THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

14.6 Pumps and Plumbing Systems 369rise that propagates as a shock wave upstream from the interrupting valve. Thesteep wavefront can be reflected numerous times back and forth throughout thevarious parts of the piping system until the energy is finally dissipated.Noise control factors which must be dealt with in order to effect noise controlinclude (a) water flow and piping characteristics, (b) radiation of sound to thebuilding structure, (c) selection and mounting of fixtures, (d) isolation of pumpsystems, and (e) water hammer noise control.Water pressure in a plumbing system influences the flow noise caused by piperuns and water supply valves. According to typical building codes, water pressureshould be maintained at least at 100 kPa (15 psi) but not more than 500 kPa (80 psi).For acceptable system performance, the supply pressure should be somewherebetween 230 and 370 kPa (35 and 55 psi), with a preference toward the lowerrange in order to minimize noise. Flow noise radiation from pipes can be lessenedby minimizing the number of pipe transitions (elbows, tees, y-connections, andthe like). This reduces the opportunity for the onset of turbulence and cavitation.Larger pipes are used in building design when noise control is given a higherpriority. In the U.S. 1 / 2 -inch diameter piping is generally used in domestic plumbingsystems but can be as high as 3 / 4 -inch diameter to cut down on noise by as muchas 3–5 dB.Noise resulting from water flow in pipes may be transmitted to the rooms throughwhich they pass, particularly if they are in direct contact with large radiation surfacessuch as walls, ceilings, and floors. Isolation of these pipes from the buildingstructure provides significant noise reduction. If the pipes are mounted with foamisolators, instead of being rigidly attached to the building structure, a considerablenoise reduction of 10–12 dB may be obtained. Whenever piping passes through astructure (block, stud, joist, or plate) or is in contact with a wall or masonry, resilientmaterials such as neoprene or fiberglass should be used to provide isolation.It is vital to seal with a resilient caulking around the perimeter of all pipes, faucets,and spouts that penetrate through floors, walls, and shower stalls.The impact of water hammer can rupture piping; thus, it will spring leaks, causeweakening of connections and produce damage to valves. Water-hammer pulsingassociated with the use of washing machines and dishwashers can be partiallydamped by connecting these machines to the water supply with extra-long flexiblehose. Figure 14.6 shows a schematic of a capped pipe that incorporates an airchamber for water-hammer suppression. The length of the pipe ranges from 30 to60 cm and may be of the same or larger diameter than the line it serves. The volumeof the air chamber, which serves as an air cushion, depends on the nominal pipediameter, the branch line length, and the supply pressure. But if the air chamberbecomes filled with water, it becomes ineffective. A petcock is provided alongwith a shut-off valve, so that the chamber may be drained of water and vented,thereby reactivating the unit.There are commercial devices, called water-hammer arresters, which are notsubject to the limitation of capped pipes, because a metal diaphragm separatesthe water from the air. These devices are best placed near quick-acting valves andshould also be installed at the termini of long pipe runs.