Accurate and thermodynamically consistent steam properties are provided for the IDAES framework by implementing the International Association for the Properties of Water and Steam’s “Revised Release on the IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use.” Non-analytic terms designed to improve accuracy very near the critical point were omitted, because they cause a singularity at the critical point, a feature which is undesirable in optimization problems. The IDAES implementation provides features which make the water and steam property calculations amenable to rigorous mathematical optimization.


Theses modules can be imported as:

from idaes.property_models import iapws95

The Heater unit model example, provides a simple example for using water properties.

import pyomo.environ as pe # Pyomo environment
from idaes.core import FlowsheetBlock, MaterialBalanceType
from idaes.unit_models import Heater
from idaes.property_models import iapws95

# Create an empty flowsheet and steam property parameter block.
model = pe.ConcreteModel()
model.fs = FlowsheetBlock(default={"dynamic": False})
model.fs.properties = iapws95.Iapws95ParameterBlock(default={

# Add a Heater model to the flowsheet.
model.fs.heater = Heater(default={
  "property_package": model.fs.properties,

# Setup the heater model by fixing the inputs and heat duty

# Initialize the model.

Since all properties except the state variables are Pyomo Expressions in the water properties module, after solving the problem any property can be calculated in any state block. Continuing from the heater example, to get the viscosity of both phases, the lines below could be added.

mu_l = pe.value(model.fs.heater.control_volume.properties_out[0].visc_d_phase["Liq"])
mu_v = pe.value(model.fs.heater.control_volume.properties_out[0].visc_d_phase["Vap"])

For more information about how StateBlocks and PropertyParameterBlocks work see the StateBlock documentation.


The iapws95 property module uses SI units (m, kg, s, J, mol) for all public variables and expressions. Temperature is in K. Note that this means molecular weight is in the unusual unit of kg/mol.

A few expressions intended to be used internally and all external function calls use units of kg, kJ, kPa, and K. These generally are not needed by the end user.


These methods use the IAPWS-95 formulation for scientific use for thermodynamic properties (Wagner and Pruss, 2002; IAPWS, 2016). To solve the phase equilibrium, the method of Akasaka (2008) was used. For solving these equations, some relations from the IAPWS-97 formulation for industrial use are used as initial values (Wagner et al., 2002). The industrial formulation is slightly discontinuous between different regions, so it may not be suitable for optimization. In addition to thermodynamic quantities, viscosity and thermal conductivity are calculated (IAPWS, 2008; IAPWS, 2011).

External Functions

The IAPWS-95 formulation uses density and temperature as state variables. For most applications those state variables are not the most convenient choices. Using other state variables requires solving equations to get density and temperature from the chosen state variables. These equations can have numerous solutions only one of which is physically meaningful. Rather than solve these equations as part of the full process simulation, external functions were developed that can solve the equations required to change state variables and guarantee the correct roots.

The external property functions are written in C++ and complied such that they can be called by AMPL solvers. See the Installation page for information about compiling these functions. The external functions provide both first and second derivatives for all property function calls, however at phase transitions some of these functions may be non-smooth.

IDAES Framework Wrapper

A wrapper for the external functions is provided for compatibility with the IDAES framework. Most properties are available as Pyomo Expressions from the wrapper. Only the state variables are model variables. Benefits of using mostly expressions in the property package are: no initialization is required specifically for the property package, the model has fewer equations, and all properties can be easily calculated after the model is solved from the state variable values even if they were not used in the model. Calls to the external functions are used within expressions so users do not need to directly call any functions. The potential downside of the extensive use of expressions here is that combining the expressions to form constraints could yield equations that are more difficult to solve than, they would have been if an equivalent system of equations was written with more variables and simpler equations. Quantifying the effect of writing larger equations with fewer variables is difficult. Experience suggests in this particular case more expressions and fewer variables is better.

Although not generally used, the wrapper provides direct access to the ExternalFunctions, including intermediate functions. For more information see section ExternalFunctions. These are mostly available for testing purposes.

Phase Presentation

The property package wrapper can present fluid phase information to the IDAES framework in different ways. See the class reference for details on how to set these options. The phase_presentation=PhaseType.MIX option looks like one phase called “Mix” to the IDAES framework. The property package will calculate a phase fraction. This will bypass any two phase handling equations written for unit models, and should work with any unit model options as long as you do not want to separate the phases. The benefit of this option is that it can potentially lead to a simpler set of equations.

