Minimum Skirt Thickness et al
Minimum Skirt Thickness et al
(OP)
It is my belief that, per code, skirts on vertical pressure vessels must have a minimum thickness equal to the vessel wall. True?
Also, that unless thicker than vessel wall, OD of skirt must match OD of vessel. True?
Also, that unless thicker than vessel wall, OD of skirt must match OD of vessel. True?





RE: Minimum Skirt Thickness et al
RE: Minimum Skirt Thickness et al
False to both. The code does not specify design details for skirts.
There's no physical reason a vessel's skirt thickness should be governed by the internal pressure in the shell. A little common sense would be good here. As for matching OD's vs matching centerlines - again, just a question of load transfer and induced moment. For heavy vessels with relatively thin skirts, it makes sense to provide a load path (mid wall of shell to mid wall of skirt) without a moment arm.
The guidelines codeeng provided are reasonable. You'll find most company standards have something close to what he stated.
jt
jt
RE: Minimum Skirt Thickness et al
Thickness should be checked as follows:
Skirt bending stress=
[Weight/Area] +/- [Overturning Moment/ Z]
Thus the [ +/-] means 2 checks have to be performed, one on each side of the skirt, for at least these cases:
Check for wind.
Check for seismic.
Check for empty vessel.
Check for vessel operating).
Check for vessel under hydrotest (in which case , the
wind is lower).
Notations:
Overturning moment = due to wind and due to seismic.
Both not acting concurently.
Allowable to be used are:
S_hot from Div. II , for the tensile case
B for compressive stress
Area= 3.14* (R_av) * t = skirt area
Z = 3.14* t* (R_av)^3 = Skirt modulus of inertia
with:
t= (Skirt thickness) - (Skirt tolerance on thickness plate)
R_av = skirt average radius.
Make sure that there enough material compensation and that skirt access holes are fully compensated.
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Note also that skirt design temperature is < then vessel design temperature.This means that S and B and the Young modulus (used for calculation of deflection) of the skirt are a bit higher than the ones used for the vessel.
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It happened, that for tall columns and in order to avoid vortex, that the column own frequency has to be improved (e.g. increased), in this case a THICKER skirt than the one given above is necessary.
RE: Minimum Skirt Thickness et al
Area = TWO *3.14* (R_av) * t = skirt area
RE: Minimum Skirt Thickness et al
I just had a quote from a vendor who specified a skirt thickness of 55 mm - the same as the shell wall thickness. A bit OTT I think
RE: Minimum Skirt Thickness et al
Be aware of restriction marked in UG-4(b): material for nonpressure parts, such as skirts, supports, baffles …. , need not conform to the spec. for the material to which they are attached …. The allowable stress values for material not identified in accordance with UG-93 shall not exceed 80% of the max. allowable stress value permitted for similar material in Subsection C.