Dear hoyar ,
The design drop across the valve is sometimes expressed as a
percentage of the friction drop in the system, exclusive of
the valve. A good working rule is that 50% of this friction
drop should be available as drop across the valve. In
other words, one-third of the total system drop, including
all heat exchangers, mixing nozzles, piping is
assumed to be absorbed by the control valve.
For valves installed in extremely long or high-pressure drop
lines, the percentage of drop across the valve may be
somewhat lower, but at least 15% (up to 25% where
possible) of the system drop should be taken.