Water hammer

Water hammer . Also known as the Zhukowski pulse, it is the physical phenomenon that occurs when the pressure of a fluid in a pipe changes abruptly, motivated by the sudden closing of a tap, tap or valve; It can also be produced by starting or stopping a motor or hydraulic pump. During the sudden pressure fluctuation the liquid flows along the pipe at a speed defined as the propagation speed of the shock wave. This phenomenon deteriorates the pipes and can even cause them to burst. It is about transforming this useless energy, and even dangerous, into useful energy.

Table of Contents

Summary

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1 History
2 Concept
3 Explanation of the phenomenon
4 Calculation of water hammer
5 As can be foreseen in practice
6 Places or contexts in which this medium seems the most appropriate
7 Description of the main components of a hydraulic ram pump
8 Main advantages and disadvantages
9 Materials to make the hydraulic ram
10 In Cuba
11 Sources

History

The Russian scientist Nikolai Zhukovski ( 1847 – 1921 ) studied this phenomenon for the first time in his work On hydraulic shock, as part of his hydroaeromechanical investigations, which constituted the theoretical basis for the further understanding of the operation of the water hammer or “ hydraulic ram , which shows that physical phenomena (and natural phenomena in general) should not be assumed as negative or positive, but as laws that we must incorporate into our cognitive arsenal towards a harmonious performance of man in nature and towards creative fullness of the human being.

To take advantage of this phenomenon, easily observable in our own water systems, and which is called “water hammer” , the hydraulic ram for a house in Cheshire , England was invented in 1772 by the Englishman John Whitehurs .

In 1796 , the French inventor Joseph Michel Montgolfier ( 1740 – 1810 ) built a hydraulic ram for his paper mill in Voiron. This is an inexpensive and low-maintenance device, making it particularly suitable for use in mountainous areas. , or in rural areas that are elevated with respect to water points.

Since ancient times, man has used hydraulic energy to alleviate or replace his physical work in the toughest daily tasks: grinding grains (mill coupled to a water wheel), generating electricity (electric generator coupled to a hydraulic turbine) and to pump water with the Hydraulic Ram. With the development of internal combustion and electric engines, in the initial times of abundance of fossil fuels, when the environment did not suffer from the pollution of today, a trigger for climate change and the destruction of the biosphere, this technology was forgotten , underestimating its remarkable fuel economy and friendly relationship with the environment.

The main strengths of hydraulic ram pumps are their low maintenance needs and the absence of costs related to the presence of a motor, since the energy that allows their operation comes from falling water. They make it possible to alleviate the problems of suction pumps, whose suction height decreases with suction, so they do not allow the feeding of elevated environments due to gravity (inclined pipes through which the water falls naturally with the slope), and offer an alternative to expensive solutions based on the presence of an electric or diesel motor.

Concept

The elastic deformations in the liquid and in the walls of the pipeline that cause this phenomenon are considered undesirable because they cause frequent breaks in the hydraulic networks of cities and in indoor installations, and it is also the cause of the characteristic sounds that we hear in the pipes when we abruptly turn on or off a tap in our homes. For this reason, time-delay valves are often designed or safety devices are installed.

Explanation of the phenomenon

Due to the slightly elastic properties of the fluid when the valve suddenly closes through which a certain speed and length circulates, the fluid particles that have stopped are pushed by those that come behind and that are still in motion. situation that causes an increase in pressure that moves through the pipe at a speed that can exceed the speed of sound in the fluid. This increase in pressure has two effects: it slightly compresses the fluid, reducing its volume, and slightly expands the pipe. When all the fluid circulating in the pipe has stopped, the impulse that compressed it ceases and, therefore, it tends to expand. On the other hand, the pipe that had been slightly widened tends to resume its normal dimension.

Together, these effects cause another pressure wave in the opposite direction. The fluid moves in the opposite direction but, as the valve is closed, there is a depression with respect to the normal pressure of the pipeline. As the pressure is reduced, the fluid can become a gaseous state, forming a bubble while the pipe contracts.

From the energy point of view, the transformation of the kinetic energy of the fluid into elastic potential energy (pressure changes) and vice versa can be considered. If the pipe lacks friction and is non-deformable (ideal conditions) where there are no energy losses, the phenomenon is reproduced indefinitely. If there is friction and the pipe is elastic, part of the energy is lost and the overpressures are less and less until the phenomenon is extinguished.

In the event of a valve closing, the live force with which the water was animated would be converted into work, determining pressures higher than the initial load on the pipe walls. If the valve could be closed in a time t = 0, instantaneous closure occurs and considering that the water was incompressible and the pipeline was not elastic, the overpressure would have an infinite value.

In practice, the closure takes some time (Tv), no matter how small, and the energy to be absorbed is transformed into compression forces of the water and deformation of the pipe walls.

The overpressure is not infinite, but it has a higher or lower value depending on the closing time and the material of which the pipe is made. Temperature also influences, although not much. This overpressure originates from the valve closing, and travels through the pipe at a speed called the speed “Cs”. These overpressure waves are part of the so-called transient waves, and are usually followed by depression waves.

