WIP: feat(yaml): introduce INTERSTAGE process system type and PRESSURE_CON… by olelod · Pull Request #1453 · equinor/ecalc

olelod · 2026-03-23T12:09:37Z

Context

The existing YAML supports a MULTIPLE_STREAMS_AND_PRESSURES compressor train type for trains where additional streams are injected or extracted between stages, and where intermediate pressures must be controlled. This type is separate from COMMON_SHAFT and requires its own set of fields and mapper logic.

This PR is a design proposal for how the same physical concept — and more — can be expressed using the general PROCESS_SYSTEMS structure, without a dedicated train type.

Approach

The key design principle is full separation of equipment and operation:

PROCESS_SYSTEMS describes physical topology — compressors, how they are connected, and what anti-surge equipment is installed. No streams, no rates, no targets.
PROCESS_SIMULATIONS describes operational scenarios — what flows through the system, what pressure targets to hit, and how to achieve them.

A serial compressor train with interstage elements is expressed as a SERIAL process system containing an ordered list of named items. Six item types are supported:

Type	Description
`COMPRESSOR_STAGE`	A single compressor stage
`PRESSURE_DROP`	Any element modelled as a fixed pressure drop
`MIXER`	A stream injection point
`SPLITTER`	A stream extraction point
`SERIAL`	An ordered serial train
`PARALLEL`	A parallel arrangement of serial trains

Anti-surge equipment (INDIVIDUAL_ASV or COMMON_ASV) is declared on the SERIAL train — it is physical equipment, not an operational choice. Defaults to INDIVIDUAL_ASV.

Segment constraints

Intermediate pressure targets are expressed in a CONSTRAINTS block on the simulation, keyed by item name. Each entry defines a segment boundary: the solver resolves all items up to and including that item as one segment.

Using the train name itself as a key targets the train outlet — independent of which stage happens to be last.

Each constraint requires:

OUTLET_PRESSURE: the pressure target [bara]
PRESSURE_CONTROL (optional): defaults to DOWNSTREAM_CHOKE for the train outlet, INDIVIDUAL_ASV_RATE for intermediate segments

Consistency between PRESSURE_CONTROL and the train's ANTI_SURGE is validated at parse time.

Validation

All cross-reference checks run at parse time in YamlAsset, before any domain calculations:

Every MIXER in the target train must have a stream in MIXER_STREAMS
Every SPLITTER in the target train must have a rate in SPLITTER_RATES
Every constraint must have OUTLET_PRESSURE set
Constraint keys must be item names in the train or the train name itself
COMMON_ASV pressure control requires COMMON_ASV on the train

Backward compatibility

A SERIAL train with a single constraint continues to use the existing solver path. Multi-constraint trains are resolved by MultiPressureSolver (separate work). No existing YAML is affected.

Mapping from YAML to domain is separate work.

Example YAML

INLET_STREAMS:
  feed_stream:
    FLUID_MODEL: dry_gas
    RATE:
      VALUE: SIM1;FEED_RATE
      UNIT: SM3_PER_DAY
  injection_stream:
    FLUID_MODEL: wet_gas
    RATE:
      VALUE: SIM1;INJECTION_RATE
      UNIT: SM3_PER_DAY
  train_a_stream:
    FLUID_MODEL: dry_gas
    RATE:
      VALUE: SIM1;TRAIN_A_RATE
      UNIT: SM3_PER_DAY
  train_a_injection_stream:
    FLUID_MODEL: wet_gas
    RATE:
      VALUE: SIM1;TRAIN_A_INJECTION_RATE
      UNIT: SM3_PER_DAY
  train_b_stream:
    FLUID_MODEL: dry_gas
    RATE:
      VALUE: SIM1;TRAIN_B_RATE
      UNIT: SM3_PER_DAY

PROCESS_SYSTEMS:
  - TYPE: COMPRESSOR_STAGE
    NAME: stage_1
    INLET_TEMPERATURE: 30
    COMPRESSOR: my_compressor

  - TYPE: PRESSURE_DROP
    NAME: interstage_pressure_drop
    PRESSURE_DROP: 3

  - TYPE: MIXER
    NAME: injection_point

  - TYPE: COMPRESSOR_STAGE
    NAME: stage_2
    INLET_TEMPERATURE: 40
    COMPRESSOR: my_compressor

  - TYPE: SPLITTER
    NAME: extraction_point

  - TYPE: COMPRESSOR_STAGE
    NAME: stage_3
    INLET_TEMPERATURE: 35
    COMPRESSOR: my_compressor

  - TYPE: SERIAL
    NAME: main_train
    ANTI_SURGE: INDIVIDUAL_ASV
    ITEMS:
      - stage_1
      - interstage_pressure_drop
      - injection_point
      - stage_2
      - extraction_point
      - stage_3

