API 521 Noise Calc, Figure 18

[nhcf]

Section 5.8.10.3 provides calculations for estimating noise from a discharge pipe. The equation uses "L", sound pressure level, which is taken from Figure 18, and based on a pressure ratio = absolute pressure upstream of relief valve/absolute pressure downstream. The figure only shows ratios up to 10. What is the "L" value for conditions where the PR is > 10? For instance, 750 psi line venting to atmosphere, 764.7/14.7 is > 10. Is the calculation no longer valid?

 

[Orsiz]

Is 14.7 the absolute pressure at the PSV outlet? Did you include the pressure drop across the discharge pipe at the relief flow rate?

 

[nhcf]

Thanks for the reply.
- I referenced 'relief valve' in my post but actually this is a blowdown application - there will be a restricting orifice and not a PRV.
- The note at the bottom of the table suggests that the upstream pressure is the inlet pressure to the relief device (I assumed this would be my restricting orifice) and the downstream pressure would use atmospheric.