cv_wobbe index

Calculation of CV or Wobbe Index
From Specific Gravity
There are several standards for the calculation of specific gravity, density and calorific value for gasses from
percent volume, or mole percent volume of the individual constituents of the gas being used.
Most of these standards rely on a chromatograph to give the values of each of the constituents. CV Density or
SG is then calculated from the volume, mass or mole percent contribution of each of the constituents to the
mixture.
Common standards that rely on percent mass or volume of each constituent to calculate CV, etc are AGA8, ISO
6976 and ASTM D3588.
One of the older standards (AGA 5) gives calculations and constants to allow the calculation of CV from either
constituent fractions or SG for a hydrocarbon gas mixture.
The SG method relies on the hydrocarbon series having a fixed pattern of Carbons and Hydrogens in ascending
values from methane (CH4). The series adds one carbon and one Hydrogen molecule (H2) for each new
product.
The table below shows the CV (in BTU / Ft3) for various Hydrocarbons (up to C5). The CVs shown are
CV Ref = value stated in standards for each product. CV Std = CV calculated from SG for each product (using
AGA 5 ) and CV from Cs + Hs = CV from the total numbers of C and H in the product, using an assumed
element CV of 325 BTU/Ft3 for H2 and 363 BTU/ft3 for Carbon.
C
Methane
Ethane
Propane
Butane
Pentane
1
2
3
4
5
H
4
6
8
10
12
MW
16.04303
30.07012
44.09721
58.1243
72.15139
SG
CV From Cs + Hs
0.5539
1012.1
1.0382
1699.5
1.5225
2386.9
2.0068
3074.3
2.491
3761.7
CV Ref
1012.1
1773
2523.3
3260.7
4009.7
CV Err
0.00%
-4.15%
-5.41%
-5.72%
-6.19%
CV Std Error %
1014.45
0.23%
1775.53
0.14%
2536.61
0.53%
3297.69
1.13%
4058.61
1.22%
Whilst the simplistic approach of counting Cs and Hs gives significant error as the products get heavier, it can
be seen that by a small adjustment the error could be minimised. The AGA5 SG method of CV determination
gives far better results across the range with error again increasing as the product gets heavier. This of course
is acceptable, as the bulk of the hydrocarbon content of natural gas will be Methane and Ethane. The larger
errors noted with the heavier products will be minimised because of their small contribution to the total SG of the
mixture.
The simplistic approach of the AGA3 equation becomes apparent when non-hydrocarbon products are added to
the mixture, especially the inert (non-energy releasing) gasses and hydrogen (because of hydrogen’s highenergy release for small contribution to SG).
The standard allows correction for the impurities (or dilutents), giving coefficients for correction (by % volume
contribution) for Hydrogen, Helium, Water (Vapour), Carbon Monoxide, Nitrogen, Oxygen, Hydrogen Sulphide,
Argon, Carbon Dioxide and Air. The corrections give good results for low concentrations of each impurity.
However, if the user has to generate an analysis to give the percent volume of each of the impurities they might
just as well use an analyser to give a full analysis and calculated CV, density and SG from the analysis.
Fortunately, in many cases the supply gas has fairly constant analysis with small changes of impurities. Where
the gas constituents do change dramatically it is normally only one or two components that actually change.
Over
Continued.
In these cases the user, with a little information on the expected impurities, can modify the response of the
equation (to SG). If the impurity change is too high it may be possible to carry out an analysis of one product in
the gas which causes the main problem (vol percent H2 for example) and input this variable into the equation to
modify the output.
In many cases is should be possible to estimate CV to within 5% (of other calculation methods) from density
temperature and pressure (giving SG)
If the CV of the gas is estimated from SG then the Wobbe Index can be calculated from
CV / Root SG
Sarasota density meters and converters can be used to measure the density, pressure and temperature and then
calculate SG
Both the available converters, CM200 and the Headmount Electronics have the ability to generate functions via
a customer defined look up table, while the CM200 can also accept an external input, direct into the table to
modify the table output.
This function allows the user to convert SG into CV or Wobbe index using the AGA3 equation for CV in its
published form, or modified to suit the gas being analysed. If wobbe index is required then the function can be
modified to include the CV/Root SG
If a second variable input is used to modify the function of SG (giving CV or Wobbe Index) then this can be input
into the CDF using the USER Input (4 – 20mA). The user input is only available when using the CM200 density
converter,
Hydrogen Input
4 – 20 mA
If required.
Hydrogen Input
25%
20%
Each element of the table contains
15%
the CV or Wobbe Index
10%
relating to the Specific Gravity or
5%
Percent Hydrogen Axis
CV or Wobbe
Index Out.
0%
0.1
0.2
0.3
0.4
0.5
0.6
Specific Gravity Value Input (From Density Calcs)
SG Calculated from Period, Temperature
and Pressure
The diagram above shows a six by six element look up table where either CV or Wobbe Index for the input
conditions of SG and %H2 can be entered. The CM200 then interpolates within the table to give an output in CV
or Wobbe Index. Note that the table can have up to 121 elements in any configuration required by the user.