On-Off vs Control Valve

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Quick pick (TL;DR)

Holds a setpoint (flow/pressure/level/temp) → control valve (globe is the default; rotary for big bore / high Cv).
Size on Cv so normal flow is ~60–80% travel.
Equal-% for most flow/pressure control; linear for level.
Add 2 isolation valves + (often) a bypass around it.
Pick fail action from the safe state.

Two different jobs

Isolation = stop/allow + tight shutoff. Control = modulate. Don't swap their jobs.

Step 1 — Gather the input data

Controlled variable & setpoint.
Flow min/normal/max; P1, P2 → ΔP.
SG, viscosity, Pv, temperature.
Line size/class; corrosives; fail action.

Step 2 — Size it on Cv

Size on Cv at max flow; check normal ~60–80% travel, min > ~10%.

Worked example — boiler feed water

Given Liquid (water), SG = 0.98 Normal Q = 100 gpm , ΔP = 25 psi Step 1 — Required Cv Cv = Q × √(SG / ΔP) Cv = 100 × √(0.98 / 25) Cv = 100 × 0.198 = 19.8 Step 2 — Select rated Cv target ≈ 70% travel → rated Cv ≈ 19.8 / 0.7 rated Cv ≈ 28 → pick a trim with Cv 28–30 Step 3 — Cavitation check if ΔP > F_L²·(P1 − Pv) → choked / cavitating → use anti-cavitation / multi-stage trim

Step 3 — Flow characteristic

Characteristic	Use when
Equal-%	Most flow/pressure control.
Linear	Level & constant-ΔP.
Quick-opening	On/off-ish, fast relief.

Step 4 — Body, trim & actuator

Globe (cage) default; rotary for big bore.
Hardened/anti-cav trim for severe ΔP.
Leakage Class IV typical; V/VI tighter.
Pneumatic spring-diaphragm + positioner.

Frequently asked questions

What should I know about On-Off vs Control Valve?

Holds a setpoint (flow/pressure/level/temp) → control valve (globe is the default; rotary for big bore / high Cv). Size on Cv so normal flow is ~60–80% travel. Equal-% for most flow/pressure control; linear for level. Add 2 isolation valves + (often) a bypass around it. Pick fail action from the safe state.

Which standards or codes apply?

choose from the safe state on loss of air/signal: fail-closed (FC) stops flow (e.g., fuel), fail-open (FO) keeps cooling/relief flowing, fail-last (FL) holds position. This is a process-safety decision, set on the cause-and-effect chart.

Fail action — FC stops flow, FO keeps cooling/relief, FL holds — a safety decision.

Failure modes

Symptom	Cause & fix
Hunting near shut	Oversized → resize.
Noise / trim erosion	Cavitation → anti-cav trim.
Sticky response	Actuator/positioner → check & size.

Requisition & datasheet

Process data, Cv, % travel.
Body, characteristic, trim, materials.
Actuator, fail action, positioner.

Standards — IEC 60534 / ISA 75; FCI 70-2; ASME B16.34.