Compressor Anti-Surge and Coordinated Control

This guide explains how NeqSim represents compressor operating-envelope protection and coordinated compressor control in the dynamic compressor map example.

The focus is educational and simulation-oriented. The models illustrate common control philosophy patterns used for centrifugal compressors; they are not a replacement for vendor-certified compressor controls, machinery protection systems, or site-specific control narratives.

Control Objectives

A centrifugal compressor control scheme normally has two different objectives:

Objective Typical manipulated variable Purpose
Process performance Compressor speed, inlet guide vanes, or another load-control handle Meet discharge pressure, suction pressure, flow, or header demand efficiently
Compressor protection Anti-surge recycle valve Keep the operating point to the safe side of the surge control line

The anti-surge action is protective. It should override normal process-performance control when the compressor approaches the surge line. Recycle flow consumes power and reduces forward throughput, so the normal operating strategy is to keep recycle closed unless protection or minimum-flow handling requires it.

Operating Envelope

The compressor map defines:

NeqSim reports distance-to-surge with Compressor.getDistanceToSurge(). Positive distance means the point is to the safe side of the surge curve; negative distance means the point has crossed into the surge region for the configured map.

Anti-surge control line and recycle energy penalty

The anti-surge control line sits a fixed flow margin to the right of the surge line — it is the line the controller actually acts on. NeqSim exposes it as a first-class concept on Compressor (operating-point schema 1.1):

Method Returns
setSurgeControlMargin(double frac) / getSurgeControlMargin() control-line flow margin as a fraction of surge flow (e.g. 0.10; 0 disables)
getControlLineFlow() control-line inlet volumetric flow (m³/hr) = getSurgeFlowRate() * (1 + margin) at the current head
getDistanceToControlLine() inletFlow / controlLineFlow − 1; positive ⇒ right of the control line, so the anti-surge valve (ASV) can be closed
getRequiredRecycleFractionToControlLine() recycle fraction of total suction flow needed to hold the point on the control line (0 if already right of it)
getAntiSurgeRecyclePower(recycleFraction, unit) shaft power wasted by recycling ≈ getPower(unit) · recycleFraction
getAntiSurgeRecycleHeatDuty(recycleFraction, unit) recycle-cooler heat duty (equals the wasted shaft work at screening level)

This supports the energy-efficiency question “can the control line be moved so the ASV closes?” — recycled gas is compressed then throttled back to suction, so its compression work is pure waste:

\[P_\mathrm{wasted} \approx P_\mathrm{shaft}\,\frac{\dot m_\mathrm{recycle}}{\dot m_\mathrm{process}+\dot m_\mathrm{recycle}}\] 
compressor.setSurgeControlMargin(0.10);          // control line 10 % right of surge
double controlFlow = compressor.getControlLineFlow();       // m3/hr at current head
double distCtrl = compressor.getDistanceToControlLine();    // > 0 => ASV can be closed
double wastedKW = compressor.getAntiSurgeRecyclePower(0.10, "kW"); // at 10 % recycle

Calibrating a screening chart from a field surge test

A generated chart is a stand-in for the vendor map. Once a field surge test (measured surge points) and a gas sample (actual molecular weight) are available, CompressorChartCalibrator brings the chart closer to reality:

CompressorChartCalibrator cal = new CompressorChartCalibrator(compressor);
cal.fitSurgeCurve(flowPoints, headPoints);   // installs a SafeSplineSurgeCurve
double f = CompressorChartCalibrator.molarMassHeadCorrectionFactor(mwRef, mwActual); // = mwRef/mwActual
double margin = cal.recommendControlMargin(0.10, measuredSurgeFlow); // widens with scatter

Polytropic head scales as ~1/MW at fixed volumetric flow and speed, so a wrong assumed molecular weight mis-places the whole map. Results remain screening-level pending a qualified rotating-equipment (API 617 / API 692) review.

