When the grain season comes to an end, millers have to face ultimate confrontation – storage and handling. Grain storage and handling facility includes grain receival, grain movement, grain cleaning, reclaim, storage and possibly drying and cleaning operations. This is a major investment and the whole system must be carefully planned. Storage facilities, handling systems, and atmospheric conditions can significantly affect the quality and value of the grain. Since grains are usually stored for a longer period, storage is a particularly delicate stage in grain production that requires adequate management. After all, grains are an extremely important part in human nutrition and when the production relocates from the beautiful golden fields to storage, there is no room for any mistakes.

In many countries, grain handlers, millers, and consolidators are switching from bagged storage to bulk storage of grains. Everyone has understood that bulk storage will make them more competitive as they save a lot of money as they no longer have to spend on gunny bag purchase and bag handling activities like bagging, bag sealing, bag stacking, stack breaking, bag loading, bag unloading, etc. Besides, condition monitoring of bulk grain storage is much easier than bagged grain storage due to the ease of installation of condition monitoring and controlling equipment like temperature monitoring, carbondioxide monitoring, humidity sensing, fumigation system, aeration and ventilation systems. Moreover, bulk grain logistics is much cheaper and faster than bagged grain logistics because it can be mechanized easily.

All storage systems must be designed to adequately protect and preserve the quality of the grain. Whole grain can sprout under certain conditions and will also attract moulds, insects and rodents. In addition, the storage of grain presents several safety issues. Grain storage systems come in a range of shapes and sizes; The design of a grain storage facility should be based on:

•Length of time for storage – temporary or long-term

•Degree of segregation of different types of grain

•Identity preservation requirements

•Expected useful life of the structure

In general, grain in long-term storage should be held cool and dry. Options include smooth wall steel silos, corrugated steel silos bins, concrete silos and underground pits. Steel silos are the most common method of long-term storage for grain. In order to maximize grain storage life and preserve their yield, millers choose to store their grains into silos. Silos are high-capacity commercial or industrial structures that are used for permanent grain storage.

 Grain storage in metallic grain silos is the most cost-effective, economical and long-term storage system. Grains in a silo can be monitored in real-time for its quality and quantity very efficiently. Spillage and grain spoilage are minimum in silo storage system. Other advantages of storing grains in a silo include high-speed handling of grains, effective fumigation, less dependence on labour and very less operation cost. 

Hence buying grain silos to store grains is a big financial and strategic decision and it must be taken with a full understanding of metallic silos and its accessories. While setting up specifications for grain silos the following points must always be considered:

•Flat bottom or hopper bottom silos

•Design considerations

•Corrugation profiles

•Design specification of steel sheets

•Material of construction of metal coated steel sheets

•Grade of steel and thickness of sheets

•Galvanization standards

•Fasteners

•Aeration and perforated floor

•Vents and roof exhausters

•Temperature monitoring system

•Fumigation system

 Modern grain storage or scientific storage means real-time monitoring of quality and quantity of grains. There are many methodologies of modern grain storage, and all have advantages and disadvantages. The concept of storing grains in silos is most acceptable for long-term grain storage as it is scientific and ensures rapid handling of grains.

Silos are not static; it is dynamic equipment and must be operated properly to maintain the grain quality inside the silo. Aeration, ventilation and fumigation should be sized properly so that the environment inside the silo can be controlled to maintain the grain quality.

Grains are hygroscopic in nature. Grains absorb or desorb moisture based on temperature and relative humidity of air surrounding them and continue to absorb or desorb moisture until it attains equilibrium with the surrounding air. This final moisture content of the grain is called Equilibrium Moisture Content (EMC). This EMC value is different for different grains based on RH and the Temperature of the surrounding air. Understanding the EMC concept of grains is very important to understand how grains will behave in various temperature and relative humidity conditions. In general, relative humidity of 65% or less is considered safe for the storage of grains. The storage period of grains depends upon the moisture content of grains. As a rule of thumb 9% to 13%, moisture content of grains may give a storage life of 8-11 months, however, there still may be some infestation problems, which have to be tackled with effective fumigation. When the grains are at 14% to 18% moisture level, the storage period is reduced to only 4-8 weeks due to problems of mould growth resulting in discoloration of grains. Below 9% moisture content, grains can be stored safely for more than one year, but it may result in loss of germination.

