Water hammer in pipelines is an instantaneous pressure surge. The difference is associated with a sharp change in the speed of the water stream. Next, we learn in more detail how a hydraulic shock occurs in pipelines.
Main misconception
The result of filling with a liquid above the piston space in an engine of the corresponding configuration (piston) is mistakenly considered a water hammer. As a result, the piston does not reach the dead center and begins to compress water. This, in turn, leads to engine failure. In particular, to a fracture of the rod or connecting rod, breakage of the studs in the cylinder head, breaks in the gaskets.
Classification
In accordance with the direction of the pressure jump, a water hammer can be:
- Positive. In this case, the increase in pressure occurs due to a sharp turn on of the pump or blocking the pipe.
- Negative. In this case, we are talking about a pressure drop as a result of opening the damper or turning off the pump.
In accordance with the wave propagation time and the period of shutoff of the valve (or other shutoff valves), during which a water hammer formed in the pipes, it is divided into:
- Direct (full).
- Indirect (incomplete).
In the first case, the front of the formed wave moves in the direction opposite to the original direction of the water flow. Further movement will depend on the elements of the pipeline, which are located before the closed gate. It is likely that the wave front will pass repeatedly forward and backward directions. In case of incomplete hydraulic shock, the flow can not only begin to move in the other direction, but also partially pass further through the valve if it is not fully closed.
Effects
The most dangerous is considered a positive hydraulic shock in the heating system or water supply. If the pressure jump is too high, the line may be damaged. In particular, longitudinal cracks appear on the pipes, which subsequently leads to a split, a violation of the tightness in the stop valves. Due to these failures, plumbing equipment begins to fail: heat exchangers, pumps. In this regard, water hammer must be prevented or reduced its strength. Water pressure becomes maximum during flow inhibition during the transition of all kinetic energy to the work of stretching the walls of the line and compressing the liquid column.
Research
Experimentally and theoretically studied the phenomenon in 1899, Nikolai Zhukovsky. The researcher identified the causes of water hammer. The phenomenon is due to the fact that in the process of closing the line along which the fluid flows, or when it is quickly closed (when connecting the dead end channel with a hydraulic energy source), a sharp change in the pressure and speed of water is formed. It is not simultaneously throughout the pipeline. If in this case, to make certain measurements, it can be revealed that the change in speed occurs in direction and magnitude, and pressure - both in the direction of decrease and increase relative to the original. All this means that an oscillatory process takes place in the trunk. It is characterized by periodic decrease and increase in pressure. This whole process is transient and is caused by elastic deformations of the liquid itself and the pipe walls. Zhukovsky proved that the speed with which the wave propagates is in direct proportion to the compressibility of the water. Also important is the magnitude of the deformation of the pipe walls. It is determined by the modulus of elasticity of the material. The wave speed also depends on the diameter of the pipeline. A sharp jump in pressure cannot occur in a gas-filled line, since it is easily compressed.
Process
In an autonomous water supply system, such as a country house, a downhole pump can be used to create pressure in the line. Water hammer occurs when there is a sudden cessation of fluid intake - when the tap is shut off. The water stream that made the movement along the highway is unable to stop instantly. A column of liquid by inertia crashes into the water "dead end", which was formed when the tap was closed. In this case, the relay does not save from water hammer. It only reacts to the jump, turning off the pump after the valve is closed, and the pressure exceeds the maximum value. Shutdown, like stopping the water flow, is not instantaneous.
Examples
You can consider a pipeline with a constant pressure and fluid movement, which is of a constant nature, in which the valve was sharply closed or the valve was suddenly closed. In a borehole water supply system, as a rule, a water hammer occurs when the return gate element is higher than the static water level (9 meters or more) or has a leak, while the next valve above holds the pressure. In both cases, partial discharge occurs. The next time the pump starts, water flowing at high speed will fill the vacuum. The fluid collides with the closed check valve and the flow above it, causing a pressure surge. As a result, water hammer occurs. It contributes not only to cracking and fracture of joints. If a pressure surge occurs, the pump or electric motor is damaged (and sometimes both elements at once). This phenomenon can occur in surround hydraulic drive systems when a spool valve is used. When the spool overlaps one of the fluid injection channels, the processes described above occur.
Water hammer protection
The strength of the jump will depend on the flow rate before and after the overlap of the trunk. The more intense the movement, the stronger the shock at a sudden stop. The speed of the flow itself will depend on the diameter of the line. The larger the cross section, the weaker the movement of the liquid. From this we can conclude that the use of large pipelines reduces the likelihood of water hammer or weakens it. Another way is to increase the duration of the shut-off of the water supply or turning on the pump. For the implementation of the gradual closure of the pipe, valve-type locking elements are used. Especially for pumps, soft start kits are used. They allow not only to avoid water hammer during the switching process, but also significantly increase the operational life of the pump.
Compensators
The third option of protection involves the use of a damping device. It is a membrane expansion tank, which is able to "quench" the resulting pressure surges. Water shock absorbers work according to a specific principle. It consists in the fact that in the process of increasing pressure, the piston moves with the liquid and the elastic element (spring or air) is compressed. As a result, the shock process is transformed into an oscillatory one. Due to the dissipation of energy, the latter decays quickly enough without a significant increase in pressure. The compensator is used in the filling line. It is charged with compressed air at a pressure of 0.8-1.0 MPa. The calculation is made approximately, in accordance with the conditions of energy absorption of the moving water column from the filling tank or battery to the compensator.