Skirts thickness

The skirt thickness must be sufficient to withstand the dead-weight loads and bending moments imposed on it by the vessel it will not be under the vessel pressure. 

Ds = inside diameter of the skirt, at the base, ts = skirt thickness. 

The skirt thickness should be such that under the worst combination of wind and dead-weight loading the following design criteria are not exceeded ... 

As a first trial, take the skirt thickness as the same as that of the bottom section of the vessel, 18 mm. 

Keep the skirt thickness the same as that calculated for the cylindrical skirt. Highest stresses will occur at the top of the skirt where the values will be close to those calculated for the cylindrical skirt. Sin 80.5° = 0.99, so this term has little effect on the design criteria. 

The skirt thickness A may be predicted from an approach suggested by Guthrie and Bradshaw (G8) which in extended form (W2) yields... 

Experimental measurements of skirt thickness (B3, B5, G8, W2) show reasonable agreement with Eq. (8-18). In practice, skirts become thinner with increasing distance from the rear of the bubble or drop (B3, H5). Skirts behind bubbles are of order 50 fim thick, while the thickness of liquid skirts behind drops is of order 1 mm. 

Where the vessel wall will be at a significantly higher temperature than the skirt, discontinuity stresses will be set up due to differences in thermal expansion. The British Standard BS 5500 requires that account should be taken of the thermal discontinuity stresses at the vessel to skirt junction where the product of the skirt diameter (mm), the skirt thickness (mm), and the temperature above ambient at the top of the skirt exceeds 1.6 X 10 (mm °C). Similar criteria are given in the other national codes and standards. Methods for calculating the thermal stresses in skirt supports are given by Weil and Murphy (1960) and Bergman (1963). 

Thus, it would seem that the skirt thickness might be less than the shell thickness if the same steel is used for construction in both. However, the increase is bending stress with X tends to increase the skirt thickness so that the two influences somewhat cancel each other. As a result, the same... 

In general, the allowable deflection for vertical vessels should not exceed 6 inches per 100 feet of height. Since this ratio is exceeded in the example above, the thickness of the lower sections of the vessel, and especially the skirt thickness, should be increased to keep the total deflection within the allowable. 

It should be noted that the thickness of the skirt is important, especially for long skirts. Preferably, the skirt thickness is the same as the shell thickness. This is a relatively inexpensive way to keep the column natural frequency high. 

Skirt Thickness. The skirt loading is calculated with operating weight and full wind pressure and with test weight and full wind pressure. 

There are various methods of making the attachment weld of the skirt to the shell. The preferred method is the one in which the center line of the shell and skirt coincide. This method will minimize stresses at the juncture. Probably the most common method, however, is to make the OD of the skirt match the OD of the shell. Other methods of attachment include lap-welding, pedestal type, or a shear ring arrangement. The joint efficiency of the attachment weld also varies by the method of attachment and is usually the governing factor in determining the skirt thickness. This weld may be subject to cracking in severe cyclic service. 

Combine moments with corresponding weights at each section and use allowable stresses to determine required shell and skirt thicknesses at the elevation. 

I his thickness can be reduced b> increasing tin guscset height. Assuiiiing a gusHid height of 18 in. rather than), 2 in. will n luce the skirt thickness fo -. ... 

Consider a vessel having a diameter of 130 in. and a skirt thickness of >2 hr insulated inside and out with the skirl supporting a shell in which the bottom temperature is 700 F. Assume that the temperature distribution in the skirl is given by Eq. 10.60. Calculate the thermal stresses at the junction. 

Example 12.1. Determine the required skirt thickness and the number of bolts needed in a vessel with an outside radius / = 7.0 ft. Let empty weight IVi = 160 kips, weight of contents IV2 = 1440 kips, wind-bending moment M = 1500 ft-kips and temperature — 300 F. Assume A307 bolts and use Figure 8.11 for the external pressure chart. 

Example 12.2. In Example 12.1, it was found that 12-1 in. A307 anchor bolts were needed for a vessel with an outside radius / = 7 ft, IV) = 160 kips, M = 1500 ft-kips, and a skirt thickness of 0.375 in. If/J = 3000 psi, determine the actual stress in the concrete and bolts. 

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