Pipe noise - PEUTZ CONSULT GmbH

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Figure 4: Pressure drop in stages. Restriction orifices PAGE 6 A restriction orifice can be made with one hole or a number of holes. While the free crosssection areas are the same in both cases, the reduction of turbulence in the orifice with several holes, as compared with a single hole, can reach approximately 7.lg(n) dB(A), where n is the number of holes. Muffler In an effective muffler, gas flows through a diffuser in combination with sound absorbing material (steel wool and/or rock wool). As the muffler only reduces sound radiating at the downstream side of piping, insulation of the muffler housing itself, including the nearby turbulence generating valve, can also be necessary. INSULATION Effect limitations Acoustical insulation of sound radiating surfaces is an effective and relatively cheap way of reducing the high frequency flow <strong>noise</strong> radiated by piping, piping supports, instrumentation, etc. In practice, piping will often be constructed as a thermal insulation. However, this can only provide effective sound insulation if it is mounted in an acoustically correct way.
Sound insulating construction PAGE 7 Acoustic insulation is generally constructed using a metal outer layer or cladding (steel or aluminium) without any rigid connections with the pipe. Acoustic leaks are avoided using adequate overlaps and sealings. Between the outer layers and the pipe wall, a porous layer is generally provided, for instance mineral fibre (glass or rock) or open cell flexible plastic foam. The mass of the outer layer needs to be sufficient to obtain the required level of insulation. Table 1 gives insulation classes based on the recent standard ISO/FDIS 15665 3 . Table 1 Minimum insertion loss required for each class 3 Class A1 A2 A3 B1 B2 B3 C1 C2 C3 Range of nominal diameter (D, mm) D < 300 300 ≤ D < 650 650 ≤ D < 1000 D < 300 300 ≤ D < 650 650 ≤ D < 1000 D < 300 300 ≤ D < 650 650 ≤ D < 1000 Octave band centre frequency (Hz) 125 250 500 1 000 2 000 4 000 8 000 -4 -4 -4 -9 -9 -7 -5 -7 1 -4 -4 2 Minimum insertion loss (dB) In order to conform to a given class, the insertion loss of all seven octave bands will either exceed or be equal to the levels specified. Sound insulation at low frequencies can be negative due to the mass spring resonance and, with smaller pipe diameters, the increase of the <strong>noise</strong> radiating surface of the outer cladding. This means that in these frequency bands, sound radiation increases as a result of sound insulation. Effective insulation Effective insulation of pipe systems means that each part of the system with a SPL1m higher than the limit must be insulated. In many cases, supports and parts of the instrumentation that are linked directly to the sound radiating pipe wall also have to be insulated. In some cases, and as an alternative to the insulation of supports, vibration insulators can be applied between a pipe and its support within the acoustic insulation. If, for maintenance reasons, the insulation must be taken off and again be applied in a simple way, removable enclosures should be applied. An example of insulation in a case where the flange has to be ventilated is given in Figure 5 3 . -3 -3 2 -1 4 9 2 2 7 3 6 11 11 14 17 9 9 13 11 15 20 23 24 26 16 16 19 19 24 29 34 34 34 22 22 24 27 33 36 38 38 38 29 29 30 35 42 42 42 42 42
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Pipe noise - PEUTZ CONSULT GmbH

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