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Monday, June 8, 2009

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High frequency acoustic excitation downstream of pressure reducing device and potential of downstream piping failure on Acoustic Induced Vibration (AIV) has raised concern in many plant design. Earlier post "Extra Attention to Common Point and Similarity on AIV Failure" has discussed the common points and similarity of AIV such as typical failure location, system experienced failure in the past, failure time & period and Mach no. An engineer assessing AIV shall pay extra attention in these factors.

Earlier post "Assess AIV with "D/t-method "has presented a Sound Power level (PWL) limit line (logarithm line) to assess the possibility of Acoustically Induced Fatigue failure based on D/t method. Nevertheless, there is still some concern on the area where there is no published data shows the possibility of AIV failure (shown as pink triangle area in below image). Eisinger PWL limit line and Logarithm PWL limit line consider this area is non-failure.

Taking conservative approach, this area may be defined as area potentially failure until more real field or experiment data available. Therefore, a new Polynomial PWL limit line is proposed.

PWLlimit =C4.r4 + C3.r3 + C2.r2 + C1.r + C0

PWLlimit = Maximum allowable PWL at Pipe diameter D meter, (dB)
r = 1000D/t
D = Pipe Outside diameter (m)
T = Pipe wall thickness (mm)
C4 = 0.000000032
C3 = 0.000023
C2 = 0.0057
C1 = 0.691
C0 = 192

Refer following graph. New polynomial presented as violet line.

Note :
1) A "failure" point (0.3m, 165dB) is below curve. It is failed on bad welding. No further failure after good welding.

2) Red point are "failure" point and Blue point are "No failure" point

3) May be used for 0.2m < D < 0.9m. Use at risk for D < 0.2m and D > 0.9m

This line may be used to assess potential failure of piping downstream of pressure reducing device. Any point above this line potentially fail on AIV, piping treatment or redesign required to minimise the risk of AIV failure. This will be discussed in future...

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posted by Webworm, 8:41 PM


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