First, the calculation:
HeatRate = Fuel Flow * Fuel Heating Value / Power Output
The first question is: What are the engineering units on these values?
In the US, Heat Rate is most often shown in Btu/kWh. Fuel Flow can be in a number of different units, the most common being KPPH (thousands of pounds per hour), PPS (pounds per second) or SCFM (standard cubic feet per minute). Fuel Heating Value might be provided in Btu/SCF or Btu/lb. Power Output is nearly always in either kW or MW.
For places outside the US, Heat Rate is most often shown in kJ/kWh. Fuel Flow might be reported in m3/hr (standard cubic meters per hour) or kg/hr. Fuel Heating Value may be in GJ/kg, or GJ/m3. Power Output is still in either kW or MW.
As long as your fuel flow rate and fuel heating value are in compatible units (both mass basis or both volume basis), your units should cancel out. If not, you will need to know the density of your fuel (kg/m3) in order to convert them to a common basis. To calculated the density of the fuel, you’ll need to know the constituent analysis: for natural gas, this would mean the volume percent of Methane, Ethane, Propane, Hexane, etc. Note: Industry standards, such as ASME PTC-22, provide guidance on converting the constituent analysis to a density (as well as calculating the heating value).
The next question is: Where does the fuel heating value come from? The best answer for this question is to have your own gas chromatograph or heating value lab on site. A more common source of fuel heating value and constituent analysis is your fuel supplier. If the supplier cannot provide you the detail you need for the time frame you need, or if there are mixing stations between their reporting station and your unit, you may need to take your own fuel samples and send them to a laboratory for analysis. You’ll need to determine the source of your fuel heating value prior to calculating heat rate, just in case you do need to take your own samples. Samples need to be taken at an approved location (free from moisture or other ‘heavy’ particles that are filtered out prior to combustion in the unit), and must be transported in an approved container (an approved pressurized cylinder for natural gas) to an appropriate lab.
Once you have your fuel heating value source, the sources for the other two values must be found as well, but luckily, these two are normally easier to identify. There are normally two choices for fuel and power output: At the unit, or at the plant boundary (i.e. the billing meters). The heat rate you need to determine will define which meters to use.
For a gas turbine unit heat rate, you’ll want to record the fuel flow to the gas turbine at the meter closest to the unit and the power output from the power meter on the gas turbine generator – again, at the meter closest to the unit
For an overall plant or facility heat rate, you’ll probably want to use the billing meters for both gas and electricity – and this should therefore be a net heat rate for the facility (after all auxiliary and house loads have been accounted for).
There is still the question of uncertainty, or: How accurate do I know my calculated heat rate? For an ASME PTC level test, facility heat rate should be known with an error band of less than 1.5% (including corrections to reference conditions). When doing spot checks for heat rate using permanently installed instrumentation and a fuel suppler reported heating value, the uncertainty may be much higher – it all depends on the sources of your information.