Compressor Anti-Surge Recycle Setup

This skill explains how to protect a centrifugal compressor against surge in a NeqSim ProcessSystem by adding a recycle (spill-back) loop. It covers the preferred steady-state helper (AntiSurgeRecycleCalculator), the dynamic reverse-acting PI controller (AntiSurgeController), and the application-level supervisor (CompressorAntiSurgeApplication) that can bind directly to real NeqSim topology objects such as a compressor, hot recycle valve, cold recycle valve, cooler, mixer, recycle blocks, and ProcessSystem. It also explains how to auto-generate a compressor chart with surge and stonewall curves when no vendor chart is available, because every anti-surge option needs a surge limit to act against.

The pure-Python helper (AntiSurgeRecycleModel) is an educational planning aid that mirrors NeqSim's proportional anti-surge step so an agent can pre-estimate the recycle flow and decide whether a chart must be generated first. Real control design must use the validated NeqSim equipment classes below and a qualified rotating-equipment review (API 617 / API 692).

Complete compressor automation and P&ID studies

For a request to build a natural-gas compressor train with scrubbers, cooling, anti-surge, instrumentation, shutdown tests and professional P&IDs, use the agentic compressor workflow. It coordinates the existing process, rotating-equipment, control, safety and independent P&ID review agents. It requires an actual material graph, converged steady state, canonical settings applied/read back on native copies, fault and refinement tests, source-bound DEXPI 2.0.1/P&ID exports, and automation-engineering registers/narratives. Proposed devices and direct compressor-derived inputs are separate from executed measurement paths. A controller command alone is not recycle-flow or surge-protection evidence.

When to Use

Use this skill when:

• A NeqSim compressor operating point can fall to the left of its surge line at low throughput (turndown, start-up, trips) and needs recycle protection.
• You must build the anti-surge recycle topology (surge curve, recycle stream, discharge splitter or recycle branch, anti-surge valve, Recycle, and optional cooler/mixer).
• You need an executable dynamic application model where CompressorAntiSurgeApplication writes hot/cold recycle valve openings and optional compressor speed runback to real NeqSim units.
• No vendor compressor chart is provided and a chart with surge and stonewall curves must be generated from the compressor's design point.
• You want a screening estimate of the recycle flow required to keep the compressor off surge before running the full simulation.

Do not use this skill as a substitute for vendor performance maps, anti-surge controller tuning, dynamic surge/transient analysis, or rotating-equipment design review.

Inputs

• Compressor inlet (suction) volumetric flow at the operating point (m3/h).
• Surge-limit flow at the operating head and speed (m3/h). When no vendor chart is available, this comes from the generated surge curve.
• Whether a vendor compressor chart is provided (chart_provided).
• Existing recycle flow, if any (m3/h).
• For chart generation: a design speed, the number of speed lines, and optionally impeller diameter / number of stages for advanced corrections.

Outputs

• needs_chart_generation — whether a chart must be generated before control.
• in_surge — whether the operating point is at or below the surge flow.
• surge_margin_fraction — (inlet_flow - surge_flow) / surge_flow.
• recommended_recycle_flow — screening recycle flow to add at suction (m3/h).
• total_suction_flow — inlet flow plus recommended recycle (m3/h).
• recycle_warning — ok, recycle, or surge.
• Assumptions and limitations.

Engineering Method

A centrifugal compressor surges when the inlet volumetric flow drops below the surge flow at the current head/speed. Anti-surge control opens a recycle line from discharge back to suction to keep the total suction flow above the surge limit plus a margin.

For steady-state recycle sizing and initialization, prefer AntiSurgeRecycleCalculator for a charted compressor. It runs the compressor, checks natural inlet flow against the surge-control line, and iterates a cooled recycle stream until the target distance-to-surge margin is met. Use the older splitter-based Calculator / AntiSurgeCalculator pattern only when you need to reproduce an existing flowsheet that already uses a discharge splitter.

The legacy splitter-based topology is:

1. A surge curve on the compressor chart provides the surge flow versus head.
2. A low-flow recycle stream is added into the compressor suction.
3. A splitter on the compressor discharge creates a forward branch (getSplitStream(0)) and a recycle branch (getSplitStream(1)).
4. An anti-surge Calculator (legacy name-prefix API) or typed AntiSurgeCalculator reads the compressor and writes the splitter recycle flow each iteration. Internally it compares the inlet flow to the surge flow: far from surge (inlet_flow > 1.2 * surge_flow) it drives recycle to a minimum; otherwise it adds a proportional, capped step 0.5 * (surge_flow - inlet_flow) to the recycle branch.
5. An anti-surge valve on the recycle branch drops the discharge pressure back to suction pressure.
6. A Recycle unit closes the loop, feeding the valve outlet back into the placeholder suction recycle stream.

Generating a Compressor Chart When None Is Given

The anti-surge loop needs a surge limit, so if no vendor chart exists a chart must be generated first. NeqSim builds a chart from the compressor's design operating point and automatically populates both the surge curve and the stonewall curve:

• Compressor.generateCompressorChart("normal curves", numberOfSpeeds) builds a multi-speed chart whose lowest-flow points form the surge curve and whose highest-flow points form the stonewall curve.
• CompressorChartGenerator gives finer control (chart type, Reynolds/Mach corrections, multistage surge correction, impeller diameter) and also returns a chart with surge and stonewall curves attached.

