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Monday, November 23, 2009

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As discussed in "Measures & Technique In Eliminating / Minimizing PWL", one of the measures to reduce Sound Power Level (PWL) at sources PWL is to split the flow. Half the original flow would result approximately 3 dB reduction. This post will discuss how this 3 dB reduction is derived.



A common acceptable method to predict Sound Power Level (PWL in dB) of AIV source is as follow :



Refer to "Sound Power Level (PWL) Prediction from AIV Aspect" for more discussion on above equatio.

Above equation will provide PWLW for a stream with W mass flow. If the stream is split into two stream, the mass flow will be W/2.

Sound Power level for single half-stream (W/2),

PWLW/2 = PWLW + 10Log [(1/2)^2]

PWLW/2 = PWLW - 6


When two half-streams with PWLW/2 are mixed, the resultant PWL of two half-streams may be estimated base on method as discussed in "Calculate Combined Sound Power Level (PWL)".
From this post,

Combined PWL = Maximum PWL + PWL Adder

Both half-streams have same PWL. The PWL difference (PWL,diff) is zero (0).
PWL adder = 10 ^ (0.4771 - 0.0795 x PWL,diff)
PWL adder = 10 ^ (0.4771 - 0.0795 x 0)
PWL adder = 3 dB


Both half-streams have same PWL.
Maximum PWL = PWLW/2 = PWLW - 6

Therefore, Combined PWL is :
Combined PWL = PWLW/2 - PWL adder
Combined PWL =
PWLW - 6 - 3
Combined PWL = PWLW - 3

So, half the flow will results approximately 3 dB reduction from total flow.

Ref.
i) "Designing Piping Systems Against Acoustically Induced Structural Fatigue", E.L. Eisinger, Journal of Pressure Vessel Technology, Aug 1997.

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posted by Webworm, 6:17 AM

2 Comments:

Anonymous Anonymous said...

Hi,

Could you please share the references you have used for your study on AIV and PWL prediction?

November 23, 2009 at 2:37 PM  
Blogger Webworm said...

Included in the post.

November 24, 2009 at 5:55 AM  

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