grain elevator paper part ii

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An ASAE/CSAE Meeting Presentation Paper Number: 044146

Design Considerations for the Construction and Operation of Grain Elevator Facilities.

Part II: Process Engineering Considerations

Kurt A. Rosentrater, Ph.D., Agricultural and Bioprocess Engineer USDA, ARS, Crop and Entomology Research Unit 2923 Medary Avenue, Brookings, SD, 57006, [email protected]

Gregory D. Williams, Ph.D., P.E., S.E., Manager of Engineering Faciliy Engineering Services, PA 201 O'Hara Lane, Springdale, AR, 72762, [email protected]

Written for presentation at the 2004 ASAE/CSAE Annual International Meeting

Sponsored by ASAE/CSAE Fairmont Chateau Laurier, The Westin, Government Centre

Ottawa, Ontario, Canada 1 - 4 August 2004

Abstract. Grain elevators play a key role in U.S. agriculture, and fulfill three main functions: post-harvest handling and storing of cereal grains and oilseeds, conditioning and preserving of grain, and facilitating the delivery of grain to domestic feeding and processing, as well as overseas, end-use destinations. These facilities have evolved from mere storage sites to large, high-throughput, highly automated, processing plants. This trend has been driven by the consolidation of local country elevators, which has been due, in part, to both local economic conditions as well as changing railroad regulations. Another reason has been an increased demand for grain storage space, especially as yearly harvests have increased. Thus, grain elevators represent a key intersection in our food production chain. To date, however, information regarding the unique design requirements of these facilities has been limited. In an effort to summarize state of the art design procedures for grain elevator facilities constructed in North America, a review of these procedures and accepted standards has been assembled. With this paper, engineers and designers should become familiar with the distinctive design process for these facilities and develop an appropriate reference base from which to work.

Keywords. Facility Design, Grain Elevator, Process Engineering

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Introduction Agriculture is one of the most important industries in the United States. In 2000, farmers in the U.S. produced 9.97 billion bushels (253.2 million Mg) of corn, 2.77 billion bushels (75.4 million Mg) of soybeans, and 2.22 billion bushels (60.4 million Mg) of wheat (AG-STATS.COM, 2002). Modern farmers efficiently produce these commodities because they utilize modern technology, such as chemicals and fertilizers, mechanization, and hybrid seeds. Another advantage that American grain producers have is an extensive merchandising and transportation system that links agri-industrial sectors together across the country. One locus of this system consists of grain elevators, which are used to store the majority of cereal grain and oilseed crops produced in the U.S. These are connected by an intricate network of rail lines, highways, and inland waterways. After harvest, many farmers utilize their grain on their own operations to feed their livestock herds. Depending on a given farmers storage and financial decisions and constraints, surplus grain is typically transported and sold to a local country or terminal grain elevator. These facilities are thus the primary means by which grain is merchandized in the U.S., and, in fact, they actually serve as the primary launching points for the nations grain supply. From these facilities grain is shipped to final downstream destinations such as processing facilities, river terminals, and export terminals (Bern and Hurburgh, 1998). Recently, due to advances in genetic and biological engineering, another function that grain elevators are increasingly being relied upon to fulfill is that of preservation of the identity of specific genetic lines and hybrids. Initially, grain elevators were constructed of wood. Many from the 1800s and early 1900s still exist today, and are found in various states of either repair or disrepair (Selyem, 2004; Swanson, 2004). In 1903, however, the Canadian Pacific Railroad constructed a concrete grain elevator at Port Arthur, Ontario, using a novel construction technique: concrete forms were progressively slipped before the concrete in the forms was completely set. This new slipforming method ushered in the modern age of concrete grain elevator design and construction, which is a practice that is, in fact, still used today. Not only are modern construction methodologies similar to those of the early 20th Century, but the physical layout and material handling systems have, in all practical purposes, essentially changed little over the years (Sargent, 1979).

Modern grain elevators do, however, differ from their predecessors in many key respects. Todays grain elevators are much larger, have higher yearly throughputs, greater equipment capacities, improved safety measures and dust control systems, and utilize electronic instrumentation and control systems. Even though large, concrete facilities have been constructed for approximately a century, there exists a continual need for new facilities to service the grain industry in this country. This need exists because many older facilities have reached the end of their effective service life, and must be replaced. Also, many localities require increased storage capacity due to increased yields and harvests. Consequently, every year in North America several new facilities are designed and constructed. Each new facility, even though it may appear similar and operate comparably to those of a century ago, is unique, both in design and in operation. While a variety of storage options exists for modern commercial grain systems, including flat storage, smooth-wall steel bins, and corrugated-wall steel bins, by far the most common type of commercial grain storage facilities are concrete structures. Modern grain elevators are typically constructed according to one of three fundamental varieties, based on operational requirements and capacities: country elevators, inland terminal elevators, and river/port terminals. Design information for these facilities, while in the public domain, is mainly anecdotal, as opposed to scientific or peer reviewed, encompasses a variety of disparate sources, and has not been thoroughly compiled for several years. In fact, no work has been produced to date that comprehensively discusses all of the process engineering aspects of modern grain elevator design. Thus, the purpose of this paper is to summarize state of the art design procedures for grain elevator facilities, from a process engineering perspective. While not intended to be comprehensive, it is intended to provide standards of practice as well as sources of fundamental design and reference information for engineers and designers of these facilities.

