The Short Answer
A control valve is a throttling valve that modulates flow in response to a signal from a controller. Select it in five steps: (1) choose the valve type for the service, (2) calculate the required Cv at the design conditions, (3) select the flow characteristic (equal percentage or linear), (4) size and select the actuator and positioner for the required stroke and fail-safe position, and (5) verify the trim for cavitation, noise, and erosion. Getting any of these steps wrong — especially Cv and characteristic — produces unstable or underperforming control.
Step 1: Choose the Valve Type
| Valve Type | Best For | Avoid When |
|---|---|---|
| Globe (single seat) | General pressure and flow control, tight shutoff, most services | Very high pressure drop with cavitation; large sizes (cost) |
| Globe (double seat / cage-guided) | Higher pressure drop, larger sizes, good guiding | Tight shutoff with soft seats is not always achievable |
| Ball (segmented V-ball) | Flow control of liquids and slurries, high Cv | High-temperature metal-seat duty needs special trims |
| Butterfly (with positioner) | Large lines, economical control, water/air | Very tight shutoff at high pressure; cavitation at high ΔP |
| Angle valve | Flashing, cavitation, erosive slurries | General service (higher cost) |
| Needle valve | Small flow, instrument lines | Anything needing large Cv |
The globe valve is the default control valve in process plants; V-ball and butterfly valves compete on cost and capacity.
Step 2: Calculate the Required Cv
Cv is the number of US gallons per minute of water at 60°F that flows through the valve with a 1 psi pressure drop. Size the valve so that the calculated Cv sits between about 20% and 80% of the valve’s rated Cv at the design flow.
For liquids:
Cv = Q × √(G / ΔP)
For gases (simplified for moderate pressure drop, ΔP/P1 < 0.5):
Cv = Q / (1360 × √(ΔP × P1) / √(G × T)) — use the manufacturer’s sizing program for accuracy
Where Q = flow, G = specific gravity, ΔP = pressure drop, P1 = upstream pressure, T = absolute temperature. For flashing, choked flow, and high-pressure-drop gas, use the IEC 60534-2-1 (ISA-75.01) standard formulas or the manufacturer’s sizing software — hand calculation is not adequate.
Rule of thumb: if the calculated Cv is above about 80% of the rated Cv, the valve is undersized and will not pass design flow; if it is below about 10–20%, the valve is oversized and will be unstable at low flow.
Step 3: Select the Flow Characteristic
- Equal percentage: flow change is a constant percentage of the current flow per equal stem travel. Best for most liquid and gas loops, and for valves with high installed pressure drop. This is the default choice.
- Linear: flow is proportional to travel. Use for level control, and for processes with constant system pressure drop.
- Quick opening: maximum flow change near the seat. Use for on-off service and safety applications only.
Match the characteristic to the process loop gain: an equal-percentage trim compensates for decreasing system resistance as the valve opens, giving a near-linear installed characteristic.
Step 4: Actuator and Positioner
- Fail position: decide whether the valve must fail open (air-to-close) or fail closed (air-to-open) on loss of instrument air. This is a process-safety decision, not a preference.
- Actuator sizing: the actuator must overcome the maximum stem force at the worst case: maximum differential pressure across the valve at the closed position, plus seat friction and packing friction. Use the manufacturer’s thrust/torque tables; never guess.
- Positioner: use a smart positioner (HART, FOUNDATION Fieldbus, or Profibus) for accurate positioning, diagnostics, and characterization. A positioner converts the control signal into the correct stem position regardless of packing friction and process forces.
Step 5: Check Trim for Cavitation, Noise, and Erosion
For liquids with high ΔP, check the cavitation index (σ = (P2 − Pv) / (P1 − P2)). If σ is below about 2 at high flow, cavitation is likely — consider a multi-stage trim, an anti-cavitation trim, or an angle valve. For gas with high ΔP (choked flow), aerodynamic noise can exceed limits; use a low-noise trim or an attenuator. For erosive media, hard-face the trim or use a V-ball with hardened surfaces.
Common Selection Errors
- Oversizing. The single most common control-valve mistake. An oversized valve operates near the seat at normal flow, causing instability, hunting, and premature seat wear.
- Wrong characteristic. A linear trim on a system with falling pressure drop gives unstable control at low flows.
- Ignoring installed gain. Always evaluate the installed characteristic with the real system curve, not the inherent characteristic from the catalog.
- Weak actuator. The valve will not seat fully or will be unstable under high ΔP.
- No cavitation check. The trim is destroyed within months in flashing/cavitation service.
Selection Data Sheet Checklist
- [ ] Service description, fluid, phase
- [ ] Normal, maximum, and minimum flow
- [ ] Upstream and downstream pressure at each flow
- [ ] Temperature, density/specific gravity, viscosity
- [ ] Required shutoff class and fail-safe position
- [ ] Allowable noise level
- [ ] Body material, end connections, pressure class
- [ ] Instrument signal type and positioner protocol
Conclusion
Control valve selection is a calculation-driven process: Cv first, then characteristic, then actuator, then trim protection. Fill in the data sheet completely, size with the standard (IEC 60534 / ISA-75.01) or the manufacturer’s software, and verify the installed characteristic before purchase. A correctly sized control valve controls well; an incorrectly sized one causes process problems that no amount of tuning can fix.