For dynamic studies, the important trend is not only the instantaneous distance, but also how fast the operating point is moving toward the surge line. The newer anti-surge controller includes a predictive option that uses a filtered margin rate and a short look-ahead horizon:

\[m_\mathrm{pred} = m + \dot{m}_f \Delta t_\mathrm{pred}\]

where $m$ is current surge margin, $\dot{m}f$ is the filtered margin rate, and $\Delta t\mathrm{pred}$ is the prediction horizon.

Coordinated Control Philosophy

The dynamic compressor map notebook demonstrates three levels of control behavior:

Case Behavior
Fixed-speed turndown Feed reduction moves the operating point left toward surge on one speed line.
Pressure and anti-surge control Speed changes to meet a pressure target, while recycle opens when surge margin is low.
Coordinated pressure-speed-recycle control Anti-surge protection has priority: speed integration is held back during recycle action, speed can run back, and recycle opens faster to preserve surge margin.

The coordinated case is the closest representation of plant control philosophy in the example. The performance loop still manipulates speed, but it does not blindly increase speed while the anti-surge loop is opening recycle. This prevents the pressure controller from fighting the protective recycle action.

Example Notebook

The executable example is:

The notebook demonstrates:

The coordinated comparison intentionally plots both an independent-loop case and a coordinated-loop case. In the executed example, the independent loops cross the surge line during deep turndown, while the coordinated case uses earlier recycle action and speed runback to keep a positive minimum surge margin.

From Steady State to Dynamic Mode

Use steady state to initialize the compressor train before starting a transient anti-surge study. The topology should be the same in both modes: suction valve or boundary, suction volume, compressor with map, discharge cooling or volume as needed, splitter, recycle valve, recycle stream, and outlet valve or boundary. The mode change is a handoff from a converged initial condition to time stepping; it is not a different process model.

The recommended sequence is:

  1. Build the compressor recycle topology and install the compressor map, surge curve, and stonewall curve.
  2. Run the flowsheet with process.run() to converge pressures, flows, recycle state, and the compressor operating point.
  3. Record or check the initial point with getDistanceToSurge(), compressor inlet flow, head, speed, recycle valve opening, and discharge pressure.
  4. Set the dynamic timestep with process.setTimeStep(...).
  5. Switch supported dynamic equipment to setCalculateSteadyState(false). Typical candidates are valves, separators or volumes, compressor, cooler, and splitter.
  6. Keep algebraic convergence helpers such as Recycle in steady-state mode unless the specific class has transient state support. The recycle block should settle the loop at each timestep; the valve and equipment holdups carry the dynamics.
  7. Attach the anti-surge controller to the recycle valve and run with process.runTransient().

This distinction is important because steady-state anti-surge and dynamic anti-surge are different problems. AntiSurgeRecycleCalculator is the preferred steady-state helper for a charted compressor when you want a cooled recycle stream mixed back into suction and solved to a configured surge-control margin. The older AntiSurgeCalculator is an algebraic splitter-based helper that adjusts recycle split flow during process.run(). AntiSurgeController is a transient controller that changes the recycle valve opening during runTransient() based on the compressor surge margin, optional prediction, and actuator dynamics.

Advanced Screening and Fault-Tolerant Logic

NeqSim also provides AdvancedAntiSurgeControlSystem for public, screening-level implementations of the more advanced anti-surge ideas often described in compressor-control literature. It is intentionally separate from the basic dynamic AntiSurgeController so a normal process model can keep the simple PI/recycle-valve loop, while studies that need more instrumentation and supervisory behavior can opt in.

