Mach number for steam PSV

[Section80]

Hi, always thanks to your precious advice

I'd like to ask for your advice and knowledge for Steam PSV design. Currently I review Steam generation system and the relief pressure is around 120 barg and relief flow is around 100 t/h.

PSV discharge is sized to 6" x 28" in order to meet the mach number.

1) Our stress team oppose current design due to stress problem orignated from big discharge line and request to reduce the discharge line size. So when I reduce PSV discharge size to 24", the mach number becomes 0.89. Normally the PSV discharge mach number shall not over 0.7, but some of my colleague said Mach number under 1 is allowable if it relieve it to the ATM.
Is there anyone who have seen these criteria in specific code? I reference API 520/521 but there are no specific comment for mach number that relieve to the ATM case.

2) Some other colleague said that if the mach number is over than 0.7, noise will be the other issue for this design. In this case, is it OK to install acoustic insulation only?

It would be appreciate to advice for above issue. Thank you for reading.

 

[georgeverghese]

Velocities aproaching Mach 1.0 are not preferred, but may be accepted if you are sure of the piping system configuration, and that there are no internal choke points within this PSV tail pipe. A reduction in exit line line size must also be backed up by built up backpressure calcs to show that this is is acceptable for the type of PSV used. Beware that as you exceed 0.7Mach and approach 1.0Mach, that pressure drop rises very steeply with length and any increase in mass rate. So check that your pressure drop is still predictable and not unstable at the smaller line size for an incremental line length or an incremental mass rate or decrement in line dia beyond what the design case values ( say 10% increase in line length or 10% increase in mass rate or say a 5% decrease in pipe flow area).
Also note that at these high velocities 0.7Mach and higher, acoustic induced vibration failure can occur in this tailpipe. Where necessary, thicker wall pipe may be required - talk to an AIV expert or see if you can get hold of the prescreening thresholds for a more detailed AIV check - sorry, I dont have this.