8 replies to this topic

[Phukan]

We are having a box type furnace with natural draft having four passes. We are using John Zink low Nox burners with both oil and gas firing and using a manual damper to maintain the draft. Normally half of the burners in a single pass are oil fired and rest are gas fired. Although the oil header pr is properly maintained, there is a huge fluctuation in fuel gas pressure. Therefore to maintain the COT, oil firing has to be regulated on frequent basis. This effects the overall furnace efficiency as oxygen available in the box is fixed and (it becomes very cumbersome for the field operator to go and adjust it every time.)Moreover, during soot blowing operation, it is our normal practice to open the stack damper to about 70% in one to two steps, which causes huge fluctuation in COT due to sudden change in fuel-air mixture.
Now my question is it safe to install a motorized stack damper instead of a manual one, so that it can be operated from the console. Actually I am not sure whether such a system exits and who can be the potential suppliers in India.

Thanks

Dipankar Phukan

[Zauberberg]

Yes, there are such devices installed on natural draft heaters. They are usually provided with either software or mechanical stopper (or both), in order to prevent full closure of the damper. My suggestion is to operate it as HCV and not in AUTO or CAS mode, as all analyzer-type of controls are very slow and difficult to be tuned. In addition to that, Oxygen analyzer can get fouled which will create problems if the system is operated in AUTO/CAS mode(s).

If you are using two types of fuel for firing the heater, COT controller should be cascaded to the fuel system (gas or oil) that takes the most of the heating load, so that any changes of the other fuel system can be absorbed and compensated quickly. If you are having persistent problems with fuel gas pressure while most of the burners are fired with fuel gas, analyzer will not help you. Then you have to fix the problem at the source.

[Phukan]

Thanks for the reply. My idea was just to give the handle of damper operation to the panel operator, so that he can manually change its opening remotely. The operator will always have the option to cross check with the draft reading to comply with the oxygen reading and then take the appropriate action.

And now with your reply I think I can look for a supplier.

[breizh]

Good day ,
A resource you should consider :


http://www.heaterdes...com/design0.htm
Go to auxiliaries

Hope this helps

Breizh

Edited by breizh, 05 June 2011 - 05:04 AM.

[Phukan]

Thanks Breizh..this would definitely give me some insight

[Flame]

Generally, it is the fuel gas Control valve which regulates firing based on COT requirements. This is supposed to be better practice. It is advised that the fuel gas pressure fluctuation problems are rectified at your end.
Morever, if some provision is employed in the damper linkage, which enables gradual opening of damper along with mechanical position indication, then also your purpose will be served. Why do you want to opt for a automatic damper, any other problems?

Problems during soot blowing are totally unrelated with fuel/air mixture issues, could you please elaborate.

[Root]

Flame:

For air/fuel ratio use air register and stack damper in last stage, but for fluctuation in gas pressure you need to correct it from sources not from the heater side. Pressure fluctuation always coming from upstream units. Go and ask from fuel gas sources point why pressure fluctuation is coming, definitely they will rectify your problem.
Toor

[Phukan]

Toor':

Normally we put Fuel Gas in cascade control with the COT. Controlling the FG header pressure at a constant value is possible but it has been found to economical this way (at the cost of COT fluctuation in different units). Therefore, the Fuel Oil firing has to be changed(which is in the auto made)in order to meet the COT requirement.

Moreover, during soot blowing operations the damper has to be opened to about 70%( to prevent the draft from becoming positive during the soot blowing operation). The normal opening of the stack damper is about 20%. The change is normally not gradual(as it is done manually from the field) and it takes about 30 minutes for the furnace to cope with this enhanced fuel requirement (with FG in cascade to take the load), due to increase in amount of air in furnace. With this operation in the hands of panel operator he can automate this opening change (of the Damper) and let the heater control absorb the shock in a better way. I am not sure how the soot blowing operation is performed elsewhere, but this is how it is done here and the problem we are facing.Hope I could make some point.

[Flame]

It seems you mean that extra soot blower steam flow rate decreases draft @ heater top. But you see only one soot blower is operated at a time, does that give off enough steam to over-pressurize your furnace system?