After generation, the surge flow at the operating head is available through Compressor.getSurgeFlowRate() and the distance to surge through Compressor.getDistanceToSurge().

The AntiSurgeRecycleModel mirrors step 4 so you can estimate the recycle flow and flag whether chart generation (needs_chart_generation) is required before wiring the loop.

Python Usage Pattern

Screening estimate of the recycle flow (no NeqSim required):

from compressor_antisurge_recycle import AntiSurgeRecycleModel

model = AntiSurgeRecycleModel()
plan = model.plan(
    inlet_flow=4200.0,   # m3/h at suction
    surge_flow=5000.0,   # m3/h surge limit at operating head
    chart_provided=True,
    current_recycle=0.0,
)
print(plan.recommended_recycle_flow, plan.recycle_warning)

Generate a compressor chart (with surge and stonewall curves) when none is given, using NeqSim through the neqsim package:

from neqsim import jneqsim

# `compressor` is an existing neqsim.process.equipment.compressor.Compressor
compressor.setSpeed(8000.0)   # design speed (rpm)
compressor.run()              # solve the design point first
compressor.generateCompressorChart("normal curves", 5)  # 5 speed lines

# Surge and stonewall curves are now populated automatically.
chart = compressor.getCompressorChart()
surge_flow = compressor.getSurgeFlowRate()
distance_to_surge = compressor.getDistanceToSurge()

Finer control with the generator (optional corrections):

from neqsim import jneqsim

generator = jneqsim.process.equipment.compressor.CompressorChartGenerator(compressor)
generator.setChartType("interpolate and extrapolate")
generator.enableAdvancedCorrections(numberOfStages)  # Reynolds + Mach + multistage
chart = generator.generateCompressorChart("normal curves", 5)
compressor.setCompressorChart(chart)

Wire the anti-surge recycle loop (generic data, after a chart exists):

from neqsim import jneqsim

splitter_pkg = jneqsim.process.equipment.splitter
valve_pkg = jneqsim.process.equipment.valve
util_pkg = jneqsim.process.equipment.util

suction_pressure = compressor.getInletStream().getPressure("bara")

# 2. Low-flow placeholder recycle stream into the compressor suction.
recycle_gas = compressor.getInletStream().clone()
recycle_gas.setName("anti surge recycle gas")
recycle_gas.setFlowRate(1.0, "kg/hr")
recycle_gas.run()
process.add(recycle_gas)

# 3. Splitter on the compressor discharge: branch 0 forward, branch 1 recycle.
gas_splitter = splitter_pkg.Splitter("anti surge splitter", compressor.getOutletStream(), 2)
gas_splitter.run()
process.add(gas_splitter)

# 4. Anti-surge Calculator (name MUST start with "anti surge calculator").
anti_surge_calc = util_pkg.Calculator("anti surge calculator 1")
anti_surge_calc.addInputVariable(compressor)
anti_surge_calc.setOutputVariable(gas_splitter)
process.add(anti_surge_calc)

# 5. Anti-surge valve on the recycle branch back to suction pressure.
anti_surge_valve = valve_pkg.ThrottlingValve("anti surge valve", gas_splitter.getSplitStream(1))
anti_surge_valve.setOutletPressure(suction_pressure, "bara")
anti_surge_valve.run()
process.add(anti_surge_valve)

# 6. Recycle unit closes the loop into the placeholder suction stream.
recycle = util_pkg.Recycle("recycle anti surge")
recycle.addStream(anti_surge_valve.getOutletStream())
recycle.setOutletStream(recycle_gas)
recycle.setTolerance(1e-2)
process.add(recycle)

process.run()

The forward process continues on gas_splitter.getSplitStream(0).

Preferred steady-state helper for a charted compressor:

AntiSurgeRecycleCalculator calc = new AntiSurgeRecycleCalculator(compressor, suctionStream);
calc.setSurgeControlMargin(0.10);
calc.setRecycleCoolerTemperature(35.0, "C");
AntiSurgeRecycleCalculator.Result result = calc.solve();

Constant-Speed Compressors: Fixed-Speed Operating Point

A motor-driven machine on a fixed gear (no VFD) cannot move speed, so the speed solvers of CompressorShaft do not apply. Use CompressorShaft.evaluateFixedSpeedControl() (or CompressorFixedSpeedControl.evaluate(compressor, requiredBara) for one stage). Per stage it reports, on the real chart at the fixed speed:

Set the stage pressures with setMemberRequiredDischargePressure, call runAtFixedSpeed(rpm, null), then evaluate. Idle members (1oo2 trains) are NOT_EVALUATED. The evaluation runs a deep copy of each member, so MW correction, IGV and the efficiency model are those of the flowsheet. Tests: CompressorFixedSpeedControlTest.

