The application of bulk handling equipment in the United States dates back to 1785 when Oliver Evans designed and built screw conveyors and bucket elevators for handling grain in his father's mill. Since 1866, fans have been used to activate pipeline systems handling light materials such as dust or wood shavings. The advent of the positive-pressure blower in the mid-1920s enabled the handling of higher material volumes through smaller pipes leading to the application of pneumatic conveying systems for grain handling.
Today, while dramatic improvements have been made to materials of construction, controls and other equipment features, the need for safe application of pneumatic, screw, flight and belt conveyors and bucket elevators for handling grain remains relatively unchanged.
Dust control equipment, preventative maintenance and good housekeeping are basic requirements necessary to keep the plant areas free of dust. Belt conveyors and bucket elevators have been reported as frequent fire ignition sources due to belt slippage, hot bearing surfaces and sparks created by the striking of metal surfaces during operation.
To minimize the danger of explosions in the area of belt conveyors and bucket elevators, follow these recommendations:
• Install a belt slippage device on the bucket elevator or belt. This can be installed on the boot shaft or on a tail pulley to indicate the speed of the belt directly. An audible alarm can be connected to the belt monitoring device.
• Hot bearing sensors should be installed on all bearings operating in the presence of dust.
• A motor ammeter should be used in combination with a motor amperage overload cutout device to monitor and control the bucket elevator motor.
• Install a mechanical drive overload release in combination with the gear reducer on the bucket elevator or conveyor headshaft.
• Install a mechanical backstop or drive electric brake to prevent back rotation in the event the elevator or conveyor must be stopped when full.
• Tramp metal collectors (magnetic separators) are recommended to remove at least ferrous materials from the flow into bucket elevators and onto conveyors.
• Non-metal buckets (high-density linear polyethylene or cast nylon) are recommended for the purpose of reducing spark generation in the elevator casing.
• Static conducting belting in the bucket elevator or belt conveyor is recommended to minimize static electrical charge build-up. A belting material of low flammability is recommended.
• The bucket elevator and belt conveyor assembly — and other processing machinery — should be adequately and permanently grounded.
CONVEYOR SAFETY. Written procedures for regular inspections of all mechanical and electrical machinery and equipment are strongly recommended, and standard operating procedures should be established and implemented. In addition, employees should be trained, especially regarding lock-out and tag-out procedures when machinery is shut down for repair or replacement.
The conveyor types typically encountered in grain elevators include belt, bucket, screw, screw-lift, flight and pneumatic conveyors. The structure, application and installation of each conveyor type is critical to safe operation.
Belt conveyors, primarily used for horizontal conveying of larger grain volumes (+/- 30,000 bushels per hour), utilize endless rubber belts that form moving and supporting surfaces on which bulk materials ride. Idlers support the troughed, carrying portion of the belt as well as the return flat portion. Pulleys and terminals support and direct the belt and return strand. The drive imparts power through one or more pulleys to move both belt and load. The frame structure provides for support and alignment of the idlers, pulleys and drive.
A single belt conveyor can be designed with a "tripper" or as a "shuttle" for discharging grain at multiple points. A similar unit, with properly designed "skirts," can be fed from multiple discharge gates or feeders. A flight conveyor also can be used for these operations, offering a cleaner operation with less need for dust collection. But, as in any case, an economic evaluation is necessary to choose which is right for the application.
Bucket elevators normally used in grain processing plants can be classified into three general types. The first is the high-speed, high-capacity grain elevator leg. This should be a heavy-duty piece of equipment engineered for large grain handling installations such as terminal elevators and the larger grain handling receiving stations. Such legs are normally used with car unloading, truck unloading and loading out facilities at high speed capacity installations. Allied to this type of leg is the heavy-duty marine elevator, which operates at similar speeds.
The second type of leg commonly used is the light-duty, moderate-speed elevator for handling smaller grain capacities. Applications include dryer installations, smaller grain elevators, receiving and storage stations and as in-plant processes for handling processed grain, such as soybean meal, flour, alfalfa meal and other similar soft mill stocks.
The third type of bucket elevator is a centrifugal discharge chain elevator. The chains operate at moderate to slow speeds, making the conveyor useful for applications where belts cannot be used due to the condition of the material handled, such as handling wet grain in a malt house or spent grain in a distillery.
Screw conveyors can be furnished to specific requirements, such as reversible operation. If screw conveyor flighting is omitted – leaving bare pipe – over the last discharge opening, it will prevent carry-over of material. Opposed flight ends at hanger points are set at approximately 180° to each other to facilitate continuity of flow across the hanger space. Hanger location should clear all trough joints as well as inlet.
