5.4 PUMP NOISE

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Standard-line pumps are generally not severe noise sources when the pumps

are operated at their rated speed and capacity. Noise from pumps involves

both hydraulic and mechanical sources. Some of the sources of noise

in pumps include cavitation, fluid pressure fluctuations, impact of solid

surfaces, and dynamic imbalance of the rotor. The hydraulic sources are

usually the more important noise generators, unless there is a mechanical

problem in the pump, such as imbalance of the rotor (Heitner, 1968).

Noise may be radiated from a pump through the surrounding air or

through the piping and support structure for the pump. As was the case for

fans, the structureborne noise may be reduced to negligible levels by proper

vibration isolation of the pump.

The sound power level for airborne noise from a pump may be estimated

from the following correlation within 3 dB:

LW ј Ko ю 10 log10рhpЮ (5-18)

Noise Sources 171

TABLE 5-3 Conversion Factors CF1 (dB) to Convert from the

A-weighted Sound Power Level for an Electric Motor to the

Octave Band Sound Power Levels

Motor Size

Octave band center frequency, Hz

63 125 250 500 1,000 2,000 4,000 8,000

1 to 250 hp 16 12 8 4 4 8 12 16

300 to 400 hp 21 15 9 3 3 8 15 22

450 hp and above 19 13 7 3 3 8 14 22

Copyright © 2003 Marcel Dekker, Inc.

The quantity hp is the rated horsepower for the pump. For pumps having a

rated speed of 1600 rpm or higher, the values of the constant Ko are as

follows:

Koј 98 dB for a centrifugal pump

Ko ј 103 dB for a screw pump

Ko ј 108 dB for a reciprocating pump

For pumps operating at speeds below 1600 rpm, subtract 5dB from these

values to obtain the Ko for Eq. (5-18).

To convert from the overall sound power level to the octave band

sound power levels, the conversion factor given in Table 5-4 may be used:

LWрoctave bandЮ ј LW _CF2 (5-19)

Example 5-2. An 18 kW DRPR electric motor is used to drive a 20hp

centrifugal pump at 1800 rpm, as shown in Fig. 5-2. The directivity factor

for the system is Q ј 4 for all frequencies. The system is located in a room

having a room constant of 50m2 (538 ft2). Determine the overall sound

pressure level at a distance of 2.00m (6.56 ft) from the system.

The horsepower rating of the motor is found by conversion of units:

hp ј р18kWЮр1:3410 hp=kWЮ ј 24:14 hp

The A-weighted sound power level for the motor is found from Eq. (5-14):

LWрAЮ ј 20 log10р24:14Ю ю 15 log10р1800Ю _ 3

LWрAЮ ј 27:7 ю 48:8 _ 3 ј 73:5 dBA

The non-weighted sound power level for the electric motor (hp < 250 hp) is

found as follows:

LW;m ј LWрAЮ ю 1:1 ј 73:5 ю 1:1 ј 74:6dB

172 Chapter 5

TABLE 5-4 Conversion Factors CF2 (dB) to

Convert from the Overall Sound Power Level for a

Pump to the Octave Band Sound Power Levels

Octave band center frequency, Hz

63 125 250 500 1,000 2,000 4,000 8,000

CF2, dB 10 9 9 8 6 9 12 17

Copyright © 2003 Marcel Dekker, Inc.

The sound power level for the pump is found from Eq. (5-18):

LW;p ј 98 ю 10 log10р20Ю ј 98 ю 13:0 ј 110:0dB

The total sound power level for the combined motor and pump system is

found by combining the power levels:

LW ј 10 log10Ѕ107:46 ю 1011:00_ ј 10 log10р1:0003 _ 1011Ю ј 110:0dB

We note that the effect of the electric motor noise is negligible in this case.

The overall sound pressure level due to the pump and motor noise may

be calculated from Eq. (5-6):

Lp ј 110:0 ю 10 log10

4

50 ю р4:0Ю

р4_Юр2:00Ю2

_ _

ю 0:1

Lp ј 110:0 ю 10 log10р0:0800 ю 0:0796Ю ю 0:1 ј 110:0 _ 8:0 ю 0:1

Lp ј 102:1dB

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