Optimization of natural frequency of vertical storage tower using FEA Mr. S S. Hatwalne 1 , Prof. A. S. Dhekane 2 . Mr. Vinaay Patil 3 1. Asst.Prof., SITS Pune -41. 2.Prof., DYPCE Pune -41. 3. Head, Vaftsy CAE Pune, India. . Abstract - A vertical storage tower is typically used to store liquids or fine powders. To maximize the storage capacity, these columns are usually very tall typically over 27 meters. They are typically susceptible to wind loads, as the bending caused is much greater at these heights. The design problem is that, a vertical storage vessel is to be constructed near an agitator used in the process. The agitator is to generate vibrations at a certain frequency. If the natural frequency of the storage column matches that of the agitator, then it would result in a certain failure of the vessel at resonance. In addition to that, there is an inlet nozzle placed near the top of the column, the nozzle and the piping connected to it might also be susceptible to the vibration. The objective of the project will be to optimize the design and increase the natural frequency of tower in a manner that all failures can be avoided. I. INTRODUCTION Tall cylindrical stacks and towers may be susceptible to wind-induced oscillations as a result of vortex shedding. This phenomenon, often referred to as dynamic instability , has resulted in severe oscillations, excessive deflections, structural damage, and even failure. Once it has been determined that a vessel is dynamically unstable, either the vessel must be redesigned to withstand the effects of wind- induced oscillations or external spoilers must be added to ensure that vortex shedding does not occur. The deflections resulting from vortex shedding are perpendicular to the direction of wind flow and occur at relatively low wind velocities. When the natural period of vibration of a stack or column coincides with the frequency of vortex shedding, the amplitude of vibration is greatly magnified. The frequency of vortex shedding is related to wind velocity and vessel diameter. The wind velocity at which the frequency of vortex shedding matches the natural period of vibration is called the critical wind velocity. [1] Wind-induced oscillations occur at steady, moderate wind velocities of 20-25 miles per hour. These oscillations commence as the frequency of vortex shedding approaches the natural period of the stack or column and are perpendicular to the prevailing wind. Larger wind velocities contain high velocity random gusts that reduce the tendency for vortex shedding in a regular periodic manner. A convenient method of relating to the phenomenon of wind excitation is to equate it to fluid flow around a cylinder. In fact this is the exact case of early discoveries related to submarine periscopes vibrating wildly at certain speeds. At low flow rates, the flow around the cylinder is laminar. As the stream velocity increases, two symmetrical eddies are formed on either side of the cylinder. At higher velocities vortices begin to break off from the main stream, resulting in an imbalance in forces exerted from the split stream. The discharging vortex imparts a fluctuating force that can cause movement in the vessel perpendicular to the direction of the stream. Historically, vessels have tended to have many fewer incidents of wind-induced vibration than stacks. [1] There is a variety of reasons for this: 1. Relatively thicker walls. 2. Higher first frequency. 3. External attachments, such as ladders, platforms and piping, that disrupt the wind flow around the vessel. 4. Significantly higher damping due to: a) Internal attachments, trays, baffles, etc. b) External attachments, ladders, platforms, and piping. c) Liquid holdup and sloshing. d) Soil. e) Foundation. f) Shell material. g) External insulation. II. DESIGN PROBLEM The challenge posed is that the agitator and the vertical column are required to be on the same platform. Due to this the Column is susceptible to vibrations from the agitator. The frequency generated by the agitator is directly proportional to the rotations per minute (rpm). If the freq generated by the agitator matches with the natural frequency of the Vertical Column then such a resonance will cause the column to vibrate at max amplitude and may even result in Failure. The issue can be resolved by increasing the natural frequency of the column. Natural freq is a function of the mass (m) and the stiffness (k). If we International Journal of Engineering Research & Technology (IJERT) Vol. 1 Issue 3, May - 2012 ISSN: 2278-0181 1 www.ijert.org