Flow Characteristics
Quick-Opening Characteristic
The quick-opening inherent characteristic delivers most of the valve's capacity within the first fraction of travel — commonly modelled as Q ≈ Qmax·√(1−(1−h)²). A flat plug approaching the seat does this naturally: barely leave the seat and flow is already high.
Loop gain is very high just off the seat and nearly zero beyond half travel. That profile is wrong for continuous throttling and exactly right for duties wanting a big, fast flow change on a short, powerful stroke: two-position valves, batch drains, bypasses, startup vents, and relief-adjacent services. A quick-opening trim also minimizes the actuator stroke needed to reach full flow, which suits piston actuators and compact builds.
Why it appears where it wasn't asked for: in low-authority installations a linear valve's installed curve bends quick-opening — which is the classic "oscillates at low load, does nothing at high load" loop complaint. The remedy is characteristic selection (equal percentage), not more controller tuning. See the comparison article.