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 makes the operation trouble free. However, I have seen many people who get totally confused while deciding on Pre-Storage Silos. Maximum people get confused on the quantity of Pre-Storage Silos and the Size of Pre-Storage silos.
Since you do not have much literature about Pre-Storage Silos, hence I thought to write the article so as to make their decision making quick and easy.
Why do they need pre-storage silos?
First and foremost, requirement of pre-storage silo is to create a buffer in between receiving pit and cleaning section. This buffer ensures that conveying system as well as 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.
What type of Pre-Storage Silo? Hopper Bottom or Flat Bottom
Preferably Pre-storage silos should be Hopper Bottom Silos because hopper bottom silos are self-cleaning types and does not require any manual intervention while unloading and cleaning thereafter like Flat Bottom Silo.
However, in case the receiving is more than 2400 tons per day, one can go for flat bottom silo also to save on capital cost. The only difficulty people face in a Flat bottom silo is that 85% of its grains gets unloaded through gravity but for remaining 15% you need to run sweep auger and once the silo is fully empty you need to clean it from inside before refilling. These activities sometimes consumes a lot of time.
How many minimum Pre-storage silos you need?
Ideally there should be at least two Pre-Storage Silos, so that loading and unloading of grain does not happen simultaneously from one silo. While you are loading first silo with grains, the other silo is in unloading mode. This ensures ideal first in first out sequence and increases the life of Pre-Storage Silos.
However, due to space constraint or any other constraint if you have no other option but to go with only one silo then you must go for Hopper Bottom silo only. This hopper bottom silo should essentially be interlocked with Low Level and High-Level probes so that Silo is never too full or never too empty as it may damage the silo.
What all Grain Quality Tests influence Pre-Storage Silo quantity?
There are certain quality tests that take a longer time like detection of insect pests in a grain sample with Berlese Apparatus. Berlese Apparatus is a very simple and effective equipment that has capability to detect even larvae in a grain sample, but it takes approximately 6 hours to get the result. In case you often receive grains from a source which is prone to infestation and Pest Detection test becomes essential then you must have at least 3 pre-storage bins of sufficient capacity, wherein you can keep grains for 6 hours and only after test confirmation that grains are not infected you should transfer the grains to main storage silos.
Let me give you an example to make you understand how to calculate size of Pre-Storage Silos required for a facility that receive grains continuously during harvest season at the rate of 100 TPH;
Tonnage at which grain is being received = 100 TPH
Pest Detection Hours with Berlese Apparatus = 6 Hours
Capacity of Additional Silos = 100 x 6 = 600 MT minimum; You may add 10% capacity be to be on the safer side.
You must have at least 3 Pre-Storage Silos to make your operation smooth. One Silo would be on Filling mode, second on resting mode and third on unloading mode as shown in the table below.
All the grains that fails Pest Detection Test either should be fumigated in the Pre-Storage silos itself or should be unloaded from the Pre-Storage silo and taken to a separate fumigation chamber for fumigation.
Infested grains should never be taken into the system as it may infests the conveying system and thus all the silos. As far as possible, no infested grain should be taken inside the silo facility.
Although three Silos are sufficient to do the job but an additional pre-storage silo always helps as then you can take a decision to carry out fumigation in the pre-storage silo itself to avoid infesting the conveying system upstream to main silo storage section.
How grain varieties influence Pre-Storage Silo decision
This is a complex case as there could be many grain varieties and all of them may not be received in equal quantities. Some varieties may be received in large quantities and some in a very small quantity.
For the sake of simplicity as a rule of thumb you should have one pre-storage silo for each variety so that each variety can be stored in a different pre-storage bin. Each of these pre-storage silos should essentially be equipped with high level and low-level probe to ensure minimum wear and tear of silo walls. Sizing of pre-storage bin shall be calculated as follows.
Suppose there are 4 varieties of grains named A, B, C & D
Maximum Percentage Arrival of each quantity in a day is
A = 60%, B=20%, C=10, D=10% respectively
If Maximum quantity of foodgrain received in a day = 1200 MTs then,
Pre-Storage Silo A = 60% of 1200 = 720 MT
Pre-Storage Silo B = 20% of 1200 = 240 MT
Pre-Storage Silo C = 15% of 1200 = 180 MT
Pre-Storage Silo D = 5% of 1200 = 60 MT
Approximately 10% capacity preferably be added to each pre-storage bin to freeze as final Pre-Storage Silo capacity.
Each pre-storage silo should be unloaded one by one and taken to main storage silo designated for that variety.
However, to handle multiple varieties with ease Pre-Storage Silo unloading rate should be double the rate of pre-storage silo filling. This means that if you are filling your pre-storage silos at the rate of 100 TPH then the unloading rate of pre-storage silo should be 200 TPH. This will ensure that your -pre-storage silos are available for reuse in very less time.
How Fumigation influence Pre-Storage silo decision
Some operations demand that you must fumigate all the grains that you receive to ensure 100% pest free grains. In such cases the quantity of pre-storage silos depends on the cycle time of fumigation. The cycle time of fumigation means that the numbers of days required for fumigation activity to get over. Fumigation activity starts with silo sealing and then achieving required PPM of fumigant gas and then releasing all the fumigant to atmosphere to achieve safe concentration of Fumigant Gas. This complete process means one fumigation cycle.
Once you know the fumigation cycle time in number of days you can calculate the pre-storage silos size and its quantity as shown in the example below;
Grain receiving rate = 50 TPH
Nos of working hours in a day = 16 Hours
Fumigation Cycle Time = 7 Days
Pre-Storage silo Emptying time days = 1 days ; Time less than 24 hours shall also be counted as 1 Day
Nos of Pre-Storage Silos = Fumigation Cycle Time + Silo Emptying Time
= 7 + 1 = 8 Nos of Silos
Size of Pre-Storage Silo = Grain Receiving Rate x Nos of Working Hours
= 50 TPH x 16 Hours
= 800 MT
I think I have covered all the scenarios. In case you think that some of the scenarios are not talked about please do let me know. I would try to include those options too in this document.
I am sure now you will not face much difficulty in finalizing your pre-storage silo section.