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

14.6 Pumps and Plumbing Systems 367speed and discharge pressure, when the rate of compression is high or the inletpressure is below atmospheric, or if the temperature runs too high. An unfortunatesituation is created when the noise from the pump easily transmits through thefluid or piping to other system components.Pumps generate two types of noise: discrete tones and broadband noise. Thepump’s fundamental frequency f p is found fromwheref p = n × N c60n = pump rotational speed, rpmN c = number of pump chamber pressure cycles per revolution(14.9)In large pumps the noise emission is the loudest at this fundamental frequency.As the pump size decreases, the frequency at which the maximum noise emissionoccurs increases, often to a frequency that constitutes a third or fourth harmonic ofthe fundamental. Above 3 kHz, the noise becomes more broadband, approachingan essentially flat spectrum. This is due to phenomena such as high-velocity flowand cavitation.The total sound power level of pumps in the four octave bands of center frequencies400, 1000, 2000, and 4000 kHz can be estimated from the following:L w = 10 log hp + K p dB (14.10)where K p = pump constant which has the following values: 95 dB for the centrifugaltype, 100 dB for the screw type, and 105 dB for reciprocating type. Forrated speeds below 1600 rpm, 5 dB is to be subtracted for reciprocal pumps. Thesound power in each of the four bands may be considered to be 6 dB less than thetotal sound power L w computed from Equation (14.10).Controlling Noise in Plumbing SystemsTable 14.4 lists the sources of noise in a building’s plumbing system and theirlikelihood of being annoying. Flow can be either laminar flow, i.e., smoothlyflowing, or turbulent flow, in which occurs an irregular, random motion of the fluidparticles. The influencing factor that determines whether a flow will be turbulentor laminar is the Reynolds number Re, a dimensionless parameter defined byRe = ρ dvμwhere ρ is the density of the fluid, d is the internal pipe diameter, v is the flowvelocity, and μ is the absolute viscosity of the fluid. For Re < 2000, the flow islaminar. For transition region 2000 < Re < 4000, the flow may be either laminaror turbulent. For Re > 4000, the flow will be turbulent. Noise generated by laminarflow tends to be quite low in intensity and is usually of no concern.In most real plumbing systems, the velocities are sufficiently high to result inturbulent flow, which is a basic mechanism for noise generation within piping runs