The phase_presentation=PhaseType.LG option appears to the IDAES framework to be two phases “Vap” and “Liq”. This option requires one of two unit model options to be set. You can use the total material balance option for unit models, to specify that only one material balance equation should be written not one per phase. The other possible option is to specify has_phase_equlibrium=True. This will still write a material balance per phase, but will add a phase generation term to the model. For the IAPWS-95 package, it is generally recommended that specifying total material balances is best because it results in a problem with fewer variables.

There are also two single phase options phase_presentation=PhaseType.L and phase_presentation=PhaseType.G, these present a single phase “Liq” or “Vap” to the framework. The vapor fraction will also always return 0 or 1 as appropriate. These options can be used when the phase of a fluid is know for certain to only be liquid or only be vapor. For the temperature-pressure-vapor fraction formulation, this eliminates the complementarity constraint, but for the enthalpy-pressure formulation, where the vapor fraction is always calculated, the single phase options probably do not provide any real benefit.

Pressure-Enthalpy Formulation

The advantage of this choice of state variables is that it is very robust when phase changes occur, and is especially useful when it is not known if a phase change will occur. The disadvantage of this choice of state variables is that for equations like heat transfer equations that are highly dependent on temperature, a model could be harder to solve near regions with phase change. Temperature is a non-smooth function with non-smoothness when transitioning from the single-phase to the two-phase region. Temperature also has a zero derivative with respect to enthalpy in the two-phase region, so near the two-phase region solving a constraint that specifies a specific temperature may not be possible.

The variables for this form are flow_mol (mol/s), pressure (Pa), and enth_mol (J/mol).

Since temperature and vapor fraction are not state variables in this formulation, they are provided by expressions, and cannot be fixed. For example, to set a temperature to a specific value, a constraint could be added which says the temperature expression equals a fixed value.

These expressions are specific to the P-H formulation:

Expression that calculates temperature by calling an ExternalFunction of enthalpy and pressure. This expression is non-smooth in the transition from single-phase to two-phase and has a zero derivative with respect to enthalpy in the two-phase region.
Expression that calculates vapor fraction by calling an ExternalFunction of enthalpy and pressure. This expression is non-smooth in the transition from single-phase to two-phase and has a zero derivative with respect to enthalpy in the single-phase region, where the value is 0 (liquid) or 1 (vapor).

Temperature-Pressure-Vapor Fraction

This formulation uses temperature (K), pressure (Pa), and vapor fraction as state variables. When a single phase option is given, the vapor fraction is fixed to the appropriate value and not included in the state variable set. For single phase, the complementarity constraint is also deactivated.

A complementarity constraint is required for the T-P-x formulation. First, two expressions are defined below where \(P^-\) is pressure under saturation pressure and \(P^+\) is pressure over saturation pressure. The max function is provided by an IDAES utility function which provides a smooth max expression.

\[P^- = \max(0, P_{\text{sat}} - P)\]
\[P^+ = \max(0, P - P_{\text{sat}})\]

With the pressure over and pressure under saturated pressure expressions a complementarity constraint can be written. If the pressure under saturation is more than zero, only vapor exists. If the pressure over saturation is greater than zero only a liquid exists. If both are about zero two phases can exist. The saturation pressure function maxes out at the critical pressure and any temperature above the critical temperature will yield a saturation pressure that is the critical pressure, so supercritical fluids will be classified as liquids as the convention for this property package.

\[0 = xP^+ - (1 - x)P^-\]

Assuming the vapor fraction (\(x\)) is positive and noting that only one of \(P^+\) and \(P^-\) can be nonzero (approximately), the complementarity equation above requires \(x\) to be 0 when \(P^+\) is not zero (liquid) or \(x\) to be 1 when \(P^-\) is not zero (vapor). When both \(P^+\) and \(P^-\) are about 0, the complementarity constraint says nothing about x, but it does provide another constraint, that \(P=P_{\text{sat}}\). When two phases are present \(x\) can be found by the unit model energy balance and the temperature will be \(T_{\text{sat}}\).

