ASME PRESSURE VESSELS
The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.
Pressure Vessel Support - Skirt
A skirt support consists of a cylindrical or conical shell welded to the bottom head of the vessel. A base ring at the bottom of the skirt transmits the load to the foundations through the anchor bolts.
The skirt is generally welded to the bottom head using any of the three arrangements shown in Figure 1 – the second arrangement is usually the most preferred one.
Skirt supports do not impose concentrated loads on the vertical shell. They are particularly suitable for use with tall columns subject to wind loading. Openings must be provided in the skirt for access and any connecting pipes.
Support Skirt Design
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 vessel pressure.
The resultant stresses in the skirt will be:
The skirt thickness should be such that under the worst combination of wind and dead weight loading, the following design criteria are not exceeded:
A large factor of safety is used in the expression for allowable compressive stress because a slight out-of-roundness can have significant consequences. ASME uses an approximate factor of safety of 10.0.
The minimum thickness of the skirt should not be less than 1/4” (6 mm). Where the vessel wall will be at a significantly higher temperature than the skirt, discontinuity stresses will be set up due to the differences in thermal expansion. Methods for calculating the thermal stresses in support skirts are given by Weil and Murphy (1960) and Bergman (1963).
Anchor Bolt Design
The loads carried by the skirt are transmitted to the foundation by the skirt base ring (bearing plates). The moment produced by wind and earthquake forces will tend to overturn the vessel; this will be opposed by the couple set up by the weight of the vessel and the tensile load in the anchor bolts. For large columns, a base ring design using double ring stiffened by gussets is often employed as shown in Figure 2.
The anchor bolts are assumed to share the overturning load equally, and the bolt area required is given by:
Following guide rules are generally used while selecting the anchor bolts:
- Bolts smaller than 1 in (25 mm) diameter should not be used.
- Minimum number of bolts is eight.
- Use number of bolts in multiples of four.
- Bolt pitch should not be less than 2 ft (600 mm). If this minimum bolt pitch cannot be accommodated with a cylindrical skirt, a conical skirt should be used.
Base Ring Design
The base ring must be sufficiently wide to distribute the load to the foundation. The total compressive load on the base ring is given by:
The minimum width of the base ring is given by:
The required thickness of the base ring is found by treating the ring as a cantilever beam. The minimum thickness is given by:
1. Coulson and Richardson’s Chemical Engineering, 3rd Edition by R. K. Sinnott
Parts of this article are reprinted from ASME 2010 BPVC, Section VIII, Div. 1, by permission of the American Society of Mechanical Engineers. All rights reserved.
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