The class includes:

Feature API entry point Use
Reduced anti-surge coordinates calculateReducedCoordinates(...) and distanceToReducedControlLine(...) Represent the operating point in pressure-normalized flow/head coordinates before comparing against a surge-control line.
Fan-law invariant map coordinates calculateInvariantMapPoint(...) Compare operating points across speed lines using reduced flow and reduced head.
Transmitter range, lag, and faults InstrumentSignal Model filtered transmitter readings with range validation and stuck, bias, drift, or invalid faults.
Sensor voting vote(...) Combine redundant signals by median, average, select-low, or select-high voting and report degraded status when signals disagree or fail validation.
Fault-tolerant supervisory decision evaluateFaultTolerant(...) Vote redundant surge-margin signals, fall back to a conservative valve opening when the vote is degraded, and force recycle open when no valid signal remains.
Recycle valve and piping-volume screening sizeRecycleSystem(...) Estimate required recycle flow, a screening valve coefficient, and whether recycle piping volume can respond within a target time.
Dual hot/cold recycle command split splitDualRecycleCommand(...) Keep the normal cold recycle path active for sustained control and open a fast hot recycle command during low-margin or fast-approach events.
Advanced control policies setAlgorithm(...) with PI, PREDICTIVE_PI, FUZZY_SCREENING, or MPC_SCREENING Compare classical PI, predicted-margin PI, rule-based fuzzy screening, and one-step screening MPC valve demand.
Surge oscillation screening stepSurgeOscillation(...) Run a lumped two-state oscillation indicator for educational controller tests.

Example supervisory calculation:

AdvancedAntiSurgeControlSystem supervisor = new AdvancedAntiSurgeControlSystem();
supervisor.setAlgorithm(AdvancedAntiSurgeControlSystem.ControlAlgorithm.MPC_SCREENING);
supervisor.setMarginSetPoint(0.10);
supervisor.setPredictionHorizon(3.0);

AdvancedAntiSurgeControlSystem.InstrumentSignal signalA =
	new AdvancedAntiSurgeControlSystem.InstrumentSignal("ASC margin A", -1.0, 1.0, 0.5);
AdvancedAntiSurgeControlSystem.InstrumentSignal signalB =
	new AdvancedAntiSurgeControlSystem.InstrumentSignal("ASC margin B", -1.0, 1.0, 0.5);
signalA.update(compressor.getDistanceToSurge(), dt);
signalB.update(compressor.getDistanceToSurge(), dt);

AdvancedAntiSurgeControlSystem.FaultTolerantDecision decision =
	supervisor.evaluateFaultTolerant(
		Arrays.asList(signalA, signalB),
		AdvancedAntiSurgeControlSystem.VotingMode.SELECT_LOW,
		marginRate,
		dt);
recycleValve.setPercentValveOpening(decision.getValveOpening());

The advanced class remains a simulation and screening layer. getCertificationStatus() returns NOT_VENDOR_CERTIFIED by design. Use it to test logic, demonstrate control philosophy, compare algorithms, and build defensible example studies; do not use it as a certified machinery-protection package.

Application-Design Layer

CompressorAntiSurgeApplication wraps the lower-level anti-surge functions into an application-style simulation object. Use it when the study needs more than a single controller block: multiple compressor stages, shared suction or discharge headers, hot and cold recycle paths, startup and shutdown sequencing, commissioning checks, scan-cycle execution, and operator-facing diagnostics.

The class is intentionally deterministic and data-driven. It can be used with measured or simulated scan inputs, and stages can also bind directly to real NeqSim topology objects: compressor, hot recycle valve, cold recycle valve, aftercooler, suction mixer, recycle blocks, and the owning ProcessSystem. The scan result reports stage decisions, header decisions, recycle-valve commands, diagnostics, recommendations, sequence state, and certification status.

