Water and Steam Properties

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Standard
Species Model
Stream Properties
Calculations
Density Heat of Formation Heat of Dilution Specific Heat
(Cp)
Solubility Boiling Point Elevation pH Charge Steam and Water Sulfuric Acid

Latest SysCAD Version: 25 February 2021 - SysCAD 9.3 Build 138.28098

Introduction

As H2O (steam / water) is a common and important part of most models, the property equations for both water and steam are built into SysCAD. The user cannot change these properties.

However, the user may select the properties of H2O to be calculated using either High Fidelity (IF97), Fast IF97 Sat or Simple equations. (These are discussed in more detail below.) Obviously, the High Fidelity equations will return more accurate results for T and P, but the calculations do require more computing power. Therefore the solver speed will be less.

Notes:

1. Properties for solid H2O (ice, or water trapped in a solid crystalline structure) are not included. To include these, add a species to the Species Database in the normal way with Phase = 'Solid' but the compound must NOT be 'H2O' and the Definition must NOT be H2O1, rather use O1H2.
2. If alternate user defined properties for water or steam are required, then it is possible to "trick" SysCAD using a species with a Definition 'O1H2' and a Compound that is not "H2O".
• IMPORTANT: Caution should be used when including alternate properties for steam/water as the hardwired steam/water will also exist in the project!
3. SysCAD does a range check on streams and many units to determine if liquid water, H2O(l), is present above the critical temperature of water. If it detects water above Tc it will generate a warning message. Please also see Plant Model - RC for more information.

Equations

Method Description Notes

IF97

• The algorithms used to calculate the thermodynamic properties of steam/water, such as heat capacity and enthalpy, saturated temperature/pressure, etc, are not given in this document, as they are available in the reference.
• The implementation in SysCAD has been optimised to maximise performance (speed).
• The calculations are based on the IAPWS-IF97 standard. This is described in a paper by W. Wagner et al. called "The IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam", published in ASME J. Eng. Gas Turbines and Power, Vol. 122 (2000).

Some good internet references for further information are:

http://www.iapws.org/
http://www.cheresources.com/iapwsif97.shtml
The valid temperature range is:

Steam: 273.16 to 3000K
Water: 273.16 to 647K

IF97 Equation names:

In the SysCAD species database, equation names for full IF97 correlations are:

• For Builds 136 to 138: IF97WaterRho(), IF97WaterCp(), IF97SteamRho(), IF97SteamRho2(), IF97SteamCp() and IF97SteamCp2().
• For Builds 139: IF97WaterRho(), IF97WaterCp(), IF97SteamRho() and IF97SteamCp()

General Notes

1. Due to differences in reference points, numerical methods and implementation techniques there can be very small (generally much less than a tenth of a percent) differences between IAPWS results and SysCAD results.
• Reference: Properties of Water and Steam in SI-Units, 2nd Revised and Updated Printing, Springer 1979.

IF97 equation Version NOTES:

For Builds 136 to 138, there are two equations for Steam Density and Steam Cp: “IF97” and “IF97_2”.

• During the release of SysCAD 9.3 Build136, we have found the original "IF97" (SysCAD 9.2 version) have errors in steam Enthalpy and Density equations for region5 (Caution: The IF97 equations are applied for temperatures up to 1073.15 K. Above that temperature SysCAD calculates any steam/water properties using Region 5 properties and linear extrapolation. Thus the higher the temperature, the more inaccurate the steam properties will becomes).
• A corrected version (IF97_2) was implemented and for backward compatibility we have retained "IF97" (SysCAD 9.2 version).
• NOTE for Build 136 to Build138: SysCAD has improved equations for steam in Region 5. Projects upgraded from SysCAD 9.2 will use old IF97 (SysCAD 9.2 version), but these should be changed to use IF97_2.

For Build 139.27886 and later, the “IF97” and “IF97_2” equations have been merged into a single equation "IF97" (SysCAD 9.3 Build139 Version).

• user now only has one simple choice – namely “IF97”. This uses the correct steam Enthalpy and Density equations for region5 (ie the IF97_2 code).
• Projects and cfgfiles auto upgrade to always use “IF97”.
• “IF97_2” is removed as an option for various dropdown lists (eg: Plant Model - Species Tab).

