Process Control
Control Valve Sizing
Size Cv for liquid, gas, or steam with ISA/IEC equations, avoid choked flow, and keep the valve in its controllable stroke.
1. Sizing Principles
Control valve sizing determines the valve size needed to pass required flow at specified pressure drop. The goal: select a valve that operates in its controllable range (typically 20–80% open) at normal conditions.
Controllability
20–80% stroke
Keep normal flow in the mid-stroke zone.
ΔP budget
Rule of thirds
Split available ΔP among valve, pipe, and fittings for stable control.
Noise/cavitation
Manage early
Check F_l / x_T, size trim or multi-stage if needed.
Turn-down
Rangeability
Ball/butterfly for higher rangeability; globe for precision.
Key Sizing Objectives
- Normal operation: Valve 40–60% open at design conditions
- Maximum capacity: Valve can handle 110–120% of design flow
- Minimum controllable flow: Valve provides stable control at turndown
- Rangeability: Ratio of max to min controllable flow (typically 50:1)
2. Flow Coefficient (Cv)
Cv quantifies valve flow capacity. Defined as: gallons per minute of water at 60°F that flows through the valve with 1 psi pressure drop.
Cv vs. Kv
| Coefficient | Units | Conversion |
|---|---|---|
| Cv (US) | GPM with 1 psi ΔP | — |
| Kv (Metric) | m³/hr with 1 bar ΔP | Cv = 1.156 × Kv |
Typical Cv Values
| Valve Size | Globe (Full Port) | Ball (Full Port) | Butterfly |
|---|---|---|---|
| 1" | 10–14 | 30–40 | 35–50 |
| 2" | 40–55 | 130–170 | 150–200 |
| 4" | 160–220 | 500–700 | 700–950 |
| 6" | 350–500 | 1100–1500 | 1500–2100 |
3. Sizing Equations
Liquid Service (Non-Choked)
Gas Service (Subsonic)
Example: Liquid Sizing
Given: 500 GPM water, P1 = 150 psig, P2 = 50 psig
ΔP = 150 - 50 = 100 psi
Cv = 500 × √(1.0 / 100)
Cv = 500 × 0.1 = 50
→ Select 2" globe valve (Cv = 55 @ full open)
Example: Gas Sizing
Given: 1,000,000 SCFH natural gas (SG=0.65), P1 = 500 psia, P2 = 450 psia, T = 100°F
ΔP = 500 - 450 = 50 psi
T = 100 + 460 = 560°R, Z ≈ 0.92
Cv = 1,000,000 / (1360 × 500 × √(50/(0.65 × 560 × 0.92)))
Cv ≈ 42
4. Choked Flow
Flow becomes choked when downstream pressure reduction no longer increases flow rate. Velocity reaches sonic limit at the vena contracta.
Critical Pressure Drop
Pressure Recovery Factors
| Valve Type | FL (Liquid) | xT (Gas) |
|---|---|---|
| Globe (parabolic plug) | 0.90 | 0.70–0.75 |
| Globe (equal %) | 0.85 | 0.65–0.70 |
| Ball (V-port) | 0.80 | 0.55–0.65 |
| Butterfly (60° open) | 0.55–0.70 | 0.35–0.50 |
| Ball (full bore) | 0.60 | 0.30–0.40 |
⚠ Cavitation risk: High-recovery valves (low FL) are more prone to cavitation in liquid service. Use anti-cavitation trim or staged pressure reduction for high ΔP applications.
5. Valve Selection
Valve Types
| Type | Best For | Rangeability |
|---|---|---|
| Globe | Throttling, precise control, high ΔP | 50:1 |
| Ball (V-port) | Moderate control, slurries, fibers | 30:1 |
| Butterfly | Large flows, low ΔP, on-off with throttling | 20:1 |
| Rotary plug | High capacity, noise reduction | 50:1 |
Characteristic Selection
- Linear: Use when valve ΔP is constant fraction of system ΔP
- Equal percentage: Use when valve ΔP varies with flow (most process applications)
- Quick opening: On-off or relief service
Selection Checklist
- ☐ Calculate Cv at min, normal, and max conditions
- ☐ Check for choked flow (especially gas service)
- ☐ Select valve size where Cv at normal flow = 40–60% of rated Cv
- ☐ Verify valve can handle max flow at <90% open
- ☐ Check minimum controllable Cv for turndown requirements
- ☐ Evaluate noise and cavitation potential
- ☐ Select appropriate trim and characteristic
References
- ISA-75.01.01 – Flow Equations for Sizing Control Valves
- IEC 60534-2-1 – Industrial Process Control Valves Sizing
- API 609 – Butterfly Valves: Double-Flanged, Lug, and Wafer Type
- Fisher Control Valve Handbook (Emerson)
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