Antisurge Control

Chapter from the Instrumentation & Control Design Guide.

COMPRESSOR ANTISURGE CONTROL

Surge Fundamentals

Surge is a violent aerodynamic instability in centrifugal compressors that occurs when:
• Flow drops below the surge limit at a given speed
• The compressor cannot maintain the required pressure ratio at the reduced flow
• Flow reverses momentarily, then re-establishes → cyclic reversal

Surge Consequences:
• Rapid temperature rise in compressor
• Extreme mechanical forces on bearings, seals, impellers
• Severe vibration → potential catastrophic failure
• Thrust reversal can damage thrust bearings
• A single surge event can damage a compressor; repeated surge will destroy it

Surge Line: the locus of points (flow, head) at each speed where surge occurs
Surge Control Line: set 10-15% to the right of surge line (safety margin)
Surge Trip Line: set at or slightly left of surge line (emergency shutdown)

Antisurge Control System Components

Component	Function	Location	Critical Requirements	Typical Type	Notes
Flow Transmitter	Measure compressor inlet flow	Suction pipe	Fast response (<100 ms)	Orifice or venturi	Signal used for surge parameter
ΔP Transmitter	Measure compressor head	Suction & discharge	Fast response, ranged for full map	DP transmitter	Head = ΔP / ρ
Antisurge Controller	Calculate surge parameter, control valve	Control room / SIS	Dedicated controller, <50 ms scan	CCC, Triconex, or dedicated	NOT regular DCS PID
Antisurge Valve	Recycle gas from discharge to suction	Discharge piping	Fast stroking (<2 sec full open)	Globe or butterfly	Fail-OPEN required
Recycle Cooler	Cool recycled gas	Recycle line	Sized for full recycle flow	Air or water cooled	Prevent compressor inlet overheating
Hot Gas Bypass	Rapid depressurization of discharge	Discharge header	Very fast (<1 sec)	On-off valve	Emergency antisurge, supplements main valve

Antisurge Control Strategy

The antisurge controller continuously calculates a surge parameter that represents the compressor's operating point relative to the surge line.

Common Surge Parameters:
• Reduced flow (Qr) vs. reduced head (Hr) — normalized by speed
• ΔP/P_suction vs. h_orifice/P_suction (flow-head coordinates)
• Polytropic head vs. inlet volume flow

Control Logic:
1. Calculate surge parameter from measured flow and head
2. Compare to surge control line (set point)
3. If operating point moves toward surge control line → open antisurge valve proportionally
4. If point crosses surge control line → rapid opening of antisurge valve (step + ramp)
5. Decoupling: prevent antisurge controller from fighting process controller

Tuning:
• Proportional gain: high enough for fast response, not so high as to oscillate
• Derivative: used for fast-approaching surge (anticipatory action)
• Integral: slow reset to avoid steady-state offset
• Step response: typically 50% step on crossing SCL, then PI takes over

Source: Instrumentation_Control_Design_Guide_v1.xlsx · sheet “Antisurge Control”

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