Feed Ops: Interlocks, sensors and automation

by Fred Fairchild
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Feed Ops
Figure 1: Small grain elevator process flow diagram. 
To view the full diagram, click here.
Every processing plant or system has a path that materials follow to make a final product. This is commonly called process flow. In complicated processing systems, it is usually shown in a drawing or document called a process flow diagram. A process flow diagram for a small, simple grain elevator is shown in Figure 1 above. I will base most of the article on the features of this diagram.

 

Receiving

The diagram indicates that the grain elevator has two truck dumping pits, but both feed the same receiving conveyor. The maximum capacity this conveyor can handle is 15,000 bushels per hour (bph) as it feeds a bucket elevator that has a capacity of 15,000 bph. It is unlikely that both truck pits can be emptied at the same time unless they have gates on the receiving hoppers outlets that may be positioned to control flow rates from the two pits so that not more than 15,000 bph is delivered to the receiving conveyor.

The bucket elevator elevates the received grain and delivers it through an electric distributor to one of three destinations: conveyors to the two large grain storage bins or spouted directly to the loadout bin for truck shipment. Each of these bins is equipped with a high-level sensor that indicates the bin is nearly full. The level switch should close gates or shut down the grain source, but leave adequate room in the bin for all the equipment feeding the bin to clear out the receiving system so that the receiving system is cleared and not forced to restart under a loaded condition.

Equipment forced to restart under load (full) normally require higher horsepower than when operating under normal conditions. Starting under load can also cause damage to conveying equipment. The receiving system equipment should be electrically interlocked so that if any motor driven equipment stops, the rest of the system preceding the stopped motor will be shut down so grain is not feeding the stopped piece of equipment. The system equipment also should be interlocked so that each succeeding piece of equipment must be running before each preceding piece of equipment can be started.

Internal transferring

The elevator is also able to transfer grain between the two large grain bins or turn (recycle) the grain in one of the bins. In either case, a maximum of 15,000 bph may be handled using the reclaim conveyor under the bin being emptied or recycled because, again, the maximum capacity is controlled by the bucket elevator. Control of the grain flow out of a storage bin is controlled by the center electric unloading gate setting. This gate is used whether transferring or turning the grain in the bin until the bin is fully emptied or inverted forming grain left in the bin sloping from none at the center discharge to a point on the bin wall, the height of which is determined by the angle of repose of the grain. As an example, grain with an angle of repose of 28 degrees is being unloaded from a 60-foot diameter bin. When the grain height is zero at the center, the grain at the bin wall is 16 feet high.

After all possible grain is emptied by gravity through the center gate, the sweep auger is activated and used to sweep the remaining grain to the center outlet. With the inverted grain pile in the bin, the sweep auger is buried in the remaining grain. The sweep auger is positioned over the manual gates so that the grain covering the auger may be removed so the sweep auger can be activated.

When transferring grain to a similar sized bin or recycling an existing bin, the high level indicators may be activated. But since the grain is coming from a source that is of the same size, the high-level indicators may be overridden.

The maximum capacity for the transfer system is limited to 15,000 bph. This is controlled by the capacity of the bucket elevator and the top filling conveyors in the system.

Loadout system

The loadout system for the process flow diagram has a maximum capacity of 15,000 bph. This maximum is controlled by the capacity of the bucket elevator. The reclaim conveyors collect the grain from one or both of the large grain bins and deliver it to the bucket elevator, but the total delivery rate from one or both reclaim conveyors operating cannot exceed 15,000 bph. The discharge rate from each of the bins is again controlled by the electric outlet gate at the center of the bin.

The loadout system is for trucks only with the grain delivered to a 3,000-bushel overhead bin above one of the receiving pits. At full capacity, the bucket elevator takes only 12 minutes to fill the bin. The loadout bin has a high level indicator to stop the loadout system. It should be located at a depth in the bin that would allow the electric outlet gate(s) from the large grain bin(s) to close and leave enough room in the top of the loadout bin to clear the reclaim conveyor(s) and bucket elevator.

The interlocking system is simple for the loadout operation. The reclaim conveyors should be interlocked with the bucket elevator so that they will not run unless the bucket elevator is running. In addition, the electric discharge gate at the bin(s) center should not be able to be opened unless the reclaim conveyor(s) are running.

Feed Ops
Types of motion sensors and level detectors*
To view the graph click here.
 

*The sensors listed above aren't all that may be required to meet safety or regulartory requirements. Speed indicators for turning shafts should be installed on shafts not directly run by the motor or power source, but on the other end of the equipment to verify teh end shaft is turning. 

System upgrades

The process flow diagram for the small elevator in this article is limited as only one operation can occur at any time. When receiving grain, transferring or loadout operations cannot be done.

To improve the elevator system, I would recommend the following modifications:

1. Install a second bucket elevator with a minimum capacity of at least 7,500 bph.

2. Separate the two receiving pits and add a second receiving conveyor from the separate pit to the bucket elevators. Both receiving conveyor discharges should have two-way valves installed so the grain from each receiving conveyor can be spouted to either of the receiving conveyors.

3. Replace the existing single electric distributor with an electric double (two inlets) distributor to which each bucket elevator may be spouted. With these additions, the elevator can perform two operations simultaneously and should be able to receive or ship at any time.

Automation

With the above additions, it also may be desirable to automate the elevator operations. Automation systems are based on the process flow diagram and normally displayed in sections of the diagram on computer screens. The screens show a portion of the flow diagram for different systems in the operation such as receiving systems, transfer systems, loadout systems, etc. The automation systems will show process flow diagram portions and will light up the track of the process to show what is happening at that time and the path the grain is traveling in the elevator. The automation system can be designed to identify and record information about the incoming grain and set the path the grain will follow to reach a bin or other destination. Grain being removed from a bin can be directed to a designated destination and the system will set up the path to direct the grain to that destination. The receiving and ultimate use or shipment may be identified and record-keeping can be saved on the source and the disposition of the grain. This makes it possible to keep a record of each grain and the path it followed through the grain elevator.

To automate a system, it is necessary to equip each piece of equipment with some type of sensor to verify it is running and the path is identified as running.

In some automation systems, the fact that motors on the system equipment are running indicates the piece of equipment is fully operational. The motor may be running, but that is no proof that a piece of equipment is actually running.

The best way to prove this is to put a motion sensor on the machine that will verify it is running.

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