Dynamic Anti-Surge Control (AntiSurgeController)

The Calculator-driven loop above is a steady-state recycle solver. For a dynamic (transient) anti-surge response, NeqSim provides a dedicated reverse-acting PI controller, AntiSurgeController (neqsim.process.controllerdevice.AntiSurgeController), that reads Compressor.getDistanceToSurge() and drives a recycle ThrottlingValve open as the margin falls below the set point, then closes it again on recovery.

from neqsim import jneqsim

controllerdevice = jneqsim.process.controllerdevice

# `recycle_valve` is the anti-surge ThrottlingValve on the recycle branch.
asc = controllerdevice.AntiSurgeController("anti-surge", compressor, recycle_valve)
asc.setSurgeMarginSetPoint(0.10)   # protect a 10% distance-to-surge margin
asc.setProportionalGain(400.0)     # percent opening per unit margin error
asc.setIntegralTime(20.0)          # s
asc.setOpeningRange(0.0, 100.0)    # valve opening clamp (%) with anti-windup
asc.setActive(True)
recycle_valve.addController("anti-surge", asc)

Control law each transient step: error = setPoint - distanceToSurge, integral += Kp/Ti * error * dt, opening = clamp(Kp*error + integral) with anti-windup. A reproducible benchmark, neqsim.process.util.scenario.AntiSurgeDynamicBenchmark, drives the real controller against a transparent first-order gas-path surrogate (m_next = m - disturbance*dt + authority*(opening/100)*dt) and is the preferred way to verify or tune the control law without solver fragility.

Critical gotchas when building a full dynamic recycle flowsheet:

• A fixed-factor Splitter (setSplitFactors([0.97, 0.03])) pins the recycle fraction in dynamic mode, so the anti-surge valve has no authority over the actual recycle flow — it can reach 100% open with no effect. Let the recycle flow be set by the valve (Cv/resistance), or keep the steady-state Calculator pattern.
• Once the operating point crosses left of the surge line, getDistanceToSurge() clamps at -1.0 and the steady solver cannot recover; a flowsheet driven into deep surge will not self-heal even after the inlet is reopened. Apply gradual/ramped disturbances and keep the machine off deep surge.
• Aggressive proportional gain can slam the recycle valve to minimum opening, starve a stream, and trigger an SRK flash NaN (PhaseSrkEos:molarVolume ... NaN). Keep gains moderate.

Dynamic controller tuning and transient surge analysis still require qualified rotating-equipment review (API 617 / API 692).

Application-Level Dynamic Topology Binding

For realistic dynamic studies with coordinated pressure, speed, hot recycle, and cold recycle behavior, use CompressorAntiSurgeApplication. It is a deterministic supervisory scan layer that can write directly to real NeqSim objects and then advance the bound process one transient step.

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 the scan input 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 and cold recycle valve openings, applies the optional compressor speed command/runback, and advances the bound ProcessSystem with runTransient(). Keep Recycle blocks algebraic unless the specific class has transient inventory support; the valve, compressor, cooler, mixer, and any volume-capable equipment carry the dynamic response.

CompressorAntiSurgeApplication is still a simulation and advisory layer. Its certification status remains NOT_CERTIFIED_FOR_PROTECTION; use it for engineering studies, training, digital twins, and commissioning evidence, not as a certified machinery-protection package.

MCP runProcess Physical Topology Contract

Use two distinct JSON surfaces deliberately:

• Compressor property antiSurge configures the compressor's embedded screening control. It does not create or certify a physical recycle path.
• Root-level antiSurgeSystems binds CompressorAntiSurgeApplication to named units that already exist in the submitted process topology. It never synthesizes hidden valves, coolers, mixers, or recycle blocks.

A single-area cold-recycle declaration has this shape:

{
  "autoSizing": {"enabled": true},
  "antiSurgeSystems": [
    {
      "name": "Export anti-surge",
      "stages": [
        {
          "name": "Stage 1",
          "compressor": "Comp",
          "suctionMixer": "Suction Mixer",
          "aftercooler": "Aftercooler",
          "coldRecycleValve": "Cold ASV",
          "coldRecycle": "Cold Recycle",
          "recycleDesign": {
            "controlMargin": 0.10,
            "valvePressureDrop": 10.0,
            "pipingVolume": 1.0,
            "requiredResponseTime": 5.0
          },
          "speedControl": {
            "dischargePressureSetPoint": 40.0,
            "minimumSpeed": 5000.0,
            "maximumSpeed": 15000.0,
            "speedGain": 40.0,
            "recycleRunbackRate": 80.0
          }
        }
      ]
    }
  ]
}

The referenced names must resolve to the expected concrete types: Compressor, Mixer, optional Cooler, ThrottlingValve, and Recycle. At least one complete hot or cold recycle path is required. For a multi-area ProcessModel, every system also requires an area matching a submitted area; unqualified and unknown-area definitions return bounded per-system failures.

Every stage needs an active surge curve. Supply properties.compressorChart, enable autoSizing, or set generateScreeningMap: true on the stage. The response reports truthful map provenance:

• submitted: chart was present in the submitted canonical process JSON.
• auto_sized_screening: autoSizing generated the chart earlier in the run.
• stage_generated_screening: the anti-surge stage generated the chart.

The latter two set screeningGradeMap: true. They are estimates, not vendor maps. generatedScreeningMapCount counts all configured stages using a screening-grade map, regardless of which generation path created it.

The response antiSurgeSystems object contains configured/failed counts, per-stage physical binding and map provenance, optional speed-control status, and runCommissioningChecks() evidence. Its certification status is always NOT_CERTIFIED_FOR_PROTECTION. The canonical processDefinition preserves the declaration, and the generated Python replays that canonical post-design JSON.

