Hopper Live Bottom Connections

[jheydon2]

Hello All,

One of my products is a sludge storage hopper with live bottom screw conveyor. For this equipment, we design the hopper shell and live bottom trough as separate components. These are connected by a large bolted flange. Our typical design is to use L6x6x3/8” structural stainless steel angles welded to the side wall of both components with gasket sealing between surfaces. This joint uses many bolts and is difficult to manufacture with tight tolerances. My interpretation of AISC code for bolted connections puts this in a bit of a grey area. I have several questions for designing these joints, can you give me your opinion? For reference, we always design this a bearing connection and not slip-critical.

1. Would the use of oversized holes be allowed?

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a. If so, do plate washers need to be welded after installation?
2. Would long-slots be allowed?
3. When using short slots, can both joining flanges have slots, but in perpendicular directions?

Here is a screenshot of a typical connection.

 

[dvd]

Why do AISC bolting requirements apply? It seems like your bolting is more of a function of gasket compression. There are plenty of references (even on this website) to rectangular, gasketed bolted flanges. The attached has some discussion and lists references.

 

[human909]

Are you simply assuming it is difficult to manufacture? Or is it from past experiences.

My company regularly fabricates such items both the hoppers and the screws and we don't employ any special methods for tolerance control.

 

[lucky-guesser]

To be transparent -- I've never dealt with AISC code...that said:

1. Would the use of oversized holes be allowed?-
Unless you want to use your bolts as self tappers, you will have to oversize some.
a. If so, do plate washers need to be welded after installation?-
Not sure why they would but I suppose I'm open to correction.

2. Would long-slots be allowed?
3. When using short slots, can both joining flanges have slots, but in perpendicular directions?
Regarding 2 & 3, I would think it depends on how close you need things to line up. I assume you aren't looking for anything super precise considering its just a hopper. Regardless, linear long slots would likely be fine but I would think that perpendicular short slots gives a lot of room for slop and I would try to avoid it. I've had designs where I had included doing that but I didn't like the wide range of alignments it gave so I came up with a way to force correct alignment, rather than just making it so things will fit together wherever you set it. Slotting can definitely be a necessary evil but I avoid it when I can, but my designs are also fairly small and easier to get things to line up correctly, not something larger like what you have drawn so take the advice for what you will.

I would stick with 1/16 oversize holes (maybe 1/8 for larger bolts and ease of installation) and if the welders do their job everything should line up.

 

[TugboatEng]

If you use thicker flanges you won't need so many bolts but the drilling costs will be much higher. If you make the holes large enough you can plasma cut instead of drilling to reduce production costs. Material costs should be minimally different between 3/8 and 1" plate in the overall cost.

 

[FacEngrPE]

Bolted shipboard man-way closures have as many bolts as this assembly, we find standard clearance holes provide ample space to allow every thing to line up.
If defective/mis-aligned holes occur infrequently they can be fixed with a bridge reamer.

https://www.mcmaster.com/products/bridge-reamers/