Quanity of Sec I PRV's

[BJaffa]

A client has a question I cannot answer. Not a big surprise there. A 1950's era Wickes coal fired 475# MAWP power boiler has five PRV's. This typical 1 of 6 is on a US Army Base in Alaska. One on the SPHT at 475# is normal. The remaining are located across the top of the Steam Drum. These are rated at 455# then 460#, 465# and 470#. PG 67 (current and the 1965 Code)only requires two PRV's of adequate capacity. Why use four? Could the additional PRV's been redundency or an insurance company request? Installing all 4, what order should the PRV's be arranged across the drum? I have looked in Sec I, B31.1, Sec VII, B&W's "Steam" but not found an answer.