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FIGURE 6 Noise propagation paths in a pumping plant (Reference 9)

FIGURE 6 Noise propagation paths in a pumping plant (Reference 9)

Airborne Noise Although acoustic energy can be generated in the pumped liquid by purely fluid processes (turbulences and so on), most noise radiated to the surrounding air is the direct result of mechanical vibrations in the pump case, the pipe wall, or other structures to which the pump system is coupled by liquid or mechanical attachment. (The exceptions to purely mechanical sources are windage noise produced by the rotating coupling, by cooling air in the drive motors, and so on).

When excessive noise is encountered, the source or sources can sometimes be identified by making sound measurements at points in a grid around the suspect equipment and plotting sound level contours. A sound level contour of a typical flow control valve installation is shown in Figure 7. Octave band analyses or spectral analyses and contours can also provide clues by identifying the source of various frequency components of the noise. These components can also be compared with the information in Table 1 to aid in determining the source and location of the noise-generating mechanism.

The Hydraulic Institute standard3 gives specific details of the procedures for noise measurement around pumps, including microphone locations (Figure 8), measurement procedures, and a data sheet (Figure 9).

The approaches for reducing noise from pumps and piping systems after it is airborne generally consist of either interrupting the transmission path (barriers) or controlling the reverberation characteristics of the pump room.

A highly reflective (reverberant) pump room or enclosure can increase pump noise levels several decibels by reflections of the noise back and forth in the enclosure. The maximum reduction that can be achieved by the application of acoustic absorption material to the interior surfaces is normally about 10 dB for a highly reverberant enclosure. At most, such a treatment can reduce noise levels to those that would exist if the pump were operating in the

FIGURE 7 Contours of equal sound level (decibels) (Reference 12)
FIGURE 8 Placement of microphones on a horizontally split centrifugal pump (Hydraulic Institute ANSI/HI 2000 Edition Pump Standards, Reference 3)

open, totally free of reflecting surfaces. Quantitatively, the noise reduction NR in decibels that can be achieved is aa

ab where aa = average absorption coefficient of the surfaces after treatment ab = average absorption coefficient of the surfaces before treatment

FIGURE 9 Hydraulic Institute data sheet for measurement of airborne sound from pumping equipment (Hydraulic Institute ANSI/HI 2000 Edition Pump Standards, Reference 3)

The average absorption coefficient is defined as follows:

Ai + A2 + A3 + • • • An where a1( a2, a3, . . . are absorption coefficients of various surface areas within the enclosure and A^ A2 A3 . . . are the corresponding surface areas.

Absorption coefficients for typical building materials are given in Table 2. Note that the absorption coefficients vary with frequency, and hence calculated values of NR can be quite frequency-sensitive. Absorption data for various absorbing materials can best be obtained from the suppliers. One of the most common and most effective of such materials is fiberglass matting or the more rigid fiberglass board.

The noise reduction that can typically be attained with fiberglass piping insulation is shown in Figure 10. Note that noise reduction varies with noise frequency and with the thickness and density of the fiberglass matting. Densities of 5 to 6 lb/ft3 (80 to 96 kg/m3), found in the rigid fiberglass board, are normally more effective than the 1 to 2 lb/ft3 (16 to 32 kg/m3) found in rolled matting.

Normally, the average absorption in a room must be increased by a factor of at least three before noise improvement is discernible to the ear. Unless the absorption coefficient of the untreated room is less than about 0.3 in the frequency range of maximum noise, the

TABLE 2 Sound absorption coefficients of common construction materials

Frequency, Hz

Frequency, Hz

TABLE 2 Sound absorption coefficients of common construction materials

Material

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