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How do you calculate actual flow rate of Fuel Gas from the flow meter reading?
In our process plant we have Fuel Gas FTs which gives us reading in Nm3/hr and we want to calculate from this reading actual fuel gas flow taking into account temperature, pressure and density corrections.
Which is the most accurate formula for calculating flow rate? What are the best practices followed by other refineries in order to calculate the fuel gas flow, do they apply temperature and pressure correlations? Pressure or density which is better to take in account?
 
Answers
30/05/2019 A: Prakash Pimparkar, Environmental Consulting Services , prakash.pimparkar@yahoo.in
It is very simple for practical purposes you can neglect the pressure effect as normally the pressure in the stack would be few mm below atmospheric pressure ( 101325 vs 10000 mm water)
The temperature effect is by multiplying ( T oc + 273 / 273 ) where T is the flue gas Temperature T iin Degrees Centigrade.
Now most of the time the flue gas temperature is kept at above 120 degrees centigrade say about 130- 140 to avoid SO3 condensation and associated corrosion
So if we take flue gas temperature as 135 degrees the actual flue gas flow will be Q ( NM3/h) X 273 +135 /273 or Q X 408/ 273 = Q X 1.4945 say QX 1.5 ACM/h Actual M3/h
To get actual velocity in M/Sec one has to divide this by stack area in M2 X 3600
If you want to account for pressure the factor would be 1.4945 X 10132/10000 ) = 1.49 X 1.01 or 1.5097 close to 1.5
So just multiply your flow in NM3/h by 1.5 to get actual flow M 3/h at operating conditions
29/05/2019 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Ask any Process Engineer to recalculate the "meter factor" based on the actual gas analysis, temperature and pressure." He or she will know how to do it.