Valve Characteristic: Equal-% vs Linear

Quick pick (TL;DR)

If the valve takes only a small, varying share of system dP (long pipes, pump curve) → equal-percentage (the common default).
If valve dP is large & nearly constant (valve dominates) → linear.
On/off or fast fill → quick-opening.
The goal is an installed characteristic that is roughly linear so loop gain is steady across the range.

Step 1 — Match dP to characteristic

System dP behaviour	Choose	Why
Small & varies	Equal percentage	Compensates falling dP
Large & constant	Linear	Flow ∝ travel
On/off	Quick opening	Fast initial flow

Step 2 — Why equal-% works

Cancelling falling valve dP

The problem as the valve opens, flow rises → pipe loss rises → valve dP falls → each % of travel gives less extra flow Equal-% behaviour each equal step of travel = equal % increase in flow (not equal amount) small Cv near closed, large Cv near open The result rising Cv slope × falling dP → ≈ linear installed flow vs travel So → steady loop gain, easy to tune

Equal-% gives larger Cv steps as it opens, offsetting the shrinking dP → linear installed flow.

Step 3 — Trim & rangeability

Equal-% via contoured plug; check rangeability.
Seat leakage class per shutoff need.
Smart positioner can re-characterize.

Common mistakes

Linear on high-line-loss loop.
Quick-opening on a modulating loop.
Ignoring rangeability.

Requisition & datasheet

Variable, dP behaviour, dP ratio.
Characteristic, trim, leakage class.

Frequently asked questions

What should I know about Valve Characteristic (Equal-% vs Linear)?

If the valve takes only a small, varying share of system dP (long pipes, pump curve) → equal-percentage (the common default). If valve dP is large & nearly constant (valve dominates) → linear. On/off or fast fill → quick-opening. The goal is an installed characteristic that is roughly linear so loop gain is steady across the range.

Which standards or codes apply?

IEC 60534-2-4 (inherent flow characteristic & rangeability), ANSI/FCI 70-2 (seat leakage), ISA 75.11.

Standards — IEC 60534-2-4; ANSI/FCI 70-2; ISA 75.11.

Valve Characteristic