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Overview of Grain Handling Facilities Each grain elevator facility is unique. No two are identical, because the components which

comprise each can be assembled in an infinite variety of configurations. Country elevators are those smaller-scale facilities that generally service limited geographical areas, which often tend be to rural localities. Terminal elevators are those larger-scale facilities that generally serve as collection points for the country elevators, and also serve as distribution points for export or wholesale to food, feed, and other end-use destinations. These may or may not be located in rural areas. Even though these two types of facilities can be drastically different in terms of scale, each typically consists of five common major, or primary, operations (outlined in Figure 1), which include receiving, distribution, storage, reclaim, and loadout of grain. In country elevators, grain is typically delivered to the facility via farm-scale trucks and wagons, but seldom via rail service; in terminal facilities, grain is typically delivered via larger semi-trucks, but rail service is also common. After the grain has been received into the facility, it is then transported (via gravity, mechanical conveyor, or a combination thereof) to appropriate storage locations. It is vital that the facility have the ability to segregate inbound grain according to commodity, moisture content, genetic line, etc., and will become even more so over the next several years. After a given length of storage time (which can be dependent on needs of end-users, market conditions, and rail car or ship availability), grain is then removed from storage, transported via the reclaim system to loadout, where it is scaled (for facility accounting as well as appropriate container fill purposes), sampled (for subsequent grain quality testing), and placed on either rail cars, semi-trucks, or ships. Sometimes there may exist the need to recycle grain back to storage (e.g., if quality specifications are not met, or if grain needs to be redistributed within the facility itself). Figure 2 depicts a generic process flow diagram for either country or terminal facilities; Table 1 lists equipment commonly used within these facilities. Grain elevators can be further classified into two main sub-types: those designed for handling large grains (e.g., corn and soybeans; Figure 2), and those intended for small grains (Figure 3). The major difference between them is that facilities designed to handle large grains typically have a fewer number of bins, and typically have limited grain cleaning capabilities. Small grains elevators, on the other hand, typically have a much greater number of bins, and thus have a much greater ability to segregate grain streams. Also, they typically have an extensive grain cleaning capability which, in fact, typically comprises an entire cleaning section within the facility (Figure 4).

Process Design Considerations In order to effectively plan and construct grain elevator facilities, it is essential that

engineers and designers be cognizant of modern design options and procedures that are commonly used throughout the industry. These can be suitably categorized into primary systems and secondary systems. Beyond the physical sizing and locating of structures and equipment, a facility designer must pay attention to life safety, structural design, ADA, NFPA, and other design codes. For specific details regarding non-process related issues, the reader is referred to Williams and Rosentrater (2004).

Primary Systems and Components

The five most commonly used primary systems in any grain storage facility include grain receiving, distribution, storage, reclaim, and loadout.

Receiving

Almost all country and terminal grain elevators receive grain with wagons and trucks (and sometimes, but not as often, with rail cars) with transport capacities ranging from a few hundred

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bushels up to approximately 1200 bu for the large semi trailers. Occasionally terminal elevators are designed to receive grain via rail or ship. Providing adequate receiving capacity is essential for facility operations, especially during the harvest season. The two most common receiving options include gravity-flow pits that directly feed one or more bucket elevators (which are one stage of the distribution process), and gravity-flow pits that feed mechanical conveyors (typically belt or drag conveyors), that subsequently transport the incoming grain to one or more bucket elevators. Figure 5 illustrates both of these receiving systems.

The first consideration when designing a receiving system is the physical nature of the commodities that the facility must accommodate. Specifically, the angle of repose for these grains is a key factor for the design of these operations. Table 2 provides typical ranges for angle of repose data for several grains. When designing a grain receiving system, the limiting factor for the design of any hopper is the valley angle of that hopper (i.e., the angle with respect to a horizontal plane that results from the conjunction of two nonparallel surfaces, each at a unique angle with respect to that horizontal plane), because it must be steeper than the grains angle of repose for the material to flow properly out of the hopper. The valley angle for any hopper can be calculated as:

( ) ( )( ) ( )

+= 22

221

tantantantantan (1)

where is the valley angle that the hopper makes vis--vis a horizontal plane, is the angle of one side of the hopper relative to that horizontal plane, and is the angle the other side of the hopper relative to that horizontal plane. In practice, the five most commonly used valley angles that receiving hoppers are designed to meet include 6-on-12 (26.57o), 8-on-12 (33.69o), 9-on-12 (36.87o), 10-on-12 (39.81o), and 12-on-12 (45o). These values are also key to the design of hopper bottoms for grain storage silos themselves, and will be discussed again later.

The pit structure itself should be designed to accommodate the maximum possible physical holding capacity (i.e., volume) which, in practice, should be approximately 1200 bu; this will accommodate the largest vehicles (i.e., hopper-bottom semi trailers) that will be transporting grain to the facility. Most receiving hoppers can be approximated as truncated pyramids, and as such, their volume can be determined using the standard equation for a frustum of a pyramid:

[ ]2121 AAAA2HV ++= (2) where V is the volume of the hopper (ft3 or m3), H is the vertical height of the hopper (e.g., vertical distance from the hopper inlet to the hopper discharge; ft or m), A1 is the area of the top plane of the hopper (ft2 or m2), and A2 is the area of the discharge plane of the hopper (ft2 or m2).

Furthermore, the transport capacity (bu/hr) of the hopper discharge (e.g. orifice, gate, or inlet into a conveyor) will need to be sized appropriately. For more information, refer to ASAE standard D274.1 (ASAE, 2004). This capacity is typically specified by a facilitys owner, and, if not designed appropriately, can become a bottleneck in the facilitys ability to receive incoming grain. As a point of reference, an emptying capacity of 20,000 bu/h will completely empty a receiving pit of 1200 bu in 3.6 min, a capacity of 40,000 bu/h will completely empty the receiving pit in 1.8 min, and a capacity of 60,000 bu/h will completely empty the receiving pit in 1.2 min. Ultimately, the ability to empty the receiving pit will determine how long a truck driver or a rail car will have to wait to unload, which can become problematic and potentially expensive during the harvest season.

Additionally, the location and size of the truck scale and sampling probe, whether located adjacent to the facilitys office structure, or at a remote location, must also be considered. Several manufacturers design standard units, but the analysis of pneumatic transport systems remains

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essential, because an air velocity greater than the terminal velocity of the grain sampled must be achieved in order for the system to work properly. Table 2 provides aerodynamic recommendations for several grains.

Distribution

Grain is transported from the receiving area to the storage bins via the distribution system, which consists of multiple pieces of equipment, such as bucket elevators, distributors and gravity-flow spouting, belt conveyors, and drag conveyors (Figure 6). If bucket elevator height is not a constraining factor (due to motor size limitations), then a gravity system that primarily utilizes a distributor and spouting is generally more cost-effective for grain distribution. If, however, bucket elevator height is a constraining factor, and a distributor with spouting will not be able to fill all required bins, then a conveyor will need to be used to transport the grain from either the bucket elevator discharge, or a distributor outlet, to the appropriate storage silos. In grain elevator facilities, two types of conveyors are primarily used: belt and drag. Belt conveyors are typically more cost effective for conveying over large distances, but intermediate discharges are problematic. Drag conveyors, on the other hand, due incur more friction during operations, and will require larger motors, but can readily be used to discharge to multiple locations. When designing conveying systems for grain transport and distribution, the throughput capacity and required horsepower for each equipment to be used in the facility are of prime importance.

