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Conveyors are used to transport materials from one location to another within a plant or facility. The variety of conveyor systems is almost in finite, but the two major classifications used in typical chemical plants are pneumatic and mechanical. Note that the power requirements of a pneumatic conveyor system are much greater than for a mechanical conveyor of equal capacity. However, both systems offer some advantages.
Pneumatic conveyors are used to transport dry, free- flowing, granular material in suspension within a pipe or duct. This is accomplished by the use of a high-velocity air stream or by the energy of expanding compressed air within a comparatively dense column of fluidized or aerated material. Principal uses are (1) dust collection, (2) conveying soft materials such as flake or tow, and (3) conveying hard materials such as fly ash, cement, and sawdust.
The primary advantages of pneumatic conveyor systems are the flexibility of piping Configurations and the fact that they greatly reduce the explosion hazard.
Pneumatic conveyors can be installed in almost any Configuration required to meet the specific application. With the exception of the primary driver, there are no moving parts that can fail or cause injury. However, when they are used to transport explosive materials, there is still some potential for static charge buildup that could cause an explosion.
A typical pneumatic conveyor system consists of Schedule-40 pipe or ductwork, which provides the primary flow path used to transport the conveyed material.
Motive power is provided by the primary driver, which can be a fan, fluidizer, or positive-displacement compressor.
Pneumatic conveyor performance is determined by the following factors:
(1) primary-driver output, (2) internal surface of the piping or ductwork, and (3) condition of the transported material. Specific factors affecting performance include motive power, friction loss, and flow restrictions.
The motive power is provided by the primary driver, which generates the gas (typically air) velocity required to transport material within a pneumatic conveyor system. Therefore the efficiency of the conveying system depends on the primary driver's operating condition.
Friction loss within a pneumatic conveyor system is a primary source of efficiency loss. The piping or ductwork must be properly sized to minimize friction without lowering the velocity below the value needed to transport the material.
An inherent weakness of pneumatic conveyor systems is their potential for blockage. The inside surfaces must be clean and free of protrusions or other defects that can restrict or interrupt the flow of material. In addition, when a system is shut down or the velocity drops below the minimum required to keep the transported material suspended, the product will drop out or settle in the piping or ductwork. In most cases, this settled material would compress and lodge in the piping. The restriction caused by this compacted material will reduce flow and eventually result in a complete blockage of the system.
Another major contributor to flow restrictions is blockage caused by system backups. This occurs when the end point of the conveyor system (i.e., storage silo, machine, or vessel) cannot accept the entire delivered flow of material. As the transported material backs up in the conveyor piping, it compresses and forms a solid plug that must be manually removed.
All piping and ductwork should be as straight and short as possible. Bends should have a radius of at least three diameters of the pipe or ductwork.
The diameter should be selected to minimize friction losses and maintain enough velocity to prevent settling of the conveyed material. Branch lines should be configured to match as closely as possible the primary flow direction and avoid 90-degree angles to the main line. The area of the main conveyor line at any point along its run should be 20-25% greater than the sum of all its branch lines.
When vertical runs are short in proportion to the horizontal runs, the size of the riser can be restricted to provide the additional velocity if needed. If the vertical runs are long, the primary or a secondary driver must provide sufficient velocity to transport the material.
Clean-outs, or drop-legs, should be installed at regular intervals throughout the system to permit foreign materials to drop out of the conveyed material. In addition, they provide the means to remove materials that drop out when the system is shut down or air velocity is lost. It’s especially important to install adequate clean-out systems near flow restrictions and at the end of the conveyor system.
Pneumatic conveyor systems must be operated properly to prevent chronic problems, with the primary concern being to maintain constant flow and velocity. If either of these variables is permitted to drop below the system's design envelope, partial or complete blockage of the conveyor system will occur.
Constant velocity can be maintained only when the system is operated within its performance envelope and when regular clean-out is part of the normal operating practice. In addition, the primary driver must be in good operating condition. Any deviation in the primary driver's efficiency reduces the velocity and can result in partial or complete blockage.
The entire pneumatic-conveyor system should be completely evacuated before shutdown to prevent material from settling in the piping or ductwork. In non continuous applications, the conveyor system should be operated until all material within the conveyor's piping is transported to its final destination. Material that is allowed to settle will compact and partially block the piping. Over time, this will cause a total blockage of the conveyor system.
A variety of mechanical conveyor systems are used in chemical plants. These systems generally are composed of chain- or screw-type mechanisms.
