We are having GE Frame 6B Gas turbines in our plants. For Fuel Gas Flow Measurement, they employ two differential pressure transmitters, one for high dp and one for low dp. Furthemore, they also employ one pressure transmitter and one temperature transmitter for so called pressure and temperature compensation.
I'm interested in knowing the formula which they use to calculate the compensated Fuel Gas Flow. I had a chance to see in the Mark VI control system, where these inputs are feeded to the block called GASFLWC1 which finally outputs the Mass Flowrate QM. The block has the following inputs:
FQKGDB (0.25 inH2O)
FQKGSG (0.634502) Specific Gravity
GQKGG (2.047 n/d) Gas Constant
Can anybody tell me what formula they have employed with the above mentioned inputs to calculate the compensated mass flow rate QM?
Does the application code in the turbine control system (you didn't say what it is) not have a GASFLWCALCVn block? All of the internals of the block are shown in the graphic depiction of the block, and if the turbine control system is a Mark VI or -VIe if you right-click on the block and click on 'Item Help' a Help window will open with more information.
If the calculation is done in a macro in a Mark VI or -VIe you can dive into the macro in the Tree view window to see how the calculation is done to determine the formula.
Also, sometimes (more often than not) the formula is shown in the Control Specification, usually in Section 05.02.
If the turbine control system uses MBC (Model-Based Control) it might be done slightly differently.
It would be helpful if you told us what version of turbine control system is used on the turbine.
The control system is Mark VI. We just can't enter inside the block as it looks like blocked from GE. That is why i'm interested in knowing what formula this block uses to calculate mass flow rate.
>The control system is Mark VI. We just can't enter inside
>the block as it looks like blocked from GE.
What is the name at the top of the "block" in the main pane of the Toolbox window?
If the name is something like 'GASFLWCALCVn' then it's a gas turbine block, and you should be able to right-click on the block and in the pop-up that opens click (left-click) on 'Item Help'. If there is no Help for the block, a dialog should pop up to say "No help is available for this block" (or something to that effect).
If the block has no name like the one above at the top, or Toolbox says there's no help available for the block, then it is most likely a macro--written using "primitives" like ADD, SUB, MULT, DIV and SQRT, etc. In this case, you have to expand the macro in the Tree View pane of the Toolbox window (the tall narrow pane on the left). To expand the macro you need to click on the icon next to the macro name, and Voila! you will see, in the main pane of Toolbox, the macro magically open and display its contents to you. The passed parameter signal names will not be the same inside the macro as outside of it, but may be very similar--similar enough to follow the signals through the macro's functions to see what is happening.
Did you look in the Control Specification drawing to see if the formula is listed in the Gas Fuel section of the drawing?
OK I'm able to get into the block to find the actual equation/formula involved. I see one strange constant called O_CONST (Orifice Constant) which is mentioned as 2.047. Anyone has any idea how it could have been derived?
If I recall correctly (and it's been a very long time), the Orifice Constant was a value supplied by the orifice maker on a sheet of paper that listed all of the measurements of the orifice.
You may be able to find some information on how to calculate orifice constants on the World Wide Web; I believe it's a fairly standard calculation.
I also believe that over time, orifice plates do wear (especially the sharp edges) and that can have an effect on their accuracy.
You should know that the gas fuel flow measurement methods used by GE in their Mark* turbine control systems were not highly accurate--and did not need to be for the purpose they were intended. In other words, they are NOT revenue quality measurement methods. They were only intended to provide a number (accuracy was intended, but not important) on which to base the water or steam injected to reduce the formation of NOx emissions. The US Code of Federal Regulations (CFR) allowed the injection ratio (water-to-fuel; steam-to-fuel) for the reduction of NOx emissions to be based on fuel flow-rates. In the early days of emissions reductions, it was not possible to obtain real-time emissions data on which to base the injection ratio in order to control NOx emissions. It sometimes took more than 5 minutes to get a stable reading from an in-site Continuous Emissions Monitor System (CEMS). This meant that real-time emissions reductions during loading/unloading was not possible--but by using actual fuel flow-rates (which are good indicators of emissions) an injection ratio could be established, and that could be demonstrated with detailed emissions test to reduce NOx emissions to values slightly less than the permit requirements.
Many early plants did not have in-site stack CEMS systems--they were very expensive and prone to frequent failures and breakdowns.
Now, you are thinking that the gas fuel flow measurement must be accurate. No; it doesn't have to be. Once the required injection ratio was established for the gas fuel flow feedback to achieve the desired emissions values, the ratio would not change very much over time--unless the fuel changed considerably, or the calibration of the low- and high-range dp transmitters changed over time, or the orifice plate became excessively worn.
The flow measurement was also corrected for upstream (supply) pressure fluctuations, as well as temperature; so there was an upstream pressure transmitter and, usually, three gas fuel temperature measurement T/Cs that supplied inputs to the flow calculation block.
So, accuracy was desirable--but not required. All that was required was to establish the injection ratio required to achieve the necessary emissions values and then demonstrate the injection ratio worked by testing (while actually monitoring emissions with a portable emissions analyzer in the beginning; later with more stable and reliable in-site CEMS) the laws about emissions were satisfied.
If you're trying to compare the Mark IV's gas fuel flow measurements with any other gas flow-rate measurement (such as from the gas supply company's "revenue" meters--the meters they use to charge for the gas consumed by the turbine)--don't. That's NOT what the gas flow measurement system was designed for, and certainly not what it's capable of.
If you're having emissions troubles, it's very possible the orifice plate has worn, or, even that it was installed backwards (YES; that happens a LOT after maintenance outages if the orifice plate was removed!). And, know also that the water injection flow meters typically supplied by packagers of GE turbines all have their own individual K-factors (flow factors--similar to orifice constants), so if the flow-meters are changed the new K-factor values have to be used in the Mark IV calculations to have to same degree of accuracy as before.
And, if the unit uses steam injection for NOx emissions reductions, it measures steam flow exactly the same as gas fuel flow--using an orifice plate, which can also wear and need replacement, and which, frequently, gets re-installed backwards. (Often, it was even installed originally backwards....!)
I don't believe GE re-invented any wheel here when measuring the flow of compressible gases--they used very standard methods and formulae and constants.
And, MOST people are shocked to learn that the gas fuel flow measurement is not used by the Mark* to control the air/fuel ratio. It's not; it has nothing to do with combustion or fuel flow control. Unless the unit at your site is VERY different from the overwhelming majority of other GE-design heavy duty gas turbines.
Hope this helps! If you're having a particular problem, try explaining your situation; perhaps we can help.