When a canonical response is replayed, its exported compressor chart is now an explicitly submitted chart for that new request. Therefore its provenance is submitted; replay does not generate another map. This makes JSON round trips stable while preserving the original run's generation evidence in that run's response.

Compressor Pressure-Target Solves

For Compressor.setSolveSpeed(true), check isSpeedSolveConverged() after run() before accepting an optimization point. Read actual pressure from the outlet stream and the signed pressure error in bar from getSpeedSolvePressureResidual(). getOutletPressure() retains the requested setpoint, including after failure. The returned state uses the fixed-speed forward calculation, including anti-surge flow changes; these can leave the requested pressure unmet. setLimitSpeed(false) allows extrapolation, so pressure convergence does not establish mechanical feasibility. Check speed and operating-envelope constraints separately. ProductionOptimizer treats Compressor.SpeedSolveException as an infeasible trial and continues searching without evaluating stale outputs; unrelated exceptions propagate. Its hard compressorSpeedSolve constraint also rejects returned unconverged results even with capacity analysis enabled. Keep this gate when integrating chart-based compressor models into optimizers. See the compressor curves guide and CompressorSpeedSolveConsistencyTest for the tested contract.

Minimum-Speed Recycle Coordination (MinimumSpeedRecycleControllerStructure)

neqsim.process.controllerdevice.structure.MinimumSpeedRecycleControllerStructure coordinates pressure control between compressor speed and recycle-valve opening when the compressor reaches its minimum speed. Use it to stop opposing speed demand and recycle-valve opening in a compressor pressure loop (control-optimization tasks).

1. Split-range output: the pressure controller output is divided at a configurable transition (e.g. 75 %).
2. Speed command range: above the transition (75 % to 105 %) recycle addition is zero and speed rises linearly from minimum (75 %) to maximum (100 %).
3. Inverse recycle addition: below the transition (0 % to 75 %) speed is held at minimum while recycle addition increases as pressure output falls.
4. Latch on entry: on lower-range entry the previously selected recycle command is latched and pressure-derived recycle is added to that baseline.
5. High-selector protection: independent anti-surge and suction-pressure demands join a high selector, $R_{selected} = \max(R_{latch} + R_{add}, R_{AS}, R_{suction})$, so protection authority is never suppressed.
6. Unwind before speed increase: on rising pressure, pressure-derived recycle unwinds to the latched baseline before speed increases.
7. Dynamic saturation floor: a lower output limit (latched * 74 / 100) prevents integral windup.

import neqsim.process.controllerdevice.ControllerDeviceInterface;
import neqsim.process.controllerdevice.ControllerDeviceBaseClass;
import neqsim.process.controllerdevice.structure.MinimumSpeedRecycleControllerStructure;

ControllerDeviceInterface pressureController = new ControllerDeviceBaseClass("PIC-0205A");
ControllerDeviceInterface antiSurgeController = new ControllerDeviceBaseClass("UIC-0231");
ControllerDeviceInterface suctionController = new ControllerDeviceBaseClass("PIC-0131");

// (pressureCtrl, antiSurgeCtrl, suctionCtrl, transitionPct, maxOutputPct, minSpeedPct, maxSpeedPct)
MinimumSpeedRecycleControllerStructure coordStructure = new MinimumSpeedRecycleControllerStructure(
    pressureController, antiSurgeController, suctionController, 75.0, 105.0, 75.0, 100.0);

// In the transient loop or update step:
coordStructure.update(pressureControllerOutput, antiSurgeOutput, suctionPressureOutput);

double speedCommand = coordStructure.getSpeedOutput();             // % speed
double recycleAddition = coordStructure.getRecycleAddition();      // % recycle addition
double recycleCommand = coordStructure.getLatchedRecycleOutput();  // % latched baseline

Energy and CO2 Savings from Avoided Recycle

When recycle opens uncoordinatedly while the compressor runs at minimum or elevated speed, extra gas is re-compressed without increasing net forward production.

1. Shaft power: $P_{shaft} = \dfrac{\dot{m}{net}, h{poly}}{\eta_p (1 - \alpha_{recycle})}$, with recycle fraction $0 \le \alpha < 1$.
2. Power penalty: $\Delta P = P_0,\dfrac{\alpha}{1 - \alpha}$.
3. Fuel and CO2: $\Delta \dot{m}{fuel} = \dfrac{\Delta P}{\eta{driver},LHV}$ and $\Delta \dot{m}{CO2} = \Delta \dot{m}{fuel}, e_{CO2}$.
4. Annual saving (MNOK/yr) $= \dfrac{\Delta m_{CO2,yr},\mathrm{Tax}{CO2} + \Delta m{fuel,yr},\mathrm{Value}_{gas}}{10^6}$.

Recipe: build the recompressor with a fixed net feed, then sweep the recycle fraction by setting the suction flow to net / (1 - alpha) and re-running stream and compressor; the power difference to $\alpha = 0$ is the penalty. Quantify annual MWh, fuel-gas tonnes, CO2 tonnes and cost over the operating hours, and compare the dynamic pressure, speed, recycle, power and CO2 time series.

