OSHA standards call for specific noise levels, most of which are below 90 dB(A) for an 8 hour work day. However, it doesnt seem as though this would be applicable to your situation with PSV. You may want to investigate IEC 60534-8-3. This is a standard for predicting Aerodynamic noise produced by control valves, and may be modified to be made applicable to a PSV on a boiler. Hope this information helps.