In 9FA gas turbine,we come across the word "wobbie index". what does that mean actually. send your replies to my mail "pawanp7 [at] gmail.com"

 

Have you looked up Wobbie Index using your preferred Internet search engine?

Have you looked in the Control Specification for the Speedtronic panel, section 05.02?

If so, do you still have questions?

 

Hello Pawan

The Wobbe Index is a factor used to compare the combustion energy output of different composition fuel gases in combustion chambers. If two fuels have identical Wobbe Indices then for given pressure and valve settings the energy output will also be identical. The Wobbe index is calculated using the Heating Value of the fuel (GE typically uses the Lower heating value) and the fuel specific gravity.

WI = LHV / sqrt ( SG )
LHV - Lower Heating Value (Btu/scf)
SG - Fuel Specific Gravity

However, GE uses the modified wobbe index, in this definition the natural gas temperature is also taken into account:

WI = LHV / sqrt( SG * NG Temp )

LHV - Lower Heating Value (Btu/scf)
SG - Fuel Specific Gravity
NG Temp - Natural gas temperature (Rankine)

In the gei41040j, GE states the following about the interchangeability of gases in a gas turbine:

"Gas turbines can operate with fuel gases having a very wide range of heating values, but the amount of variation that a specific fuel system design can accommodate is limited. The fuel nozzles are designed to operate within a fixed range of pressure ratios and changes in heating value are accommodated for by increasing or decreasing the fuel nozzle area or gas temperature. A measure of the interchangeability of gas fuels for a given system design is the MWI."

A combustion system installed at a certain plant is typically designed for a given fuel gas composition and temperature (a certain MWI) and as long as the MWI does not change significantly (with respect to certain limits in gas composition), no combustion problems are expected (GE typically allows a 5% MWI variability).

More useful information can be found the following two references:

http://www.gepower.com/prod_serv/serv/env_serv/en/downloads/gei41040j.pdf

http://www.gepower.com/prod_serv/products/tech_docs/en/downloads/ger4189b.pdf