Fixed Minimum Flow Without a Numerical Recycle Loop

For steady-state screening with a prescribed minimum suction flow and a return cooled to the net-feed suction state, use neqsim.process.equipment.compressor.MinimumFlowSpill(name, netFeed). Set setMinimumInletFlow(value, unit) and configure getCompressor(). Add the assembly once to the ProcessSystem; the net forward outlet is getOutletStream(), while getSpillStream() is an internal discharge spill. getSpillFlow(unit) uses suction conditions even for actual volume. Use getPower(unit) for gross compression and getSpillCoolingDuty(unit) for ideal heat rejection. The assembly delegates capacity constraints to its compressor. Disable internal AntiSurge flow modification. Use physical valve, cooler and recycle equipment for composition changes, condensation, finite cooler approach or dynamics. See the compressor equipment guide and MinimumFlowSpillTest for the executed synthetic material and energy checks.

Validation Checklist

• The compressor has a chart with an active surge curve before the loop runs; if not, generate one and confirm getSurgeCurve().isActive() is true.
• For steady-state recycle initialization, use AntiSurgeRecycleCalculator when practical; if using the legacy splitter path, the anti-surge Calculator name starts with "anti surge calculator" or the typed AntiSurgeCalculator is used.
• For the legacy splitter path, the Calculator input is the Compressor and the output is the discharge Splitter.
• The anti-surge valve sits on getSplitStream(1) and drops to suction pressure.
• The Recycle outlet stream is the placeholder suction recycle stream.
• After process.run(), the total suction flow exceeds the surge flow plus the intended margin (getDistanceToSurge() is positive).
• Recycle convergence tolerance is set (for example setTolerance(1e-2)).
• For CompressorAntiSurgeApplication topology binding, hot/cold recycle valves are real ThrottlingValve units, speed-control limits are bounded, and runDynamicStep(...) is used only after the process has a converged initial state.
• For MCP runProcess, use root-level antiSurgeSystems only after all named physical units are present in process; require area for every multi-area system and inspect configuredCount, failedCount, mapProvenance, screeningGradeMap, commissioning checks, and certification status.

Common Mistakes

Fixed-speed chart calculations reject nonfinite or negative head and efficiencies outside (0, 100] percent before the PH flash. If this diagnostic appears, check actual inlet flow, speed, and map range; do not treat polynomial extrapolation to negative map values as a thermodynamic solver failure or clamp it into feasibility. CompressorChartOperatingPointTest covers rejection and accepted-outlet preservation.

• Naming the legacy calculator anything that does not start with "anti surge calculator" — the legacy anti-surge logic then never triggers.
• Wiring the anti-surge valve to the forward branch getSplitStream(0) instead of the recycle branch getSplitStream(1).
• Running the loop without a surge curve, so there is no surge flow to compare against (generate a chart first when no vendor chart is given).
• Using a zero or negative placeholder recycle flow that the Recycle cannot converge from; start from a small positive flow (for example 1 kg/hr).
• Reading the surge flow at the wrong head or speed — the surge flow must match the operating head and speed.
• Treating the generated chart as a vendor-validated map; generated charts are estimates for modelling, not design certification.
• For dynamic studies, leaving a fixed-factor Splitter on the recycle branch so the AntiSurgeController valve has no authority over the recycle flow.
• Driving a dynamic recycle flowsheet straight into deep surge, where getDistanceToSurge() clamps at -1.0 and the steady solver cannot recover.
• Loading a VENDOR map into the default chart. Compressor's default "fan law" CompressorChart fits one quadratic of H/N^2 vs Q/N through all speed lines, which on a real vendor map shifts discharge pressure by 10-15 % at the outer speed lines (and moves the surge flow with it). Call compressor.setCompressorChartType("interpolate and extrapolate") BEFORE getCompressorChart().setCurves(...); Pump.setPumpChartType(...) has the same option. Units for setCurves: actual inlet m3/hr, head kJ/kg or meter, polytropic efficiency in PERCENT.
• Checking anti-surge valve capacity with the default ThrottlingValve sizing. The default method uses xT = 0.137 without choking and under-rates a globe anti-surge valve by a factor ~2 at pressure ratio 4. Set getMechanicalDesign().setValveSizingStandard("IEC 60534"), then on getValveSizingMethod() call setxT(0.70), setFL(0.90) (or the vendor's values) and setAllowChoked(True) before setCv(cv, "US").
• Setting the control-line margin from habit instead of the machine's anti-surge write-up. Vendor controllers store the surge-control line as a function generator in (Q^2, DP/Ps) coordinates; the flow margin is sqrt(Q^2_SCL / Q^2_SLL) - 1 at equal DP/Ps (~20 % in Q^2 = ~10 % in flow is typical), then compressor.setSurgeControlMargin(margin).

Limitations

• Screening logic only; it does not tune an anti-surge controller or set recycle valve Cv, response time, or surge control line offset.
• The Python helper does not perform dynamic surge, ESD, or transient recycle analysis; for dynamic response use NeqSim's AntiSurgeController / AntiSurgeDynamicBenchmark under qualified review.
• Generated compressor charts are model estimates, not vendor performance maps.
• It does not replace API 617 / API 692 rotating-equipment design and review.
• Results require qualified human review before any design or operating use.

