Anonymous
Also, it depends on turndown of the unit, because this it can be fixed by turndown of the furnace (generally greater to 60% of its design) , As the discharge pressure in that pumps are over 500 psig, the minimum flow for these pumps are over 60%, Thus 2 pumps designed to the 100% possibly it makes operating under this condition impossible(turndown condition), unless that you install a recirculation line (minimum flow; You have to take care in this design, in order to avoid the high vibration on control valve/RO/etc, depending what are you going to install in order to reduce the differential pressure in these system)
In the case that you install 3 pump to 50% you have to review/to assure that when operating, only one pump, it has the capacity to operate to the minimum flow of a furnace
Also having 2 (50%) pumps operating, in case of fault of one pump is easier to avoid the shutdown for low flow in the heater, since in some cases a single pump allows to operate to minimum flow of the furnaces (in the case of the unit it has 2 furnaces)
Also it is necessary to consider if they will be electrical or of turbine
I beg you pardon for my English