Control Valves

Chapter from the Instrumentation & Control Design Guide.

CONTROL VALVE SELECTION & DESIGN

Control Valve Type Selection

Valve Type	Body Style	Rangeability	ΔP Capability	Best Application	Limitations	Standards
Globe	Single seat, cage-guided	50:1	High (Class V shutoff)	Throttling, most process control	Large sizes expensive, high ΔP	ISA/IEC 60534
Globe	Double seat	50:1	Moderate	Large flow, balanced forces	Leakage (Class II typical)	ISA/IEC 60534
Butterfly	Disc in flow path	30:1	Moderate	Large flow, low ΔP applications	Torque at shutoff, limited ΔP	ISA 75.02
Ball	Full or V-port ball	100:1 (V-port)	High	On-off or high rangeability	Slurry can pack cavity	ISA/IEC 60534
Eccentric Plug	Rotary plug, offset axis	50:1	High	Erosive/cavitating service	Limited sizes	ISA/IEC 60534
Three-Way	Mixing or diverting	30:1	Moderate	Bypass, blending, heat exchange	Complex, limited ΔP	ISA/IEC 60534

Multi-Step Valve Applications

When to Use Multi-Step (Split-Range) Valve Arrangements:

1. Very high rangeability (>100:1 turndown)
• Two valves: small valve (0-50% signal) + large valve (50-100% signal)
• Each valve sized for its portion of the flow range

2. Different failure modes needed
• Example: heating/cooling duty with one valve fail-open and one fail-closed

3. High ΔP at low flow, low ΔP at high flow
• Small valve handles high ΔP at low flow (restricted trim)
• Large valve opens for high flow at lower ΔP

4. Severe service + normal service combined
• Severe service valve (anti-cavitation trim) for high-ΔP conditions
• Standard valve for normal conditions

Split-Range Signal Configuration:
• Sequential: 4-12 mA drives valve 1, 12-20 mA drives valve 2
• Overlapping: small overlap zone where both valves are partially open
• Ensure smooth transition — no dead band or bump at handoff point

Source: Instrumentation_Control_Design_Guide_v1.xlsx · sheet “Control Valves”

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