Neqsim Compressor Antisurge Recycle

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).

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)).

1. 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.

1. An anti-surge valve on the recycle branch drops the discharge pressure

back to suction pressure.

1. 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();

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.

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.

Common Mistakes

• 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.

Limitations

• Screening logic only; it does not tune an anti-surge controller or set

recycle valve Cv, response time, or surge control line

…

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: equinor
• Source: equinor/neqsim-community-skills
• License: Apache-2.0
• Homepage: https://equinor.github.io/neqsimhome/

Install and usage instructions live in the source repository linked above.