Water hammer

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Water hammer (or, more generally, fluid hammer) is a pressure surge or wave caused when a fluid (usually a liquid but sometimes also a gas) in motion is forced to stop or change direction suddenly (momentum change). Water hammer commonly occurs when a valve closes suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe. It's also called hydraulic shock.

This pressure wave can cause major problems, from noise and vibration to pipe collapse. It is possible to reduce the effects of the water hammer pulses with accumulators and other features.

Rough calculations can be made either using the Joukowsky equation,[1] or more accurate ones using the method of characteristics.

Contents

Cause and effect

If the pipe is suddenly closed at the outlet (downstream), the mass of water before the closure is still moving forward with some velocity, building up a high pressure and shock waves. In domestic plumbing this is experienced as a loud banging resembling a hammering noise. Water hammer can cause pipelines to break if the pressure is high enough. Air traps or stand pipes (open at the top) are sometimes added as dampers to water systems to provide a cushion to absorb the force of moving water to prevent damage to the system.

In hydroelectric generating stations, the water travelling along the tunnel or pipeline may be prevented from entering a turbine by closing a valve. But if there is, say, 14km of tunnel of say 7.7m diameter, full of water travelling at say 3.75 m/sec[2], that represents a very large amount of kinetic energy that must be arrested. This is frequently achieved by a surge shaft[3] open at the top, into which the water flows. As the water rises up the shaft, converting kinetic energy into potential energy, it decelerates the water in the tunnel. At some HEP stations, what looks like a water tower is actually one of these devices, known in these cases as a surge drum.

In the home, water hammer may occur when a dishwasher, washing machine, or toilet shuts off water flow. The result may be heard as a loud bang, repetitive banging (as the shock wave travels back and forth in the plumbing system), or as some shuddering.

On the other hand, when an upstream valve in a pipe closes, water downstream of the valve attempts to continue flowing, creating a vacuum that may cause the pipe to collapse or implode. This problem can be particularly acute if the pipe is on a downhill slope. To prevent this, air and vacuum relief valves, or air vents, are installed just downstream of the valve to allow air to enter the line and prevent this vacuum from occurring.

Other causes of water hammer are pump failure, and check valve slam (due to sudden deceleration, a check valve may slam shut rapidly, depending on the dynamic characteristic of the check valve and the mass of the water between a check valve and tank).

Related phenomena

Expansion joints on a steam line that have been destroyed by steam hammer

Steam distribution systems may also be vulnerable to a situation similar to water hammer, known as steam hammer. In a steam system, water hammer most often occurs when some of the steam condenses into water in a horizontal section of the steam piping. Subsequently, steam picks up the water, forms a "slug" and hurls it at high velocity into a pipe fitting, creating a loud hammering noise and greatly stressing the pipe. This condition is usually caused by a poor condensate drainage strategy.

Where air filled traps are used, these eventually become depleted of their trapped air over a long period of time through absorption into the water. This can be cured by shutting off the supply, opening taps at the highest and lowest locations to drain the system (thereby restoring air to the traps), and then closing the taps and re-opening the supply.

Water hammer during an explosion

When an explosion happens in an enclosed space, water hammer can cause the walls of the container to deform. However, it can also impart momentum to the enclosure if it is free to move. An underwater explosion in the SL-1 nuclear reactor vessel caused the water to accelerate upwards through 2.5 feet (0.76 m) of air before it struck the vessel head at 160 feet per second (49 m/s) with a pressure of 10,000 pounds per square inch (69,000 kPa). This pressure wave caused the 26,000 pounds (12,000 kg) steel vessel to jump 9 feet 1 inch (2.77 m) into the air before it dropped into its prior location.[4]

Mitigating measures

Water hammer has caused accidents and fatalities, but usually damage is limited to breakage of pipes or appendages. An engineer should always assess the risk of a pipeline burst. Pipelines transporting hazardous liquids or gases warrant special care in design, construction, and operation.

The following characteristics may reduce or eliminate water hammer:

Typical pressure wave caused by closing a valve in a pipeline

The magnitude of the pulse

One of the first to successfully investigate the water hammer problem was the Italian engineer Lorenzo Allievi.

Water hammer can be analyzed by two different approaches—rigid column theory, which ignores compressibility of the fluid and elasticity of the walls of the pipe, or by a full analysis that includes elasticity. When the time it takes a valve to close is long compared to the propagation time for a pressure wave to travel the length of the pipe, then rigid column theory is appropriate; otherwise considering elasticity may be necessary.[5] Below are two approximations for the peak pressure, one that considers elasticity, but assumes the valve closes instantaneously, and a second that neglects elasticity but includes a finite time for the valve to close.

Instant valve closure; compressible fluid

The pressure profile of the water hammer pulse can be calculated from the Joukowsky equation [6]

\frac{\delta P}{\delta t} =\rho a \frac{\delta v}{\delta t}

So for a valve closing instantaneously, the maximum magnitude of the water hammer pulse is:

\Delta P =\rho a \Delta v

where ΔP is the magnitude of the pressure wave (Pa), ρ is the density of the fluid (kgm−3), a is the speed of sound in the fluid (ms−1), and Δv is the change in the fluid's velocity (ms−1). The pulse comes about due to Newton's laws of motion and the continuity equation applied to the deceleration of a fluid element.[7]

Equation for wave speed

As the speed of sound in a fluid is the \sqrt{\frac{\text{effective bulk modulus}} {\text{density}}}, the peak pressure depends on the fluid compressibility if the valve is closed abruptly.

a = \sqrt{\frac{K/\rho} {(1+V/a)[1+(K/E)(D/t)c]}}

where

Slow valve closure; incompressible fluid

When the valve is closed slowly compared to the transit time for a pressure wave to travel the length of the pipe, the elasticity can be neglected, and the phenomenon can be described in terms of inertance or rigid column theory:

F = m a = P A = \rho L A {dv \over dt}.

