3 replies to this topic

[HLO]

Dear all, In API 560, section 14.1.7, it is required a maximum heat release as a percentage of normal heat release at design conditions (understood, process design conditions). Maximum heat release per burner requirements are based on safety concerns? For instance, if a fired heater is equipped with a burner which maximum heat release exceeds 14.1.7 requirements, other safety measures should be taken to limit burner maximum heat release to percentages of the aforementioned paragraph? I've submitted a Request for interpretation (RFI)to API with this question, but API answered that "Due to limited committee resources, API cannot respond to questions seeking the rationale for requirements in its standards", which is logical. I saw numerous DSs with maximum heat releases exceding this requirement, but my doubts lasts. Which are your opinions? Thanks and regards, Hernan

PS: Paragraph 14.1.7 is the following: "All burners shall be sized for a maximum heat release at the design excess air based on the following: a) five or fewer burners: 120 % of normal heat release at design conditions; six or seven burners: 115 % of normal heat release at design conditions; c) eight or more burners: 110 % of normal heat release at design conditions."

Edited by HLO, 25 February 2013 - 06:33 PM.

[ankur2061]

HLO,

 

The API committee seems to have a standard response to queries raised to them. This has been also encountered during an explanation for API STD 2000.

 

Let us examine the gudielines for burner maximum heat release for some operating companies. Both Chevron and Mobil describe in their company design standards the burner maximum heat release as 125% of the normal heat release at design excess air.

 

Chevron provides some logic for why burner maximum heat release needs to be restricted. One argument they provide is that at very high release rates intense noise is generated which they term as "combustion roar". They also suggest that "combustion roar" can be suppressed using acoustically absorbent air intake plenums.

 

Another reason they mention for not operating the burner above maximum heat release at design draft is that the burner will not have enough combustion air above the maximum heat release even with the air register wide open. Thus the flame pattern will be one of that with insufficient air and result in poor firing.

 

Hope this helps.

 

Regards,

Ankur.

[kkala]

Firing without excess air is not safe.  Actually combustion controls aim at eliminating fuel rich mixtures with air, especially in transition periods. When firing is increased, airflow shall increase before fuel increase;  when decreased, fuel shall be cut down before air decrease (lead-lag system).

Consider a forced (or even balanced) draft system, where maximum combustion fan flow has been specified for e.g. 15% firing over the maximum normal; this means combustion air flow (stoichiometric + excess) enough for burning 15% more fuel (than the max normal). Inversely, total burners fuel flow shall not exceed 1.15 x max normal, to avoid formation of mentioned fuel rich mixtures (having caused several accidents). If all burners are operating, this is realized by restricting heat release of each of them to 1.15 x individual max normal. 

It is understood that burners shall have relevant limits on their heat release, in case that API 560 is applied, e.g. by limiting travel of fuel valves through a pneumatic signal (applied here to boilers, but for minimum turn down flow).

Not having access to API 560, it  would be interesting to know its recommendations on combustion fan flow rate (if any) for forced draft heaters, probably there is some similarity to burners.

Consider a heater with 5, 7, 8 burners, where one burner is not operating; total heat release is 96%, 99%, 96% of max normal (remaining burners operate at max permissible heat release). Probably API 560 practically means "do not exceed max normal capacity, if one burner is out of operation". The restriction is more strict for less than 5 burners, while for numerous burners it can be slightly exceeded, e.g. 20 burners would "permit" just 4.5% over max normal (if one burner is not operating). Rationale can be heat flux uniform enough  in the radiant section.

Above interpretation is subject to comments / additions, experience on fired heaters is limited to elementary theoretical level. Two more notes:

1. API Std 560 has been recently applied for a new furnace in a local refinery.

2.  A two burner boiler could operate at a capacity approaching max normal with only one burner operating (much higher than 60%). Relevant restrictions of API 560 do not seem to apply.

[HLO]

Ankur, Kostas,

 

Thank you very much for your valuable answers. I could understand that, at the end, it depends on the burner's supplier to establish wich is the maximum heat release.

An API's response would be useful, since Std. 560 don't give any chance to override the requirement, based on the rangelability of a particular burner.

 

Kostas, you are right: in Std. 560 in Annex E.2.1.b, it is requested for fans in forced draft heaters, the following: "The fan rated point shall include the flow required (including all surpluses for excess air, system leakage and design safety factor) to meet the design heat release."

 

Thanks and regards,

 

Hernan