How to resolve water line noise

I am sure everyone has experienced the clanging of loose water lines, a distant yet persistent rattle every time you close the kitchen faucet, or a loud, jarring thwack every time the washing machine cycles. At some point in time most of us have been rudely awakened by these noises, or called in to eliminate them.
Whether you live in an old house or a new house, single family abode or a multi-unit high rise, noisy banging pipes will have been a nuisance you have learned to live with, or a curse you were driven or tasked to exorcize. “Water hammer” is the term used to identify this common problem and it can lead to much more than just a poor night’s sleep.
 Water hammer is a pressure wave that occurs inside a water line when the fluid in motion is abruptly stopped or forced to change direction due to the rapid closing of a valve or faucet (see Figure 1). Rapid-closing valves are pretty much the norm nowadays with very few slow-closing globe style valves used anymore.
Solenoid valves in clothes and dishwashers are particular culprits. Flowing water contains considerable energy, kinetic force, which needs to be released somewhere. Water is a non-compressible fluid so it cannot simply absorb this kinetic energy. Instead it bounces as a wave back and forth within the piping system until all the energy is either released into a larger water main, or into a vessel, such as a water tank. If there are loose pipes, this force will shake and rattle them, creating the “hammer” noise.
Just how much force are we talking about here? If you are a budding John Siegenthaler and you love mathematical equations, check out the formulas in the sidebar. If you prefer to look at the back of the book for the answers and simply trust the big foreheads who figured this stuff out for us already, consider this: the pressure rise in copper pipe due to a sudden change in velocity is approximately 60 times the original flow velocity in feet per second (fps).
Your typical 1/2″ supply line, flowing at six fps, can generate a pressure rise of 360 psig above and beyond the flow pressure. That means that if you have an average water pressure of 60 psi, the water surges can be creating “hammer hit” forces equal to 420 psi. Have you ever wondered why most of the pipe, fittings and components we use in our plumbing systems have test pressure ratings that far exceed the working pressure rating? This is one of the reasons.
Larger pipe sizes and longer pipe runs will see even higher spikes. With this kind of force at work you can understand why water hammer is more than an annoying nuisance. It can be a serious and potentially destructive issue.  Virtually every component in a plumbing system is susceptible to damage from the repeated shocks of water hammer. Hangers and supports can be loosened over time. Relief valves, solenoid valves, faucet cartridges and all manner of piping connections can all be stressed to the point of failure. Severe water hammer has even been implicated in the premature failure of hot water storage tanks.
The most effective means of mitigating the disruptive forces of water hammer is through the installation of an engineered water hammer arrester. There are several types, most of which employ a diaphragm, a bellows or a piston to permanently separate the water from a measured, compressible cushion of air or gas. When a valve closes abruptly, the water shock wave column is absorbed up into the arrester, compressing the permanent air charge until all the momentum of the moving water is safely dissipated. They are basically hydraulic shock absorbers.
The piston style seems to be by far the most common, at least in the residential market. These should be installed within six feet of the rapid closing valve they are serving and should be on both the hot and cold lines.
In recent years we have seen most plumbing codes adopt new or stricter regulations regarding the installation of water hammer arrestors. The Unified Plumbing code  (UPC/IAMPO) and the International Plumbing Code (IPC), as well as the National Plumbing Code of Canada, all require the installation of ASSE 1010 Certified Water Hammer Arresters on all quick-closing valves to control water hammer in both residential and commercial plumbing applications (see Figure 2).
Gone are the days when we could simply install vertically capped air chambers, as these have been proven ineffective. As is the case with most regulations, enforcement comes down to individual local authorities and may vary across the country. Regardless of whether or not these requirements are being enforced in your area, adopting them is just good practice and should be standard part of every job.

Steve Goldie learned his trade from his father while working as a plumber in the family business. After 21 years in the field, he joined the wholesale side of the business in 2002. He is frequently asked to troubleshoot systems and advise contractors. He can be reached at sgoldie@nextsupply.ca.

How much force are we talking about?

Kinetic Energy = ½ mass x velocity²      
When considering KE within a piping system, the mass (m) can be substituted with the physics characteristics of water within a cylindrical pipe (i.e. specific weight, cross-sectional area, length, plus gravitational constant). Velocity (v) is calculated in feet per second (fps), which can easily be converted from the known gpm and pipe size. For a plumbing or piping system, the kinetic energy formula can be expressed as this:

KE  =  .97 x A x L x v²
Where:
A = Cross sectional area of pipe I.D. in square feet
L = Length of effective pipe in feet
v = Velocity of flowing water in feet per second
Since we can calculate the kinetic energy, we can also calculate the actual “pressure rise” within a piping system by using Joukowsky’s Formula:

pr = wav/144g.
Where:
pr = Pressure rise above flow pressure (psig)
w = Specific weight of liquid (water = 62.4 lbs/ft³)
a = Velocity of pressure wave in feet per second (fps)
(4000 – 4500 fps in metal pipe)
v = Change in flow velocity in feet per second (fps)
g = Gravitational constant (32.2 fps²)