Flow Phenomena
Choked Flow
Choked (critical) flow is the condition in which increasing the pressure drop across a valve no longer increases the flow rate. The flow becomes limited by physics at the vena contracta, not by the downstream pressure.
In liquids, choking occurs when the vena contracta pressure falls to the fluid's vapour pressure: vapour forms and blocks further flow increase. The sizing pressure drop is capped at ΔPchoked = FL²(P1 − FFPv), where FL is the recovery coefficient and FF the liquid critical-pressure-ratio factor. Using the full actual ΔP beyond this point overstates the capacity — a classic sizing error.
In gases, choking occurs when the vena contracta reaches sonic velocity; the limit is set by the expansion factor xT and the pressure-drop ratio x = ΔP/P1.
Operationally, choked liquid flow is inseparable from cavitation and flashing: if you size into the choked region, you are also buying vapour-phase damage. Try the liquid Cv calculator — it detects and reports the choked condition.