Sizing & Selection

Recovery Coefficient (FL)

F_L (liquid pressure-recovery coefficient) states how much of the pressure drop a valve recovers after the vena contracta — equivalently, how deep the pressure dips inside the trim. It is defined as F_L = √(ΔP / (P₁ − P_vca,min)), determined by test per IEC 60534-2-1, and it ranges from near 1.0 (low-recovery trim: the pressure barely dips, e.g. multi-stage cages) down to ~0.5 (high-recovery rotary bodies: a deep dip and a strong rebound).

Its central role is the choked-flow and cavitation calculation: ΔP_choked = F_L²(P₁ − F_FP_v). A low-recovery valve (high F_L) tolerates more pressure drop before choking — and before cavitation damage — which is why severe liquid services choose multi-stage low-recovery trim even at a cost in capacity per size.

Practical notes: F_L varies with travel (use the value at the operating travel, from the manufacturer's curves); it is a property of the whole internal geometry, not a material; and vendors' published values are test conditions — your installation's piping modifies the effective recovery (F_LP factor). Our liquid Cv calculator uses F_L for its choked-flow check.

Related glossary terms

• Flow Coefficient (Cv, Kv)
• Rangeability
• Valve Authority