fix: what fluid to recirculate when there is no rate entering a compressor stage in a multiple streams compressor train at a given time step

src/ecalc/libraries/libecalc/common/libecalc/core/models/compressor/train/fluid.py

@@ -35,8 +35,6 @@ def __init__(
             existing_fluid: Initialize FluidStream from an existing (NeqSim) fluid. Warning: Be mindful of keeping fluid_model and existing fluid consistent. If not the fluid properties may be incorrect.
         """
         self.fluid_model = fluid_model
-        self.initial_temperature_kelvin = temperature_kelvin
-        self.initial_pressure_bare = pressure_bara
 
         if not temperature_kelvin > 0:
             raise ValueError("FluidStream temperature needs to be above 0.")
@@ -60,8 +58,8 @@ def __init__(
         self._enthalpy_joule_per_kg = _neqsim_fluid_stream.enthalpy_joule_per_kg
 
         if (
-            pressure_bara == UnitConstants.STANDARD_PRESSURE_BARA
-            and temperature_kelvin == UnitConstants.STANDARD_TEMPERATURE_KELVIN
+            self._pressure_bara == UnitConstants.STANDARD_PRESSURE_BARA
+            and self._temperature_kelvin == UnitConstants.STANDARD_TEMPERATURE_KELVIN
         ):
             self.standard_conditions_density = _neqsim_fluid_stream.density
             self.molar_mass_kg_per_mol = _neqsim_fluid_stream.molar_mass
@@ -72,8 +70,8 @@ def __init__(
                 pressure_bara=UnitConstants.STANDARD_PRESSURE_BARA,
                 eos_model=self.fluid_model.eos_model,
             )
-
             self.standard_conditions_density = _neqsim_fluid_at_standard_conditions.density
+            self.molar_mass_kg_per_mol = _neqsim_fluid_stream.molar_mass
 
     @property
     def pressure_bara(self) -> float:
@@ -201,7 +199,10 @@ def set_new_pressure_and_temperature(
             new_temperature_kelvin=new_temperature_kelvin,
             remove_liquid=remove_liquid,
         )
-        return FluidStream(existing_fluid=fluid_stream, fluid_model=self.fluid_model)
+        return FluidStream(
+            existing_fluid=fluid_stream,
+            fluid_model=self.fluid_model,
+        )
 
     def set_new_pressure_and_enthalpy_change(
         self, new_pressure: float, enthalpy_change_joule_per_kg: float, remove_liquid: bool = True
@@ -277,7 +278,5 @@ def mix_in_stream(
 
         return FluidStream(
             existing_fluid=mixed_neqsim_fluid_stream,
-            temperature_kelvin=temperature_kelvin,
-            pressure_bara=pressure_bara,
             fluid_model=dto.FluidModel(composition=mixed_fluid_composition, eos_model=self.fluid_model.eos_model),
         )

src/ecalc/libraries/libecalc/common/libecalc/core/models/compressor/train/variable_speed_compressor_train_common_shaft_multiple_streams_and_pressures.py

@@ -91,6 +91,10 @@ def __init__(
             else:
                 self.outlet_stream_connected_to_stage[stream.connected_to_stage_no].append(i)
 