An auxiliary end thrust is recommended for long conveyors or for conveyors handling heavy materials. The preferred location is at the drive end, but it can be located at the opposite end if required by layout limitations.
Standard troughs of listed lengths should be used wherever possible. End flanges should be provided with feet, if support arrangements permit, to allow removal of trough ends without disturbing trough alignment. Connecting flanges should be equipped with a foot whenever support spacing permits. Otherwise, use saddles as required.
At the trough ends, seal glands or seal arrangements should be employed to protect and preserve the end bearings when handling detrimental materials. When a countershaft end is used with an end thrust, it should be installed at the same end, if possible.
Screw-lift conveyors can effectively elevate a large variety of substances and may solve difficult elevating problems. It consists of split tubing, is comparable to a pipe of similar diameter and installation requires little space. The tubing may be installed in crowded quarters, taking material to or from processing equipment or from receiving hopper to storage.
This type of conveyor may be used for re-cycling operations or reclaiming from storage, delivering to processing and then to packing. It should be noted, however, that this unit is not self-cleaning.
Another type of conveyor possessing en-mass action is the "Super-Flo" or flight conveyor, which consists of a series of flights or paddles attached to a chain and enclosed within a standard "U"-shaped trough. Its mechanism consists of a chain, sprockets, flights, shafts, flange bearings, take-ups and other parts. The flights are made of a single piece of smooth, non-metallic material that will not chip or break. They are shaped to fit the trough and exert a wiping action that cleans the trough interior.
The operation is smooth and quiet, and does little damage to the material handled. The return strand, as well as the conveying strand, is encased within a single trough, eliminating the need for double troughing. The power factor is low and capacity is high, keeping operating costs low.
There have been new and significant developments in the use of air as a conveying medium for grain. In fact, pneumatic conveying should be considered and evaluated for all conveying problems, along with conventional screw conveyors, bucket elevators, belt conveyors and other types of mechanical conveyors.
Equipment for conveying dry solid materials by air may be classified into three general classes, according to the method of operation. There are dilute phase systems using high velocity streams of low material concentration; dense phase systems using relatively low velocity streams with high concentration; and air-activated gravity systems.
In recent years, advances in construction materials and design improvements resulting in higher efficiencies have created new and extended fields of use for the first two classifications. Explosions, fires and personal injuries have been greatly reduced with the introduction of pneumatic conveyors. The product-to-air ratios in the conveying tubes are normally not explosive, and static electrical charges present in the tube can be safety and easily grounded.
However, for one reason or another, conventional mechanical conveyor systems may still be preferred. If mechanical conveyors are chosen for grain handling, mill managers must provide for safe operation by formulating a Conveyor Safety Strategy (CSS) considering:
• Study and analysis of available conveyor standards.
• Typical conveyor and conveyor-system hazards, risks and consequences.
• Elimination of hazards with the provision of appropriate guarding.
• Use of warning signs as required.
• Development and application of training and instruction programs.
• Use of appropriate personal protection equipment.
The most comprehensive conveyor safety standards presently available in the United States are issued by the American Society of Mechanical Engineers, ASME B20.1 — Safety Standards for Conveyor and Related Equipment. These standards have been issued periodically since 1947, covering conveyor safety in general and individually addressing the safety needs of belt, screw and flight conveyors and bucket elevators.
The ANSI B20.1, issued in 1972, is a specification standard and is highly recommended as a reference source to aid in preparation of a CSS. All other issues of ANSI B20.1, released in 1947, 1952, 1976, 1984, 1985, 1987, 1990, 1993 and 1995, specify performance-based standards. While there are U.S. Occupational Safety and Health Administration requirements covering grain dust fires and explosion safety, the agency has not published standards covering conveyor safety specifically. Recently, a text covering conveyor safety was published by the American Society of Safety Engineers (ASSE) that addresses safety in the design and operation of conveyors and conveyor systems.
It is the conveyor designer and manufacturer's responsibility to provide functional equipment which will perform to the owner's expectations and be safely operated and serviced. It is the owner's responsibility, however, to establish the conveyor's objectives, operations, maintenance and CSS plan.
George A. Schultz is president of Siebert Engineers, Inc., Lombard, Illinois, U.S., and a member of the American Society of Safety Engineers. Mr. Schultz's book, Conveyor Safety: Safety in the Design Operation of Material Handling Systems, was recently published by the ASSE. Visit the association's web site at www.asse.org for more information.