As I read PG 61.5,the boiler mfr is to determine the max sustained pressure that will be present at the inlet to the steam generator ( ie economizer inlet) when the unit is generating maximum design steam flow AND the final element of the steam generator ( ie main steam outlet header) is at its MAWP.
The forced flow steam generator also has a design pressure that varies over the length of the steam generator; there is not one "master stamping pressure" for all of the pressure parts as is done for a drum-type boiler, so there is an incentive to minimize the design pressure of the main steam outlet header to allow minimizing the specified design pressure of the upstream components and of the BFP.
There is one other unusual aspect to this:
===> in the old days, before the use of the "combined oxygenated feedwater treatment " method became prevalent, the pressure drop thru the furnace waterwall tubing would increase quickly due to accumulations of deposits of magnetite that was spalled off from the carbon steel tubed feedwater heaters. For some units, the pressure at the economizer inlet would increase rapidly over time due to this progressive increase in furnace frictional pressure drop. Annual acid cleaing of the furnace was common for one brand of forced flow steam generator ( 0.75" ID tubes) , but other brands with larger ID turbes could wait as long as 3-5 years between acid cleanings. As the boiler mfr rarely provides the BFP, the boiler mfr would primarily be obligated to advsie the EPC the max performance requirements for teh BFP and assocaited piping , and advise the operator when the unit would need to be scheduled for acid cleaning due to excessive pressure at the BFP outlet.