+        # in rare cases we can end up with trying to mix two streams with zero mass rate, and need the fluid from the
+        # previous time step to recirculate. This will take care of that.
+        self.fluid_to_recirculate_in_stage_when_inlet_rate_is_zero = [None] * len(self.stages)
+
     @staticmethod
     def _check_intermediate_pressure_stage_number_is_valid(
         _stage_number_intermediate_pressure: int,
@@ -837,7 +841,7 @@ def calculate_compressor_train_given_rate_ps_speed(
                 mass_rate_this_stage_kg_per_hour = mass_rate_previous_stage_kg_per_hour
             # take mass out before potential mixing with additional ingoing stream
             # assume that volume/mass is always taken out before additional volume/mass is potentially added, no matter
-            # what order the streams are defined in in the yaml file
+            # what order the streams are defined in the yaml file
             for stream_number in self.outlet_stream_connected_to_stage.get(stage_number):
                 mass_rate_this_stage_kg_per_hour -= inlet_stream.standard_rate_to_mass_rate(
                     std_rates_std_m3_per_day_per_stream[stream_number]
@@ -853,15 +857,27 @@ def calculate_compressor_train_given_rate_ps_speed(
                     mass_rate_additional_inlet_stream_kg_per_hour = additional_inlet_stream.standard_rate_to_mass_rate(
                         float(std_rates_std_m3_per_day_per_stream[stream_number])
                     )
+                    if (mass_rate_this_stage_kg_per_hour > 0) or (mass_rate_additional_inlet_stream_kg_per_hour > 0):
+                        inlet_stream = additional_inlet_stream.mix_in_stream(
+                            other_fluid_stream=inlet_stream,
+                            self_mass_rate=mass_rate_additional_inlet_stream_kg_per_hour,
+                            other_mass_rate=mass_rate_this_stage_kg_per_hour,
+                            temperature_kelvin=additional_inlet_stream.temperature_kelvin,
+                            pressure_bara=additional_inlet_stream.pressure_bara,
+                        )
+                    mass_rate_this_stage_kg_per_hour += mass_rate_additional_inlet_stream_kg_per_hour
 
-                    inlet_stream = additional_inlet_stream.mix_in_stream(
-                        other_fluid_stream=inlet_stream,
-                        self_mass_rate=mass_rate_additional_inlet_stream_kg_per_hour,
-                        other_mass_rate=mass_rate_this_stage_kg_per_hour,
-                        temperature_kelvin=additional_inlet_stream.temperature_kelvin,
-                        pressure_bara=additional_inlet_stream.pressure_bara,
+            if mass_rate_this_stage_kg_per_hour == 0:
+                fluid_to_recirculate = self.get_fluid_to_recirculate_in_stage_when_inlet_rate_is_zero(
+                    stage_number=stage_number
+                )
+                if fluid_to_recirculate:
+                    inlet_stream = fluid_to_recirculate
+                else:
+                    raise ValueError(
+                        f"Trying to recirculate fluid in stage {stage_number} without defining which "
+                        f"composition the fluid should have."
                     )
-                    mass_rate_this_stage_kg_per_hour += mass_rate_additional_inlet_stream_kg_per_hour
 
             stage_result = stage.evaluate(
                 inlet_stream_stage=inlet_stream,
@@ -879,7 +895,9 @@ def calculate_compressor_train_given_rate_ps_speed(
             )
 
             mass_rate_previous_stage_kg_per_hour = mass_rate_this_stage_kg_per_hour
-
+            self.set_fluid_to_recirculate_in_stage_when_inlet_rate_is_zero(
+                stage_number=stage_number, fluid_stream=inlet_stream
+            )
         return CompressorTrainResultSingleTimeStep(
             stage_results=stage_results,
             speed=speed,
@@ -1122,19 +1140,27 @@ def _update_inlet_fluid_and_std_rates_for_last_subtrain(
                     float(std_rates_std_m3_per_day_per_stream[stream_number])
                 )
                 # mix streams to get inlet stream for first compressor stage
-                inlet_stream = additional_inlet_stream.mix_in_stream(
-                    other_fluid_stream=inlet_stream,
-                    self_mass_rate=mass_rate_additional_inlet_stream,
-                    other_mass_rate=inlet_mass_rate,
-                    temperature_kelvin=self.stages[0].inlet_temperature_kelvin,
-                    pressure_bara=additional_inlet_stream.pressure_bara,
-                )
+                # if rate is 0 don't try to mix,
+                if (inlet_mass_rate > 0) or (mass_rate_additional_inlet_stream > 0):
+                    inlet_stream = additional_inlet_stream.mix_in_stream(
+                        other_fluid_stream=inlet_stream,
+                        self_mass_rate=mass_rate_additional_inlet_stream,
+                        other_mass_rate=inlet_mass_rate,
+                        temperature_kelvin=additional_inlet_stream.temperature_kelvin,
+                        pressure_bara=additional_inlet_stream.pressure_bara,
+                    )
                 inlet_std_rate += std_rates_std_m3_per_day_per_stream[stream_number]
                 inlet_mass_rate = inlet_stream.standard_rate_to_mass_rate(inlet_std_rate)
 
