Water Hammer

Water hammer is a shock wave, usually generated by the rapid closure of a valve, but occasionally caused by a pressure wave resonance within a pipe. In the case of an isolated occurrence, it produces a violent slamming, which physically moves the pipe and causes a loud banging noise. In the case of a resonance the passage of a shock wave back and forth results in a very loud low-frequency buzzing sound.

Water hammer creates a dramatic effect, which if allowed to continue, can result in damage to the pipe. Control is effected by exposing the fluid stream to a confined air pocket, which allows the high pressure shockto expand into apressure release surface. These devices, called water hammer arrestors, contain a gas trapped in a flexible bladder that encircles the fluid or is contained in a separate branch on a threaded T-fixture. In residential structures such devices are very effective when mounted to the outlet pipe of the water heater and near any quick-closing valves, such as automatic sprinkler systems. Occasionally they may also be necessary to control resonances in long runs of pipe or near a particularly troublesome valve.

Waste Stacks

Waste stacks, which carry water and effluent from toilets and other fixtures, are a frequent cause of noise complaints, particularly when plastic (PVC or ABS) pipe is used. This lightweight pipe transmits annoying levels of noise when water flows through it. When a toilet is flushed upstairs and the water rushes through a plastic pipe, levels as high as 60 to 65 dBA have been measured in downstairs units. This is loud enough that it makes normal conversations difficult and is certainly a show-stopper for the party guests. The cause is the lightweight plastic material, which vibrates due to the turbulent flow created by the passage of the waste water, and any direct coupling of the pipe to the structure. Consequently plastic waste stacks should never be used in residential buildings or sensitive commercial structures such as offices, classrooms, hospitals, studios, or theaters even for short runs such as P-traps.

Instead of plastic, cast iron is the preferred material for waste stacks due to its mass. Even with cast iron problems may arise if the pipe is not properly isolated. Waste stacks must be vibrationally isolated just as supply pipes were isolated. The inherent mass of cast iron and the relatively slow fluid velocity makes the isolation requirements less stringent than similarly sized supply pipes. Vertical risers should be supported on type WSW neoprene waffle pads as shown in Fig. 15.19. Horizontal runs may be isolated with 1/2" (13 mm) felt or neoprene pads. Frequently waste stacks exceed 2.5" (64 mm) in diameter, and when they are wrapped with a felt isolation material the combination may not fit within a single stud space without binding. Thus it is prudent to allow at least a 2 x 6 stud for all walls housing waste stacks. This permits a neatly drilled round hole of sufficient size to allow the pipe to pass through the base plate or stud without touching the structure.

When treating an existing condition where plastic pipe has been used as a waste stack, noise levels have been reduced to inaudibility by decoupling the pipe from the structure, wrapping it with 2 lb/sq ft (9.8 kg/sq m) foam lead sheets, installing R-11 batt insulation in the airspace, and applying two layers of 5/8" (16 mm) drywall on resilient channel to the stud. In some cases the replacement of plastic pipe with cast iron pipe may be less expensive. Note that if the plastic pipe is replaced it should be removed completely for its entire length.

Tubs, Toilets, and Showers

Tubs, toilets, and showers should be installed over a floor-ceiling, which is equivalent to that constructed in the rest of the residence. In wood construction if the floor has lightweight concrete it should extend under the tub or shower. Where fiberglass tubs are installed, it is good practice to pour additional lightweight concrete around the bottom of the tub to add mass and damping.

Below the tub, where the piping penetrates the floor, builders sometimes leave a sizable opening in the floor for ease of installation. This must be closed for both acoustical and fire


code reasons using at least two layers of 5/8" (16 mm) drywall as in Fig. 15.20. Where the P-trap is supported on a crosspiece the pipe should rest on a neoprene waffle pad isolator consistent with its size.

Since the noise generated by supply piping is velocity dependent, low-flow devices like an aerated showerhead can reduce supply-pipe noise. Upstream noise from the pipe serving a toilet can be reduced by restricting the flow locally at the wall valve. Careful selection of quiet fixtures such as ball valves can prevent noise creation at a critical location.

Pump and Piping Vibrations

When a pump is located in a residential or other noise-sensitive structure the pulsations imparted to both the pipe and the driven fluid can create noise both by vibrational and airborne transmission paths. Control of pump vibrations begins with the location of the pump in a nonsensitive area such as a basement; however, it is not uncommon to find rooftop installations in large condominium complexes. Both the pump and the pipe must be properly isolated, beginning with a concrete housekeeping pad and inertial base such as that shown in Fig. 13.6. Any piping attached to isolated equipment, which includes not only pumps but cooling towers, compressors, boilers, refrigeration equipment, and fan coil units, must itself be isolated with an isolator having the same deflection as the isolated equipment for a distance of 50 feet or 50 pipe diameters, whichever is greater. At the first point of support an isolator having twice the pump deflection can be used, since when the pipe is drained it weighs considerably less than when it is full. The pump isolators are installed when the pipe is empty, and when it is filled the pump and pipe assembly drops down under the increased load. This motion may crack the pipe unless it is given the freedom to move. All piping must be resiliently supported either from the pump's inertial base or with separate vibration isolators. In addition to isolating the pipe, flexible couplings can be installed near the pump. These serve to compensate for minor pipe misalignments and provide some vibrational decoupling for impulses transmitted along the pipe.

Fluid Pulsations

Occasionally the pulsations induced by a pump impeller will travel from the pump along the pipe and into a structure, through mechanical coupling between the pipe and a wall or floor. In existing buildings some reduction can be obtained by inserting a flexible coupling in the pipe between the pump and the receiver.

In high-pressure lines the impeller impulses may also be transmitted within the fluid so that a pulse dampener is recommended on the supply side of the pump after the flexible coupling. Pulse dampeners work in much the same way as a water hammer arrestor with an encapsulated gas-filled bladder. The pressure surge is allowed to expand into the bladder, which removes it from the liquid in the pipe. These devices are commercially available and are quite effective for attenuating vibrations in high-pressure lines including hydraulic lines.

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  • Marie
    What is water hammer transmission?
    4 years ago

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