Bucket elevators are the primary mechanism used to transport grain vertically (e.g., from the receiving pit to the top of the elevator for subsequent distribution to appropriate storage bins). The volumetric capacity of a bucket elevator can be determined as:

2

1rsfc

CCVCCCC

Q= (3)

where Q is the volumetric capacity of the bucket elevator (bu/hr), Cc is the capacity of each cup (in3/cup), Cf is the fill of each cup (%, expressed as a decimal), Cs is the linear spacing of cups per unit length of belt (number of cups/ft), Cr is the number of cup rows across the width of the belt, V is the linear belt speed (ft/min), C1 is a conversion factor of 60 (min/h), C2 is a conversion factor of 2150.42 (in3/bu). The power required to drive a bucket elevator can be determined according to Bloome et al. (1978):

121 R1

2490Q

CCHBDQ1.1P

+

= (4)

where P is the power required to drive the elevator shaft (hp), Q is the volumetric capacity of the elevator (ft3/h), BD is the bulk density of the grain (lb/ft3), H is the total vertical distance between the head and tail shafts of the elevator (ft), C1 is a conversion factor of 33,000 (ft.lb/min/hp), C2 is a conversion factor of 60 (min/h), and R1 is the efficiency of the motor speed reducer (which typically ranges from 0.85 to 0.95). This equation has been empirically adapted to:

121

fw

RCCSHTQP = (5)

where P is the power required to drive the elevator shaft (hp), Q is the volumetric capacity of the elevator (bu/h), Tw is the grain test weight (lb/bu), H is the total vertical distance between the head and tail shafts of the elevator (ft), Sf is an empirical service factor of 1.1(-), C1 is a conversion factor of 33,000 (ft.lb/min/hp), C2 is a conversion factor of 60 (min/h), and R1 is the efficiency of the motor speed reducer (which typically ranges from 0.85 to 0.95). Many times an additional 10% is added

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to the Equation 5 above to empirically account for friction losses within the elevator. Power consumption has been further simplified by using empirical relationships:

( ) 000036.0DHQP a += (6) where P is the power required to drive the elevator (hp), Q is the volumetric capacity of the elevator (bu/h), H is the total vertical distance between the head and tail shafts of the elevator (ft), and Da is an additional distance of 5 ft, which empirically accounts for frictional losses within the elevator.

Belt conveyors offer the simultaneous advantages of high throughput and relatively low required power for transport over substantial travel distances. Belt conveyors used in modern grain facilities are typically entirely enclosed, which improves dust control when compared to the open belt conveyors that have historically been used. The capacity of a belt conveyor can be determined according to CEMA (1994):

1CAVQ = (7) where Q is the volumetric capacity (i.e., throughput or volumetric flowrate [bu/min]), V is the belt speed (ft/min), A is the cross-sectional area of grain on the belt (which typically assumes the shape of a combination of trapezoidal area with a circular segment area [ft2]), and C1 is a conversion factor of 0.8036 (bu/ft3). The power required to drive a belt conveyor can be determined according to CEMA (1994):

( ) ( )[ ]V

RCTTTHKLWW015.0WKKKL

P11

acampymbbyxt ++++++= (8)

where P is the power required to drive the conveyor head shaft (hp), L is the conveyor length, defined from head pulley shaft to tail pulley shaft (ft), Kt is the temperature correction factor, which is related to belt thermal expansion and flexibility (-), Kx is the frictional resistance factor that accounts for the interaction between the belt and the idlers (lb/ft), Ky is the factor that accounts for the resistance of the grain load to flexure as it passes over the idlers (-), Wm is weight of the grain per unit length of belt (lb/ft), Wb is the weight of the belt per unit length (lb/ft), H is the change in vertical height from the tail shaft to the head shaft (ft), Tp is the tension force resulting from the belts resistance to bending around the head and tail pulleys (lb), Tam is the tension force resulting from the force necessary to accelerate grain as it is fed onto the belt (lb), Tac is the tension force resulting from conveyor accessories such as plows, trippers, and belt cleaning devices (lb), V is the linear belt speed (ft/min), C1 is a conversion factor of 33,000 (ft.lb/min/hp), and R1 is the efficiency of the motor speed reducer (which typically ranges from 0.85 to 0.95). This equation has been empirically adapted to a somewhat simpler form:

( ) ( )[ ]V

RC225HL035.0W58.0W05.0W00068.0LP

11

mbm ++++= (9)

where P is the power required to drive the conveyor (hp), L is the conveyor length, defined from head pulley shaft to tail pulley shaft (ft), Wm is weight of the grain per unit length of belt (lb/ft), Wb is the weight of the belt per unit length (lb/ft), H is the change in vertical height from the tail shaft to the head shaft (ft), V is the linear belt speed (ft/min), C1 is a conversion factor of 33,000 (ft.lb/min/hp), and R1 is the efficiency of the motor speed reducer (which typically ranges from 0.85 to 0.95).

Drag conveyors offer the advantage of multiple discharges along their length, although frictional resistance is considerably greater, and therefore motor sizes are also appreciably greater, than belt conveyors with comparable capacities. The capacity of a belt conveyor can be determined as:

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1CwhVQ = (10) where Q is the volumetric throughput (bu/min) of the conveyor, V is the linear chain speed (ft/min), h is the height of the grain mass at a given cross section inside the conveyor (ft), and is a function of the height of the flighting used, w is the width of the conveyor (ft), and C1 is a conversion factor of 0.8036 (bu/ft3). The power required for a drag conveyor can be determined according to:

