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Wednesday, November 18, 2009

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Earlier post "Measures & Technique In Eliminating / Minimizing PWL" discussed about several measures can be considered in eliminating and minimizing PWL for pressure reduction devices. In similar post, Sound Power Level attenuation due to extended "strengthened" pipe has also been presented. This post will focus on approach to estimate combined sound power level (PWL mix) for two or more streams.

In process plant, there will be scenario for two and/or more pressure reduction devices (PRD) downstream piping discharge to a common header. Typical example is blowdown / restriction orifice to flare header. During plant wide total plant blowdown, all blowdown valves may be opened simultaneously or opened in group. Different PRD will results different level of PWL. Now the queries will be :
  • How these PWL are interact between each and other ?
  • How to estimate resultant PWL ?
  • Are different PWL addition arithmetically ?
When two Sound power sources are combined, it is understood that the total combined Sound Power Level will increase due to two energy stream are combined. However these energy streams are transmitted in wave form, the resultant Sound Power level will not be added arithmetically i.e. 1+1=2. The total combined Sound Power Level is equal to "PWL adder" (which estimated base on PWL difference between both stream and from several experience equations ) plus maximum PWL out of both streams.

Mathematically,

Combined PWL = Maximum PWL + PWL Adder




PWL Adder
PWL adder is estimated from PWL difference between both stream and the following equations.

If PWL Difference (PWL, diff)
  • between 0 to 6 dB, PWL adder = 10 ^ (0.4771 - 0.0795 x PWL,diff)
  • between 6 to 10 dB, PWL adder = 10 ^ (0.5651 - 0.0942 x PWL,diff)
  • between 10 to 14 dB, PWL adder = 10 ^ (0.5432 - 0.092 x PWL,diff)
  • more than 14 dB, PWL adder = 10 ^ (0.7794 - 0.1088 x PWL,diff)
Example
Two Pressure control valves with PWL of 160 dB and 166 dB discharging to a flare header. Calculate combined PWL.

Assumed PWL attenuation due to piping is ignored.
PWL,diff = 166 - 160 = 6 dB

PWL adder = 10 ^ (0.4771 - 0.0795 x 6) = 1 dB

Combined PWL = Maximum PWL + PWL Adder
Combined PWL = 166 + 1 = 167 dB.


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posted by Webworm, 5:55 AM

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