Assuming constant deceleration of the water column (dv/dt = v/t), gives:

P = \rho v L/t.

where:

The above formula becomes, for water and with imperial unit: P = 0.0135 V L/t. For practical application, a safety factor of about 5 is recommended:

P =0.07 V L/t +P_1

where P1 is the inlet pressure in psi, V is the flow velocity in ft/sec, t is the valve closing time in seconds and L is the upstream pipe length in feet.[8]

Expression for the excess pressure due to water hammer

When a valve with a volumetric flow rate Q is closed, an excess pressure δP is created upstream of the valve, whose value is given by the Joukowsky equation:

\delta P = Z_h \, Q

In this expression[9]:

The hydraulic impedance Zh of the pipeline determines the magnitude of the water hammer pulse. It is itself defined by:

Z_h = \frac{\sqrt{\rho \, B_\mathit{eff}}}{A}

with:

The latter follows from a series of hydraulic concepts:

Thus, the effective compressibility modulus is:

\frac{1}{B_\mathit{eff}} = \frac{1}{B_l} + \frac{1}{B_{eq}} + \frac{1}{B_g}

As a result, we see that we can reduce the water hammer by:

Dynamic equations

The water hammer effect can be simulated by solving the following partial differential equations.

 \frac{\partial V}{\partial x}+ \frac{1}{B_m}\frac{\partial P}{\partial t}=0\,
 \frac{\partial V}{\partial t}+ \frac{1}{\rho}\frac{\partial P}{\partial x}+\frac{f}{2D}V|V|=0\,

where V is the fluid velocity inside pipe, \rho is the fluid density and B_m is the equivalent bulk modulus, f is the friction factor.

Column separation

Column separation is a phenomenon that can occur during a water-hammer event. If the pressure in a pipeline drops rapidly to the vapor pressure of the liquid, the liquid vaporises and a "bubble" of vapor forms in the pipeline. This is most likely to occur at specific locations such as closed ends, high points or knees (changes in pipe slope). When the pressure later increases above the vapor pressure of the liquid, the vapor in the bubble returns to a liquid state, which leaves a vacuum in the space formerly occupied by the vapor. The liquid either side of the vacuum is then accelerated into this space by the pressure difference. The collision of the two columns of liquid, (or of one liquid column if at a closed end,) results in Cavitation and causes a large and nearly instantaneous rise in pressure. This pressure rise can damage hydraulic machinery, individual pipes and supporting structures. Many repetitions of cavity formation and collapse may occur in a single water-hammer event.[10]

Simulation software

Most water hammer software packages use the method of characteristics [7] to solve the differential equations involved. This method works well if the wave speed does not vary in time due to either air or gas entrainment in a pipeline. The Wave Method (WM) is also used in various software packages. WM lets operators analyze large networks efficiently. Many commercial and non commercial packages are available.

Software packages vary in complexity, dependent on the processes modeled. The more sophisticated packages may have any of the following features:

  • Multiphase flow capabilities
  • An algorithm for cavitation growth and collapse
  • Unsteady friction - the pressure waves dampens as turbulence is generated and due to variations in the flow velocity distribution
  • Varying bulk modulus for higher pressures (water becomes less compressible)
  • Fluid structure interaction - the pipeline reacts on the varying pressures and causes pressure waves itself

Applications

See also

References

  1. ^ Kay, Melvyn (2008). Practical Hydraulics (2nd ed.). Taylor & Francis. ISBN 0-415-35115-4. http://books.google.com/books?isbn=0415351154&pg=PA120.
  2. ^ http://communities.bentley.com/products/hydraulics___hydrology/f/5925/p/60896/147250.aspx#147250
  3. ^ http://cr4.globalspec.com/thread/73646
  4. ^ Flight Propulsion Laboratory Department, General Electric Company, Idaho Falls, Idaho (November 21, 1962), Additional Analysis of the SL-1 Excursion: Final Report of Progress July through October 1962, U.S. Atomic Energy Commission, Division of Technical Information, IDO-19313, http://www.id.doe.gov/foia/PDF/IDO-19313.pdf; also TM-62-11-707
  5. ^ Bruce, S.; Larock, E.; Jeppson, R. W.; Watters, G. Z. (2000), Hydraulics of Pipeline Systems, CRC Press, ISBN 0-8493-1806-8
  6. ^ Thorley, A. R. D. (2004), Fluid Transients in Pipelines (2nd ed.), Professional Engineering Publishing, ISBN 0-79180210-8[page needed]
  7. ^ a b c Streeter, V. L.; Wylie, E. B. (1998), Fluid Mechanics (International 9th Revised ed.), McGraw-Hill Higher Education[page needed]
  8. ^ "Water Hammer & Pulsation"
  9. ^ Faisandier, J., Hydraulic and Pneumatic Mechanisms, 8th edition, Dunod, Paris, 1999 (ISBN 2100499483)
  10. ^ Bergeron, L., 1950. Du Coup de Bélier en Hydraulique - Au Coup de Foudre en Electricité. (Waterhammer in hydraulics and wave surges in electricity.) Paris: Dunod (in French). (English translation by ASME Committee, New York: John Wiley & Sons, 1961.)

External links