( ) ( ) ( )( )[ ] VRC

h044.0sincosFWsincosFWsincosFWL1.1P11

2ccmmcc

+++++= (11)

where P is the power required to drive the conveyor (hp), L is the conveyor length, from head shaft to tail shaft (ft), Wc is the weight of the chain and flighting per unit length of conveyor (lb/ft), Fc is the coefficient of kinetic friction between the chain and flights and the conveyor floor (-), is the slope of the conveyor relative to a horizontal plane (o), Wm is the weight of the grain per unit length of conveyor (lb/ft), Fm is the coefficient of kinetic friction between the grain and the conveyor floor (-), h is the average depth of the grain in the conveyor (ft), V is the linear chain speed (ft/min), C1 is a conversion factor of 33,000 (ft.lb/min/hp), and R1 is the efficiency of the motor speed reducer (which typically ranges from 0.85 to 0.95). For horizontal use with soybeans, this equation has been empirically adapted to a simpler form:

56000LQP = (12)

where P is the power required to drive the conveyor (hp), Q is the volumetric capacity of the conveyor (bu/h), and L is the length of the conveyor (ft). For horizontal use with corn or small grains, it has been empirically adapted to:

75000LQP = (13)

where P is the power required to drive the conveyor (hp), Q is the volumetric capacity of the conveyor (bu/h), and L is the length of the conveyor (ft). If the conveyor is at an incline, then Equations 12 and 13 must be empirically modified to account for the change in potential energy in the system. This is accomplished by increasing them by adding an additional power requirement:

30000HQPa= (14)

where Pa is the additional power required due to the conveyors incline (hp), Q is the volumetric capacity of the conveyor (bu/h), and H is the change in elevation from the conveyors tail shaft to head shaft (ft).

Spouting is used in a grain elevator facility to transfer grain between various equipment components, unit operations, and storage locations, and as such, it is essential to the functionality of the overall facility. The two types of spouting that are used include unlined round spouting (often referred to as well-casing) and square spouting, which is typically constructed in a u-trough shape. Square spouting typically is constructed with removable lids, to provide access to various spout liners (such as urethane sheeting or ceramic tiles) which can be installed on the inner surfaces to prevent wear from flowing grain. As with liquids in pipelines, the volumetric flowrate of bulk materials, such as grain, through a spout can be calculated according to the continuity equation:

222111 AVQAVQ === (15)

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where Q1 is the volumetric flowrate of grain entering a spout (ft3/min), V1 is the average velocity of grain entering a spout (ft/min), A1 is the cross sectional area (perpendicular to the direction of flow) of a spout inlet (ft2), Q2 is the volumetric flowrate of grain exiting a spout (ft3/min), V2 is the average velocity of grain exiting a spout (ft/min), and A2 is the cross sectional area (perpendicular to the direction of flow) of a spout discharge(ft2). Unlike liquids, however, grain flow does not always fill the entire cross section of a spout. Because of this channel-flow situation, a designer must not just consider average volumetric flowrate, as described above, but must also consider the grain flux through a given spout cross section. Flow flux is commonly referred to as spout capacity per unit cross sectional area (bu/h/in2). Fluxes vary from 60 bu/h/in2 to over 100 bu/h/in2, depending on the size, shape, angle, length, and liner of the spout, as well as the physical properties of the grain kernels themselves (e.g., length, bulk density, moisture content, etc.). The flux at a given point in a spout will significantly affect the spouts volumetric flowrate capacity, as shown in Figures 7 and 8. In practice, most designers use a flux value between 50 and 60 bu/h/in2 for flow from receiving pits, between 70 and 80 bu/h/in2 for bin discharges, as well as for general spouting, and between 100 and 110 bu/h/in2 for rail loadout spouts. In order for grain to flow properly through a spout, the primary consideration is the angle of installation, in other words, the flow angle. It is common practice to use an angle of 9-on-12 (approximately 37o) as an absolute minimum for whole grain flow through a spout. Preferably, though, spouts should be installed at angles between 10-on-12 to 12-on-12 (approximately 40o to 45o), to ensure adequate material flow.

Storage

No two facilities are identical; the combinations and permutations for the layout of grain storage structures are vast. Even so, Figure 9 illustrates several commonly used bin plans. An essential consideration when designing these structures is bin capacity, which will ultimately affect the ability of a facility to isolate and store each inbound commodity. Silo bottoms are generally flat, but can be built up with sand-fill and coated with a layer of concrete to form a hopper (e.g., sand-filled slick-coated hoppers), or the hoppers can be suspended from the bin walls by forming steel or concrete hoppers. For a round, flat-floored, level full bin, the storage volume can be determined using the standard equation for a cylinder:

HD4

V 2 = (16) where V is the volume of the silo (ft3 or m3), D is the diameter of the silo (ft or m), and H is the height of the silo (ft or m). In reality, however, most bins never achieve a state approaching level full, due to the angle of repose of the grain (Table 2), but will actually produce a cone shape at the top of the silo. If a round silo is filled at a single location at the center of the bin, then the effective capacity of the grain cone can be determined using the equation for a standard cone:

HD12

V 2 = (17) where V is the volume of the grain cone (ft3 or m3), D is the diameter of the silo (ft or m), and H is the height of the cone (ft or m). Considering the angle of repose will reduce the effective storage capacity of a given silo. If a silo is filled by a single location, but off-center, then the grain cone actually becomes an irregular cone, but the effective capacity can still be determined using the above equation for a standard cone. If, on the other hand, a silo is filled at multiple locations, regardless of their relation to the bins center, then determining the effective volume occupied by the grain will become substantially more complex. In this case, solid modeling (Figure 10; a and b) can facilitate the accurate determination of effective grain volume. Solid modeling is also extremely useful for determining storage volumes of interstice bins (Figure 10, c and d).

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Not only will the grains angle of repose affect the effective storage capacity of a given silo, but compaction between the individual grain kernels will also need to be considered. In practice, it is common to use a compaction factor of 7% for bin diameters up to 39 ft, 8% for bin diameters between 40 to 59 ft, and 9% for bins with diameters greater than 60 ft.

Reclaim

Reclaim systems are used to remove grain from storage, so that it can be transported to another location within the facility (e.g., other storage bins), to the loadout system, or to turn the bin, which effectively means to move grain out of a given storage bin, and then return the grain back to that original bin, in order to mix the grain, redistribute moisture, and as a result reduce the potential for degradation and spoilage. In practice, one of the most commonly used reclaim options for grain elevator facilities includes the use of multiple-discharge, flat bin floors (Figure 11,a). Because of the grains angle of repose, however, in order to completely empty this type of bin, either a bin sweep auger, or a door large enough to accommodate a skid-steer loader must be provided. Another commonly used configuration is the sand-filled, slick-coated conical hopper bin bottom (Figure 11, b). Flat-floored silos vis--vis hopper-bottom bins typically require more mechanical components and thus consume more operational power than hopper bottom bins, but offer greater effective bin storage volume. Using hopper bottoms minimizes power consumption and mechanical systems, but reduces effective storage volume due to the physical structure (e.g., sand and concrete) required to construct the conical hoppers.

