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Multistage restriction orifice

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Multistage restriction orifice

(OP)
Dear colleagues,
I'm working in acoustic induced vibration studies and I have a doubt about how I can calculate a PWL(source).
According to guidelines from Energy Institute, if I had a single restriction orifice,RO, (pressure drop) with a high value of PWL (T2.7.3), I could reduce the AIV risk using a multi-stage device. This is a common corrective action applied in some situation.

However, i don't know how I should calculate the new PWL source value, lower than single RO. Most suppliers calculate the value of dBA, which is different from dB. Sometimes it is neccesary a quantitative evaluation.

We assume two restriction orifices arranged in series. Up to know, I have applied two times the basic equation with an intermediate pressure: P0,P1(intermediate) and P2. I use the same flowrate in both cases P0,P1 and P1,P2.
PWLa=10log10(((P0-P1)/P0)^3.6 * W^2 * (Te/Mw)^1.2)+126.1+SFF
PWLb=10log10(((P1-P2)/P1)^3.6 * W^2 * (Te/Mw)^1.2)+126.1+SFF
And then, assuming simultaneous work (conservative way) : PWLt= 10log10(10^(PWLa/10) + 10^(PWLb/10))

The gain obtained is not significant (1-2dB) and this fact causes me a doubt if it should be the correct calculation. What do you think about?

I will appreciate any consideration which contributes to clarify the methodology.
Thanks in advance to everybody.

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