  - TYPE: COMPRESSOR_STAGE
    NAME: train_a_stage_1
    INLET_TEMPERATURE: 30
    COMPRESSOR: my_compressor

  - TYPE: MIXER
    NAME: train_a_injection_point

  - TYPE: COMPRESSOR_STAGE
    NAME: train_a_stage_2
    INLET_TEMPERATURE: 40
    COMPRESSOR: my_compressor

  - TYPE: SPLITTER
    NAME: train_a_extraction_point

  - TYPE: COMPRESSOR_STAGE
    NAME: train_a_stage_3
    INLET_TEMPERATURE: 35
    COMPRESSOR: my_compressor

  - TYPE: SERIAL
    NAME: train_a
    ANTI_SURGE: COMMON_ASV
    ITEMS:
      - train_a_stage_1
      - train_a_injection_point
      - train_a_stage_2
      - train_a_extraction_point
      - train_a_stage_3

  - TYPE: COMPRESSOR_STAGE
    NAME: train_b_stage
    INLET_TEMPERATURE: 30
    COMPRESSOR: my_compressor

  - TYPE: SERIAL
    NAME: train_b
    ANTI_SURGE: INDIVIDUAL_ASV
    ITEMS:
      - train_b_stage

  - TYPE: PARALLEL
    NAME: parallel_trains
    ITEMS:
      - train_a
      - train_b

PROCESS_SIMULATIONS:
  - NAME: sim_serial
    TARGET: main_train
    INLET_STREAM: feed_stream
    MIXER_STREAMS:
      injection_point: injection_stream
    SPLITTER_RATES:
      extraction_point:
        VALUE: SIM1;SPLIT_RATE
        UNIT: SM3_PER_DAY
    CONSTRAINTS:
      injection_point:
        OUTLET_PRESSURE: 50
        PRESSURE_CONTROL: DOWNSTREAM_CHOKE
      stage_2:
        OUTLET_PRESSURE: 80
      main_train:
        OUTLET_PRESSURE: 120

  - NAME: sim_parallel_individual
    TARGET: parallel_trains
    STREAM_DISTRIBUTION:
      METHOD: INDIVIDUAL_STREAMS
      INLET_STREAMS:
        - train_a_stream
        - train_b_stream
    MIXER_STREAMS:
      train_a_injection_point: train_a_injection_stream
    SPLITTER_RATES:
      train_a_extraction_point:
        VALUE: SIM1;TRAIN_A_SPLIT_RATE
        UNIT: SM3_PER_DAY
    CONSTRAINTS:
      train_a_injection_point:
        OUTLET_PRESSURE: 50
      train_a:
        OUTLET_PRESSURE: 120
        PRESSURE_CONTROL: COMMON_ASV
      train_b:
        OUTLET_PRESSURE: 120

  - NAME: sim_parallel_common
    TARGET: parallel_trains
    STREAM_DISTRIBUTION:
      METHOD: COMMON_STREAM
      INLET_STREAM: feed_stream
      SETTINGS:
        - RATE_FRACTIONS: [0.6, 0.4]
          OVERFLOW:
            - FROM_REFERENCE: train_a
              TO_REFERENCE: train_b
        - RATE_FRACTIONS: [0.5, 0.5]
        - RATE_FRACTIONS: [0.4, 0.6]
    MIXER_STREAMS:
      train_a_injection_point: train_a_injection_stream
    SPLITTER_RATES:
      train_a_extraction_point:
        VALUE: SIM1;TRAIN_A_SPLIT_RATE
        UNIT: SM3_PER_DAY
    CONSTRAINTS:
      train_a_injection_point:
        OUTLET_PRESSURE: 50
      train_a:
        OUTLET_PRESSURE: 120
        PRESSURE_CONTROL: COMMON_ASV
      train_b:
        OUTLET_PRESSURE: 120

jsolaas · 2026-03-25T07:57:43Z

src/libecalc/presentation/yaml/yaml_types/components/yaml_process_system.py

+class YamlInterstageProcessSystem(YamlBase):
+    type: Literal["INTERSTAGE"]
+    name: ProcessSystemReference
+    items: list[YamlInterstageItem] = Field(
+        default_factory=list,
+        title="ITEMS",
+        description="Ordered list of choke, mixer, and splitter units at this interstage location.",
+    )


Do we actually need this? Can we instead just have a list of units in addition to CompressorStages when setting up the train?