Implications:

• May affect results if the project was using the old equation with steam at high temperatures.
• Cfg files are upgraded in Build139, the files are not backward compatible, so if they are re-used in earlier build there will be load errors/warnings.
Method Function Equation Notes

LowFid

(Fitted IF97 @ SatP)

SysCAD 9.3 has a new set of lower fidelity equations called "FastIF97Sat" for water and steam Density, Cp, Enthalpy and Entropy. These equations are based on accurate curve fits to the IF97 data along the Saturation pressure line. The equations are therefore a function of temperature only, pressure is ignored and it is assumed that the pressure is the saturation pressure. These equations are significantly faster and for many operating conditions, especially for water, are suitable for most simulation requirements where the full accuracy of IF97 is not required. Please refer to Fast IF97 Sat Properties for more detail.
Water Density special density function LiqH2ODensity()
See Fast IF97 Sat Properties.
The equation for water density is an equation fitted to IF97 water density along the saturation line. The valid temperature range is 260K to 647.096K.
If the temperature is outside of the valid range then the minimum or maximum temperatures are used to calculate the density.
In the SysCAD species database, any species can be assigned this density by use of a special density function LiqH2ODensity().
Water Cp special Cp function LiqH2OCp().
See Fast IF97 Sat Properties.
The equation for water Cp is an equation fitted to IF97 water heat capacity along the saturation line. The valid temperature range is 260K to 573.15K.
In the SysCAD species database, any species can be assigned this Cp by use of a special Cp function LiqH2OCp().
Steam Density special Density function VapH2ODensity().
See Fast IF97 Sat Properties.
The density is calculated using the IdealGasDensity equation with the temperature ranged between 260K and 4000K.
In the SysCAD species database, any species can be assigned this Density by use of a special Density function VapH2ODensity().
Steam Cp special Cp function VapH2OCp().
See Fast IF97 Sat Properties.
The equation for steam Cp is an equation fitted to IF97 steam heat capacity along the saturation line.
In the SysCAD species database, any species can be assigned this Cp by use of a special Cp function VapH2OCp().
Method Function Equation Notes

Simple

(less accurate)

The "Simple" equations were called "Low Fidelity" in SysCAD 9.2. These are retained for backward compatibility. For Water Density, Water Cp and Steam Cp the new "FastIF97Sat" equations should be used instead as faster more accurate equations valid for larger temperature ranges.
Water Density
(less accurate)
$\cfrac{a_0+a_1T+a_2T^2+a_3T^3+a_4T^4+a_5T^5}{1+b_1T}$
• T is in degrees centigrade.
• The valid temperature range is -30°C to 300°C.
• If the temperature is outside of the valid range given above, then the minimum or maximum temperatures are used to calculate the density.
For example, if the stream temperature is 400°C, then the water density will be calculated using the maximum temperature of 300°C.

In the SysCAD species database, any species can be assigned this density by use of a special density function LiqH2ORho().

Water Cp
(less accurate)

HSC_Cp(a,b,c,d)

$C_p = a + b.10^{-3} T + \cfrac{c.10^5}{T^2} + d.10^{-6} T^2\,$

where

a = 16.749
b = 62.120
c = 32.798
d = 90.391
T = temperature in Kelvin
This equation is used in the range from 273.15 to 500K.

In the SysCAD species database, a species can be assigned this heat capacity as HSC_Cp(16.749, 62.12, 32.798, 90.391):Range(K, 273.15, 500).

Steam Density
(less accurate)
$\rho = \cfrac{PM}{RT}$

where

P = Pressure
R = universal gas constant
T = Temperature in Kelvin
M = molecular weight

The density is calculated using a rearrangement of the Ideal gas law. This equation is used for all temperatures and pressures.

Steam Cp
(less accurate)
CRC_Cp(a,b,c,d)
$C_p = a + 10^{-3}b T + \cfrac{10^5c}{T^2} + 10^{-6}dT^2\,$
where
a = -38.14955
b = 463.4602
c = 6.24e-5
d = -762.3604

T = temperature in Kelvin This equation is used in the range from 298.15 to 500K.

vapour pressure of H2O
(less accurate)
$\rho = 0.1333224\times10^{\tfrac A T +B\log_{10} \;(T)+CT+D}$

(0.1333224 converts pressure from mm Hg to kPa.)

where

A = -3433.74
B = -12.0063
C = 0.004782
D = 41.1767
T = Temperature in Kelvin, this equation is used from 0 to 1000K.

See IF97.

Fast IF97 Sat

See LowFid.

Please refer to Fast IF97 Sat Properties for more detail.

Water and Steam Properties in PGM

The user may obtain some water and steam values in PGM code using the Species Database Class.

Some of the values that are available to users are:

• Saturated Temperature;
• Saturated Pressure;
• Density;
• Heats of Formation; and
• Enthalpy.