If the discharge from a hopper-bottom bin is centrally located, determining the effective storage volume of the hopper is straightforward, because the standard equation describing the frustum of a cone (which is the same as that used to calculate the volume of a frustum of a pyramid) can be used (Equation 2). If, however, the discharge is eccentric, located in either the floor or the bin wall (i.e., a side tap), then volumetric analysis becomes problematic as the cone shape of the hopper intersects (i.e., Boolean) with the cylinder shape of the silo wall. Solid modeling can greatly facility the determination of effective hopper volumes (Figure 12) in these cases.

The discharge capacity (bu/hr) of the hopper exit (e.g. orifice, gate, or inlet into a conveyor) must be sized appropriately to meet facility throughput requirements. More information regarding orifice flow can be found in ASAE standard D274.1 (ASAE, 2004). Moreover, the conveyors which are used to transport the reclaimed grain must be adequately sized as well, so that potential throughput bottlenecks are eliminated.

Loadout

For larger grain elevator facilities, it is becoming common to dedicate the loadout system almost entirely to rail cars, or ships in the case of facilities adjacent to waterways, and if present at all, truck loadout has been relegated to secondary status. Over the last several years, railroad regulations have become more stringent. More emphasis is being placed on accommodating 110-car unit trains, which must be loaded in 15 h or less. Assuming an average train car size of 4000 bu, this will result in a total of 440,000 bu, or 29,333 bu/h required loadout capacity. For an average train car size of 5000 bu, this would result in a total of 550,000 total bu, or 36,667 bu/h required loadout capacity. To account for time between car fillings (i.e., rail car progressioning), however, systems are typically designed with 50,000 to 60,000 bu/h loadout capacities. For a 50,000 bu/h batch loadout scale, common operation requires 100 fill/empty cycles per hour, with each draft at 30,000 lb (625 ft3, using 48 lb/ft3 as a reference for whole grains). Figure 13 illustrates the three most common types of high-throughput rail loadout systems currently used in practice. Single-car, gravity flow, internal bulk scale (Figure 13, a), and single-car, mechanical fill, external bulk scale (Figure 13, b), are typically used in large-grains facilities. Multiple-car, gravity

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flow/mechanical fill, external bulk scale (Figure 13, c) loadout systems are typically used in small-grains facilities, especially in Canada. Many times the bottleneck in a loadout system is not the bulk scale itself, but rather the ability to provide grain to the scale. In the case of gravity systems, that would entail providing large enough surge bin capacity (bu) above the scale. In the case of mechanical systems, that would entail providing a bucket elevator or conveyor with a large enough rate of material throughput (bu/h). Other design considerations include sampling of the outbound grain (for quality control, as well as FGIS assessment), operator access to the rail car inlets, rail clearance regulations, and the potential need to reclaim and recycle, or even to receive, grain from the rail cars.

Secondary Systems and Components

The six most commonly used secondary systems in grain storage facilities include cleaning, aeration, drying, dust control, sampling, and instrumentation and control. Because not all facilities utilize these systems, these topics will only be covered cursorily, and the reader is referred to other sources for more information.

Cleaning

Some grain elevators utilize cleaning equipment; others do not. Facilities handling large grains typically use gravity flow screeners (Figure 14, a) to remove materials smaller than the grain (e.g., fines and broken grains), and scalpers (Figure 14, b) to remove materials larger than the grain (e.g., foreign materials such as stems, pods, leaves, and insects). Facilities handling small grains typically implement a more sophisticated cleaning system (Figure 3) that utilizes equipment such as aspirators, destoners, universal cleaners, and rotary indent cleaners, in order to segregate the various small grains from other grains, weed seeds, and foreign materials and other materials of similar size. Design of these systems is typically undertaken with the manufacturers of the various equipment, and is dependent on the physical properties (e.g., size, shape, and mass) of the individual grain kernels (Table 3). For further information, the reader is referred to Watson (2002).

Aeration

Aeration systems are used to maintain the viability of grain while in storage, primarily by mitigating moisture migration problems and increasing allowable storage times (Bern and Hurburgh, 1998). These systems are composed of fans, plenums (non-perforated ducts), and discharge areas, which are large perforated steel sheets, to distribute air into the grain mass. In these sheets, perforation diameters of 0.094 in are typically used for large grains, while diameters of 0.05 in are typically used for small grains. These systems can be designed in a variety of configurations, several of which are illustrated in Figure 15, a. These systems also require roof-mounted, gravity-flow exhaust vents (Figure 15, b), and may also utilize roof-mounted exhaust fans (Figure 15, c). Systems can be designed to provide between 0.01 to 1.0 cfm/bu of grain in a given storage silo (large terminal facilities opting for no drying systems often use higher air flow rates). When designing aeration systems, an air velocity between 1500 and 2000 ft/min within the plenums and ducts is recommended. To achieve this, 1 ft2 of duct cross-sectional area is typically provided for each 1500 cfm of airflow. At the face of the perforated sheet (i.e., discharge into the grain mass), it is recommended to provide an airstream velocity of 30 ft/min. This equates to 1 ft2 of perforated surface area for every 30 cfm. Roof vents are typically designed using 1 ft2 of vent opening for every 1000 cfm of airflow into the bin (each roof vent has approximately 1.8 ft2 of opening, so multiple vents are typically used on the elevator roofs). More information regarding aeration design can be found in ASAE standard D272.3 (ASAE, 2004) and MWPS (1999).