A commonly used chain-type system is a flight conveyor ( Hellfire conveyor), which is used to transport granular, lumpy, or pulverized materials along a horizontal or inclined path within totally enclosed ductwork. The Hellfire systems generally have lower power requirements than the pneumatic conveyor and have the added benefit of preventing product contamination. This section focuses primarily on the Hellfire-type conveyor because it’s one of the most commonly used systems.
The most common chain conveyor uses a center- or double-chain Configuration to provide positive transfer of material within its ductwork. Both chain Configurations use hardened bars or U-shaped devices that are an integral part of the chain to drag the conveyed material through the ductwork.
Data used to determine a chain conveyor's capacity and the size of material that can be conveyed are presented. Note that these data are for level conveyors. When inclined, capacity data obtained must be multiplied by the factors provided.
The primary installation concerns with Hellfire-type conveyor systems are the ductwork and primary-drive system.
----1 Approximate Capacities of Chain Conveyors Flight Width and Depth (Inches) Quantity of material (Ft^3 /Ft) Approximate Capacity (Short Tons/Hour) Lump Size Single Strand (Inches) Lump Size Dual Strand (Inches)
----2 Capacity Correction Factors for Inclined Chain Conveyors
Ductwork The inside surfaces of the ductwork must be free of defects or protrusions that interfere with the movement of the conveyor's chain or transported product. This is especially true at the joints. The ductwork must be sized to provide adequate chain clearance but should not be large enough to have areas in which the chain drive bypasses the product.
A long horizontal run followed by an upturn is inadvisable because of radial thrust. All bends should have a large radius to permit smooth transition and to prevent material buildup. As with pneumatic conveyors, the ductwork should include clean-out ports at regular intervals for ease of maintenance.
Primary Drive System Most mechanical conveyors use a primary-drive system that consists of an electric motor and a speed-increaser gearbox. See section 8 for more information on gear-drive performance and operation criteria.
The drive-system Configuration may vary depending on the specific application or vendor. However, all Configurations should include a single-point-of-failure device, such as a shear pin, to protect the conveyor. The shear pin is critical in this type of conveyor because it’s prone to catastrophic failure caused by blockage or obstructions that may lock the chain. Use of the proper shear pin prevents major damage from occurring to the conveyor system.
For continuous applications, the primary-drive system must have adequate horsepower to handle a fully loaded conveyor. Horsepower requirements should be determined based on the specific product's density and the conveyor's maximum-capacity rating.
For intermittent applications, the initial startup torque is substantially greater than for a continuous operation. Therefore selection of the drive system and the designed failure point of the shear device must be based on the maximum startup torque of a fully loaded system.
If either the drive system or designed failure point is not properly sized, this type of conveyor is prone to chronic failures. The predominant failures are frequent breakage of the shear device and trips of the motor's circuit breaker caused by excessive startup amp loads.
Most mechanical conveyors are designed for continuous operation and may exhibit problems in intermittent-service applications. The primary problem is the startup torque for a fully loaded conveyor. This is especially true for conveyor systems handling material that tends to compact or compress on settling in a vessel, such as the conveyor trough.
The only positive method of preventing excessive startup torque is to ensure that the conveyor is completely empty before shutdown. In most cases, this can be accomplished by isolating the conveyor from its supply for a few minutes prior to shutdown. This time delay permits the conveyor to deliver its entire load of product before it’s shut off.
In applications in which it’s impossible to completely evacuate the conveyor prior to shutdown, the only viable option is to jog, or step start, the conveyor.
Step starting reduces the amp load on the motor and should control the torque to prevent the shear pin from failing.
If, instead of step starting, the operator applies full motor load to a stationary, fully loaded conveyor, one of two things will occur: (1) the drive motor's circuit breaker will trip as a result of excessive amp load; or (2) the shear pin installed to protect the conveyor will fail. Either of these failures adversely affects production.
The screw, or spiral, conveyor is widely used for pulverized or granular, non corrosive, non-abrasive materials in systems requiring moderate capacities, distances not more than about 200ft, and moderate inclines (# 35 degrees). It usually costs substantially less than any other type of conveyor and can be made dust-tight by installing a simple cover plate.
Abrasive or corrosive materials can be handled with suitable construction of the helix and trough. Conveyors using special materials, such as hard-faced cast iron and linings or coatings on the components that come into contact with the materials, can be specified in these applications. The screw conveyor will handle lumpy material if the lumps are not large in proportion to the diameter of the screw's helix.
