Pre 1978 Safety Replacement

[engrpiper]

I am told that the relief calculation changed in 1978. I have been asked to investigate this due to that fact that a safety on a boiler steam drum needs to be replace in the furture. The vendor's quote for an identical replacement valve at the original design inlet conditions now has an increase in the outlet capacity of ~20%.

I am looking to understand the details of the change in calc. The drum was constructed in 1973 and I do not have a copy of any code pre 1986 (I or VIII). Any and all help would be greatly appreciated.

Also, if someone has replaced a pre 1978 safety valve I would am interested in know how the increase in relief capacity was handled.

Thanks!