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Monday, April 6, 2009

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Earlier discussion in "Acoustic Induced Vibration (AIV) Fatigue" has discussed the generation of high frequency acoustic excitation downstream of pressure reducing device and potential of downstream piping failure due to Acoustic Induced Vibration (AIV). Acoustic energy can lead to circumferential and longitudinal excitation. Read more in "Piping Excitation When Expose to Acoustic Energy" to see how piping is excited by acoustic energy. Piping downstream of pressure reducing device shall be designed sufficiently strong to resist AIV fatigue. One of the common acceptable criteria to indicate piping resistivity to AIV fatigue is ensuring Sound Power Level (PWL) allowable limit of downstream piping higher than the PWL generated by the pressure reducing device.

Sound Power Level Generated by Pressure Reducing Device
Acoustic energy generated by a pressure reducing device may range from 0 to 10 MW. It is more convinient to express this energy in another term called Sound Power Level (PWL) in logarithmic scale with reference to a most common acceptable reference power of 10^(-12) watts (W). Thus, PWL is 10 times the logarithm to the base 10 of the ratio of acoustic enerygy to reference power of 10^(-12) watts. A common acceptable method to predict Sound Power Level (PWL in dB) is as follow :



Example
A Depressuring valve open to pass 100,000 kg/h of gas with molecular weight (MW) of 22. The inlet condition is 87 barg and 50 degC and estimated backpressure is about 7 barg.

PWL = 10 x Log [((87-7) / (87+1.01325))^3.6
x (100,000 / 3600)^2
x ((50+273.15)/22)^1.2]
+ 126.1

PWL = 167.5 dB


Allowable Sound Power Level (PWL) of Piping
A piping will have it stiffness and resistivity against vibration. It is very much subject to piping diameter, material properties, wall thickness, distribution of masses, pipe support, etc. Allowable sound power level and/or dynamic stress of piping can be a measurement of this resistivity of piping against excitation due to AIV. Several allowable PWL curves for piping which derived from different method i.e. "D-method", "D/t-method ", "E-method", etc have been used. Besides PWL related method, there are other method i.e. "Dynamic stress method", "Likelihood of Failure (LOF) method", etc which adopting allowable stress level also been used. This may be discussed in future post.

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, 12:52 AM

2 Comments:

Anonymous Anonymous said...

Hi..

I happened to be doing some estimations on noise, may I find out from you where is the source of the above equation?

Thanks in advance.

May 27, 2009 at 8:34 PM  
Blogger Webworm said...

Reference included.

June 3, 2009 at 12:00 PM  

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