Water hammer in pipelines is an instantaneous pressure surge. The difference is associated with a sharp change in the speed of the water flow. Next, we will learn more about how hydraulic shock occurs in pipelines.
Main delusion
The result of liquid filling of the over-piston space in the engine of the corresponding configuration (piston) is erroneously considered to be hydraulic shock. As a result, the piston does not reach the dead center and begins to compress the water. This, in turn, leads to engine failure. In particular, to a break in the rod or connecting rod, breakage of the studs in the cylinder head, rupture of the gaskets.
Classification
According to the direction of pressure surge, water hammer can be:
- Positive. In this case, the increase in pressure occurs due to a sharp turn on of the pump or blockage of 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.
According to timewave propagation and the period of closing the valve (or other shutoff valves), during which a water hammer formed in the pipes, it is divided into:
- Straight (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 valve. It is quite probable that the wave front will pass repeatedly forward and backward direction. With an incomplete water hammer, the flow can not only begin to move in the other direction, but also partially pass further through the valve if it is not completely closed.
Consequences
The most dangerous is considered to be a positive water hammer in the heating or water supply system. If the pressure surge 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 valves. Due to these failures, plumbing equipment begins to fail: heat exchangers, pumps. In this regard, water hammer must be prevented or reduced. The water pressure becomes maximum in the process of flow deceleration when all kinetic energy is transferred to the work of stretching the walls of the main and compressing the liquid column.
Research
Experimentally and theoretically studied the phenomenon in 1899 Nikolai Zhukovsky. The researcher has identifiedcauses of hydraulic shock. The phenomenon is due to the fact that in the process of closing the line through which the fluid flows, or when it is quickly closed (when a dead-end channel is connected to a source of hydraulic energy), a sharp change in pressure and water velocity is formed. It is not all over the pipeline at the same time. If in this case certain measurements are made, then 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 initial one. All this means that an oscillatory process takes place in the line. It is characterized by a periodic decrease and increase in pressure. This whole process is characterized by transience and is caused by elastic deformations of the liquid itself and the walls of the pipe. Zhukovsky proved that the speed at which a wave propagates is directly proportional to the compressibility of water. The amount of deformation of the pipe walls is also important. It is determined by the modulus of elasticity of the material. Wave speed also depends on the diameter of the pipeline. A sudden pressure surge cannot occur in a line filled with gas, as it compresses quite easily.
Process progress
In an autonomous water supply system, such as a country house, a borehole pump can be used to create pressure in the line. Water hammer occurs when fluid consumption suddenly stops - when a tap is turned off. A stream of water moving alonghighway, unable to stop instantly. The column of liquid by inertia crashes into the plumbing "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 surge, turning off the pump after the valve is closed and the pressure exceeds the maximum value. Shutdown, like stopping water flow, is not immediate.
Examples
One can consider a pipeline with a constant pressure and fluid movement of a constant nature, in which a valve was abruptly closed or a gate valve was suddenly closed. In a downhole water supply system, water hammer typically occurs when the check valve is higher than the static water level (9 meters or more) or is leaking while the next valve above is holding pressure. In both cases, a partial discharge occurs. The next time the pump is started, the high velocity water will fill the vacuum. The liquid impinges on the closed check valve and the flow above it, causing a pressure surge. The result is water hammer. It contributes not only to the formation of cracks and the destruction of joints. When a pressure surge occurs, the pump or the electric motor (and sometimes both elements at once) is damaged. This phenomenon can occur in positive displacement hydraulic drive systems when a spool valve is used. When one of the discharge channels is blocked by a spoolliquid arise processes described above.
Protection against water hammer
The strength of the surge will depend on the flow rate before and after the line is blocked. The more intense the movement, the stronger the impact when suddenly stopped. The speed of the flow itself will depend on the diameter of the line. The larger the cross section, the weaker the fluid movement. From this it can be concluded that the use of large pipelines reduces the likelihood of water hammer or weakens it. Another way is to increase the duration of shutting off the water supply or turning on the pump. Valve-type shut-off elements are used to gradually close the pipe. Especially for pumps, soft start kits are used. They allow not only to avoid water hammer during switching on, but also significantly increase the operating life of the pump.
Compensators
The third protection option involves the use of a damper device. It is a membrane expansion tank, which is able to "extinguish" the resulting pressure surges. Water hammer compensators work according to a certain principle. It lies in the fact that in the process of increasing pressure, the piston moves with liquid and the elastic element (spring or air) is compressed. As a result, the shock process is transformed into an oscillatory one. Due to energy dissipation, the latter decays fairly quickly without a significant increase in pressure. The compensator is used in the filling line. It's being chargedcompressed air at a pressure of 0.8-1.0 MPa. The calculation is made approximately, in accordance with the conditions for absorbing the energy of the driving column of water from the filling tank or accumulator to the compensator.