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Drying

Although moisture control is essential to the preservation of any grain after harvest (Table 4), not all grain elevator facilities have drying operations. This is primary true of the larger, high-throughput facilities that ship grain on a weekly or biweekly basis. Because of the relatively short residence time of the grain in storage, owners of these facilities cannot justify the capital expenditure of drying systems, so many instead rely on aeration systems to maintain the viability of the grain. Most smaller country elevators do have drying systems, however, because shipping of grain may not occur with regular frequency, and the facility may not be completely emptied on a regular basis. Most grain drying systems that are currently installed have continuous flow operation, as opposed to the older batch-operated machines, and are selected based drying capacity (i.e., rated in terms of moisture removal points/h). Grain drying theory is beyond the scope of this paper, but several works do provide excellent background information: Bern and Hurburgh (1998), MWPS (1987), and Loewer et al. (1994). Typically, though, the selection and design of these systems is undertaken with the manufacturers of the various equipment.

Dust Control

Most elevators utilize some type of dust control system. Some facilities use an oil suppression system where mineral oil is sprayed onto incoming grain. Some facilities segregate dust from the grain stream, using inlet hoods and ducting, and then either reintroduce the dust into the grain at a downstream location, or use cyclone or bag house filters to completely remove dust from the grain, and then place it in a contained storage location for subsequent disposal or other end use. When designing systems to pneumatically transport dust, it is important to produce an airspeed that will provide adequate dust conveyance. To accomplish this, inlet hoods are typically designed to provide, at a minimum, 150 to 200 cfm of airflow for each 1 ft2 of pit grate surface area (i.e., open area over the pit), 100 cfm of airflow for each 1 ft2 of inlet hood opening (e.g., an air velocity of 100 ft/min), and the duct work is typically designed to provide an air velocity of 4000 ft/min. Further information regarding dust control systems can be found in EPA (2004) and McDaniel (1994).

Sampling and Inspection

Inbound grain at an elevator facility is sampled in order to assess the quality of that grain (e.g., moisture content, fines, foreign material, etc.) and thus, based on current market conditions, establish the monetary value for that grain. Inbound grain is generally sampled using a pneumatically-operated truck probe (Figure 16, a). Several manufacturers produce standard equipment for this purpose. Outbound grain is also sampled to assess value. Selecting a sample that is representative of the grain, especially for high-capacity loadout systems, requires a large-scale in-stream sampler, with a subsequent automated divider for producing viable samples (Figure 16; b, c). Several manufacturers produce standard equipment for this purpose. If the outbound grain is destined for export, federal law requires an official grading by an inspector of the Federal Grain Inspection Service (GIPSA, 2004), which is why many modern facilities, especially terminal elevators, typically allocate specific space near the loadout area for these inspectors to perform their analyses.

Instrumentation and Control

Most facilities are controlled using computer-based PLC systems, which provide real-time monitoring and control of all electrical equipment in a facility. An advantage to this type of system is that it provides the ability to utilize sensors and instrumentation to monitor operations. Many sensors are available for use in a grain elevator: proximity sensors (Figure 17, a) and continuous flow sensors (Figure 17, b) to test for plugged conditions; continuous (Figure 17, c) and discrete

12

(Figure 17, d) bin level sensors to monitor the amount of grain in specific bins; motion detection sensors (Figure 17, e), bearing temperature sensors (Figure 17, f), belt alignment sensors (Figure 17, g), and chain slack sensors (to ensure proper conveyor operation). Design, implementation, and installation of these components are typically undertaken with the manufacturers of the various sensor and control systems and the suppliers of the equipment to which they are to be mounted.

Process Design Methodology Although design philosophies are unique to each individual engineer, and are typically

developed over time and with experience, conducting the process design for a grain elevator facility typically involves four common major steps. These are delineated in Figure 18. Prior to initiating a design, it is vital that a facility designer compile and organize project-specific data. This includes understanding the needs, goals, constraints, and preferences of the client. Specific information is included in Tables 5 and 6, which provide a basis for establishing this information. It is also important in this stage to review all applicable design codes and standards, several of which have been discussed by Williams and Rosentrater (2004). The next phase of process design is the most critical aspect of the entire design process: amalgamating all collected information and translating it into a comprehensive process flow diagram for the facility. Many facility design and construction projects have encountered difficulties, sometimes to a considerable, and often expensive extent, due primarily to a faulty or inadequate process flow diagram. After this stage, but before process design can proceed, a facility layout must be developed which, in fact, will become the basis for the design of all subsequent systems in the facility. Tasks associated with this step include determining the necessary number of bins and bin sizes (and therefore bin capacities as well as the facility roof height), bin layouts (i.e., bin plans; see Figure 9 for several common examples), and thus the overall facility layout, which will affect building locations, traffic flow patterns, equipment locations and orientations, etc. Finally, the actual process design must be undertaken, and all of the process engineering considerations mentioned earlier in this paper, for both primary as well as secondary systems, must be implemented, subject to the requirements discerned from Tables 5 and 6. Throughout the entire life cycle of this design methodology, however, process engineers must work closely with structural engineers, especially during facility and equipment layout tasks. Although these are separate disciplines, the design activities of each are not mutually exclusive, but rather are closely dependent upon each other, and can either synergistically complement or impair each other. Process and equipment changes affect structural design, and vice-versa.

Conclusions and Recommendations This paper has reviewed common practices and procedures related to the planning, design,

construction, and operation of modern grain elevators. Specifically, process engineering of these facilities was considered, so designers and educators should find this paper useful. Although no single work currently exists that summarizes all necessary information required for process engineering of these facilities, the reader is encouraged to consult several resources for further details regarding this topic that, while not all-inclusive in themselves, will provide useful additional information. These include, but are not limited to: AACC (1992), AFIA (1994), Bern and Hurburgh (1998), Boumans (1985), GEAPS (1995),GEAPS (1996),GEAPS (1997),GEAPS (1998),GEAPS (1999),GEAPS (2000),GEAPS (2001),GEAPS (2002a), GEAPS (2002b), GEAPS (2003), GEAPS (2004), MWPS (1987), NGFA (1979), NGFA (1985).

References AACC. 1992. Storage of Cereal Grains and Their Products. ed. D. B. Saucer. St. Paul, MN: American

Association of Cereal Chemists. AFIA. 1994. Feed Manufacturing Technology IV. Arlington, VA: American Feed Industry Association.

13

AG-STATS.COM. 2002. Statistical data for agribusiness and the study of agriculture. AG-STATS.COM. Available online: http://www.ag-stats.com. [Accessed 27 February 2002].

