Water hammer (Water)
Contents
Water hammer
Definition
Water hammer is a term commonly used to explain the unsteady flow situations in pipes or channels that carry water or any other fluid. More generally, it is a pressure surge or wave resulting when a fluid in motion is forced to stop or change when there is a momentum change. This phenomenon commonly occurs when a valve is closed suddenly at an end of a pipeline system and a pressure wave propagates in the pipe.
Most commonly, this occurs in home dishwashers, washing machines or toilets where taps shut off water flow, resulting in a loud bang or banging sound. A hydro-pneumatic device similar in principle to a shock absorber, called a ‘Water Hammer Arrestor’, is installed between the water pipe and the machine that will absorb the shock and stop the banging.
The same phenomenon may occur in steam distribution system where in the phenomenon is known as ‘steam hammer’. This occurs when some of the steam in the system 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 thereby greatly stressing the pipe. This condition is often caused by a poor condensate drainage strategy.
Let us first examine some of the common things that accompany the phenomenon, and later explore more on the associated attributes. Basically, this article explains the fundamental questions like what, where, why and when of the entire phenomenon, including its prevention.
What is it?
The term ‘Water hammer’ has originated from two words – Water and Hammer. Thus, this term is used to describe a phenomenon that involves water and the latter part, hammer, to describe the sound accompanied with the phenomenon.
It is a pressure surge or wave that occurs in any water conductor-like pipe or open channel when the momentum of flowing liquid is disturbed.
Since this phenomenon can occur in a system carrying something other than water, it is also known as ‘Fluid hammer’.
Where does it occur?
The change in momentum of a flowing liquid occurs in the entity carrying the fluid, and water hammer can therefore occur in a pipe, open channel or any water/fluid conductor system. Furthermore, the flowing liquid in the conductor system could be under pressure, and water hammer can occur in pumping systems or gravity systems. However, since the disturbance in flow in pumping systems is more aggravated or intense, it is more frequent in pumping systems than gravity systems.
Most commonly, water hammer is observed in regular pipes in the home when the tap at the end of the pipe is opened suddenly or for the first time in the day. This activity is generally accompanied with the ‘thud’ or the ‘knocking’ sound.
Why does it occur?
Water hammer basically occurs due to the sudden change in the flow conditions. This change may be intentional or accidental. Following are some of the general causes:
- Sudden valve closure;
- Pump failure; and
- Valve malfunction (valve slam that shuts rapidly depending on the dynamic characteristic)
What happens and why should it be prevented?
Whenever the flowing liquid in a pipe is disturbed by either closing or opening of a valve at the end of the pipe, there are pressure fluctuations. Depending on the time of closure and the way the valve is closed and the actual pipeline profile, it may prove to be detrimental if such pressures created exceed the pressure for which the pipeline is designed. Any positive pressure that exceeds the pipeline design pressure can result in the bursting of the pipeline. Any negative pressure less than the design negative pressure can cause crumbling of the pipeline. For the reasons mentioned above, the pipeline burst or collapse should therefore be prevented.
Magnitude of water hammer
The magnitude of water hammer pulse can be estimated from the well known Joukowsky equation as below:
- ?P = ?*a*?C ------------------------------------------(1)</dd>
where, ?P is the magnitude of the pressure wave (Pa), ? is the density of the fluid (kg/m3), a is the speed of sound in the fluid (m/sec) or the water hammer wave velocity, and ?C is the change of the fluid velocity (m/sec).
Dynamic equations
The following partial differential equations are used to calculate the magnitude of water hammer:
- ?V/?x + 1/Bm * ?P/?t = 0 --------------------------(2)</dd>
- ?V/?t + 1/? * ?P/?x + f/2d *v* |v| = 0 -------------(3)</dd>
where, V is the fluid velocity inside pipe, ? is the fluid density and Bm is the bulk modulus, f is the friction factor.
Water hammer mitigating measures
The following (one or a combination) reduce water hammer in any water conductor system:
- Flywheel on Pumps (Increased moment of inertia of pumps);
- Reduced velocity of flow in the pipeline;
- Slow closure of valves;
- Adopting correct pump start-up and shut down conditions;
- Air chambers (Air vessels) at pumps;
- One-way Surge tanks; and
- Air valves
Software
Many commercial and non-commercial software packages exist to analyze water hammer. Generally, all software use ‘Method of Characteristics’ to solve the unsteady partial differential equations (Ref. Equations (1) and (2)).
The software packages generally use several boundary conditions that are encountered in pumping/gravity systems:
- a) Reservoir;</dd>
- b) Pumps;</dd>
- c) Pipeline;</dd>
- d) Air Valves;</dd>
- e) Pipe Junction (converging/diverging);</dd>
- f) Flow Control Valve (Check, BFV, PRV, Sluice Valve, Gate Valve, etc.);</dd>
- g) Dead End. </dd>
Many of the computer programs also handle the surge protective devices like:
- a) Air Chamber (Air Vessel); </dd>
- b) Pressure Regulating Valve (PRV); and </dd>
- c) Surge Tanks; </dd>
Some of the more specialized programs also cater to the following boundary conditions and capabilities:
- a) Intakes; </dd>
- b) Condenser; </dd>
- c) Hydro-turbine; </dd>
- d) Intakes; and </dd>
- e) Multiphase flow capabilities. </dd>
References
- Streeter, V.L., Wylie, E.B., and Bedford, K.W., 1998. Fluid Mechanics,.McGraw-Hill Higher Education International, 9th Revised Edition. ISBN: 0070625379
- Thorley, A.D.R., 2004. Fluid Transients in Pipelines, 2nd Edition, Professional Engineering Publishing. ISBN: 0791802108
Further Reading
- Plast-O-Matic Valves, Inc. Tech Brief: The Effects of Water Hammer And Pulsations.
- Wikipedia. Water hammer.
