(70dy) Harmonic Analysis of Mass-Flow Bunkers and Silos with Reference to Thermal Power Plants

Bulk solids handling operations perform a key function in large number and variety of industries. Animal feed, cereals, coal, dry chemicals, food grains, ores, pharmaceutical pills, sand, shale are some of the bulk solids handled in industry. Bulk solids handling operations include storage, gravity flow, feeding and conveying. The industries such as mining, mineral processing and thermal power stations are heavily engaged in bulk solids handling operations. Thermal power stations handle millions of tons of coal annually through bins, hoppers, bunkers and silos, utilizing gravity force to cause flow of coal and it is very important that these handling systems be designed and operated with a view to achieving maximum efficiency and reliability.

The noise, which generally produced during silo emptying, is referred as ?Silo Honking'. This noise is connected to significant vibrations of silo walls. If a frequency of pulsation of granular fill during flow meets any one of natural vibrations of silo structure results in resonance. This resonance condition can be avoided if bunkers and silos are operated at appropriate frequencies.

As a part of their research work, the authors have estimated natural frequencies of circular and wedge shaped coal bunkers, in empty as well as different discharge conditions. Natural frequencies (transverse, longitudinal and torsion) corresponding mode shapes are found. ANSYS, the finite element analysis programme is used for modeling and analysis purpose. The frequencies obtained by varying constraints and coal-filling heights are analysed for circular and wedge bunkers. Typical graph shows comparison of natural frequencies between circular and wedge shape bunker is shown in fig.1 below.

The coal level in the bunker has great impact on the natural frequencies since the equivalent density changes. As the coal level decreases, the natural frequencies go on increasing. Bunker fixation (constraints) plays an important role in evaluating natural frequencies. These values increase with decreasing level of coal. Based on the results, authors conclude that resonance conditions can be avoided if bunkers are properly constrained to ensure that the natural frequencies of the overall structure must lie above the range of frequencies. And also felt that this analysis gives guidance to engineers for the design of a bunkers structure against dynamic loading conditions.