In Engineering it is very important to determine the magnitude of this overpressure in order to design the pipes with enough resistance to withstand it. In valves operated by discretion, the overpressure is not very great because it is tried that Tv is great (slow closing). But in the equipment operation outputs (pump stop, valve damage, etc.) the overpressure can be very large, so it is tried to reduce it with relief valves , pneumatic chambers , balance chimneys , etc.

Water hammer calculation

If the closing or opening of the valve is abrupt, that is, if the closing time is less than the time it takes for the wave to travel the pipe back and forth, the maximum overpressure is calculated as shown: C × V ₀ ÷ g

Formulas

where:

C is the wave speed (relative velocity with respect to the fluid) of overpressure or depression,
r ₀is the average speed of the fluid, in regime, g is the acceleration of gravity (9.81m / s ² ).

In turn, the wave speed is calculated as: C = ((K ÷ r ₀ ) ÷ (1 + K × (D) ÷ (E × e)) ^½

where:

K is the elastic modulus of the fluid,
r ₀is the density of the fluid,
E is the modulus of elasticity (Young’s modulus) of the pipe which naturally depends on the material of the pipe.

same,

e is the thickness of the pipe walls,
D is the diameter of the pipe.

For the particular case of having water as a fluid:

r ₀= 1000kg / m ³
K = 2.074E + 0.9N / m ²

This expression is reached by Allievi’s formula:

C = 9900 ÷ (43.7 + λ × D / e) ½

where a variable (lambda) that depends on the material of the pipe is introduced, and as a reference the following value is given:
λ = 0.5

The water hammer problem is one of the most complex problems in hydraulics, and it is generally solved using mathematical models that allow the behavior of the system to be simulated.

As can be foreseen in practice

Limiting the speed in pipes.
Slow closing of valves or registers. Construction of parts that do not allow very fast clogging. Use of valves or special mechanical devices. Relief valves , whose discharges prevent excessive pressure values.
Taking into account the overpressure admitted in the calculation of the thickness of the pipes.
Construction of oscillation wells, capable of absorbing water hammer, allowing the oscillation of the water. This solution is adopted provided that the topographic conditions are favorable and the geometric heights are small. Oscillation wells should be located as close as possible to the powerhouse.
Installation of compressed air chambers that provide shock absorption. The maintenance of these devices requires certain care, so that the compressed air is kept in the chambers.

Places or contexts in which this medium seems the most appropriate

It is a particularly suitable method for areas located at a certain altitude and close to a pond or a water source. This technology makes it possible to supply water to isolated rural facilities located at a certain altitude.

Description of the main components of a hydraulic ram pump

The battery valve (also called “shock valve”): It is a generally metallic part that allows water hammer to be caused when it is closed due to water pressure. It determines the performance of the hydraulic ram, and especially that of the pump; therefore, it is advisable that its installation be carried out by a qualified technician.
The battery tube (also called a ‘drive line’): Connects the pump to the reservoir.
The pump body: The pump body receives the water from the power supply from the battery tube and transmits it to the battery valve and the discharge valve.

The water hammer occurs in the body of the pump, which is why it must be made of a material capable of resisting pressure variations and possible chemical attack from the water coming from the power supply.

The discharge valve (also called the discharge clapper): It has a specific role in each phase of operation. During the overpressure phase it is open and allows the passage of water from the body of the pump to the pneumatic tank. During the underpressure, it is closed and prevents the tank from emptying into the pump body.
The pneumatic tank (also called air chamber or air tank): Receives water in periods of overpressure and releases it to the discharge tube in periods of underpressure in the body of the pump. The tank is essential for the proper functioning of the pump, and allows to increase performance and prevent the pump body, the drive pipeline or the tank itself from exploding as a result of water hammer.
The breather: It is a small conditioned orifice below the discharge valve in the body of the pump. It allows feeding the pneumatic tank with air, necessary to release the water in the discharge pipe. It is only installed on the most advanced hydraulic rams, in order to avoid having to bleed the air reservoir.
The check valve: It allows to prevent that, in case of stop of the pump, the water of the discharge conduit does not reach the tank.
The discharge tube: It is connected to the pneumatic tank and to the tank located in height, where the water is collected.

Main advantages and disadvantages

Advantage

Zero energy cost (electricity, gasoline)
Limited maintenance.
Useful life around ten years.
Reduced volume.

Disadvantages

Limited performance (significant loss at shock valve level )
Sensitivity to water impurities .
Little known and widespread procedure due to its limited commercialization
Manufacture in limited series, few suppliers.

Materials to make the hydraulic ram

There is the hydraulic ram completely made of stainless steel, as this would give a much longer useful life to the entire device, but:

It rises to the cost of materials
Increase the weight of the ram.

PVC is a widely used material in the manufacture of hydraulic ram in smaller devices.

It is lighter.

Generally, galvanized steel is currently being used for hydraulic ram due to:

Its low weight compared to stainless steel ,
It is a material resistant to corrosion and abrasion ,
Provides great durability
It provides good resistance to the great pressures that will occur inside the hydraulic ram.
It has the advantages in that the pipe material directly influences the speed.