Grain Storage in metallic grain silos is most cost effective, economical and long-term storage system. Grains in silo can be monitored real time for its quality and quantity very efficiently. Spillage and grain spoilage is minimum is Silo Storage system. Other advantage of storing grains in silo includes high speed handling of grains, effective fumigation, less dependence on labor and very less operation cost. Hence buying Grain Silos to store grains is a big financial and strategic decision and its must taken with full understanding of metallic silos and its accessories. While setting up specifications for Grain Silos the following points must always be considered.
Flat Bottom Vs Hopper Bottom Types
This decision is very important considering the fact that Flat Bottom Silos always work on “First in Last Out” principle. In case you are looking for Fist in First Out kind of operation then Hopper Bottom Silos are only choice. Flat Bottom Silos are good for large storages from 500MT to 20000 MT of grain in one silo. Bottom Hoppers are generally small silos and maximum capacity may go to 1500 MT only, however more 1000 MT capacities in bottom hopper silos are not economical.
Further, wherever a constant feed is required, Hopper Bottom Silos are preferred as Mill Feeding Silo or Process Silo because in Hopper Bottom Silos 100% grain flows through the discharge gate by gravity. In Flat Bottom Silos only 85% flow is gravity flow and rest is by Sweep Auger.
Always specify the bulk density and Angle of Repose of grain in Stock pile condition of the grain which is to be stored inside a silo. Maximum Wind Velocity and Seismic zone category are other most important design criteria and must always be specified before buying any silo. Maximum and Minimum temperature difference must also be considered as it helps designing the ventilation system better. Improper ventilation in silo results in condensation problems in side silo.
GI Corrugated Sidewall sheets are generally available in 2.66 “ and 4” corrugation sizes. This aspect does not matter much as each company has its own design. Only thing that needs to be understood here is that 4” corrugation is much flatter from inside than 2.66” corrugation sizes.
Design Specifications of Steel Sheets
Specifications of Steel Sheets are the most important criteria as cost of silo is majorly dependent on this aspect. The most popular steel grades are ASTM A-653 – Grade 50 and ASTM A-653 – Grade 40.
ASTM A-653-Grade 50 has Yield strength 50000 psi, 340 Mpa Tensile Strength – 65000 psi, whereas
ASTM A-653-Grade 40 has Yield Strength 40000 psi, 275 Mpa, and Tensile Strength 55000 psi.
Material of Construction of Metal Coated Steel Sheets
Various types of Metal Coated Steel sheets are used to fabricate grain silos. Most popular among them is GI, Galfan, Magnelis and Zincalume. While selecting the material of metal-coated steel sheet one must consider the following parameters
· Corrosion Resistant Properties of Metallic Sheets – Salt Spray Test, which is the accelerated corrosion test, is most acceptable criteria to test corrosion resistant of metal-coated steel sheets. It is measured in numbers of hours a steel sheet takes to rust by 5%. More the number of hours mean better the corrosion resistance property. For Example a GI Sheet of G115 Grade can withstand Salt Spray Test of 500 Hrs and Zincalume Steel Sheet of AZ150 grade can withstand salt spray test of 2000 Hrs
· Solar Heat Reflectivity of Metallic Sheets – This is a also very important criteria one must look for. More solar heat reflectivity means less heat absorption and hence cooler grains. Cooler grains mean, lesser Aeration and fumigation requirements thus overall saving on power cost and fumigation cost. Besides cooler grains have much lesser problems of quality deterioration and discolouration. Zincalume Steel absorbs only 4-5% heat as compared to 22-26% heat absorption by GI Steel Silos.
Grade of Steel and Thickness of Sheets in gauge from bottom to top must be verified while evaluating the quotations from various vendors. Also, after the purchase, one must verify if he gets what he has specified. I would recommend Shriram Institute of Research for sample analysis.
Galvanisation Standards of GI Steel as per ASTM Standards are as follows;
G60, G90,G115,G140,G165,G185, G210 etc. This refers to Galvanisation in oz/ft2 which means G115 means 115/100=1.15 oz/ft2
Similarly in metric system Z180, Z275, Z350, Z425, Z500, Z565.Z650 In this case last three digits refers to galvanization in gms/m2. It means Z350 means 350 gms/m2.
For Indian atmospheric G165 or Z500 is more than enough except in some coastal area like where you may go in for G210.
In case you prefer lower galvanisation standard you feel that galvanization is giving up. You may paint the silo with White food grade paint. Rusted Silos may collapse any time and must not be used unless rested for leftover thickness of corroded sheets. Once Silo starts rusting, the rust propagates very fast and reduces the strength of silo thereby reducing its storage capacity.
In Zincalume Silos, AZ150 grade is more than suitable as it can last more than 35 years. AZ200 is also available and it will give more life.
Fasteners are very important part of any silo. One must not go for Nuts and Bolts of grade lesser than 8.2. All fasteners should be able to withstand at least 72 hours of salt spray test.
Aeration and Perforated Floor
Aeration Fans are also one of the most important parts of any silo as it helps in quality management of grain inside silos. It removes hot spots, decrease mould growth, decreases foul smell and more importantly keeps the grain temperature cool and uniform.
Air Flow of Aeration Fans must be selected based on silo volume and type of grain being stored. Aeration Fan must have static pressure enough to overcome resistance offered by grain depth. Perforated floor inside must be strong enough to withstand the grain load in long run.
Vents and Roof Exhausters
Vents and Roof Exhausters must be sized according to Air Flow of all the aeration fans combined together. As per research it must be 2.5 times the flow of all aeration fans put together. Roof exhausters must at least have exhaust flow sufficient enough to expel humid air out of the silo so that condensation does not happen inside the silo and they must be mounted as close to silo inlet as possible practically.
Temperature Monitoring System
Temperature Monitoring Cables are there to monitor temperature inside the silo. Some cables are thermocouple based and some are thermistor based. Both have their own advantages. Thermocouples ones are robust and Thermistor ones are maintenance friendly. Once you select the cables insist on 1 card per cable as than you can immediately diagnose which cable is not performing and maintenance would be easy.
Moreover, Temperature Cable strength must be decided based on Grain depth. Generally Temperature cables are available to withstand tensile load of 3000Kgf to 5000Kgf.
Silos Must Maintain Grain Quality
Silos are not static; it is dynamic equipment and must be operated properly to maintain the grain quality inside silo. Aeration, Ventilation and Fumigation should be sized properly so that the environment inside the silo can be controlled to maintain the grain quality.