An alternative approach is sometimes useful. If you know for certain that you have two phases, the complementarity constraint can be deactivated and a \(P=P_{\text{sat}}\) or \(T=T_{\text{sat}}\) constraint can be added.

Using the T-P-x formulation requires better initial guesses than the P-H form. It is not strictly necessary but it is best to try to get an initial guess that is in the correct phase region for the expected result model.


Unless otherwise noted, the property expressions are common to both the T-P-x and P-H formulations. For phase specific properties, valid phase indexes are "Liq" and "Vap"

Expression Description
mw Molecular weight (kg/mol)
tau Critical temperature divided by temperature (unitless)
temperature Temperature (K) if PH form
temperature_red Reduced temperature, temperature divided by critical temperature (unitless)
temperature_sat Saturation temperature (K)
tau_sat Critical temperature divided by saturation temperature (unitless)
pressure_sat Saturation pressure (Pa)
dens_mass_phase[phase] Density phase (kg/m3)
dens_phase_red[phase] Phase reduced density (\(\delta\)), mass density divided by critical density (unitless)
dens_mass Total mixed phase mass density (kg/m3)
dens_mol Total mixed phase mole density (kg/m3)
flow_vol Total volumetric flow rate (m3/s)
enth_mass Mass enthalpy (J/kg)
enth_mol_sat_phase[phase] Saturation enthalpy of phase, enthalpy at P and Tsat (J/mol)
enth_mol Molar enthalpy (J/mol) if TPx form
enth_mol_phase[phase] Molar enthalpy of phase (J/mol)
energy_internal_mol molar internal energy (J/mol)
energy_internal_mol_phase[phase] Molar internal energy of phase (J/mol)
entr_mol_phase Molar entropy of phase (J/mol/K)
entr_mol Total mixed phase entropy (J/mol/K)
cp_mol_phase[phase] Constant pressure molar heat capacity of phase (J/mol/K)
cv_mol_phase[phase] Constant pressure volume heat capacity of phase (J/mol/K)
cp_mol Total mixed phase constant pressure heat capacity (J/mol/K)
cv_mol Total mixed phase constant volume heat capacity (J/mol/K)
heat_capacity_ratio cp_mol/cv_mol
speed_sound_phase[phase] Speed of sound in phase (m/s)
dens_mol_phase[phase] Mole density of phase (mol/m3)
therm_cond_phase[phase] Thermal conductivity of phase (W/K/m)
vapor_frac Vapor fraction, if PH form
visc_d_phase[phase] Viscosity of phase (Pa/s)
visc_k_phase[phase] Kinimatic viscosity of phase (m2/s)
phase_frac[phase] Phase fraction
flow_mol_comp["H2O"] Same as total flow since only water (mol/s)
P_under_sat Pressure under saturation pressure (kPA)
P_over_sat Pressure over saturation pressure (kPA)


This provides a list of ExternalFuctions available in the wrapper. These functions do not use SI units and are not usually called directly. If these functions are needed, they should be used with caution. Some of these are used in the property expressions, some are just provided to allow easier testing with a Python framework.

All of these functions provide first and second derivative and are generally suited to optimization (including the ones that return derivatives of Helmholtz free energy). Some functions may have non-smoothness at phase transitions. The delta_vap and delta_liq functions return the same values in the critical region. They will also return real values when a phase doesn’t exist, but those values do not necessarily have physical meaning.

There are a few variables that are common to a lot of these functions, so they are summarized here \(\tau\) is the critical temperature divided by the temperature \(\frac{T_c}{T}\), \(\delta\) is density divided by the critical density \(\frac{\rho}{\rho_c}\), and \(\phi\) is Helmholtz free energy divided by the ideal gas constant and temperature \(\frac{f}{RT}\).