Multi-Body Compressor Trains on One Shaft (common speed)

When several compressor bodies sit on one driver shaft (a single gas turbine or motor, often through a gearbox) they must all turn at the same speed. A 2- or 3-body recompression string is the classic case. Model it with neqsim.process.equipment.compressor.CompressorShaft, NOT by letting each body solve its own speed.

Degrees of freedom. A shared shaft has exactly one mechanical degree of freedom — the common speed — and exactly one controlled target: the string's final discharge pressure. Every intermediate inter-body pressure is a result, not a spec, and must be allowed to float off the charts.

Pattern. Put each body in fixed-speed, chart-forward mode and iterate the one common speed until the reference (last) body hits the target discharge:

CompressorShaft = jneqsim.process.equipment.compressor.CompressorShaft
shaft = CompressorShaft("recompression shaft (single GT)")
shaft.addCompressor(body1)   # LP body (lowest suction)
shaft.addCompressor(body2)
shaft.addCompressor(body3)   # HP body = reference
shaft.setSpeedBounds(8000.0, 16000.0)
# re-run the whole flowsheet between speed guesses so inter-body streams,
# scrubbers and mixers update:
run_proxy = jpype.JProxy("java.lang.Runnable", dict(run=lambda: process.run()))
shaft.solveSpeed(body3, 49.0, "bara", run_proxy)   # one speed -> 49 bara at HP discharge
rpm = shaft.getSpeed()
mw = shaft.getTotalPower() / 1e6

Shared pressure nodes. Where another unit ties into an interstage (e.g. a 2nd-stage separator gas that joins the recompressor between bodies), model it as a pressure equality, not a second spec: slave that unit's pressure to the floating interstage discharge (a setter each iteration) or drop a small adapting valve there. This removes a DOF rather than adding one. A Mixer at the join already resolves to the lowest inlet pressure, so a small let-down is automatic.

Fixed-/single-speed drivers. If the driver is a constant-speed motor (no variable-speed drive) speed is not a DOF — do not iterate it. Use shaft.runAtFixedSpeed(rpm, run_proxy): the discharge floats off the chart at the locked speed and any pressure spec is met by anti-surge recycle, suction throttling, or inlet guide vanes — never by moving speed. Inlet guide vanes are a first-class fixed-speed control on Compressor: comp.setInletGuideVaneOpening(f) (f=1 fully open) or comp.setGuideVaneAngle(deg) closes the vanes to reduce head and efficiency and lower the surge flow (via a configurable InletGuideVaneModel), so getDistanceToSurge() reflects the shifted surge line — distinct from moving speed. If you have vendor IGV-position maps (a full chart per vane position), attach a CompressorChartIGV family with comp.setInletGuideVaneChart(family); it interpolates the positions into the active chart at each opening (and bypasses the parametric model). Use the parametric InletGuideVaneModel for screening / fixed-speed discharge control, and CompressorChartIGV to supply real per-position vendor maps.

Single body. A CompressorShaft with one compressor also works: solveSpeed just finds that one machine's speed for its discharge target. So export / re-injection / gas-lift machines (one body each on their own shaft) use the same API — separate shafts keep separate speeds.

Anti-surge coexists. The per-body anti-surge loops (recycle splitter, valve, Recycle, AntiSurgeCalculator) stay attached and keep protecting each body; they adjust recycle flow, while the shaft sets speed. Apply the shaft solve after the charts and anti-surge are active. solveSpeed uses a bracketed false-position (Illinois) secant — the speed↔discharge map is smooth and monotonic (higher speed → higher discharge), so it converges superlinearly. Each iteration re-solves the whole flowsheet, so on a large multi-train plant make the shaft solve opt-in and solve each train's shaft in turn.

Parallel machines and multi-stage strings. Real duties often use two parallel bodies (A/B) rather than one machine: split the feed 50/50, give each body its own anti-surge loop, and commingle with a Mixer. Splitting one large machine into two halves the per-machine gas load (a suction scrubber that reads ~168 % of design on one body drops to ~80 % on two). For a multi-stage duty (e.g. re-injection), commingle the parallel 1st-stage discharges then feed a common 2nd stage at an interstage pressure. Route a recompression train's HP discharge back to the gas header, not straight into the export suction.

Process-integrated control (CompressorShaftCalculator). Instead of the external solveSpeed callback, add a CompressorShaftCalculator to the ProcessSystem; it converges the one common speed inside process.run() (one damped secant step per pass, like AntiSurgeCalculator) so the shaft speed converges together with the recycles in a single run — no separate full-field solves. Add it after the compressor bodies / anti-surge.

