Grain intake methods by ship, rail, and road
April 01, 1999
by Teresa Acklin
By David Sugden
A typical grain intake flow from a ship, rail car or truck to a grain elevator or silo (see illustration on Page 47) involves first discharging the grain into a hopper that can hold up to 100 tonnes (the amount that most trucks or rail cars carry). To determine the tare and gross weights for purchasing requirements, the truck or rail car must first pass over a weighbridge before unloading.
As there is no such weighbridge for a ship, the grain is emptied from the vessel into the hopper by way of ship unloading equipment and conveyors, without these weight measurements.
From the hopper, the grain is moved by a band, screw or chain conveyor to a bucket elevator, which lifts the grain vertically and drops it into an in-line weigher. The weigher, which also is used for inventory or stock control of the grain elevator or silo, records the weight and compares it with the ship's documents or the rail car or truck weighbridge records.
The grain then passes through a magnet and preliminary cleaning intake separator to remove any coarse rubble (metal, straw, wood, paper or any matter larger than the wheat). This machine is often a drum separator or, in the case of this illustration, a vibratory or semi-rotary sieve in the horizontal plane.
Aspiration takes place at the tail end of the intake separator via a dust collector and fan. The dust is mixed with the dockage or screenings from the mill for sale in millfeed. The exhaust system also draws air by suction from every other machine, each bin and all conveyors in order to maintain a dust-free atmosphere, to aid hygiene and to minimize the risk of dust explosion.
The grain is then conveyed, elevated and distributed to the main silo or elevator bins via preselected slides. Each bin has a continuous level indicator for remote monitoring by the operator.
The entire process is run by a programmable logic controller (PLC), with minimal manpower. Indeed, the operator will have a number of other jobs in the process.
It is important to keep equipment in good running order. Follow the equipment suppliers' operating and maintenance manual recommendations and make daily inspections of the magnet and sieve screens.
Dust explosion is a risk in any elevator or silo, especially when static electricity and dry humidity combine. Overload and malfunction instruments will stop the intake process. Other safety features include elevator belt alignment and underspeed rotary detectors. Explosion relief panels on bin tops, bucket elevator legs and dust collectors are required in some jurisdictions and by some insurance companies.
Grain intake methods.
Some grain intake methods used worldwide include a grab by crane, a vertical then horizontal screw from the hold, chain Z conveyor, bucket elevator or pneumatic discharge. Pneumatic discharge is known to be the cleanest, most dust-free method, and while it is the most efficient in terms of average tonnage throughput, it also is the most expensive in running costs.
Silencers can be incorporated. Variations on most methods include floating systems, such as from ship-to-ship or ship-to-shore; mobile to access different quays and vessel sizes; and fixed installations on rails discharging to permanent gantry conveyors.
In the hold of the larger ships or barges it is common to find a “bobcat” a small tractor with a hydraulic shovel attached to clean up the remaining grain at the bottom.
The bucket elevator method is particularly power efficient, combining high throughput and low power consumption for the first three-quarters of a cargo. The bucket elevator also is used in tandem with a pneumatic final clean up.
Pneumatic discharge comes in two types: by use of a suction fan or with a Roots blower but in suction mode. The latter is thought to be about 20% more efficient in both power and average tonnage moved. Energy consumption is about 0.6 kilowatts per metric tonne shifted, depending on the distance involved.
Pneumatic ship intake plants can cost up to U.S.$2 million for 600 tonnes per hour, nominal capacity, depending on the complexity. Discharge capacities range from 100 tph or less up to 2,000 tph for heavy port installations.
Ship or barge sizes range from a few hundred tonnes up to Panamax vessels of 40,000 dwt or Cape vessels at 170,000 dwt with a draught of up to 59 feet.
Rail cars typically carry around 100 tonnes of grain. Modern grain rail cars are equipped with internal hoppers to ensure full, automatic and central discharge. The more sophisticated rail sidings shunt rail cars individually onto a railed weighbridge to gross and tare each boxcar for recording purposes.
Some rail cars without hoppers are still in use, shored up with disposable wood at the point of exit. At discharge the disposable wood “barrier” is broken and the entire boxcar is hydraulically tipped sideways to pour the grain into a sufficiently large hopper.
The benefit of grain intake by road (trucks) is the flexibility it provides both the supplier and the miller. Trucks used to transport grain come in all shapes and sizes, and several discharge variations are available. Some truck trailers have central openings, much like rail cars with hoppers. In others, the grain is discharged out the end by screw augers. Another method is to hydraulically raise the trailer on one end, allowing the grain to flow out the other end. In some cases, the truck is driven onto a fixed tipping platform, which obviates the need for the truck to have its own tipping gear.
Weighbridges are common in the rail or road intake methods for both gross and tare measurement.
Other intake benefits.
Many mills incorporate automatic sampling and laboratory testing at the grain intake point. After testing the grain, the laboratory issues a bar-coded card that is used to select and activate designated bins and routes according to any measured qualities of wheat.
Wheat can be stored for periods of several months if it is dry enough (14% moisture or, preferably, less). Wet harvests will produce wheat of up to 20% moisture, which is unacceptable if the wheat is to be kept fresh for any length of time. As a result, many mills install grain dryers or bin-heated grain can result.
Bin-heated grain can be recognized by blackened or darkened endosperm and heat-damaged devitalized gluten in the case of wheat. The grain is unsuitable for any use other than feed, and even then palatability may have been seriously impaired. Bin temperature monitors installed at close intervals are an excellent way to detect any rise in temperature of the stored grain. In this case, grain must be turned over.
Grain refrigeration and aeration are practiced in many geographic areas to keep grain fresh and to avoid hot spots. Refrigeration takes the form of a fixed or mobile refrigerator unit, which reduces the air temperature as it is forced into the base of the bin or store. Aeration follows the same principle but without refrigeration.
In dry climates or in locations that mill dry wheat, the grain can be dampened on a preliminary basis in the elevator. In particular is wheat of 11% moisture content or less. The benefit of dampening is to obviate the need for a double damping before milling in the tempering or conditioning run proper.
In designing any intake system, a “weather eye” is required on likely future trends of suppliers, be they direct from the farm or from sources such as cooperatives and traders. In particular, demurrage is increasingly payable on slow discharge at the point of intake.
Reception facilities of enough capacity to mitigate this factor are needed. That means not only the conveying capacity of grain but also the speed of laboratory testing while retaining accuracy. That, of course, is all very well in a case where the elevator is already full. Then there is a problem.
The longest running and most common difficulty because of its commercial implications is the issue of weight reconciliation between buyer and seller. If the seller's weight is final, the buyer may feel out of pocket if the measurement shows slightly short weight, and vice versa. Even 0.25% of 100,000 tonnes over time is 250 tonnes. It pays to watch and weigh carefully.