When considering adding DP flowmetering to existing piping, how does one determine the effect of the restriction on system operation?

How does one determine the optimum orifice size, and/or can the orifice size be maximized to minimize the pressure drop? What are the
tradeoffs?

We'll be metering fairly clean water. Other technologies will be considered (and probably chosen), but I'd like to know more about DP
anyway.

Know of any websites that might help?

 

Permanent loss of dp type meters is dependent on the meter type and its Beta ratio (throat diameter divided by inlet diameter). For
instance, a standard venturi type meter will only have about 10 percent of the differential as permanent loss. For a 100-inch differential,
you have less than a foot of permanent pressure loss. However for an orifice plate, it can suffer around 65 percent permanent loss.
Choosing and sizing flowmeters properly can be a career all by itself. Get a copy of ASME Fluid Meters. It'll show you what you're looking
for.