ASAE. 2004. ASAE Standards 2004: Standards, Engineering Practices, Data. St. Joseph, MI: ASAE. Baker, K. D., R. L. Stroshine, G. H. Foster, and K. J. Magee. 1985. Performance of a pressure pneumatic

grain conveying system. Applied Engineering in Agriculture 1(2): 72-78. Bern, C. J. and C. R. Hurburgh, Jr. 1998. Managing Grain After Harvest: AE 469/569 course notes. Ames,

IA: Iowa State University Bookstore CourseWorks. Bloome, P. S. Harp, and J. Garton. 1978. Bucket elevators. OSU Extension Facts No. 1106. Stillwater,

OK: Oklahoma State University. Boumans, G. 1985. Grain Storage and Handling. New York, NY: Elsevier Science Publishing Co., Inc. CEMA. 1994. Belt Conveyors for Bulk Materials. Manassas, VA: Conveyor Equipment Manufacturers

Association. CEMA. 1995. Classification and Definitions of Bulk Materials. Manassas, VA: Conveyor Equipment

Manufacturers Association. Edison, A. R. and W. L. Brogan. 1972. size measurement statistics of kernels of six grains. ASAE Paper

No. 72-841. St. Joseph, MI: ASAE. EPA. 2004. . Food and Agriculture Industries, AP-42, Chapter 9. Available online:

http://www.epa.gov/ttn/chief/ap42/ch09/index.html. [Accessed 1 May 2004]. GEAPS. 1995. Proceedings of GEAPS Exchange 1995. Available online:

http://www.geaps.com/proceedings/1995/. [Accessed 1 May 2004]. GEAPS. 1996. Proceedings of GEAPS Exchange 1996. Available online:






http://www.geaps.com/proceedings/2001/. [Accessed 1 May 2004]. GEAPS. 2002a. Facility Design Conference Proceedings. Minneapolis, MN: Grain Elevator and Processing

Society. GEAPS. 2002b. Proceedings of GEAPS Exchange 2002. Available online:



http://www.geaps.com/proceedings/2004/. [Accessed 1 May 2004]. GIPSA. 2004. Authorizing Legislation and the Regulations Under the Laws. Available online:

http://www.usda.gov/gipsa/lawsandregs/lawsregs.htm. [Accessed 1 May 2004]. Loewer, O. J., T. C. Bridges, and R. A. Bucklin. 1994. On-Farm Drying and Storage Systems. St. Joseph,

MI: ASAE. McDaniel, G. L. 1994. Dust collection systems. In Feed Manufacturing Technology IV, pp. 200-208.

Arlington, VA: American Feed Industry Association. MWPS. 1987. Grain Drying, Handling and Storage Handbook. Ames, IA: Midwest Plan Service. MWPS. 1999. Dry Grain Aeration Systems Design Handbook. Ames, IA: Midwest Plan Service. NGFA. 1979. A Practical Guide to Elevator Design. eds. R. C. Gordon and J. Maness. Washington D.C.:

National Grain and Feed Association.

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NGFA. 1985. Retrofitting and Constructing Grain Elevators. ed. R. C. Gordon. Washington D.C.: National Grain and Feed Association.

Noyes, R. T. and W. E. Pfieffer. 1985. Design procedure for pneumatic conveyors in agriculture. ASAE Paper No. 85-3507. St. Joseph, MI: ASAE.

Sargent, L. 1979. General layout and structural design. In A Practical Guide to Elevator Design, 14-41, eds. C. Gordon and J. Maness. Washington D.C.: National Grain and Feed Association.

Selyem, B. 2004. The Country Grain Elevator Historical Society. Available online: http://www.country-grain-elevator-historical-society.org. [Accessed 1 May 2004].

Swanson, L. 2004. Endangered Species: The Prairie Grain Elevator. Available online: http://www.woodengrainelevators.com. [Accessed 1 May 2004].

Watson, K. 2002. Small Grains Cleaning Systems. In Proceedings of GEAPS Facility Design Conference. July 28-31. St Charles IL: Grain Elevator and Processing Society.

Williams, G. D. and K. A. Rosentrater. 2004. Design Considerations for the Construction and Operation of Grain Elevator Facilities. Part I: Planning, Structural, and Life Safety Considerations. ASAE Paper No. 044143. St. Joseph, MI: ASAE.

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Table 1. Equipment list for a typical large grains country or terminal grain elevator.

Equipment No. Equipment Description Equipment No. Equipment Description

Receiving Storage

001 Grain truck 201 Scalpings & screenings bin002 Receiving hopper 202 Grain storage003 Receiving conveyor 203 Grain storage004 Receiving leg 204 Grain storage005 Truck sample probe 205 Grain storage006 Sample delivery vacuum box 206 Grain storage007 Sample collection cabinet008 Dust collector Reclaim009 Fan010 Airlock 301 Reclaim conveyor

302 Bin discharge gateDistribution 303 Bin discharge gate

304 Bin discharge gate101 Two-way valve 305 Bin discharge gate102 Grain scalper 306 Bin discharge gate103 Grain screener 307 Bin discharge gate104 Distribution conveyor105 Gate Loadout106 Gate107 Gate 308 Two-way valve108 Gate 309 Shipping leg109 Gate 310 Shipping bulk weigh scale

311 Grain sampler312 Grain divider313 Rail car314 Sample delivery vacuum box315 Sample collection cabinet401 Manlift

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Table 2. Physical properties of various grains. * Transport Angle of Repose (o) Bulk Density (lb/ft3) Velocity (ft/min) Grain Range Average Range Average Range Barley 24 35 28 36 48 42 3300 Corn 21 26 23 45 3150 3300 Milo 20 33 26 40 45 43 3300 Oats 24 32 28 25 35 30 2400 2700 Rice 31 41 36 32 36 34 3300 Sorghum 27 33 29 32 46 39 3300 Soybeans 22 35 28 45 50 48 3600 Wheat (durum)

22 25 23 45 48 47 3300

Wheat (red)

19 38 25 45 48 47 3300

* Based on AFIA (1994), Baker et al. (1985), CEMA (1995), MWPS (1987), and Noyes and Pfieffer (1985)

Table 3. Physical attributes of individual grain kernels. * Diameter Grain Major (mm) Intermediate (mm) Minor (mm) Kernel Mass (g) Barley 8.76 3.15 2.51 0.33 Corn 12.01 8.15 5.18 0.33 Oats 10.84 2.67 2.03 0.02 - 0.03 Rye 6.65 2.21 2.11 Soybeans 7.29 6.43 5.38 0.08 0.17 Wheat 6.02 2.79 2.54 0.04 * Based on Edison and Brogan (1972)

