics, that is, positive water hammer. On the contrary, when a closed valve is suddenly opened or the water pump is started, water hammer will also occur, which is called negative water hammer, but it is not as big as the former. The pressure impact will cause the pipe wall to be stressed and produce noise, just like a hammer hitting the pipe, so it is called the water hammer effect.
The instantaneous pressure generated by water hammer can reach dozens or even hundreds of times of the normal operating pressure in the pipeline. Such large pressure fluctuations can cause strong vibration or noise in the pipeline system and may damage valve joints. It has a very damaging effect on the piping system. In order to prevent water hammer, the pipeline system needs to be designed correctly to prevent the flow rate from being too high. Generally, the designed flow rate of the pipe should be less than 3m/s, and the valve opening and closing speed needs to be controlled.
Because the pump is started, stopped, and valves are opened and closed too quickly, the speed of the water changes drastically, especially the water hammer caused by the sudden stop of the pump, which can damage pipelines, water pumps, and valves, and cause the water pump to reverse and reduce the pressure of the pipe network. The water hammer effect is extremely destructive: if the pressure is too high, it will cause the pipe to rupture. On the contrary, if the pressure is too low, it will cause the pipe to collapse and damage the valves and fixings. In a very short time, the water flow rate increases from zero to rated flow rate. Since fluids have kinetic energy and a certain degree of compressibility, huge changes in flow rate in a very short period of time will cause high and low pressure impacts on the pipeline.
There are many reasons for water hammer. Common factors are as follows:
1. The valve suddenly opens or closes;
2. The water pump unit suddenly stops or starts;
3. A single pipe transports water to a high place (the height difference of the water supply terrain exceeds 20 meters);
4 . The total lift (or working pressure) of the water pump is large;
5. The water flow velocity in the water pipeline is too large;
6. The water pipeline is too long and the terrain changes greatly.
7. Irregular construction is a hidden danger in water supply pipeline projects
(1) For example, the production of cement thrust piers for tees, elbows, reducers and other joints does not meet the requirements.
According to the “Technical Regulations for Buried Rigid Polyvinyl Chloride Water Supply Pipeline Engineering”, cement thrust piers should be installed at joints such as tees, elbows, reducers and other pipes with a diameter of ≥110mm to prevent the pipeline from moving. “Concrete thrust piers” It should not be lower than C15 grade, and it should be cast on-site on the excavated original soil foundation and trench slope.” Some construction parties do not pay enough attention to the role of thrust piers. They nail a wooden stake or wedge an iron prong next to the pipeline to act as a thrust pier. Sometimes the volume of the cement pier is too small or is not poured on the original soil. On the other hand, some thrust piers are not strong enough. As a result, during pipeline operation, the thrust piers cannot function and become useless, causing pipe fittings such as tees and elbows to be misaligned and damaged.
(2) The automatic exhaust valve is not installed or the installation position is unreasonable.
According to the principle of hydraulics, automatic exhaust valves should be designed and installed at the high points of pipelines in mountainous areas or hills with large undulations. Even in plain areas with small undulating terrain, the pipelines must be artificially designed when digging trenches. There are ups and downs, rising or falling in a cyclical manner, the slope is not less than 1/500, and 1-2 exhaust valves are designed at the highest point of each kilometer.
Because during the process of water transportation in the pipeline, the gas in the pipeline will escape and accumulate in the raised parts of the pipeline, even forming air blockage. When the flow rate of water in the pipeline fluctuates, the air pockets formed in the raised parts will continue to be compressed and expanded, and the gas will be The pressure generated after compression is dozens or even hundreds of times greater than the pressure generated after water is compressed (public account: Pump Butler). At this time, this section of pipeline with hidden dangers may lead to the following situations:
• After water is passed upstream of the pipe, dripping water disappears downstream. This is because the air bag in the pipe blocks the flow of water, causing water column separation.
• The compressed gas in the pipeline is compressed to the maximum limit and rapidly expands, causing the pipeline to rupture.
• When water from a high water source is transported downstream at a certain speed by gravity flow, after the upstream valve is quickly closed, due to the inertia of the height difference and flow rate, the water column in the upstream pipe does not stop immediately. It still moves at a certain speed. The speed flows downstream. At this time, a vacuum is formed in the pipeline because the air cannot be replenished in time, causing the pipeline to be deflated by the negative pressure and damaged.
(3) The trench and backfill soil do not meet the regulations.
Unqualified trenches are often seen in mountainous areas, mainly because there are many stones in certain areas. The trenches are dug manually or blasted with explosives. The bottom of the trench is seriously uneven and has sharp stones protruding. When encountering this, In this case, according to relevant regulations, the stones at the bottom of the trench should be removed and more than 15 centimeters of sand should be paved before the pipeline can be laid. However, the construction workers were irresponsible or cut corners and directly laid the sand without paving sand or symbolically paving some sand. The pipeline is laid on the stones. When the backfill is completed and the water is put into operation, due to the weight of the pipeline itself, the vertical earth pressure, the vehicle load on the pipeline, and the superposition of gravity, it is supported by one or several sharp raised stones at the bottom of the pipeline. , excessive stress concentration, the pipeline is very likely to be damaged at this point and crack along a straight line at this point. This is what people often call the “scoring effect.”
There are many protective measures for water hammer, but different measures need to be taken according to the possible causes of water hammer.
