Water hammer, also known as water hammer, is a hydraulic transition phenomenon in pressure pipelines caused by sudden changes in water flow velocity, also known as transient flow. In pressure pipelines, when there is a sudden change in flow velocity due to certain reasons, the pressure of the liquid inside the pipeline increases or decreases due to the inertia of the fluid. It poses great harm to water pump units and pipeline systems. In general, water hammer does not occur during normal operation and shutdown of the water pump.
From different perspectives, water hammer phenomena can be divided into four categories:
(1) According to the relationship between the duration of valve closure and water hammer, it can be divided into direct water hammer phenomenon and indirect water hammer phenomenon;
(2) According to the fluctuation of water hammer, it can be divided into continuous water hammer phenomenon and separated water hammer phenomenon;
(3) According to the hydraulic characteristics of water hammer, it can be divided into rigid water hammer phenomenon and elastic water hammer phenomenon;
(4) According to the external conditions causing water hammer, it can be divided into startup water hammer phenomenon, valve closing water hammer phenomenon, and pump stopping water hammer phenomenon.
What are the hazards of pump shutdown water hammer:
Pump shutdown water hammer is a type of water hammer phenomenon, which occurs in the pump station and pipeline system due to the sudden power outage of the pump unit caused by misoperation by pump station staff, external power grid accidents tripping, and natural disasters (strong winds, lightning strikes, earthquakes).
According to the investigation, many water hammer accidents belong to pump shutdown water hammer accidents, which pose a great threat to the safety of pump rooms and pipelines. Several pump rooms in China have experienced pump shutdown water hammer, resulting in pump room flooding or pipeline rupture. Although the duration of water hammer phenomenon is short, the engineering accidents it causes cannot be ignored, ranging from vibration and hydraulic impact noise generated by water pump units; In severe cases, the water pump unit may be damaged by vibration, and the pipeline may crack due to hammering, resulting in a water outage accident. If the pump is stopped due to an accident and causes water column separation and flow interruption in the pipeline to bridge the water hammer, its destructive power will be more severe.
How to protect against water hammer when stopping the pump
Due to the possibility of serious accidents in pump stations and water delivery systems caused by pump shutdown water hammer, it is necessary to take corresponding measures according to the specific situation to eliminate pump shutdown water hammer or reduce water hammer pressure. So far, there have been various measures to prevent water hammer during pump shutdown,
It can be roughly divided into:
1. Water replenishment (gas injection) and pressure stabilization can prevent water column separation or high pressure interruption to bridge water hammer. There are bidirectional pressure regulating chambers, unidirectional pressure regulating chambers, and pressure tanks belonging to this type.
(1) Bidirectional pressure regulating tower: Built near the pump station or at an appropriate location on the pipeline, the water surface height of the bidirectional pressure regulating tower should be higher than the water surface height of the receiving pool at the end of the water transmission pipeline, taking into account the head loss along the pipeline. The pressure regulating tower will replenish water or release excessive pressure in the pipeline as the pressure changes, effectively avoiding or reducing water hammer pressure.
(2) Unidirectional pressure regulating tower: Built near the pump station or at an appropriate location in the pipeline, the height of the one-way pressure regulating tower is lower than the pipeline pressure at that location. When the pressure inside the pipeline is lower than the water level inside the tower, the regulating tower replenishes water to the pipeline to prevent the water column from breaking and to avoid bridging water hammer.
(3) Pressure tank: There is not much experience in using it domestically, but it is widely used abroad. It works using specific laws of gas volume and pressure. As the pressure in the pipeline changes, the pressure tank replenishes water to the pipeline or absorbs excessive pressure in the pipeline, similar to a bidirectional pressure regulating tower.
2. Water release and pressure reduction can prevent a sudden increase in pressure
This type includes pump stop water hammer eliminators, slow closing check valves, rupture discs, etc.
(1) There are three main types of pump stop water hammer eliminators: bottom opening pump stop water hammer eliminators, self closing pump stop water hammer eliminators, and automatic reset pump stop water hammer eliminators. They work similarly in principle, that is, when the outlet pressure drops to a certain value during the pump shutdown process, the eliminator opens. When the water hammer pressure rise wave returns to the water pump, the eliminator discharges water outward to eliminate the water hammer. The length of the main pipe protected by the water hammer eliminator generally does not exceed 800m.
(2) Slow closing check valve is a type of check valve that eliminates water hammer through slow closing. It has many forms, is simple and feasible, and has been widely used. There are two types of slow closing check valves: heavy hammer type and accumulator type. This type of valve can adjust the valve closing time within a certain range as needed. Generally, the valve closes 70% to 80% within 3-7 seconds after a power outage, and the remaining 20% to 30% of the closing time is adjusted according to the situation of the water pump and pipeline, usually within the range of 10-30 seconds.
(3) Blasting diaphragm is similar to installing fuses on circuits. When the pressure in the pipeline exceeds the predetermined value due to water hammer, the diaphragm automatically ruptures, causing water flow to leak out and eliminating water hammer.
3. Other types
(1) Appropriately increasing the diameter and wall thickness of the pipeline and reducing the flow velocity of the water transmission pipeline can to some extent reduce the water hammer pressure.
(2) Reduce pipeline length from one pump station to two pump stations, and connect the two pump stations with suction wells.
(3) Choosing a water pump unit with a large moment of inertia or installing a flywheel with sufficient inertia can reduce the water hammer value to a certain extent.
(4) Change the longitudinal section layout of the pipeline. When arranging water pipelines, efforts should be made to avoid sudden changes in slope.