There are tons of articles, videos, and books about water hammer. But what the heck is it? Simply put, it's the pressure surge that occurs in a piping system when the fluid within is forced to stop or change direction suddenly, and is often accompanied by knocking or banging sounds. While it may sound harmless, or even like amusing poltergeist activity, the force exerted on the system due to the fluid's momentum can generate pressure surges that wildly surpass standard operating pressure. The abrupt changes in flow, coupled with high and low pressure fluctuations, have the capacity to inflict considerable harm on the entire system. Damage can be caused from a single incident or multiple incidents over time.
Neglecting to deal with the causes of water hammer will eventually lead to the breakdown of your fluid transfer system. Continued exposure to water hammer can result in:
- Pump and flow system damage
- Leaks
- Pipe ruptures
- External property damage
- Injury to personnel
- Downtime/maintenance
The Many Names for Water Hammer
Similar to how you hear a sonic “boom” or the “crack” of a whip, the term “water hammer” describes the audible noise heard in pipelines as the pressure shock waves reverberate throughout the system. One very important item to note - “water hammer” occurs in all fluid piping systems, not just those flowing water.
The term “surge” is typically used to reference the sudden rise in pressure. Interestingly enough, it’s also used in the electrical world when referencing abrupt changes in electrical voltage caused by lightning or the starting and stopping of large appliances.
In the electrical world this is called transient voltage, which leads us to our next term - “hydraulic transient”. Before expanding on this next term however, it’s worth noting that the term “surge” doesn’t do justice to describe the pressure shock waves of water hammer.
Surge in a pump system describes both positive and negative changes in pressure. But since the word "surge" is widely associated with "increase", people often miss or don’t account for negative surge pressures.
The term “hydraulic transient” describes the pressure wave front where on one side is steady state flow and pressure and the other side is unsteady state flow and pressure. Think of this as a message that is quickly passed from one end of the system to another - as it is passed, the state changes.
For example, imagine a tightly packed marching band, marching in linear step. Suddenly, the guy with the tuba trips and falls into the band members next to him. The “transient” in this scenario is the wave or domino effect of falling band members. The transient is over when the band regains its steady step.
The Physics
Water hammer physics—this is the best part! It’s like a special sauce that turns an everyday nugget into something amazing. The secret ingredients are two equations: (1) the instantaneous water hammer equation and (2) the wavespeed equation.
The instantaneous water hammer equation (also commonly known as the Joukowski Equation) provides a way of calculating the initial rise, or fall, in pressure when a change in fluid velocity occurs faster than the communication time (i.e. the time it takes the pressure wave to reverberate out and back). This equation is a factor of the fluid density, the quantified change in velocity and the system’s wavespeed, which is the second secret ingredient.
In many cases, the Joukowski Equation appropriately estimates the worst case change in pressure. However, unmitigated water hammer can lead to other secondary and even worse scenarios—such as the super-imposing of multiple transient pressure waves, line pack, and cavitation. For this reason, the equation should only be used as a starting point, not the end all/be all assessment. The next step would be transient analysis... something to be discussed in future blog posts.
The second secret ingredient is the wavespeed equation. This equation is a bit more complex, but essentially, quantifies the rigidity of the system as a whole—pipe and fluid considered.
Note: the wavespeed, also called "celerity", doesn’t just represent how fast the transient pressure wave moves through the system. It also directly impacts the degree of pressure rise or fall as seen in the instantaneous water hammer equation.
Water Hammer is Not a Rarity
Sorry folks, no "phenomenon" could be more justifiably dismissed. Water hammer, whether large or small, is truly just everyday physics. The challenge is that water hammer often goes undetected, unmeasured, and unmitigated. But make no mistake—water hammer is impacting all fluid piping systems and will, in time, make itself known one way or another.
Want to learn how you can tackle this problem once and for all?
Watch Blacoh’s webinar series all about water hammer! Over the course of three sessions, we’ll discuss everything water hammer, including changes you can make to extend the life of your pump system.
Click here to watch our Surge Vessels webinar.
Click here to watch our Demystifying Water Hammer webinar.
Click here to watch our Transient Monitoring webinar.
References:
- Walters, T.W., and Leishear, R.A., 2018, “When the Joukowsky Equation Does Not Predict Maximum Water Hammer Pressures”, ASME PVP Conference, July 15-20, 2018, Prague, Czech Republic, ASME PVP2018-84338
- Wood, D.J., Lingireddy, S, and Boulos, P.F., 2005, Pressure Wave Analysis of Transient Flow in Pipe Distribution Systems, MWH Soft, Pasadena, CA.
Books:
- Fluid Transients in Pipeline Systems by ARD Thorley
- Applied Hydraulic Transients by M. Hanif Chaudry
- Fluid Transients by E. Benjamin Wylie and Victor L. Streeter.
- Surge Analysis and The Wave Plan Method by Dr. Srinivasa Lingireddy and Dr. Don J. Wood
Technical Papers:
- Arris Tijsseling
- Robert A. Leishear
- Trey Walters
- Amy Marroquin