         # update inlet fluid for the subtrain with new composition
-        updated_inlet_fluid = FluidStream(fluid_model=inlet_stream.fluid_model)
-
+        if inlet_mass_rate == 0:
+            fluid_to_recirculate = self.get_fluid_to_recirculate_in_stage_when_inlet_rate_is_zero(stage_number=0)
+            if fluid_to_recirculate:
+                updated_inlet_fluid = fluid_to_recirculate
+            else:
+                raise ValueError("Trying to recirculate unknown fluid in compressor stage")
+        else:
+            updated_inlet_fluid = FluidStream(fluid_model=inlet_stream.fluid_model)
         updated_std_rates_std_m3_per_day_per_stream = [inlet_std_rate]
         updated_std_rates_std_m3_per_day_per_stream.extend(
             [
@@ -1284,13 +1310,48 @@ def find_and_calculate_for_compressor_train_with_two_pressure_requirements(
             + compressor_train_results_to_return_last_part.stage_results
         )
 
+        for stage_number in range(len(self.stages)):
+            self.set_fluid_to_recirculate_in_stage_when_inlet_rate_is_zero(
+                stage_number=stage_number,
+                fluid_stream=compressor_train_first_part.get_fluid_to_recirculate_in_stage_when_inlet_rate_is_zero(
+                    stage_number=stage_number
+                )
+                if stage_number < stage_number_for_intermediate_pressure_target
+                else compressor_train_last_part.get_fluid_to_recirculate_in_stage_when_inlet_rate_is_zero(
+                    stage_number=stage_number - stage_number_for_intermediate_pressure_target
+                ),
+            )
+
         return CompressorTrainResultSingleTimeStep(
             speed=speed,
             stage_results=compressor_train_results_to_return_stage_results,
             failure_status=compressor_train_results_to_return_first_part.failure_status
             or compressor_train_results_to_return_last_part.failure_status,
         )
 
+    def set_fluid_to_recirculate_in_stage_when_inlet_rate_is_zero(
+        self, stage_number: int, fluid_stream: FluidStream
+    ) -> None:
+        """Keep track of what fluid passes through each compressor stage in the compressor train at a given calculation
+        step. This is done in case of the possibility of having a zero inlet rate at the next calculation step when
+        adding/subtracting ingoing/outgoing streams.
+
+        Args:
+            stage_number: The stage number (zero index)
+            fluid_stream: The fluid stream passing through compressor stage number <stage_number>
+        """
+        self.fluid_to_recirculate_in_stage_when_inlet_rate_is_zero[stage_number] = fluid_stream
+
+    def get_fluid_to_recirculate_in_stage_when_inlet_rate_is_zero(self, stage_number: int) -> FluidStream:
+        """Retrieve the fluid that passed through a given compressor stage in the compressor train at the previous
+        calculation step.
+
+        Arge:
+            stage_number: The stage number (zero index)
+
+        """
+        return self.fluid_to_recirculate_in_stage_when_inlet_rate_is_zero[stage_number]
+
 
 def split_rates_on_stage_number(
     compressor_train: VariableSpeedCompressorTrainCommonShaftMultipleStreamsAndPressures,
@@ -1366,4 +1427,16 @@ def split_train_on_stage_number(
         ),
     )
 
