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Have you heard the noise? You may describe it as an ominous clank or thud and it seems to happen every time you turn off the faucet or flush the toilet. It clearly has something to do with the water in the piping system, but you’re not quite sure what to call it. You have no idea how to fix it. If this is the case, you may have a problem called water hammer. Water hammer can appear in the home, in agriculture and in industrial systems. Left unaddressed, it could spell problems for pumps, piping and entire water systems.

Water hammer is a pressure shockwave that is caused by an abrupt change in the water’s flow rate and velocity. The abrupt change in kinetic energy is what impacts your system.

Water Hammer Conditions

A few conditions can produce water hammer, depending on the type of system. Thermal shock occurs when water collapses on steam bubbles, causing the bubbles to implode and exert force against the container. As velocity and pressure increase, the impact of the force increases as well. This causes the phenomenon known as cavitation.

Differential shock occurs in biphase systems where liquid and gas are moved in the same pipe. For instance, when steam and condensate flow in the same line at difference velocities, increased gas flow will create surface waves on the liquid. The waves increase in height until they meet the walls of the pipe, in effect forming a seal. The seal water pocket acts as a piston shooting through the pipe. The pocket increases in mass and velocity until it slams into the first surface it meets.

Hydraulic Shock

The third condition that causes water hammer is known as hydraulic shock. This is the most common condition that causes water hammer in home water and wastewater systems. It is likely the cause of that noise that causes your neighbors to complain and keeps you up at night (for the purposes of this blog we will be discussing hydraulic shock and its effects).

Of course, an unpleasant noise is merely the most noticeable symptom of the disease. The real, costly damage comes in the form of degradation, cracks and major breaks. It can necessitate you spending valuable time and money on infrastructure upgrades or replacements. It costs municipalities around the country thousands of dollars in repairs, manpower and lost productivity every year. Water hammer is even powerful enough to propel a 13-ton nuclear vessel nine feet in the air (hopefully you won’t have to worry about anything on that order of magnitude).

Diagnosing the existence of water hammer in your system and taking measures to prevent it from consuming your system are both critical exercises. Before you can do that, you should first understand what causes water hammer in the first place.

What Causes Water Hammer?

When a valve is closed abruptly it causes hydraulic shock water hammer. Picture water traveling from a pump through a pipe. At some point, the water will reach a valve, such as a swing check valve. If and when the flap on that check valve snaps shut abruptly, the water that has not yet passed the critical point will slam against the flap with all of the water’s mass and velocity.

The slam creates a shockwave effect that reverberates throughout the pipe, increasing pressure and sending the water back in the direction from whence it came. But remember that the running pump is still moving water. The water being pushed back from the valve and the water moving from the pump will collide.

The collision creates more shockwaves, which in turn increases the pressure in the system. In essence, water hammer is a cascading phenomenon in which each change in flow rate and velocity builds upon itself.

Water, for the most part, cannot be compressed; it will continue to move and radiate in all directions. The resulting waves of force from the water collision hits the pipe and create the distinctive water hammer sound. This effect continues until friction loss dissipates the pressure and the water settles.

Another common cause of water hammer is improper pump cycling or rather, the pump turning on and off suddenly. Similarly to the abrupt closing of a valve, a sudden pump start can produce its own shock waves. Longtime fans of our blog know how much we here at Pump Products harp on the importance of proper pump cycling. Pump burnout is just one problem.

A few variables specific to your system determine the magnitude of the pressure shock, such as the length and diameter of the piping, the time it takes to stop the water (i.e. how many seconds it takes for the valve to snap shut), the velocity of the water being pumped. The construction materials of the pipes, valves, elbows, et al can also impact the magnitude of the waves.

The Impact of Water Hammer

As mentioned above, the constant slamming effect of water hammer could drastically damage the structural integrity of your pump, pipes and water system. The kinetic energy of the moving water essentially acts as a projectile force. At a certain point, something’s gotta give – and that something is often an important fixture of your home water system.

Water hammer can result in the loosening or collapse of the piping system. The same goes for elements of the system such as valves, elbows, tees, etc. The casing of the pump, as well as its internal components like the impeller, can crack and deteriorate. In some rare cases, it is even possible for water hammer to cause a pump to explode.

For pumps specifically, water hammer can send what is known as a water column traveling back down the discharge pipe or outlet. This backflow of water reverses the impeller, causing it to spin in the opposite direction. The reverse impeller effect creates an abrupt shift in the water flow rate in the pump – meaning that there is now effectively water hammer inside the pump.

On a lighter note, the sound force from water hammer has been used to produce a percussive musical instrument known as a hydraulophone, seen and this video.

How to Prevent Water Hammer

There are several methods you can use to address water hammer:

–The simplest method might be to decrease the pressure or velocity of the water entering the house or building. You can try adding a pressure regulator to the system.

–If possible, you can retrofit the piping system with slow-closing valves. Such valves reduce the abrupt flow and velocity change caused by valves that snap shut quickly.

–A popular method is to add a hydropneumatic device like a water hammer arrester to the system. Water hammer arresters include a pressurized air chamber that is separated from the main line. The chamber asborbs excess water and pressure, thereby blunting the pressure shock of water hammer.

–An expansion tank or surge tank could absorb and contain any excess pressure.

–Changing the piping, whether installing stronger piping or shorter runs of straight pipe, can reduce the risk of water hammer. Consider that this method could be costly and time intensive.

Soft Starter

For commercial and industrial applications, installing a variable frequency drive (VFD) with a soft starter to regulate the pump could be a cost-effective solution.

The drive slowly starts the pump, ramping the motor up or down. Pumps that have the drive shaft directly coupled to the impeller often produce great starting torque on startup, so the soft starter reduces the voltage power when the pump is ready to activate. The reduced power leads to less starting torque and a more gradual start up and shut down.

The gradual ramp up process vastly decreases the the opportunity for sudden changes in pressure, water flow rate and velocity. In addition, electrical usage and costs can decrease, making the soft starter a cost-effective option in the long run.

The Goulds Aquastart series is a combination variable frequency drive (VFD) soft starter that works for centrifugal pumps and submersible pumps. It is easily compatible with factory pre-set pump parameters. The user can custom program the time it takes for the pump to ramp up or ramp down. Sophisticated monitoring systems also allow for thorough and consistent monitoring.

Any of these methods can work in combating water hammer. Consider the investment of time and money you are willing to make, as well as the pre-existing conditions of your water system. The next time you hear that ominous noise in your pipes, you will be well-equipped to end that infernal racket once and for all.