Rear door Davit

gerat

We have a 400HP CB Scotch Marine Firetube boiler (96"shell).

The rear door is supported by a david arm when in the open position. These doors are very heavy given the steel, brick and refractory.
During a repair job we added some extra thickness of refractory and the extra weight makes me a bit worried about the capacity of the davit arm. The local CB rep has been unable to provide data regarding the factor of safety in the design of the davit and I wonder if someone here could comment on this or on factor of safety in davit design in general.

Thanks

Jamie Hall

Any lifting equipment must have a factor of safety of at least 8 in the in-use condition. That's the whole thing -- the davit, support structure, pivots, the chain or cable, the sheaves, the hooks, the lot. Anything in the load path.

Now what the existing davit was designed for I wouldn't care to say without detailed inspection and analysis. Nor would I care to venture an opinion on the new weight of the door, and thus the safety factor.

Older equipment was often over designed -- but you can't count on that. Then there is wear on anything which moves -- the pivots, and shafts, whatever -- plus any possible material wasting.

Honestly you would be best off -- particularly if this is a commercial enterprise -- engaging a structural engineer to evaluate the arrangement.

EBEBRATT-Ed

Those davit arms are pretty rugged. I can't thing that adding some weight will make it sag much although getting the bolts to line up with a standard door can be problematic........... think floor jack.

How much weight did you add and why?

Besides, now you added the weight and it's still hanging..... the bolts will hold it when you close it up.

ScottSecor

We used to work on many 200 to 300hp units and a few 400 hp boilers. Many of these thirty to fifty year old boilers had similar davits that you describe. Most of these davits were worn and allowed the doors to sag.

Our solution was to use a rolling floor jack to support these heavyweight doors. We would keep the jack under the bottom of the door as we opened the doors and did our work. When we went to reseal the boiler, oftentimes we had to raise the jack when we went to tighten the bolts.

To be honest, I never examined the cause of the sagging doors. I suspect the heat and perhaps added weight might have contributed. I just googled boiler davit arm and this CB part came up. Maybe they can be rebuilt and or repaired easily? https://parts.cleaverbrooks.com/Product/807-00325-000?criteria=96" boiler davit

retiredguy

I can not see any reason to add extra refractory to the back access door on any "dry back" scotch boiler CB or otherwise. That said, I never saw any of those arms fail. You could call Clever brooks direct at 800-250-5883 and ask them for their recommendation and input.

gerat

Thanks for the replies. The lower half of the door refractory was in very poor condition. We chipped out the loose and rotten material and made a form using the old cleaver brooks instuction book and intended to pour a fresh outer cap and hope it would adhere. The form bulged and it resulted in a thicker addition than intended.
These doors have always been a struggle to close as they sag when opened as described in your replies.
As we struggled to close it up this time my thoughts regarding the additional weight (maybe 500 to 1000 lbs) makes me worry that at next opening (when I plan to remove the excess) the david system could fail.

It seems to me that these systems must have capacity above the intended load (factor of safety) but my local CB distributor has not been able to get me this info.