Does anyone have any experience of operating a water tube steam boiler which is normally balanced draft i.e. has FD and ID fan under natural draft only?

Both our fan turbines require major repairs and I've been asked what load could be achieved out of the boiler merely under natural draft.

I would be pleased to hear any tips/advice.

You could get a couple inches water draft in your firebox on natural draft.  Determine how much open area you have under the burners that could be used to pull in air.  Calculate your max flow through those openings at that draft and set flow rate based on minimum acceptable oxygen content.  You will want to run higher excess oxygen on natural draft versus forced.

A lot of it depends on the height of your stack or chimney.  Without the 'stack effect' at the back end you won't be able to pull much through the pressure loss caused by the tube banks and whatever else you have between your furnace and the back end.

I have seen solid fuel boilers that had tall stacks operated at low, idling type loads (weekend operation for example) with all the fans off.  But they wouldn't do much when a real load was applied.

rmw

I wouldn't expect to run a boiler without the FD fan running. You will get away without the FD fan for a low load but at some point you will be smoking up the boilerroom. Balanced units are not designed for pressurised furnace operation. If the smoke doesn't bother you, hot spots on the external casing can get you fired.

Bottom line: Try it but don't push it

It might not be a simple decision normally,since;

1)The combustion air flow& temperature are key factors in  smooth and secure Burner Management systems& there bye for boiler operation.

2)The Efficient heat extraction from outgoing flue gases is ensured against incoming air through air pre-heaters etc. which form resistance to airflow only overcome through the FD fans.

3)The resultant extra work load on the ID fan (without FD Fan in use) can severely hamper overall capacity and safe combustion operation.

4)Thus slow but sure deposition of soot and other incomplete combustion products,consequential decrease in ID fan's speed due to heaviness and system clogging.

5)Not only consequent greatly reduced boiler capacity but Even the Boiler's operational Safety,Boiler Room's staff& Operating,nearby staff Life safety becomes in total jeopardy.You should appreciate.   

Best Regards
Qalander(Chem)

If it is not for maintaining boiler temperature so as to shorten the next start up time, I would not recommend doing so because:

1. The natural draft might be possible to keep the fire going but the amount of heat generated can only keep the boiler itself warm but can produce barely any steam.

2. For combustion with insufficient air it is hard to maintain CO emission level within safe range.

I assume that you've got a gas- or oil-fired boiler here since most solid fuel boilers are going to require at least one FD fan to move or support the solid fuel in the furnace.

This seems like a very bad idea to me.  For starters, I don't see how you can effectively accomplish a purge of the unit prior to light-off.  I also would expect that you'd have to jumper across some of the limits in your burner management system to even light the pilot burners.  And lastly, assuming you broke the rules and managed to light off the unit, I would expect that you cannot achieve much of a load at all.  

I've seen the consequences first hand of a boiler that was lit off without its FD Fan running.  Let's just say they had to replace the entire boiler (the steam drum landed several blocks away).  Unfortunately, the operating personnel killed in the explosion were impossible to replace.   

Hazards are equally "Great" even for "Gas fired" or "Oil Fired" since in
case(1) Gas poses greater risk/danger of explosion(s).

case(2) Oil there is insufficient air flow& possibly temperature to have risks/dangers of inadequate stochiometeric ratio and non-sustainable combustion thus finally leading to possible explosion or flame extiguishment.

Best Regards
Qalander(Chem)

if you had even one working fan will accomplish 30% but the 2 are broken,
you can not keep even the pilot on.
if the boilers were started normally with fans, fans down then the stack is hot and flowing you could get 10 - 20% output w/nat draft  checking draft and combustion meassurements within acceptable limits.
but what good is it. quite dangerous.  

Thanks For confirmation of my concerns shown.

Best Regards
Qalander(Chem)