Don’t come a knockin’

Water hammer implications, complications and troubleshooting

BY CHUCK LOTT,

contributing writer

Water hammer has been around ever since man began forcing non-compressible liquids (typically water) through non-expandable conduits (name your pipe). The Romans and their aqueducts never had to deal with the problem, but as man began routing water through pipe in ever more complex ways, the destructive force of water hammer soon followed. The problem arises each time a flow of water is abruptly halted by a quick closing valve in a confined space.

All fluids in motion contain a tremendous amount of energy (hydroelectric power being the foremost example). This latent kinetic energy moves peacefully through a piping system along with the flow of water until it is required to stop. If flow is eased to a stop by a slow closing valve, the associated energy is slowly dissipated and easily absorbed by the system. If flow is abruptly halted by the slamming shut of a quick closing valve, the energy generated by the closure becomes destructive if not otherwise put in check. Pressure spikes of several hundred pounds are not uncommon in systems nominally measured at 60 to 80 psi. Constant exposure to pressure spikes of this magnitude will wreck havoc on fittings and fixtures anywhere in proximity. In addition to potential system damage, water hammer is often accompanied by a very audible and disturbing “BANG.”

So, what to do? Years ago a common solution was to install a simple standpipe in the vicinity of the quick acting valve. This upright standpipe (commonly referred to as an air chamber) simply retained a pocket of air when the system was filled. As water hammer was generated, the shock wave found this compressible air pocket allowing the water to expand into the chamber, thus dissipating its energy. The downside to this solution is that in a matter of weeks air chambers fill with water, completely eliminating any efficacy they may have originally provided.

Today’s solution to effectively manage water hammer is engineered water hammer arrestors. They are typically a nitrogen-charged, piston-style design, though you may still see bellows type arrestors on commercial projects. Though both designs are conceptually similar to air chambers (a nitrogen pre-charge to absorb energy), both the piston and bellows design differentiate themselves from the old air chamber in one critical area……….the dry nitrogen charge and system fluid are separated by a physical barrier. Barring catastrophic failure, piston arrestors will rarely lose their nitrogen charge, fill with water and lose their effectiveness. In fact, some manufacturers warrant their piston-style water hammer arrestors for the life of the plumbing system.

Installing water hammer arrestors is a fairly simple exercise, provided you remember two critical elements: The arrestor must be properly sized for the job, and placement must be in immediate proximity to the quick closing valve. Sizing for multiple fixtures on commercial projects is easily determined by use of the standardized Plumbing and Drainage Institute’s (pdi) water hammer arrestor sizing guide. Here you will find fixture types and the weight in fixture units for each valve. It’s then a simple matter of adding fixture units to determine water hammer arrestor size (A through F). Sizing residential arrestors (AA) is simpler yet, in that they are designed for single valve applications only; one valve equals one water hammer arrestor. Regarding location, most manufacturers agree that if you’re much further than six to eight feet away from the shock source, you have lost much of the protection the water hammer arrestor was designed to provide.

In troubleshooting water hammer complaints, probably the most vexing question is determining whether you have a water hammer problem at all. If any inexplicable noise can be detected anywhere throughout a plumbing system, I guarantee you it will be diagnosed as water hammer. In fact, most noisy piping systems are noisy because they have not been properly tied down. The kinetic energy I referred to earlier is present when flow begins in a system just as it is when it stops. If pressure piping is not adequately secured during flow conditions, it will want to move, just as a lightweight garden hose would with the hose bib wide open in your backyard. A water hammer arrestor will not remedy this. Secure pipe will. Water hammer is a specific, repeatable occurrence. It manifests itself only when a quick acting valve closes in a pressure piping system. Damage and noise will be localized upstream of the shock source and can be repeated each time the valve closes. Under these conditions, a properly sized water hammer arrestor placed just before the problem valve will provide the perfect solution to your water hammer dilemma.                    

Chuck Lott is director of sales & marketing, Precision Plumbing Products.