Hi Sam, opinions are like armpits - everybody's got them and they all stink. Anyway, here's mine:
The pressure is your controlling device and the flow meter is there, nifty to have, and is for your information. You can't control the pump VSDs via input from a flow meter while maintaining a constant pressure. When demand is low, the flow meter will sense the flow reduction and tell the pumps to pick up speed! How sensible would that be?
So specify a 0-60 or 0-80 psig pressure transducer, locate it one-half to three-quarters down the main run of pipe, or at a location where you know you need 40 psig. The automatic control software should take the 40# as an analog in, and stage the speed drives appropriately. (Rough) sample of staging language:
"When the system is enabled via the facility automation system, pump P1 initially starts at minimum VFD setting and ramps up to maintain pressure at transducer PT-1. If P1 operates at greater than 95% for five minutes (adj.), P2 shall start at minimum VFD and slowly ramp up to maintain setpoint. When P1 and P2 operate in parallel, both VFDs shall be synchronized. If P1 and P2 operate at greater than 95% for five minutes (adj.), P3 shall start and run in parallel with P1 and P2..."
Add similar language for unloading each pump at 45% speed (adj.). Add in standby function of P4; note the analog-in reading from the flow meter to provide indication on the facility automation system, but not to perform a control function.
Good luck, -Chas