Understanding Water Hammer: Beating to Death a Commonly Misunderstood Term
The term “water hammer” seems to suggest the use of water as a kind of hand tool. Although a water hammer is not necessarily used for driving nails, the name is appropriate.
We'll discuss the flow of steam and liquids through pipes, how water hammer happens, and design methods to avoid water hammer damages.
Kinetic Energy and Liquids
When liquid flows through a pipe it has energy. To be more specific, because the water is moving, it has “kinetic energy.” The kinetic energy of a moving object (as in a volume of water) has to do with the mass and the velocity of the moving object. The kinetic energy of a moving object is calculated by multiplying half the mass and the velocity squared together. With respect to liquid flowing through a pipe, the important thing to remember is that the kinetic energy increases with the amount of liquid, and it increases a lot more with its velocity.
The Showerhead Example
You experience the energy of water when taking a shower as the water sprays onto you from the showerhead. If you place your hand over the showerhead, you can feel this energy being dissipated as a force on your hand. If you have ever heard the pipes rattle in the walls when you turn off the shower or when your washing machine stops filling, you have witnessed a “water hammer.”
When the water going to the showerhead is quickly shut off, the energy of the flowing water must go somewhere. Bringing all this water quickly to a stop creates a force that is applied to the pipe, causing it to rattle around in the wall. Chances are your home has devices installed on the pipes as part of the plumbing system that absorb the energy transfer from the water. The effect of these devices is to reduce the forces on the pipes. In all cases, these devices increase the amount of time over which the water is slowed to a stop within the pipes.
The Relationship between Energy Transfers and Time
Balls, Bricks, and Showers
When you have energy in something and it must transfer to something else, time becomes the factor that matters most. If a hammer is swung at a large rubber ball, the energy of the hammer is transferred to the ball in about a second, the hammer stops, and then the ball transfers some of that energy back to the hammer, pushing it away. The ball doesn’t break. Now let’s consider that same hammer hitting a brick. The transfer of energy here happens over a tiny fraction of a second. A brick can withstand the weight of a tractor-trailer when the load is applied by rolling tires. But, a child can shatter that same brick with a small hammer. What we have here is a simple but recognizable example of energy transfer at work, and the critical element is time. When a shower is turned off quickly, or the energy of the hammer is transferred to the brick in a tiny fraction of a second, the energy transferred has the ability to cause damage.
A Boiler and Circulating Steam
Consider now an industrial application or a home heating system that involves a boiler and circulating steam. A part of the system is piping that carries water, that has changed from steam into liquid, back to the boiler. This water is called “condensate.” After the steam gives its heat away, it condenses to a liquid. Special devices called “traps” collect the condensate and pass it through pipes back to the boiler.
When Water Hammer Happens
Due to reasons beyond the scope of this article, know that most condensate pipes carry both liquid and steam. In general, a condensate return pipe is about half-filled with flowing liquid. The other half of the pipe is filled with steam. But, the steam is flowing a lot faster than the condensate. When a sudden change in flow happens, the condensate tends to slosh around in the pipe, filling the pipe, and blocking the passage of steam. Unfortunately, liquids don’t compress like gasses such as steam and air. When the steam is blocked, pressure builds behind the condensate, bunching it up and accelerating it to even higher speeds. This bunched-up, fast-flowing condensate is called a “slug.”
A slug in a twenty-foot-long two-inch pipe can travel fast enough to have energy approaching that of a bullet fired from a hunting rifle. When this slug strikes a point of resistance such as a valve or even an elbow in the pipeline, water hammer occurs. At the very least, you will hear it as the pipes bang around. It is easy to imagine the damage a water hammer like this can cause to large vessels, pipes, and equipment.
Avoiding Water Hammer
The best way to avoid water hammers in any piping system is to prevent sudden changes in flow. However, sudden changes in flow cannot always be eliminated. A piping designer should anticipate events that could result in a water hammer and design methods of increasing the time for energy transfer or reducing the effects by slowing everything down. For condensate return systems, an effective method is to increase the overall pressure in the system. Increasing the pressure causes the steam in the pipe to want to condense, which has the effect of eliminating steam in the line. Further downstream, where sudden changes in flow are less likely, the pressure can be relieved.
New technologies are being invented to slow down the process of turning the water on and off to your appliances so as to reduce water hammers. However, if your shower is one of those equipped with a push-off/pull-on valve, this valve is under your control. Should you push such a valve to shut off the water, take care and slow down—or one day you may find yourself wondering how a child with a hammer could have caused such a mess!