there are 2 interpretations of "header".
For the case of a "header type boiler config" , the "header' is the common steam transfer pipe from the multiple boilers to the steam user. In that case, the header size is based on limiting the frictional pressure drop and /or noise , vibration and wet steam erosion/corrosion. For frictional pressure loss, you would first define the allowable pressure drop ( 3% of inlet pressure, for example), and then calculate the frictional pressure drop for various pipe sizes, and select the smallest pipe size that also meets the noise and vibration limits. The noise and vibration limits might require the the max velocity not be greaer than ( 15% ) soundspeed.
The other interpretation of the term "header" is the large distribution /collection pipe that feeds steam to the individual heat exchangers in the boiler, such as for example the primary superheater. That header is sized to limit the tube to tube flow unbalance to the ensemble of tubes in the heat exchanger, so that the tube to tube temperature unbalance at the outlet of the heat exchanger is below allowable metallurgical limits. That sizing calculation is a bit more complicated to describe, and not as easily summarized as the first header term.
