High Inflamability: Safety in Grain Dryer Handling

In the grain industry, dryers (mostly stationary) with different drying capacities are used to reduce the moisture content of the goods to be stored (e.g. maize, wheat, rape) by introducing warm or hot air. This process is necessary because moisture in organic substances promotes biological activity during storage and thus reduces the maximum possible storage time and may also cause fermentation or spontaneous combustion in silo systems and piles.

Every grain dryer basically consists of¹:
– a column with a variable number of drying and cooling elements
– supply and exhaust air ducting
– air heater
– air technology
– control and regulation technology

1 _ Frank Hauert and Siegfried Radandt: Brand- und Explosionsschutz in Mühle, Mischfutterwerk und Landhandel (Fire and Explosion Protection in Grain Mills, Compound Feed Plants and Agricultural Trade); AgriMedia GmbH, ISBN 978-3-86037-325-5

In general, the entire drying plant is sucked over by one (or more) exhaust fans.

The material to be dried is transported into the storage container above the dryer column via the wet material elevator. From there, it trickles gravity-induced into the lower part of the column, from where it is discharged. The dryer is filled and emptied discontinuously at regular intervals, depending on the characteristics of the material to be dried.

Air is sucked through the dryer column perpendicularly to the direction the material movement (from the hot air hood into the exhaust air hood) by means of the exhaust air fan.

More or less regularly, so-called dryer fires or fires of the material to be dried occur during these drying processes, in the course of which the entire dryer is damaged. Some estimates say that there are about 100 dryer fires per year in Germany¹.

The high replacement value of such grain dryers and the business interruption losses resulting from such fires cause the insurance industry high annual costs. Furthermore, fires in drying plants cause damage to the storage facilities, as these usually have acceptance contracts with farmers who deliver the goods to them during the campaign, but due to the lack of drying (or at least reduced drying capacity in the case of redundantly arranged drying plants) further processing cannot take place or can only take place to a limited extent.

In case of a dryer fire, there is actually no possibility for the operating personnel and the alarmed fire brigade to reach the origin of the fire and thus prevent its spread within the dry material. This is due to the closed design of the conventional grain dryers. Furthermore, there is currently only a limited possibility for the operating personnel to detect a starting fire, as there are no adequate facilities available for early fire detection.

In most cases, thus, a total loss of the grain dryer occurs.

Fig. 1: Apparently “minor” thermal damage on the outside

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 2: View of the drying column from the exhaust air ducting

Fig. 3: View into the burnt-out drying column = total loss

For this reason, the R+V‘s risk engineering department has been thoroughly investigating dryer fires for many years and has identified the following protective measures for safer operation of grain drying plants:

Location of the Drying Plant

For new plants, an outdoor installation site with sufficient safety distance to existing buildings (5 m minimum) should always be chosen. If the exterior wall of an existing building is flammable (e.g. wood), the safety distance should be increased to at least 20 m due to the heat radiation to be anticipated.

“Clean” Fresh Air Intake

It is of paramount importance to ensure the intake of clean fresh air, taking into account wind drifts and turbulence caused by delivery vehicles. If necessary, the fresh air intake must be modified.

Fig. 4: Deposits to be avoided in front of the suction intake

Catwalks in the Supply and Exhaust Hoods

In order to make it safe to reach the cascades (also with regard to industrial safety), catwalks (gratings) must be provided in the supply and the exhaust air hood of the dryer.

Fig. 5: External access ladder

Fig. 6: Access ladder and grids within the exhaust air hood

Cleaning of the Material to be dried

In order to avoid a restriction of the product flow in the drying column due to impurities (e.g. stalks or cobs) and the dropping out of inflammable trimming (in particular cuticles in maize drying), the material to be dried must be pre-cleaned.

Dust Trap Mats and Spark Arrester

To prevent flammable materials from being sucked in via the outside air intake, dust trap mats must be installed or spark traps must be installed behind the burner.

Fig. 7: Spark arrester installed in the supply air shaft

Fire Detection Device

In general, classical fire detection devices and those offered by the dryer manufacturers are based on temperature measurement. However, the detection time is strongly dependent on the distance between the fire area and the temperature sensor and on the fire intensity. In order to detect a fire even earlier and independent of the location of the fire, successful (fire-) tests for fire gas detection were carried out with the company GTE Industrieelektronik GmbH. As soon as the technology is available on the market, the installation of the same should be preferred.

Stationary Extinguishing System

A stationary extinguishing pipe must be provided for supply by the fire brigade in the event of a fire (riser pipe with at least 4 extinguishing nozzles in the roof of the drying column (or perforated pipe) and “B”  connection in the floor area).

Fig. 8: “B” -connection on the grain dryer

Fig. 9: Fire-fighting water input/distribution into the drying column

Fig. 10: Extinguishing water distribution before installation in the dryer head

The protection objective of this extinguishing device is the thermal protection of the dryer construction (cooling) and the prevention of the spread of fire within the bulk material.

Emergency discharge

An emergency discharge device must be provided to enable the drying column to be emptied into a safe area in the event of a fire.

Fig. 11: Emergency discharge device

Supervision during Operation

A permanently supervised operation is required in order to be able to detect symptoms of a fire, such as the escape of product, smell of fire or malfunction messages, in a timely manner and to initiate countermeasures.

Adherence to Inspection and Cleaning Intervals

Combustion Plant

Combustion plants are subject to wear. This is particularly the case with drying plants, as some of these are only in operation for a few weeks per year and are exposed to the effects of the weather during the remainder of the year. In order to rule out technical defects in the burner, the furnace equipment must be maintained by a specialist company before the start of the drying campaign (including checking the heat exchanger for defects) and regularly checked for irregularities by employees during the drying process.

Cleanliness

The cleaning intervals specified by the manufacturer must be observed before, during and after the drying campaign, especially when changing products.

Instruction of Employees

Employees who operate the system or are present on the premises during drying must be instructed on possible risks and how to behave in the event of a fire. They need to have sufficient knowledge of the operation of the plant and the function/commissioning of existing protective devices.

Explosion Protection:

Some literature sources indicate that there is a risk that dust explosion may occur in grain drying plants and their components due to turbulence and deposition of grain dust. However, investigations of the damage to drying plants show that no explosions have occurred as a result of or as a cause of dryer fires, or that such events have been documented  (retrospective consideration).

Spark and Hot Spot Detection

In order to prevent the spread of hot spots into potentially explosive storage areas, spark/hot spot detection must be provided in the discharge area with automatic shutdown of the conveyor system. This measure does not primarily serve the fire and explosion protection of the drying plant, but the downstream plant components.

Outlook

Due to the increasing performance of harvesting machines and the rising efficiency of land use (yield/hectare), ever greater drying requirements can be expected in ever shorter time.

An expected further increase in energy costs will lead to a centralisation of drying capacities to fewer and fewer, but larger and more efficient plants. Due to this trend, it is foreseeable that if one plant fails, ever larger quantities of drying material will have to be distributed to other plants. Since harvest times coincide regionally, it can be assumed that drying plants located in the immediate vicinity are working at full capacity and therefore transport to plants located further away is necessary.

For reasons of property value protection, it is therefore to be expected that the cost of a claim caused by a single fire, with a concomitant interruption of operations will increase in the future due to rising values of the individual plant, rising raw material prices and additional transport costs.

Accordingly, there is no prospect of a risk reduction without implementation of the measures listed above.

The author:

 

Torge Brüning
Dipl.-Ing. Sicherheitstechnik
Brand- und Explosionsschutz