4. Introduction

Self supporting tall equipments are widely used in chemical process industries. Tall vessels may or may not be designed to be self supporting. Distillation column, fractionating columns, absorption tower, multistage reactor, stacks, chimneys etc. comes under the category of tall vertical vessels. In earlier times high structure (i.e. tall vessels) were supported or stabilized by the use of guy wires. Design of self supporting vertical vessels is a relatively recent concept in equipment design and it has been widely accepted in the chemical industries because it is uneconomical to allocate valuable space for the wires of guyed towers. In these units ratio of height to diameter is considerably large due to that these units are often erected in the open space, rendering them to wind action. Many of the units are provided with insulation, number of attachments, piping system etc. For example distillation and absorption towers are associated with a set of auxiliary equipments i.e. reboiler, condenser, feed preheater, cooler and also consists of a series of internal accessories such as plates or trays or variety of packings. Often the vertical vessels/columns are operated under severe conditions, and type of the material these columns handles during operation may be toxic, inflammable or hazardous in other ways. Structural failure is a serious concern with this type of columns. As a result the, the prediction of membrane stresses due to internal or external pressure will not be sufficient to design such vessels. Therefore, special considerations are necessary to take into account and predict the stresses induced due to dead weight, action of wind and seismic forces.

5. STRESSES IN THE SHELL (TALL VERTICAL VESSEL)

Primarily the stresses in the wall of a tall vessel are: a) circumferential stress, radial stress and axial stress due to internal pressure or vacuum in the vessel, b) compressive stress caused by dead load such as self weight of the vessel including insulation, attached equipments and weight of the contents.

Dead load is the weight of a structure itself, including the weight of fixtures or equipment permanently attached to it; Live load is moving or movable external load on a structure. This includes the weight of furnishing of building, of the people, of equipment etc. but doesn't include wind load. If the vessels are located in open, it is important to note that wind load also act over the vessel. Under wind load, the column acts as cantilever beam as shown (Figure 6.1). Therefore while designing the vessel stresses induced due to different parameters have to be considered such as i) compressive and tensile stress induced due to bending moment caused by wind load acting on the vessel and its attachments; ii) stress induced due to eccentric and irregular load distributions from piping, platforms etc. iii) stress induced due to torque about longitudinal axis resulting from offset piping and wind loads and iv) stress resulting from seismic forces. Apart from that, always there are some residual stresses resulting due to methods of fabrication used like cold forming, bending, cutting, welding etc.

 

Figure 6.1 : Bending moment diagram under wind load