1. Reducing the flow rate of water pipelines can reduce water hammer pressure to a certain extent, but it will increase the diameter of water pipelines and increase project investment. When laying out water pipelines, consideration should be given to avoiding humps or drastic changes in slope to reduce the length of the water pipeline. The longer the pipeline, the greater the water hammer value when the pump is stopped. From one pumping station to two pumping stations, a water suction well is used to connect the two pumping stations.
Water hammer when pump is stopped
The so-called pump-stop water hammer refers to the hydraulic shock phenomenon caused by sudden changes in flow velocity in the water pump and pressure pipes when the valve is opened and stopped due to a sudden power outage or other reasons. For example, failure of the power system or electrical equipment, occasional failure of the water pump unit, etc. may cause the centrifugal pump to open the valve and stop, resulting in water hammer when the pump is stopped. The size of the water hammer when the pump is stopped is mainly related to the geometric head of the pump room. The higher the geometric head, the greater the water hammer value when the pump is stopped. Therefore, a reasonable pump head should be selected based on the actual local conditions.
The maximum pressure of water hammer when a pump is stopped can reach 200% of the normal working pressure, or even higher, which can destroy pipelines and equipment. General accidents cause “water leakage” and water outage; serious accidents cause the pump room to be flooded, equipment to be damaged, and facilities to be damaged. damage or even cause personal injury or death.
After stopping the pump due to an accident, wait until the pipe behind the check valve is filled with water before starting the pump. Do not fully open the water pump outlet valve when starting the pump, otherwise a large water impact will occur. Major water hammer accidents in many pumping stations often occur under such circumstances.
2. Set up water hammer elimination device
(1) Using constant voltage control technology
A PLC automatic control system is used to control the pump with variable frequency speed and to automatically control the operation of the entire water supply pump room system. Since the pressure of the water supply pipeline network continues to change with changes in working conditions, low pressure or overpressure often occurs during system operation, which can easily cause water hammer, leading to damage to pipelines and equipment. A PLC automatic control system is used to control the pipe network. Detection of pressure, feedback control of the start and stop of the water pump and speed adjustment, control of flow, and thus maintain the pressure at a certain level. The water supply pressure of the pump can be set by controlling the microcomputer to maintain constant pressure water supply and avoid excessive pressure fluctuations. The probability of water hammer is reduced.
(2) Install water hammer eliminator
This device mainly prevents water hammer when the pump is stopped. It is generally installed near the outlet pipe of the water pump. It uses the pressure of the pipe itself as power to realize low-pressure automatic action. That is, when the pressure in the pipe is lower than the set protection value, the drain port will automatically open to drain water. Pressure relief is used to balance the pressure of local pipelines and prevent the impact of water hammer on equipment and pipelines. Eliminators can generally be divided into two types: mechanical and hydraulic. Mechanical eliminators are manually restored after action, while hydraulic eliminators can be automatically reset.
(3) Install a slow-closing check valve on the large-diameter water pump outlet pipe
It can effectively eliminate water hammer when the pump is stopped, but because a certain amount of water will flow back when the Api 609 valve is activated, the water suction well must have an overflow pipe. There are two types of slow-closing check valves: hammer type and energy storage type. This kind of valve can adjust the valve closing time within a certain range as needed (welcome to follow: Pump Butler). Generally, the valve closes 70% to 80% within 3 to 7 seconds after a power outage. The remaining 20% to 30% closing time is adjusted according to the conditions of the water pump and pipeline, generally in the range of 10 to 30 seconds. It is worth noting that when there is a hump in the pipeline and water hammer occurs, the role of the slow-closing check valve is very limited.
(4) Set up a one-way pressure regulating tower
It is built near the pumping station or at an appropriate location on the pipeline, and 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 pressure regulating tower replenishes water to the pipeline to prevent the water column from breaking and bridge the water hammer. However, its pressure-reducing effect on water hammer other than pump-stop water hammer, such as valve-closing water hammer, is limited. In addition, the performance of the one-way valve used in the one-way pressure regulating tower must be absolutely reliable. Once the valve fails, it may cause a large water hammer.
(5) Set up a bypass pipe (valve) in the pump station
When the pump system is operating normally, the check valve is closed because the water pressure on the pressure side of the pump is higher than the water pressure on the suction side. When the accidental power outage suddenly stops the pump, the pressure at the outlet of the water pump station drops sharply, while the pressure on the suction side rises sharply. Under this differential pressure, the transient high-pressure water in the water suction main pipe pushes open the check valve valve plate and flows to the transient low-pressure water in the pressure water main pipe, causing the low water pressure there to increase; on the other hand, the water pump The water hammer pressure rise on the suction side is also reduced. In this way, the water hammer rise and pressure drop on both sides of the water pump station are controlled, thereby effectively reducing and preventing water hammer hazards.
(6) Set up a multi-stage check valve
In a long water pipeline, add one or more check valves, divide the water pipeline into several sections, and install a check valve on each section. When the water in the water pipe flows back during water hammer, each check valve is closed one after another to divide the backflush flow into several sections. Since the hydrostatic head in each section of the water pipe (or backflush flow section) is quite small, the water flow rate is reduced. Hammer boost. This protective measure can be effectively used in situations where the geometric water supply height difference is large; but it cannot eliminate the possibility of water column separation. Its biggest disadvantage is: increased power consumption of the water pump during normal operation and increased water supply costs.
Post time: Sep-18-2023