tower structure

[picasa]

One of the most typical shapes for a refinery tower has a larger lower section and a smaller upper section connected to each other through a cone. When it comes to erection of this vessel, the skirt can be easily attached to the lower section.

Can a tower have a smaller lower section as compared to the upper section? In that case, is the skirt attached to the lower section? I would imagine such a structure would be more unstable than the one with a bigger lower section. Please comment, and suggest related reading.

 

[jte]

It's not unusual at all to have a skirt diameter which is smaller than the largest shell diameter. The design of such a skirt is no different than the design of any other skirt: Determine the loads (forces and moments), check the tensile stress (never seen this one govern...), check the compressive stress, iterate as required.

Suggested reading would start with any text which deals with the design of skirts and VIII.

Don't forget to check the top of the skirt (lower forces and moments) at the design temperature of the vessel and the bottom of the skirt (highest forces and moments) at (typically) ambient temperatures.

The same would apply to flared (conical) skirts. These are unusual, but the same concepts would apply.

jt

jt

 

[JStephen]

Single-pedestal water towers are commonly built in this shape.

If the tower is too tall/thin, you need to check p-delta deflection in it.

 

[vesselguy]

When a proper design is done, a column with a smaller diameter bottom shell courses is no more nor no less unstable than a same height of column with a larger diameter bottom shell course. It's all in the design when you do the shell/skirt thicknesses calculation to support the compressive/tensile load due the wind & self weight. You will see such design common on fractionation columns because Process Engineers need to maintain condensate flow rates on smaller trays in lower section.

Yes, the POV is higher for the smaller diameter lower section design. However, if your anchor sizes are starting to exceed 2.5" diameter than you can always flare the skirt. Please read a standard vessel design book from such authoritative books by Bedner or Brownell & Young. They'll explain it all.