Pyomo Function C Function Returns Arguments
func_p p pressure (kPa) \(\delta, \tau\)
func_u u internal energy (kJ/kg) \(\delta, \tau\)
func_s s entropy (kJ/K/kg) \(\delta, \tau\)
func_h h enthalpy (kJ/kg) \(\delta, \tau\)
func_hvpt hvpt vapor enthalpy (kJ/kg) P (kPa), \(\tau\)
func_hlpt hlpt liquid enthalpy (kJ/kg) P (kPa), \(\tau\)
func_tau tau \(\tau\) (unitless) h (kJ/kg), P (kPa)
func_vf vf vapor fraction (unitless) h (kJ/kg), P (kPa)
func_g g Gibbs free energy (kJ/kg) \(\delta, \tau\)
func_f f Helmholtz free energy (kJ/kg) \(\delta, \tau\)
func_cv cv const. volume heat capacity (kJ/K/kg) \(\delta, \tau\)
func_cp cp const. pressure heat capacity (kJ/K/kg) \(\delta, \tau\)
func_w w speed of sound (m/s) \(\delta, \tau\)
func_delta_liq delta_liq liquid \(\delta\) (unitless) P (kPa), \(\tau\)
func_delta_vap delta_vap vapor \(\delta\) (unitless) P (kPa), \(\tau\)
func_delta_sat_l delta_sat_l sat. liquid \(\delta\) (unitless) \(\tau\)
func_delta_sat_v delta_sat_v sat. vapor \(\delta\) (unitless) \(\tau\)
func_p_sat p_sat sat. pressure (kPa) \(\tau\)
func_tau_sat tau_sat sat. \(\tau\) (unitless) P (kPa)
func_phi0 phi0 \(\phi\) idaes gas part (unitless) \(\delta, \tau\)
func_phi0_delta phi0_delta \(\frac{\partial \phi_0}{\partial \delta}\) \(\delta\)
func_phi0_delta2 phi0_delta2 \(\frac{\partial^2 \phi_0}{\partial \delta^2}\) \(\delta\)
func_phi0_tau phi0_tau \(\frac{\partial \phi_0}{\partial \tau}\) \(\tau\)
func_phi0_tau2 phi0_tau2 \(\frac{\partial^2 \phi_0}{\partial \tau^2}\) \(\tau\)
func_phir phir \(\phi\) real gas part (unitless) \(\delta, \tau\)
func_phir_delta phir_delta \(\frac{\partial \phi_r}{\partial \delta}\) \(\delta, \tau\)
func_phir_delta2 phir_delta2 \(\frac{\partial^2 \phi_r}{\partial \delta^2}\) \(\delta, \tau\)
func_phir_tau phir_tau \(\frac{\partial \phi_r}{\partial \tau}\) \(\delta, \tau\)
func_phir_tau2 phir_tau2 \(\frac{\partial^2 \phi_r}{\partial \tau^2}\) \(\delta, \tau\)
func_phir_delta_tau phir_delta_tau \(\frac{\partial^2 \phi_r}{\partial \delta \partial \tau}\) \(\delta, \tau\)


The IAPWS-95 property functions do provide initialization functions for general compatibility with the IDAES framework, but as long as the state variables are specified to some reasonable value, initialization is not required. All required solves are handled by external functions.


International Association for the Properties of Water and Steam (2016). IAPWS R6-95 (2016), “Revised Release on the IAPWS Formulation 1995 for the Properties of Ordinary Water Substance for General Scientific Use,” URL: http://iapws.org/relguide/IAPWS95-2016.pdf

Wagner, W., A. Pruss (2002). “The IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use.” J. Phys. Chem. Ref. Data, 31, 387-535.

Wagner, W. et al. (2000). “The IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam,” ASME J. Eng. Gas Turbines and Power, 122, 150-182.

Akasaka, R. (2008). “A Reliable and Useful Method to Determine the Saturation State from Helmholtz Energy Equations of State.” Journal of Thermal Science and Technology, 3(3), 442-451.

International Association for the Properties of Water and Steam (2011). IAPWS R15-11, “Release on the IAPWS Formulation 2011 for the Thermal Conductivity of Ordinary Water Substance,” URL: http://iapws.org/relguide/ThCond.pdf.

International Association for the Properties of Water and Steam (2008). IAPWS R12-08, “Release on the IAPWS Formulation 2008 for the Viscosity of Ordinary Water Substance,” URL: http://iapws.org/relguide/visc.pdf.

Convenience Functions

Iapws95StateBlock Class

Iapws95StateBlockData Class

Iapws95ParameterBlock Class

Iapws95ParameterBlockData Class