jsolaas · 2026-03-25T07:58:35Z

src/libecalc/presentation/yaml/yaml_types/components/yaml_process_system.py

    type: Literal["SERIAL"]
    name: ProcessSystemReference
-    items: list[YamlItem[YamlCompressorStageProcessSystem]]
+    items: list[YamlItem[YamlCompressorStageProcessSystem | YamlInterstageProcessSystem]]


Suggested change

items: list[YamlItem[YamlCompressorStageProcessSystem | YamlInterstageProcessSystem]]

items: list[YamlItem[YamlCompressorStageProcessSystem | YamlInterstageChoke | YamlInterstageSplitter | YamlInterstageMixer]]

jsolaas · 2026-03-25T07:59:42Z

src/libecalc/presentation/yaml/yaml_types/components/yaml_process_system.py


 class YamlProcessConstraints(YamlBase):
-    outlet_pressure: YamlExpressionType | None = Field(
+    discharge_pressure: YamlExpressionType | None = Field(


Should we use discharge instead of inlet? inlet/outlet seems to work for streams, is it different for pressure?

jsolaas · 2026-03-25T08:07:47Z

src/libecalc/presentation/yaml/yaml_types/components/yaml_process_system.py

+        title="DISCHARGE_PRESSURE",
+        description="Target discharge pressure [bara].",
+    )
+    pressure_control: YamlPressureControl | None = Field(


Will constraint, anti-surge and pressure control always go hand-in-hand?

I think we also should consider introducing a type for constraint, maybe switch to a list. That gives us more flexibility in case we need to introduce different constraints.

If combining constraint and 'how to solve' like this we should also consider a different name than constraint

All types live in yaml_process_system.py. Topology types first, simulation types after. Process system topology (PROCESS_SYSTEMS): - COMPRESSOR_STAGE: single compressor stage with inlet temperature - PRESSURE_DROP: interstage pressure drop element - MIXER: stream injection point (topology only, one mixer for each stream injected) - SPLITTER: stream extraction point (topology only, one splitter for each stream extracted) - SERIAL: ordered list of named item references; carries ANTI_SURGE (INDIVIDUAL_ASV or COMMON_ASV, defaults to INDIVIDUAL_ASV) Interstage types (PRESSURE_DROP, MIXER, SPLITTER) are new — they allow expressing the legacy multiple streams compressor trains in the new Yaml Process simulation (PROCESS_SIMULATIONS): TARGETS: list of YamlSimulationTarget, each with: - TARGET: reference to a SERIAL process system - CONSTRAINTS: map of item name / train name -> YamlSegmentConstraint (constraints carry both OUTLET_PRESSURE and PRESSURE_CONTROL and they allow expressing the legacy multiple pressures compressor train in the new Yaml. Now also with more than one interstage pressure) - MIXER_STREAMS: map of mixer name -> inlet stream ref - SPLITTER_RATES: map of splitter name -> extraction rate STREAM_DISTRIBUTION: required; COMMON_STREAM or INDIVIDUAL_STREAMS (both accept StreamRef | YamlInletStream) Serial = 1 target, parallel = N targets — implicit from count. Validators in YamlAsset (parse-time): - Every simulation target must reference a known SERIAL process system - Every train must have an outlet constraint (keyed by train name) - Every MIXER in a train must have a MIXER_STREAMS entry per target - Every SPLITTER in a train must have a SPLITTER_RATES entry per target - Constraint keys must be item names in the train or the train name itself - Constraints must appear in topological order (matching SERIAL.items) - UPSTREAM_CHOKE only valid on the first constraint in topological order - DOWNSTREAM_CHOKE only valid on the last constraint in topological order

olelod force-pushed the feat/yaml-interstage-types branch from 2f01beb to 507f040 Compare March 23, 2026 12:18

jsolaas reviewed Mar 25, 2026

View reviewed changes

olelod force-pushed the feat/yaml-interstage-types branch 2 times, most recently from 9fa2c2a to 043a36d Compare April 7, 2026 08:20

olelod force-pushed the feat/yaml-interstage-types branch from 043a36d to 08d553e Compare April 8, 2026 11:03

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WIP: feat(yaml): introduce INTERSTAGE process system type and PRESSURE_CON…#1453

WIP: feat(yaml): introduce INTERSTAGE process system type and PRESSURE_CON…#1453
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olelod commented Mar 23, 2026 •

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	items: list[YamlItem[YamlCompressorStageProcessSystem \| YamlInterstageProcessSystem]]
	items: list[YamlItem[YamlCompressorStageProcessSystem \| YamlInterstageChoke \| YamlInterstageSplitter \| YamlInterstageMixer]]

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olelod commented Mar 23, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Context

Approach

Segment constraints

Validation

Backward compatibility

Example YAML

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olelod commented Mar 23, 2026 •

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