Instrumentation Engineering Curriculum
Module 3 - Control Valves
Module from the Instrumentation Engineering Curriculum curriculum.
CONTROL VALVE SIZING & SELECTION · Learning Objectives · 1. Calculate Cv using ISA/IEC 60534 sizing equations for liquid and gas service 2. Select valve characteristics (linear, equal %, quick opening) based on process requirements 3. Evaluate cavitation and flashing potential using FL factor 4. Specify valve type, size, materials, and actuator for process conditions 5. Understand inherent vs installed characteristics and rangeability · Control Valve Types — Comparison
| Valve Type | Cv Range | Characteristics | Best Application | Rangeability | Shutoff Class | Notes |
|---|---|---|---|---|---|---|
| Globe (single-seat) | 0.01-2000 | Linear or equal % | Throttling control, most process applications | 50:1 | ANSI IV-VI | Most common control valve; predictable performance |
| Globe (double-seat / cage) | 10-5000 | Linear, equal %, or quick-open | High capacity, high ΔP applications | 50:1 | ANSI II-IV | Lower shutoff than single-seat; balanced trim option |
| Butterfly (high-performance) | 50-25000 | Modified equal % | Large lines (6"+), low ΔP, on-off or throttling | 20:1 to 40:1 | ANSI V-VI | Cost-effective for large sizes; limited ΔP capability |
| Ball (segmented/V-notch) | 10-10000 | Equal % (V-notch characteristic) | Slurry, viscous, fibrous fluids, mining, pulp | 100:1 to 300:1 | ANSI V-VI | Self-cleaning action; excellent for erosive service |
| Rotary plug | 5-3000 | Equal % | General service, alternative to globe in some applications | 50:1 | ANSI IV-V | Low torque, compact design |
| Three-way (mixing/diverting) | 5-2000 | Linear | Temperature control (mix hot/cold), bypass service | 30:1 | ANSI IV | Not for tight shutoff applications |
| Eccentric rotary plug | 10-5000 | Equal % | High-pressure drop, erosive/cavitating service | 100:1 | ANSI V-VI | Cavitation-resistant design |
| Cv Sizing — Key Formulas (ISA/IEC 60534) | ||||||
| Service | Formula | Variables | Typical Values | Calculation Notes | Reference | |
| Liquid (non-cavitating) | Cv = Q × √(SG / ΔP) | Q = flow (GPM), SG = specific gravity, ΔP = psi | Size valve for 60-80% open at normal flow | Check FL factor for cavitation: ΔP < FL² × (P1-Pv) | ISA 75.01 / IEC 60534-2-1 | |
| Liquid (cavitating) | Apply FL factor: ΔP_allowed = FL² × (P1 - FF × Pv) | FL = liquid pressure recovery factor, FF = critical pressure ratio | FL: globe=0.9, butterfly=0.55-0.7, ball=0.6-0.8 | If ΔP > ΔP_allowed, valve is cavitating — use anti-cavitation trim | ISA 75.01 | |
| Gas (subcritical) | Cv = Q / (N × Fp × P1 × Y × √(x × M / (T × Z))) | N = numerical constant, Y = expansion factor, x = ΔP/P1 | Size for x < xT (subcritical); if x ≥ xT, flow is choked | Use xT from valve data sheet; Y = 1 - x/(3×Fk×xT) | IEC 60534-2-1 | |
| Gas (choked/critical) | Use xT as maximum x value | xT = pressure drop ratio at choked flow | xT: globe=0.7, butterfly=0.3-0.5 | Flow cannot increase beyond choked flow regardless of ΔP increase | ISA 75.01 | |
| Steam | Cv = W / (N × Fp × Y × √(x × γ₁ × P1)) | W = mass flow (lb/hr), γ₁ = specific heat ratio | Superheated: use gas equations; Saturated: use special factors | Account for desuperheating, two-phase conditions | ISA 75.01 | |
| Source: FOS Chief Files — Control Valve Handbook (Fisher), Control_Valve_Sizing_Calculator_v1.xlsx, IPE-EP-12-1-14 (Control Valves) |
Source: Instrumentation_Engineering_Curriculum_v1.xlsx · Sheet: Module 3 - Control Valves
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