5.2 When two or more PSVs are used to protect one piece of equipment and the
required capacity is provided by one PSV only one device need be set at or
below the MAWP. In no case shall a PSV be set at a pressure greater then
105% of the MAWP for ASME VIII vessels. For ASME I Power Boilers the
maximum set pressure of additional PSVs shall not exceed the MAWP by
more than 3%.
The complete pressure range of all saturated steam valves
shall not exceed 10% of the highest pressure to which any valve is set.
ASME IV Heating Boilers additional valves may have a set range not to
exceed 6 PSI above the MAWP of a boiler up to and including 60 PSI and 5%
for those having a MAWP exceeding 60 PSI. The set pressure shall include
the effects of static head and constant back pressure.