CompressorShaftCalculator = jneqsim.process.equipment.util.CompressorShaftCalculator
shaft = CompressorShaft("recompression shaft (single GT)")
shaft.addCompressor(body1); shaft.addCompressor(body2); shaft.addCompressor(body3)
shaftCalc = CompressorShaftCalculator("shaft speed", shaft, body3, 49.0, "bara")
shaftCalc.setSpeedBounds(8000.0, 16000.0)
process.add(shaftCalc)
process.run()          # shaft speed converges with the recycles
rpm = shaftCalc.getSpeed()

Feasibility result + pressure control (eCalc-style). A single-speed string cannot make an arbitrary pressure — the max-speed curve is a ceiling and the min-speed curve is a floor. Both solvers saturate and flag instead of crashing, and expose a CompressorShaft.SolveResult (read with getLastSolveResult(), or the shortcut isFeasible()). The two speed-bound bracket evaluations give the min-/max-achievable discharge pressures for free.

shaft.solveSpeed(body3, 49.0, "bara", run_proxy)
r = shaft.getLastSolveResult()
if not r.isFeasible():
    # r.getStatus(): PRESSURE_ABOVE_MAX_SPEED / PRESSURE_BELOW_MIN_SPEED / OVER_POWER / STONEWALL / SURGE
    ceiling = r.getMaxAchievablePressure()   # most the string can make
# too-low target -> shed the surplus head instead of flagging infeasible:
PressureControl = jneqsim.process.equipment.compressor.CompressorShaft.PressureControl
shaft.setPressureControl(PressureControl.DOWNSTREAM_CHOKE)   # or UPSTREAM_CHOKE / ASV_RECYCLE

CompressorShaftCalculator carries the same getLastSolveResult() / isFeasible() / setPressureControl(...), so an optimizer or evaluate() loop can gate on shaftCalc.isFeasible() directly. Downstream, Mixer.isPressureMismatch() flags where an unmet pressure collapses a commingling node to the lowest inlet — the signal that an upstream machine missed spec. A worked end-to-end example (three-stage separation + shared-shaft recompression + control) is in the NeqSim repo notebook examples/notebooks/CompressorShaft_ThreeStageSeparation.ipynb.

Common-Shaft Trains Across Platforms, Power-Limited Drivers and Ambient Derating

CompressorShaft (since 3.26) covers the cases a plain "one shaft, one target" model misses. All of it is reached through the same object, and the members may live in different ProcessSystems (the Runnable you pass to solveSpeed runs every system in order and copies the inter-platform stream).

shaft = CompressorShaft("HT 3rd+4th+5th stage (one GT)")
shaft.addCompressor(ka23003)            # OSA, ratio 1.0
shaft.addCompressor(ka23004, 1.0)       # OSD; second argument = gear ratio (member rpm per shaft rpm)
shaft.addCompressor(ka26001, 1.2)
shaft.setSpeedBounds(rpmMin, rpmMax)    # driver speed band; keep it inside the chart-valid region
shaft.setGasTurbineUnitDriver(gt)       # site-rated GasTurbineUnit (power vs ambient, exhaust-T limit)
shaft.setAmbientTemperature(24.0, "C")  # moves the driver rating; also feeds a CompressorDriver
shaft.setMemberRequiredDischargePressure(ka26001, 160.0, "bara")   # margins in the report
# controlling member sets the speed; the others float on their charts at r_i * N:
shaft.solveSpeedToTarget(ka23003, CompressorShaft.ControlTarget.DISCHARGE_PRESSURE, 24.0, "bara", run_both)
# other targets: SUCTION_PRESSURE (bara) and INLET_FLOW ("kg/hr" or "m3/hr"); direction is not assumed
report = shaft.getOperatingReport()     # per member: speed, power, surge/stonewall distance,
                                        # requiredRecycleFraction (to control line), flags; JSON via toJson()

