necessity of continious running of pilots

[AIM]

Good day all,
Can anybody please tell me what code prohibits one from running a heater' main burners without the pilot burner. The pilot burner will still be used for start up of the heater and for ignition of the main burner, but when the main burner is active, the pilot burner will be "killed".
Please comment.

 

[hotfoot]

I don't believe there is a code prohibiting the pilot from being turned off during normal burner operation. A well designed and reliable process burner does not depend on the pilot sustain the combustion reaction and maintain flame stability. I am aware of several process plants that run without the pilots. Some burners in operation do not have a pilot, and require ignition bya torch. However, the pilot must be considered when designing a safety shutdown system. For instance, one such permissive will not allow a main burner fuel valve to be opened if the pilot valve is closed. This assures there is an ignition source for the main flame to prevent dumping unburned fuel gas into the heater. But, the pilot may be shut off after the burner is lit. Some systems also utilize flame scanners to detect pilot and main burner flame to verify ignition.

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[jeyachandra]

I hope by “main burner active” , you mean main burner flame established and stabilized. If you are using flame scanner/scanners to ensure this condition, then the pilot ignitor can be taken out of service after establishing main burner flame. Under this condition, that is when the main burner is in operation with out pilot ignitor, if “loss of flame “ is detected by the flame scanner associated with the main burner,then the fuel shut off valve of the main burner has to be closed to prevent accumulation of un-burnt fuel in the furnace.

The Codes, Standards, and recommended practice vary based on the following;
1. Quantity of main burner- Single burner or multiple burners.
2. a) Fuel fired- Single burner firing multiple fuel example: oil and gas
b) Oil fired or pulverized coal fired burners
c) Gas fired burners
G.Jey
Powerpl_tr@yahoo.com

http://www.powerplusengrs.com

 

[geo57]

There are various codes and standards on this issue. The one most widely quoted is NFPA 85C. It basically states that if the igniter (pilot) supplies less than 4% of the heater duty then is turned off after the lighting of the main burner.
There is basically two schools of thought on this. The first is that as long as there is a flame in the heater, then no large amounts of unburned gas can accumulate, so it is better to leave the pilots on.
The other is that unless you have a stable and self supporting flame then all gas going to the heater should be turned off.
I personally belong to the second school of thought for two reasons. First is by turning the pilots off, I can use the flame scanner to detect the main burner flame, and insure it is stable and self supporting.
Second is that it is possible for unburned gas to accumulate in a corner of the heater until it reaches an ignition source and ignites, which can cause damage to equipment, and people.
That same NFPA 85C states that if the pilots are to be left on they must supply at least 10% of the heater duty, and unles you bought the heater with Class 1 ignitors, then they most likely do not supply the required heat. The thinking here is that a large flame allows less unburned gas into the heater.
George Griggs
Sr Process Controls Engineer