Key concepts:

Capability API entry point What it represents
Multiple stages addStage(...) Stage-level anti-surge supervision for each compressor.
Shared headers addHeader(...) Header coordination using the most limiting connected compressor margin.
Hot/cold recycle split StageDecision.getValveCommand() Fast hot recycle for low-margin or fast-approach events, normal cold recycle for sustained control.
Startup sequencing startStartupSequence() and scan(...) Permissive check, recycle pre-open, pressurization, speed ramp, load acceptance, and normal control state progression.
Shutdown and trip behavior startShutdownSequence() and forceTripMode() Conservative recycle opening during shutdown or trip/coastdown simulation.
Commissioning checks runCommissioningChecks() Stage configuration, recycle response, valve stroke, transmitter redundancy, header coordination, and non-certified boundary statement.
Real-time/advisory scan scan(ScanInput, dt) One deterministic application scan cycle for digital twin, operator-training, or dynamic simulation studies.
Direct topology writeback StageApplication.bindTopology(...) and runDynamicStep(...) Writes hot/cold recycle openings and optional speed commands to real NeqSim units, then advances the bound process one transient step.
Diagnostics ScanResult.getDiagnostics() Alarm/degraded/trip-demand codes with operator recommendations.

Example application scan:

CompressorAntiSurgeApplication application = new CompressorAntiSurgeApplication("export compression");
CompressorAntiSurgeApplication.StageApplication lpStage = application.addStage("LP");
CompressorAntiSurgeApplication.StageApplication hpStage = application.addStage("HP");

lpStage.setDesignBasis(11000.0, 9000.0, 30.0);
hpStage.setDesignBasis(9000.0, 8800.0, 40.0);
application.addHeader("shared suction", "LP", "HP");
application.setRunningMode();

CompressorAntiSurgeApplication.ScanInput scanInput = new CompressorAntiSurgeApplication.ScanInput();
scanInput.putStageInput("LP", new CompressorAntiSurgeApplication.StageScanInput(
	0.18, 0.0, 11000.0, 9000.0, 30.0));
scanInput.putStageInput("HP", new CompressorAntiSurgeApplication.StageScanInput(
	0.03, -0.01, 9000.0, 8800.0, 40.0));

CompressorAntiSurgeApplication.ScanResult result = application.scan(scanInput, 0.25);
CompressorAntiSurgeApplication.StageDecision hpDecision = result.getStageDecisions().get(1);
double hotRecycleOpening = hpDecision.getValveCommand().getHotValveOpening();
double coldRecycleOpening = hpDecision.getValveCommand().getColdValveOpening();

Direct topology binding for an executable dynamic model:

CompressorAntiSurgeApplication application = new CompressorAntiSurgeApplication("export compression");
CompressorAntiSurgeApplication.StageApplication stage = application.addStage("K-101");

CompressorAntiSurgeApplication.TopologyBinding binding = stage.bindTopology(
	process,
	compressor,
	hotRecycleValve,
	coldRecycleValve,
	recycleCooler,
	suctionMixer,
	hotRecycle,
	coldRecycle);
binding.enableSpeedControl(95.0, 25.0, 8500.0, 12500.0, 150.0);

application.setRunningMode();
application.runDynamicStep(null, 0.25);

When scanInput is null, the stage reads the bound compressor margin and inlet flow where available, falls back to the stage design basis where needed, writes the hot/cold recycle valve openings, applies the optional compressor speed command, and then runDynamicStep(...) advances the bound process with process.runTransient(). Keep algebraic Recycle blocks as recycle convergence helpers; the valve, compressor, cooler, mixer, and any volume-capable equipment carry the dynamic response.

The application layer does not replace AntiSurgeController in every transient process model. For simple dynamic examples, AntiSurgeController can still be attached directly to the recycle valve. For studies that need stage coordination, startup states, hot/cold recycle split, diagnostics, speed runback, direct topology writeback, or commissioning evidence, the application layer provides the extra supervisory context.

Certification remains outside the open simulation model. CompressorAntiSurgeApplication.getCertificationStatus() returns NOT_CERTIFIED_FOR_PROTECTION; runCommissioningChecks() creates evidence useful for review and testing, but it does not certify the logic as a safety instrumented function or machinery protection package.

Practical Modeling Notes