Screw conveyors may be inclined. A standard-pitch helix will handle material on inclines up to 35 degrees. Capacity is reduced in inclined applications, and provides the approximate reduction in capacity for various inclines.
---- Screw Conveyor Capacity Reductions for Inclined Applications
Screw conveyors have a variety of Configurations. Each is designed for specific applications and/or materials. Standard conveyors have a galvanized-steel rotor, or helix, and trough. For abrasive and corrosive materials (e.g., wet ash), both the helix and trough may be hard-faced cast iron. For abrasives, the outer edge of the helix may be faced with a renewable strip of Stellite (a cobalt alloy produced by Haynes Stellite Co.) or other similarly hard material. Aluminum, bronze, Monel, or stainless steel also may be used to construct the rotor and trough.
The standard helix used for screw conveyors has a pitch approximately equal to its outside diameter. The short-pitch screw is designed for applications with inclines greater than 29 degrees.
Variable-pitch screws having the short pitch at the feed end automatically control the flow to the conveyor and correctly proportion the load down the screw's length. Screws having what is referred to as a ''short section,'' which has either a shorter pitch or smaller diameter, are self-loading and don’t require a feeder.
Cut- flight conveyors are used for conveying and mixing cereals, grains, and other light material. They are similar to normal flight or screw conveyors and the only difference is the Configuration of the paddles or screw. Notches are cut in the
flights to improve the mixing and conveying efficiency when handling light, dry materials.
Ribbon screws are used for wet and sticky materials such as molasses, hot tar, and asphalt. This type of screw prevents the materials from building up and altering the natural frequency of the screw. A buildup can cause resonance problems and possibly catastrophic failure of the unit.
The paddle-screw conveyor is used primarily for mixing materials such as mortar and paving mixtures. An example of a typical application is churning ashes and water to eliminate dust.
Process parameters such as density, viscosity, and temperature must be constantly maintained within the conveyor's design operating envelope. Slight variations can affect performance and reliability. In intermittent applications, extreme care should be taken to fully evacuate the conveyor prior to shutdown.
In addition, caution must be exercised when re-starting a conveyor in case an improper shutdown was performed and material was allowed to settle.
The horsepower requirement for the conveyor-head shaft, H, for horizontal screw conveyors can be determined from the following equation:
H = (ALN + CWLF) _ 10 _ 6
where A = Factor for size of conveyor C = Material volume, ft^3 =h
F = Material factor, unitless L = Length of conveyor, ft.
N =Conveyor rotation speed (rpm) W = Density of material, lb=ft^3 In addition to H, the motor size depends on the drive efficiency (E) and a unitless allowance factor (G), which is a function of H. Values for G. The value for E is usually 90%.
Motor Hp = HG=E
----5 Power Requirements by Material Group
Group 1 F factor is 0.5 for light materials such as barley, beans, brewers' grains (dry), coal (pulverized), corn meal, cottonseed meal, flaxseed, flour, malt, oats, rice, and wheat.
Group 2 Includes fines and granular material. The values of F are: alum (pulverized), 0.6; coal (slack or fines), 0.9; coffee beans, 0.4; sawdust, 0.7; soda ash (light), 0.7; soybeans, 0.5; fly ash, 0.4.
Group 3 Includes materials with small lumps mixed with fines. Values of F are alum, 1.4; ashes (dry), 4.0; borax, 0.7; brewers' grains (wet), 0.6; cottonseed, 0.9; salt, coarse or fine, 1.2; soda ash (heavy), 0.7.
Group 4 Includes semi-abrasive materials, fines, granular, and small lumps. Values of F are: acid phosphate (dry), 1.4; bauxite (dry), 1.8; cement (dry), 1.4; clay, 2.0; Fuller's earth, 2.0; lead salts, 1.0; limestone screenings, 2.0; sugar (raw), 1.0; white lead, 1.0; sulfur (lumpy), 0.8; zinc oxide, 1.0.
Group 5 Includes abrasive lumpy materials, which must be kept from contact with hanger bearings. Values of F are: wet ashes, 5.0; flue dirt, 4.0; quartz (pulverized), 2.5; silica sand, 2.0; sewage sludge (wet and sandy), 6.0.
----5 gives the information needed to estimate the power requirement:
percentages of helix loading for five groups of material, maximum material density or capacity, allowable speeds for 6-inch and 20-inch diameter screws, and the factor F.