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Table 4. Recommended storage moisture contents for various grains. * Grain Moisture Content (w.b., %) Barley

Up to 6 mo 14 Greater than 6 mo 13

Corn Up to 12 mo 14 Greater than 12 mo 13

Oats Up to 6 mo 14 Greater than 6 mo 13

Soybeans Up to 12 mo 12 Greater than 12 mo 11

Wheat Up to 6 mo 14 Greater than 6 mo 13

* Based on MWPS (1987)

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Table 5. Essential data necessary for process design of grain elevator primary systems. Category Criteria Required Data Receiving Capacity (inbound) bu/h; container sizes; time to load;

loading rate; peak and off-season Primary receiving focus rail; truck Receiving frequency train/truck arrivals Hopper design bu; above or below ground; gravity

or mechanical transfer Distribution Transfer capacity bu/h Type of transfer gravity; conveyor Destinations (bin locations) Bin inlet locations number; size; capacity (bu/h) Inbound scaling capacity bu/h Spout design round; square; lining; capacity

(bu/h) Storage Distinct commodities number; type; properties Required storage volume bu Nominal roof height ft Bin loading locations number; size; capacity (bu/h) Bucket elevator discharge height ft Support tower height ft Reclaim Bin hopper bottom central; offset; side draw; flat floor Bin main discharge size; capacity (bu/h); number;

square; rectangular Bin cleanout single outlet; multiple outlet; bin

sweep; skid loader door Type of reclaim gravity; conveyor Destinations (loadout transfer) Capacity (loadout transfer rate) bu/h Destinations (internal recycling) Capacity (internal recycling rate) bu/h Loadout Capacity (outbound) bu/h; container sizes; time to load;

loading rate; peak and off-season Outbound scaling capacity bu/h Primary loadout focus rail; truck Loadout frequency train/truck arrivals Hopper design bu; mechanical fill; gravity fill

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Table 6. Essential data necessary for process design of grain elevator secondary systems. Category Criteria Required Data Cleaning Capacity (inbound) bu/h; efficiency Capacity (outbound) bu/h; efficiency Type of cleaner screener, scalper, indent,

universal, destoner, etc. Aeration Capacity cfm/bu; total cfm Aeration floor design layout configuration Bin venting (roof) size; number; type; rate (cfm) Drying Capacity moisture removal points/bu;

efficiency Type of dryer large grain, small grain; tower,

stack Dust Control Capacity cfm Type of dust system cyclone, bag filter; pit baffle;

mineral oil Locations Pick-up points Sampling Capacity (inbound) bu/h; kg/sample Capacity (outbound) bu/h; kg/sample Instrumentation and Control Types of sensors to use Locations of sensors Motor soft starts Variable-speed motors Total number of motors Total electricity draw kW; V; A Largest single motor hp; kW Largest single electricity draw kW; V; A Maximum facility demand kW; V; A Maximum line electricity available kW; V; A MCC size ft2 MCC location on site

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Figure 1. Major components of a typical country or terminal grain elevator.

Figure 2. Process flow diagram for a typical large grains country or terminal elevator.

Receiving

Distribution

Storage

Reclaim

Loadout

Incoming Grain

Outgoing Grain

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Figure 3. Process flow diagram for a typical small grains country or terminal elevator.

Figure 4. Process flow diagram for a typical small grains elevator cleaning floor.

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(a)

(b)

Figure 5. Gravity flow (a) and mechanical conveyor (b) receiving systems.

23

(a)

(b)

(c)

(d)

Figure 6. Distribution systems generally entail combinations of bucket elevators (a), distributors and spouting (b), belt conveyors (c), and/or drag conveyors (d).

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0.0

20000.0

40000.0

60000.0

80000.0

100000.0

120000.0

140000.0

0 4 8 12 16 20 24 28 32 36 40 44

Diameter (in)

Spou

t Cap

acity

(bu/

h)50 bph/in260 bph/in265 bph/in270 bph/in280 bph/in290 bph/in2100 bph/in2

Figure 7. Volumetric flow capacities for round spouting.

0.0

20000.0

40000.0

60000.0

80000.0

100000.0

120000.0

140000.0

160000.0

180000.0

0 4 8 12 16 20 24 28 32 36 40 44

Width (in)

Spou

t Cap

acity

(bu/

h)

50 bph/in260 bph/in265 bph/in270 bph/in280 bph/in290 bph/in2100 bph/in2

Figure 8. Volumetric flow capacities for square spouting.

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Figure 9. Common grain elevator layouts (bin plans).

(a)

(c)

(b)

(d)

Figure 10. Geometric models of a round grain silo with multiple inlets: plan view (a), side view (b); and interstice bin: plan view (c), side view (d).

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(a) (b)

Figure 11. Flat-floor, multiple discharge (a) and side-tap gravity flow (b) reclaim systems.

(a)

(b)

Figure 12. Geometric model of side-tap gravity flow hopper and discharge: plan view (a), side view (b).

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(a)

(b)

(c)

Figure 13. Single-car, gravity flow, internal bulk scale (a), single-car, mechanical fill, external bulk scale (b), and multiple-car, gravity flow/mechanical fill, external bulk scale (c) loadout

systems.

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(a)

(b)

Figure 14. Large-grains gravity flow screener (a) and scalper (b) grain cleaners.

(a)

(b)

(c)

Figure 15. Common silo aeration duct patterns (a), roof-mounted gravity vent (b) and roof-mounted exhauster (c).

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(a)

(b)

(c)

Figure 16. Common grain receiving truck probe (a), loadout sampler (b), with sample divider (c).

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(a)

(b)

(c)

(d)

(e)

(f) (g)

Figure 17. Common sensors used for flow (a, b), silo level (c, d) and conveyor operation (e, f, g).

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Figure 18. General procedure for conducting grain elevator process design.

Flow Diagram

Bin Layout/Arrangement Bin Sizes/Capacities

Facility Layout

Process Design

Primary Systems

Secondary Systems

Client Req. Building Codes Standards

Structural Design

Final Design

grain elevator paper part ii

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