Temperature and relative humidity monitoring are the two very important aspects of grain management inside a silo. A rate of change of temperature of more than 5.5 Deg C per week indicates that there is spoilage inside the grain silo. The respiration rate of grains increases exponentially when grains start to spoil, and mould/insects develop inside the silo. An increase in respiration produces additional moisture, which further deteriorates the situation. Hence continuous monitoring of temperature and relative humidity inside the grain silo is very important to keep the grains safe.

Infestation inside grain silo is not something that people are not aware of. The infestation does happen inside the silo and Fumigation must be carried out to kill pests and insects etc. to save the grains inside the silo. Most insects enter grain silos from the top and stay there within 1-2 meters for weeks.

Fumigation is a most essential activity for the safe storage of grain inside the silo. A grain storage silo must have a properly designed fumigation system. Phosphine or Hydrogen Phosphide (pH3) is the most commonly used fumigant to kill pests in a grain silo. Phosphine or Hydrogen Phosphide (pH3) is available in solid form and emits phosphine gas. Phosphine is an inexpensive gas and not an ozone-depleting gas, hence considered environmentally safe. Pure phosphine is odorless however due to impurities it smells garlic-like or decomposing fish. Phosphine has a density of 1.45 Kg/m3 at 15oC. Phosphine gas may self-ignite at a concentration over 17900 ppm hence it must be handled carefully. Exposure to humans to more than 0.3 ppm is not at all advisable. Metals like copper, gold, silver, and brass must not be exposed to phosphine gas as it reacts with them.

Grains are live, they breathe in oxygen and breathe out Carbon dioxide. This makes the atmosphere inside grain silo rich in Carbon dioxide. Carbon dioxide is acidic and when it reacts with concrete, which is alkaline. This process is called carbonation of concrete and it destroys the ability of concrete to protect the reinforcing steel from corrosion. It is seen that 200 mm thick concrete is carbonated in 50-years time in such carbon dioxide rich atmosphere. Minor cracks in concrete further aggravate this carbonation problem in concrete. Pure carbon dioxide carbonates concrete to a depth of 20-30 mm in only 4-weeks time. Never use concrete with pulverized fuel ash (A power plant by-product) for grain silo storage or foundations. Concrete having pulverized fuel ash has very low resistance to carbonation, hence should never be used for grain silos.

Grain Cleaner is a very important equipment to clean the grain before storage (Pre-Cleaner) and before milling (Fine-Cleaner). When we pass the grains through Pre-Cleaner before transferring the grains to Storage Silos it helps in achieving the following benefits:

• Aeration costs reduce as grain dust and impurities tend to generate high-density areas which generate resistance to aeration flow.

• Reduces in-storage shrink loss of grains and grain damage also decreases due to infestation. Grain dust is highly susceptible to get infested.

• Storage at higher moisture content possible due to lesser dust particles which develops into mould growth and foul smell in high moisture grains.

• Cleaning Reduces transportation costs. 

• Cleaning grains makes the density inside the silo uniform which improves the effectiveness of fumigation.

• Reduce the requirement to turn-over the grains.

Flat bottom silos have a flat concrete base having a row of discharge openings along its diameter to reclaim grain from the silo. The grain from these discharge openings is received by grain unloading conveyors that are installed just below these discharge openings in a tunnel. 

A tunnel is an integral part of all flat bottom silos foundations. These tunnels are designed to house unloading gates & unloading conveyors. Besides, tunnels provide access to operate, maintain, and clean this equipment in safe manner. 

 Pre-Storage is a set of single or multiple silos which is located in between grain receiving pits and cleaning section of a grain storage facility. Pre-Storage silos bring efficiency to the storage system and make the operation trouble-free. The Rrequirement of the pre-storage silo is to create a buffer between receiving pit and the cleaning section. This buffer ensures that conveying system, as well as the cleaning section, can get continuous and uniform feed. Conveyors, elevators, cleaners and rotary sieves etc. perform at their highest efficiency when the feed is continuous and uniform. Besides, uniform and continuous feed saves us a lot of electric power too.

Preventing dust explosions, the grains have an inherent characteristic to generate dust whenever it is conveyed or transported from one point to another, and this grain dust is highly combustible in nature. It is even more combustible than coal dust. As per available data, dust explosions can occur where any grain dust cloud is available in high-enough concentrations in the atmosphere i.e., within lowest concentration limits of 0.050-0.100 kg/m3 and highest concentration limits of 2.0-3.0 kg/m3. Once this concentration condition is met with grain dust cloud can ignite at a temperature as low as 300 C.

Grain storage occupies a vital place in the organizations (whether it is small or big) of developed and developing countries. Well, the truth is that silos are the best solution for preserving grain quality for a longer period.