+    for stage_no in range(len(compressor_train.stages)):
+        if stage_no < stage_number:
+            compressor_train_first_part.set_fluid_to_recirculate_in_stage_when_inlet_rate_is_zero(
+                stage_number=stage_no,
+                fluid_stream=compressor_train.get_fluid_to_recirculate_in_stage_when_inlet_rate_is_zero(stage_no),
+            )
+        else:
+            compressor_train_last_part.set_fluid_to_recirculate_in_stage_when_inlet_rate_is_zero(
+                stage_number=stage_no - stage_number,
+                fluid_stream=compressor_train.get_fluid_to_recirculate_in_stage_when_inlet_rate_is_zero(stage_no),
+            )
+
     return compressor_train_first_part, compressor_train_last_part

src/ecalc/libraries/libecalc/common/tests/core/models/compressor_modelling/test_variable_speed_compressor_train_multiple_streams.py

@@ -219,6 +219,41 @@ def variable_speed_compressor_train_two_compressors_two_streams(
     )
 
 
+@pytest.fixture
+def variable_speed_compressor_train_two_compressors_ingoning_and_outgoing_streams_between_compressors(
+    variable_speed_compressor_train_stage_dto,
+    dry_fluid,
+    rich_fluid,
+    mock_variable_speed_compressor_train_multiple_streams_and_pressures,
+) -> VariableSpeedCompressorTrainCommonShaftMultipleStreamsAndPressures:
+    """Train with only two compressors, and standard medium fluid, on stream in per stage, no liquid off take."""
+    fluid_streams = [
+        FluidStreamObjectForMultipleStreams(
+            fluid=FluidStream(rich_fluid),
+            is_inlet_stream=True,
+            connected_to_stage_no=0,
+        ),
+        FluidStreamObjectForMultipleStreams(
+            is_inlet_stream=False,
+            connected_to_stage_no=1,
+        ),
+        FluidStreamObjectForMultipleStreams(
+            fluid=FluidStream(dry_fluid),
+            is_inlet_stream=True,
+            connected_to_stage_no=1,
+        ),
+    ]
+    return VariableSpeedCompressorTrainCommonShaftMultipleStreamsAndPressures(
+        streams=fluid_streams,
+        data_transfer_object=mock_variable_speed_compressor_train_multiple_streams_and_pressures.copy(
+            update={
+                "stages": [variable_speed_compressor_train_stage_dto] * 2,
+                "pressure_control": dto.types.FixedSpeedPressureControl.DOWNSTREAM_CHOKE,
+            }
+        ),
+    )
+
+
 @pytest.fixture
 def variable_speed_compressor_train_two_compressors_one_ingoing_and_one_outgoing_stream(
     variable_speed_compressor_train_stage_dto,
@@ -649,3 +684,26 @@ def test_adjust_energy_usage(
         np.asarray(result_comparison_intermediate.energy_usage) + energy_usage_adjustment_constant,
         result_intermediate.energy_usage,
     )
+
+
+def test_recirculate_mixing_streams_with_zero_mass_rate(
+    variable_speed_compressor_train_two_compressors_ingoning_and_outgoing_streams_between_compressors,
+):
+    result = variable_speed_compressor_train_two_compressors_ingoning_and_outgoing_streams_between_compressors.evaluate_rate_ps_pd(
+        rate=np.asarray(
+            [
+                [3000000, 3000000, 3000000, 3000000, 3000000, 3000000],
+                [0, 3000000, 2500000, 3000000, 3000000, 3000000],
+                [0, 0, 500000, 0, 1000000, 0],
+            ]
+        ),
+        suction_pressure=np.asarray([30, 30, 30, 30, 30, 30]),
+        discharge_pressure=np.asarray([150, 150, 150, 150, 150, 150]),
+    )
+    np.testing.assert_almost_equal(result.power[0], result.power[1], decimal=4)
+    np.testing.assert_almost_equal(result.power[2], result.power[3], decimal=4)
+    np.testing.assert_almost_equal(result.power[4], result.power[5], decimal=4)
+    assert result.recirculation_loss[0] < result.recirculation_loss[1]
+    assert result.recirculation_loss[2] < result.recirculation_loss[3]
+    assert result.recirculation_loss[4] < result.recirculation_loss[5]
+    assert result.power[0] < result.power[2] < result.power[4]  # more and more of the heavy fluid