Supports skirt

Skirt supports are recommended for vertical vessels as they do not impose concentrated loads on the vessel shell they are particularly suitable for use with tall columns subject to wind loading. 

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  

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. Methods for calculating the thermal stresses in skirt supports are given by Weil and Murphy (1960) and Bergman (1963). 

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). 

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Small vertical vessels may be supported by angle support legs, as shown in Figure 12-11. Larger vertical vessels are generally supported by a skirt support, as shown in Figure 12-12. At least two (2) vent holes, 180" apart, should be provided at the uppermost location in the. skirt to prevent the accumulation of gas, which may create explosive conditions. Horizontal vessels are generally supported by a pair of saddle type supports. 

Note for vessels on a skirt support (see Section 13.9.2), the weight of the liquid to fill the vessel will be transferred directly to the skirt. 

Wind loading will only be important on tall columns installed in the open. Columns and chimney-stacks are usually free standing, mounted on skirt supports, and not attached to structural steel work. Under these conditions the vessel under wind loading acts as a cantilever beam, Figure 13.19. For a uniformly loaded cantilever the bending moment at any plane is given by ... 

Skirt support, height 100 sieve plates, equally spaced 3 m... 

Figure 13.23. Typical skirt-support designs (a) Straight skirt (b) Conical skirt...

Design a skirt support for the column specified in Example 13.3. 

The main load carried by the brackets will be the weight of the vessel and contents in addition the bracket must be designed to resist the load due to any bending moment due to wind, or other loads. If the bending moment is likely to be significant skirt supports should be considered in preference to bracket supports. 

Figure 18.14. Methods of supporting vessels, (a) Saddle supports for horizontal vessels, usually of concrete, (b) Bracket or lug supports resting on legs, for either vertical or horizontal vessels, (c) Bracket or lug supports resting on steel structures, for either vertical or horizontal vessels, (d) Straight skirt support for towers and other tall vessels the bearing plate is bolted to the foundation, (e) Flared skirt for towers and other tall vessels, used when the required number of bolts is such that the bolt spacing becomes less than the desirable 2 ft.

The method used to support a vessel will depend on the size, shape, and weight of the vessel the design temperature and pressure the vessel location and arrangement and the internal and external fittings and attachments. Horizontal vessels are usually mounted on two saddle supports see Figure 13.20. Skirt supports are used for tall, vertical columns see Figure 13.21. Brackets, or lugs, are used for all types of vessels ... 

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