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Why does COT of a furnace go down when fuel gas supply pressure goes down (at constant fuel gas composition, though fuel gas main control valve opening increases to maintain COT and fuel gas flow to furnace also increases) and constant furnace throughput?
 
Answers
23/04/2013 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
I am speculating that the actual typical fuel gas header pressure in the spurs near the fuel gas control valves is very low. Consequently, further reductions are "digging into" the burner design deltaP. So actual fuel gas flow to the burner is dropping even though fuel gas control valve is trying to compensate.
Best practice calls for MINIMUM fuel gas header pressure in spurs ahead of burner control valves of about 4 barg or 60 psig.
In refineries with central common fuel gas amine contactors, the combination of certain lower pressure source reflux drums and tower pressure controls and distribution header losses can lead to relatively low actual local header and spur pressures in the vicinity of fired heaters.
Practical program of fuel gas source pressure and tower control system reviews can help raise fuel gas contactor pressure and fuel gas distribution header needs to be assessed for bottlenecks.
23/04/2013 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Time lag? Does it recover at the lower pressure?