March 25, 2015

25/032015: MOISTURE CONTROL IN STORAGE

http://issuu.com/gfmt/docs/mag1502_w1/40
by Louise Smith and Nick Carter, Bentall Rowlands, UK

First published in Milling and Grain, February 2015


Bentall Rowlands Storage Systems Limited is a leading UK manufacturer in complete storage and processing equipment solutions for the agricultural and industrial markets.

We offer a wide range of galvanised steel silos and hoppers, water tanks, catwalks and platforms, material handling equipment, cleaning and grading and weighing and drying systems that are assembled worldwide.

 With the capabilities to design, manufacture, supply, and install storage systems from an extensive range of products, we provide a comprehensive end-to-end solution, which can be designed to any specific clients’ requirements.

We have designed and installed silos worldwide in countries that include the UK, Kenya, Thailand, Holland, France, Germany, Ukraine, Malawi, New Zealand and many more.

Kevin Groom, Technical Director says, “Our storage systems are individually designed for all clients. Each project has a bespoke design that is sure to match, if not exceed clients’ expectations. We are extremely proud of the projects that we have undertaken in these geographically challenged areas, proving that whatever the specification, we are sure to provide the most suitable design necessary.”

Moisture levels and consequences
Getting the moisture levels right in a silo can be challenging but it is essential that the target level is reached within the shortest possible time. If this does not occur, the results would be the formation of mycotoxin and quality degradation. The main causes of spoilage in stored grain are fungi, insects and mites.

Fungi is one of the main consequences of a variety of different moisture contents and temperatures stored in grain.  In order to control this, a principal method known as drying and cooling needs to be put in place.  No storage fungi will grow below a moisture content of 14.5 percent but they do continue to grow slowly at near 0°C. This means that cooling alone is not sufficient but the lower the temperature, the slower the rate of growth.

Another nuisance is storage mites which breed rapidly under favourable conditions and will cause direct damage to the grain by hollowing out oilseeds or eating the germ. Physical control methods are used for mites. If the grain is dried to 14.5 percent moisture content then the mites are unable to breed. If you cool the grain to 5°C, this can also help to prevent the build up of them. However, if you are storing oilseed rape, this is less susceptible to insect attack than cereals.
This will protect the grain bulk, but during winter, the moisture content on the surface of the grain may increase, meaning that mites can become a problem in the surface layer.
 
A final problem relating to moisture control is insect presence and infestation problems. These can occur where bad hygiene is present. Good store hygiene is therefore an important first step in eliminating these pests. Both the building structure and the stored grain should be monitored using traps. Traps within the grain bulk should be positioned approximately 5 – 10cm below the surface to monitor any insect species with different behaviours.
  

http://issuu.com/gfmt/docs/mag1502_w1/40
Stores should be thoroughly cleaned prior to the intake of product. It is extremely important for eliminating any sources of contamination from storage fungi, insects and mites. Store preparation is a key stage in ensuring the safe storage of grain. Whether the grain is being stored temporarily, or for a longer period of time, this is a necessary step that needs to be followed. Good store preparation needs to work in conjunction with obtaining and maintaining the target temperature and moisture content.  This will ensure the safe storage of grain.

There are a number of key features of a good grain store, including:
•    Clean
•    Dry
•    Well ventilated
•    Correctly functioning equipment
•    Proofed against rodent and bird entry
•    Watertight roof
•    No physical contaminants
•    Secure

How best to store your grain
A steel grain storage silo is a fully bolted vessel and while not being airtight they are water-tight. On all the joints, sidewall and roof, a sealing mastic is used to prevent against the ingress of water. The roof sheets overhang the eaves to ensure snow and rain cannot gain access. At the peak of the silo the roof sheets fit under the collar or petal and are sealed with blanking plates. As a manufacturer of silos we will give advice on how to seal the silo at base level.
 All of these design features, tried and tested, over many years of product development are in place to stop the external moisture from reaching the grain.  The level of moisture and temperature of grain in a storage silo comes from good housekeeping. It is very important that the operators of storage systems, both on-farm and industrial stores understand the levels required to maintain the quality of the grain being stored.

There are a number of good technical papers available and it is good working practice to re-view.  As the grain comes into the system it is important to know the level of moisture. From this the operator will know if the grain will require drying. There are many forms of grain dryers such as in-bin systems or continues mixed flow. The in-bin systems tend to use gas as a fuel and can be limited on the hourly capacity whereas the mixed flow dryer can run on gas, fuel oil and solid fuels.
 
Different types of fungi live at different moisture contents and temperatures in stored grain; Storage fungi can grow on cereals from about 14.5 percent moisture content upwards and may cause heating and loss of germination. Once the grain enters the storage silo or flat floor storage system it is important that the checking of grain does not stop. Most modern silos are supplied with ventilation systems. The concept of these systems is very simple and has been used for thousands of years.

 By passing air through grain it is possible to not only reduce the temperature of grain but also to reduce the moisture content. There are two main types of ventilation systems in silos: either a trench system or full floor. These systems allow low volumes of air to be pushed into the silo with a ventilation fan either Axial or Centrifugal.

The fans are connected to either the silo base for a trench system or to the silo sidewall for a full floor system. The pressurised air then moves up through the grain and thus lowers the temperature of the grain. This action will also cool air dry the grain and lowers its moisture content. Within the silo it is possible to have a number of temperature monitoring cables. These cables have a series of sensors which will measure the temperature of the grain in a given area. The system will allow the operator to see what is happening within the silo.
As the air moves through the grain it will evaporate water from the grain, helping to reduce the moisture content of the grain.

The moisture, which has been absorbed by the air, then passes into the open roof area of the silo. It is important with silos to ensure that there is good free air movement around this area. This will allow the moisture-laden air to simply vent to the atmosphere. The design of the roof vent is very important. Not only should it allow good airflow but must stop birds, rodents, snow and rain getting in. As you can see from the photograph this vent is designed for free movement of air but by being triangular it prevents rubbish collecting around its face. This is a common problem with roof vents and you can see areas of rust building up in this area.
 
http://issuu.com/gfmt/docs/mag1502_w1/40
Another way to ensure good airflow around the internal open area is to use roof exhaust fans. These are used to equalise the temperature of the air within the internal area and atmosphere. By using the design shown in the photograph they can easily be reached for maintenance or to be closed when using a fumigation system.

On our range of silos we use a dimpled eave-retaining clip. This clip gives a 2mm gap between the roof sheets and the sidewall sheets. Tucked well under the eaves it is designed not only to help with air movement around the internal area but also to allow any beads of condensation which may have formed on the inside of the roof structure to simply run off.  

www.bentallrowlands.com

Read the magazine HERE.
 

The Global Miller
This blog is maintained by The Global Miller staff and is supported by the magazine GFMT
which is published by Perendale Publishers Limited.


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