Pre-storage silos: How many and what size?
by PK Bhardwaj, Grain Storage, Handling and Processing Expert, Icepco Grainiks, India
Pre-storage is a set of single or multiple silos which are 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. However, I have seen many people who get totally confused while deciding on Pre-storage silos. Maximum people get confused concerning the quantity of Pre-storage silos and the capacity of Pre-storage silos.
Since there is not much literature readily available concerning Pre-storage silos, I thought to write this article to make decision making quick and easy.
Why do you need pre-storage silos?
First and foremost, pre-storage silos are required to create a buffer in-between the receiving pit and cleaning section. This buffer ensures that the conveying system, as well as the cleaning section, can receive 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 also saves a lot of electrical power.
What type of Pre-storage silo is best for me? Hopper bottom or flat bottom?
Preferably, Pre-storage silos should be of the hopper bottom type because hopper bottom silos are self-cleaning and do not require any manual intervention while unloading and cleaning like flat bottom silos typically do.
However, in case the receiving material exceeds 2,400 tonnes (i.e. 1200mt x 2) per day, one can also opt for flat bottom silos to save on capital cost. The only difficulty people face in a flat bottom silo is that 85 percent of its grains are unloaded through gravity but, for the remaining 15 percent, you will need to run a sweep auger and, once the silo is fully empty, you will need to clean it from inside before refilling. These activities are unproductive and time consuming.
How many minimum Pre-storage silos you need?
Ideally there should be at least two Pre-storage silos, so that the loading and unloading of grain does not happen simultaneously from one silo. While you are loading the first silo with grains, the other silo is in unloading mode. This ensures the ideal, first-in first-out sequence and increases the life of Pre-storage silos.
However, due to space constraints, if you have no other option but to go with only one silo then you must go for the hopper bottom silo only. Hopper bottom silos should essentially be interlocked with low level and high-level probes so that the silo is never too full or never too empty, as this may damage the silo.
Will Pre-storage silos affect grain quality tests?
There are certain quality tests that take a longer time as a result of using Pre-storage silos, such as for the detection of insect pests in a grain sample using Berlese apparatus. Berlese apparatus are very simple and effective equipment that have the capability to detect even larvae in a grain sample, but it takes approximately six hours to complete the test and get the results.
In case you often receive grains from a source prone to infestation and pest detection tests become essential, then you must have at least three pre-storage bins of sufficient capacity; wherein you can keep grains for six hours and, only after test confirmation that grains are not infected, should you transfer grains to the main storage silos.
To give an example of how to calculate size of Pre-storage silos required for a facility that receive grains continuously during harvest season at the rate of 100tph, one would use the following system:
- Tonnage at which grain is being received = 100tph
- Pest detection hours with Berlese apparatus = Six hours
- Capacity of additional silos = 100 x 6 = 600mt minimum; You may add 10 percent capacity be to be on the safer side.
You must have at least three Pre-storage silos to ensure your operation runs smoothly. One silo would be filling, the second on resting and the third being unloaded.
All the grains that fail the pest detection test either should be fumigated in the Pre-storage silo 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 main storage system as they may infest the conveying system and, thus, all the silos. As much as possible, no infested grain should be taken inside the silo facility.
Although three silos are sufficient to do the job, an additional pre-storage silo always helps as then one can decide whether to carry out fumigation in the pre-storage silo itself to avoid infesting the conveying system upstream into the main silo storage section.
Pre-storage silos and grain varieties
This topic can be complex as there could be many grain varieties being stored, some of which may not be received in equal quantities. For the sake of simplicity, you should have one Pre-storage silo for each variety of grain, so that each variety can be stored in a different Pre-storage bin. Each of these Pre-storage silos should, essentially, be equipped with a high-level and low-level probe to ensure minimum wear and tear of silo walls. Sizing of the Pre-storage bin shall be calculated as follows:
Suppose there are four varieties of grains named A, B, C & D.
- Maximum percentage arrival of each quantity in a day is A=60 percent, B=20 percent, C=10, D=10 percent respectively
- If maximum quantity of food grain received in a day = 1200mts then Pre-storage silo A=60 percent of 1200 = 720mt, Pre-storage silo B=20 percent of 1200 = 240mt, Pre-storage silo C=15 percent of 1200 = 180mt and Pre-storage silo D=5 percent of 1200 = 60mt.
Approximately 10 percent capacity should be added to each Pre-storage bin to arrive at final capacity. Each Pre-storage silo should be unloaded one by one and taken to the main storage silo designated for that variety. However, to handle multiple varieties with ease, the Pre-storage silo unloading rate should be at least one-and-a-half or double the rate of pre-storage silo filling. This means that if you are filling your pre-storage silos at the rate of 100tph then the unloading rate of pre-storage silos should be kept in between 150tph to 200tph. This will ensure that your Pre-storage silos are available for reuse in less time. This also provides operators with additional free time to carry out cleaning after each variety transfer and time to attend to potential minor breakdowns/malfunctions.
Pre-Storage silos and fumigation
Some operations demand that you must fumigate all the grains that you receive to ensure 100 percent 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 to be complete. Fumigation cycle time starts with silo sealing and then achieving the required levels of PPM fumigant gas, followed by releasing all the fumigants to achieve a safe concentration of fumigant gas. This complete process is one full fumigation cycle.
Once you know the fumigation cycle time and the number of days required, you can calculate the Pre-storage silos size, as shown in the example below:
- Grain receiving rate = 50tph
- No. of working hours in a day = 16 hours
- Fumigation cycle time = Seven days
- Pre-storage silo emptying time = One day ; (Any time less than 24 hours shall also be counted as one day)
- No. of Pre-storage silos = Fumigation cycle time + silo emptying time (= 7+1=8 silos)
- Size of Pre-storage silo = Grain receiving rate x No. of working hours (= 50tph x 16 hours= 800mt)