• HT 3rd stage sets the speed (until decoupling). Make the 3rd stage the controlling member with its own flow and discharge-pressure target; the 4th/5th stage run at the same speed, their flow follows from the process, and the report gives the margin to the discharge need ("excess head (throttle or recycle)", "speed too low for the discharge need") and the recycle fraction anti-surge must carry (requiredRecycleFraction, recyclePowerW). After the 3rd stage gets its own motor (DG4), call shaft.removeCompressor(ka23003) and give it a new CompressorShaft with setMaxShaftPower(5, "MW") or setElectricSupplyLimit(MW, "MW", motorEff).
• Always-coupled trains (LT 1st+2nd stage recompressors) are one CompressorShaft for all years.
• Decoupling dates and per-year modes. Keep the membership per year (or per half-year when the date falls inside a year) in a table, run each part with its own shaft set and report the time weight. Details, the efficiency-calibration and chart-similarity pitfalls: neqsim-rotating-equipment-design, section "Chart gas similarity, efficiency calibration and chart-speed basis".
• Idle members. setMemberActive(c, false) locks the compressor inactive (flowsheet skips it), removes it from the power sum and limit checks; with no active member solveSpeed returns 0 rpm and status SHAFT_IDLE (feasible). The idle compressor's outlet is not updated, so give an idle train a bypass line or zero flow in the flowsheet.
• Power limit. getAvailableShaftPower = explicit setMaxShaftPower, else a CompressorDriver curve, else GasTurbine rating, else GasTurbineUnit.getAvailablePowerAtAmbient(T); setElectricSupplyLimit caps it by the smaller value. solveSpeed flags OVER_POWER; findMaxSpeedWithinPowerLimit(run) gives the highest speed the driver holds; DriverLimitedThroughputSolver(shaft, ref, targetP, "bara", flow -> ..., run) .solve(minFlow, maxFlow) gives the highest throughput and the binding limit (DRIVER_POWER, MAX_SPEED, SURGE, STONEWALL, NONE).
• Fuel and exhaust. After a solve shaft.updateDriverOperatingPoint() runs the GasTurbineUnit at the shaft demand; read getFuelMassFlowKgPerHr(), getExhaustTemperatureK(), getWasteHeatRecoveryDutyW(stackK, effectiveness).
• Gotchas: chart evaluation outside the map throws ChartOperatingPoint at the bracket ends of a speed solve, so set speed bounds inside the chart-valid region; the discharge-pressure solve of one member assumes pressure rises with speed (suction/flow targets only need a bracket). Details and the gap list: neqsim-rotating-equipment-design section "Compressor trains on a common shaft".
• Modelling traps found in the Oseberg HT-shaft rework (Python port eot_ofc_common.shaft; they apply to the Java class too). (1) Do not take gear ratios from datasheet rated speeds. The digitised charts of KA23003 and KA23004 share one speed grid (70-108 % of 10200 rpm) and neither reproduces its Hatch rated speed (10037 / 10710 rpm); the effective member rpm per power-turbine rpm, read from measured pressure / temperature / recycle-inclusive flow, was 2.18 for both - one ratio, not 0.937. Derive the ratio per machine from a measured window (required_member_speed), keep the other window as a check, and read an ASV-recycled machine at its machine flow (net flow x measured recycle), not at the net flow. (2) The controlling member sets N only while it needs the highest speed. A follower that must reach its discharge pressure above its surge control line imposes a minimum speed; use N = max(N_control, N_min,follower) (min_shaft_speed_on_control_line) and report which one binds. Followers have no flow target: their flow is the chart flow at N (capacity), the surplus is recycled / let down, a shortfall is a surge or stonewall flag. (3) A controlling member below its control line is held on it by the ASV: evaluate its head at max(Q, Q_surge (1 + margin)) or its speed demand is understated by 10-15 %. (4) Keep the flow of a follower out of the solve. If the follower flow is an input (table or measured recycle), the shaft speed only checks it; to make the volume an output, drop the measured recycle and iterate N and the flow in the outer loop. (5) Loop gain across platforms. A recycle of flash gas between two coupled simulators (NGL stabiliser -> LP compression -> DPC -> stabiliser) can amplify; compare the one-way coupled result with the converged loop before blaming the composition.
• Wet-gas recompression chains: validate the speed set by the 3rd stage against the measured PT speed of both trains (an under-loaded train - net flow small against surge - is read at its machine flow, and its speed is then set by the 4th stage).

Related NeqSim Functionality

• neqsim.process.equipment.compressor.CompressorShaft — groups several compressor bodies on one shaft at a single common speed; addCompressor(...), solveSpeed(reference, targetP, unit, runnable) (iterate one speed to the final discharge, intermediates float), runAtFixedSpeed(rpm, runnable) (constant-speed drivers), setSpeed(...), setSpeedBounds(...), getSpeed(), getTotalPower().
• neqsim.process.equipment.compressor.Compressor — generateCompressorChart(...), getCompressorChart(), getSurgeFlowRate(), getDistanceToSurge(), setSpeed(...).
• neqsim.process.equipment.compressor.CompressorChart — getSurgeCurve(), getStoneWallCurve(), generateSurgeCurve(), generateStoneWallCurve(), checkStoneWall(...).
• neqsim.process.equipment.compressor.CompressorChartGenerator — setChartType(...), enableAdvancedCorrections(...), generateCompressorChart(...).
• neqsim.process.equipment.compressor.AntiSurgeRecycleCalculator — preferred steady-state recycle helper for charted compressors (setSurgeControlMargin, setRecycleCoolerTemperature, solve).
• neqsim.process.equipment.compressor.CompressorAntiSurgeApplication — application-level supervisor with StageApplication.bindTopology(...), TopologyBinding.enableSpeedControl(...), scan(...), and runDynamicStep(...) for direct writeback to real hot/cold recycle valves and compressor speed.
• neqsim.process.equipment.splitter.Splitter — discharge split into forward and recycle branches.
• neqsim.process.equipment.util.Calculator — steady-state anti-surge engine (runAntiSurgeCalc) triggered by the "anti surge calculator" name prefix.
• neqsim.process.controllerdevice.AntiSurgeController — dynamic reverse-acting PI controller on getDistanceToSurge() driving a recycle valve (setSurgeMarginSetPoint, setProportionalGain, setIntegralTime, setOpeningRange, setActive).
• neqsim.process.util.scenario.AntiSurgeDynamicBenchmark — reproducible surrogate benchmark for verifying/tuning the dynamic control law.
• neqsim.process.equipment.valve.ThrottlingValve — anti-surge recycle valve.
• neqsim.process.equipment.util.Recycle — closes the recycle loop.

The total_suction_flow (net forward flow + recycle) computed here is the actual compressor throughput. When you go on to compute polytropic head/efficiency or shaft power from measured conditions, use this actual throughput — not the metered net/export flow. Using net flow while the recycle is open understates suction volume and power and mis-locates the operating point on the surge map (a common error when diagnosing compressor degradation at turndown).

In Python these classes are reachable through the neqsim package (for example from neqsim import jneqsim).

References

• NeqSim: https://github.com/equinor/neqsim
• NeqSim Community Skills: https://github.com/equinor/neqsim-community-skills
• API Standard 617 — Axial and Centrifugal Compressors.
• API Standard 692 — Surge Control for Centrifugal Compressors.