What is a hydraulic shock? Causes of hydraulic shock in pipes

Hydraulic shock in pipelines is an instantaneous pressure jump. The drop is associated with a sharp change in the velocity of the water flow. Further on, we learn in more detail how a hydraulic shock occurs in pipelines.

The main error

It is mistakenly considered a hydraulic impact the result of filling the liquid with a super-piston space in the engine of the corresponding configuration (piston). As a result, the piston does not reach the dead center and starts squeezing the water. This, in turn, leads to engine failure. In particular, to break the rod or connecting rod, the breakage of the studs in the cylinder head, the rupture of the gaskets.

Classification

In accordance with the direction of the pressure jump, the hydraulic shock can be:

• Positive. In this case, the increase in pressure occurs due to a sharp turn on the pump or shutting off the pipe.
• Negative. In this case, we are talking about the pressure drop due to the opening of the flap or the pump off.

In accordance with the time of wave propagation and the period of overlapping of the gate valve (or other shut-off valves), during which a hydraulic shock was generated 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 valve. It is likely that the front of the wave will pass repeatedly direct and reverse direction. With an incomplete hydraulic impact, the flow not only can begin to move to the other side, but also partially pass further through the valve if it is not closed to the end.

Effects

The most dangerous is a positive hydraulic shock in the heating system or water supply. If the pressure drop is too high, the line may be damaged. In particular, longitudinal cracks appear on the pipes, which subsequently leads to a split, a breach of the tightness in the stop valves. Because of these failures, the water supply equipment begins to fail: heat exchangers, pumps. In this regard, hydraulic shock must be prevented or reduced its strength. The water pressure becomes maximum during the braking of the flow when all kinetic energy is transferred to the work of stretching the walls of the main and compressing the column of liquid.

Research

Experimentally and theoretically studied the phenomenon in 1899. Nikolai Zhukovsky. The researcher identified the causes of the hydraulic shock. The phenomenon is due to the fact that during the closure of the main line, which is the flow of liquid, or when it is quickly closed (when the dead-end channel is connected to a source of hydraulic energy), a sudden change in the pressure and speed of water is formed. It is not simultaneously along the entire pipeline. If in this case to make certain measurements, it can be determined that the change in speed occurs in direction and magnitude, and pressure - both in the direction of decrease and increase with respect to the initial. All this means that the oscillatory process takes place in the main line. It is characterized by a periodic decrease and an increase in pressure. This entire process is characterized by transience and is caused by elastic deformations of the liquid itself and the walls of the tube. Zhukovsky proved that the speed with which the wave propagates is directly proportional to the compressibility of the water. The value of the deformation of the pipe walls is also important. It is determined by the modulus of elasticity of the material. The speed of the wave depends on the diameter of the pipeline. A sharp pressure jump can not occur in the gas-filled line, since it is compressed quite easily.

Process Flow

In an autonomous water supply system, for example a country house, a downhole pump can be used to create pressure in the pipeline. Hydraulic shock occurs when a sudden stop of fluid consumption - when the crane is closed. The water stream that made the traffic along the highway is unable to stop instantly. The pillar of liquid, by inertia, cuts into the water "dead end", which was formed when the crane was closed. From a hydraulic shock the relay in this case does not save. It just reacts to the jump by shutting down the pump after the tap is closed and the pressure exceeds the maximum value. The shutdown, as well as stopping the water flow, is not instantaneous.

Examples

It is possible to consider a pipeline with a constant head and the movement of a liquid of a constant nature, in which the valve was suddenly closed or the valve was suddenly closed. In a downhole water supply system, as a rule, a hydraulic shock occurs when the return gate element is located higher than the static water level (by 9 meters or more) or has a leak, while the next higher pressure valve keeps the pressure. In either case, partial discharge occurs. In the next pump start, high-speed water will fill the vacuum. The liquid collides with a closed check valve and a stream above it, causing a pressure surge. As a result, a hydrostatic shock occurs. It contributes not only to the formation of cracks and the destruction of compounds. If a pressure surge occurs, the pump or motor (or both) is damaged. This phenomenon can occur in systems of volumetric hydraulic drive, when a spool valve is used. When the spool is closed by one of the fluid injection channels, the processes described above occur.

Protection against hydraulic shocks

The force of the jump will depend on the flow velocity before and after the main line is closed. The more intense the movement, the stronger the impact when a sudden stop. The velocity of the flow itself will depend on the diameter of the line. The larger the cross section, the weaker the motion of the liquid. From this it can be concluded that the use of large pipelines reduces the likelihood of a water hammer or weakens it. Another way is to increase the duration of the overlapping of the water pipe or the pump. To carry out gradual overlapping of the pipe, valve type shut-off elements are used. Specially for pumps, sets are used for smooth starting. They allow not only to avoid water hammer during the switching on, but also significantly increase the service life of the pump.

Compensators

The third option involves the use of a damper device. It is a membrane expansion tank, which is able to "quench" the resulting pressure jumps. Hydraulic hammer compensators work according to a certain principle. It consists in the fact that in the process of increasing pressure, the piston moves the liquid and compresses the elastic element (spring or air). As a result, the shock process is transformed into an oscillatory process. Due to the dissipation of energy, the latter decays rapidly 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 for absorbing the energy of the moving water column from the filling tank or battery to the compensator.