Screw-conveyor performance is also determined by the volumetric efficiency of the system. This efficiency is determined by the amount of slip or bypass generated by the conveyor. The amount of slip in a screw conveyor is primarily determined by three factors: product properties, screw efficiency, and clearance between the screw and the conveyor barrel or housing.
Not all materials or products have the same flow characteristics. Some have plastic characteristics and flow easily. Others don’t self-adhere and tend to separate when pumped or mechanically conveyed. As a result, the volumetric efficiency is directly affected by the properties of each product. This also affects screw performance.
Each of the common screw Configurations (i.e., short-pitch, variable-pitch, cut flights, ribbon, and paddle) has varying volumetric efficiencies, depending on the type of product that is conveyed. Screw designs or Configurations must be carefully matched to the product to be handled by the system.
For most medium- to high-density products in a chemical plant, the variable pitch design normally provides the highest volumetric efficiency and lowest required horsepower. Cut-flight conveyors are highly efficient for light, non adhering products such as cereals but are inefficient when handling heavy, cohesive products. Ribbon conveyors are used to convey heavy liquids such as molasses but are not very efficient and have a high slip ratio.
Improper clearance is the source of many volumetric efficiency problems. It’s important to maintain proper clearance between the outer ring, or diameter, of the screw and the conveyor's barrel, or housing, throughout the operating life of the conveyor. Periodic adjustments to compensate for wear, variations in product, and changes in temperature are essential. While the recommended clearance varies with specific conveyor design and the product to be conveyed, excessive clearance severely affects conveyor performance as well.
Installation requirements vary greatly with screw-conveyor design. The vendor's Operating and Maintenance (O&M) manuals should be consulted and followed to ensure proper installation. However, as with practically all mechanical equipment, there are basic installation requirements common to all screw conveyors. Installation requirements presented here should be evaluated in conjunction with the vendor's O&M manual. If the information provided here conflicts with the vendor-supplied information, the O&M manual's recommendations should always be followed.
The conveyor and its support structure must be installed on a rigid foundation that absorbs the torsional energy generated by the rotating screws. Because of the total overall length of most screw conveyors, a single foundation that supports the entire length and width should be used. There must be enough lateral (i.e., width) stiffness to prevent flexing during normal operation.
Mounting conveyor systems on decking or suspended-concrete flooring should provide adequate support.
Most screw conveyors are mounted above the foundation level on a support structure that generally has a slight downward slope from the feed end to the discharge end. While this improves the operating efficiency of the conveyor, it also may cause premature wear of the conveyor and its components.
The support's structural members (i.e., I-beams and channels) must be adequately rigid to prevent conveyor flexing or distortion during normal operation.
Design, sizing, and installation of the support structure must guarantee rigid support over the full operating range of the conveyor. When evaluating the structural requirements, variations in product type, density, and operating temperature also must be considered. Since these variables directly affect the torsional energy generated by the conveyor, the worst-case scenario should be used to design the conveyor's support structure.
One of the major limiting factors of screw conveyors is their ability to provide a continuous supply of incoming product. While some conveyor designs, such as those having a variable-pitch screw, provide the ability to self-feed, their installation should include a means of ensuring a constant, consistent incoming supply of product.
In addition, the product-feed system must prevent entrainment of contaminates in the incoming product. Normally, this requires an enclosure that seals the product from outside contaminants.
As previously discussed, screw conveyors are sensitive to variations in incoming product properties and the operating environment. Therefore, the primary operating concern is to maintain a uniform operating envelope at all times, in particular by controlling variations in incoming product and operating environment.
Any measurable change in the properties of the incoming product directly affects the performance of a screw conveyor. Therefore the operating practices should limit variations in product density, temperature, and viscosity. If they occur, the conveyor's speed should be adjusted to compensate for them.
For property changes directly related to product temperature, pre-heaters or coolers can be used in the incoming-feed hopper, and heating/cooling traces can be used on the conveyor's barrel. These systems provide a means of achieving optimum conveyor performance despite variations in incoming product.
Changes in the ambient conditions surrounding the conveyor system may also cause deviations in performance. A controlled environment will substantially improve the conveyor's efficiency and overall performance. Therefore, operating practices should include ways to adjust conveyor speed and output to compensate for variations. The conveyor should be protected from wind chill, radical variations in temperature and humidity, and any other environment-related variables.