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Reprinted From Powder and Bulk Engineering, April 2004
All unload! Next stop, Emptyville
A company installs a new bulk railcar unloading system to improve its pelleted PET unloading operation.
Amcor PET Packaging, a global packaging company headquartered in Manchester, Mich., operates 48 manufacturing sites in 18 countries worldwide. At its plant in Mississauga, Ontario, the company manufactures polyethylene terephthalate (PET) preforms from pelleted PET resin that arrives at the plant in bulk railcars and is conveyed to large storage silos before being processed. The company recently added more injection molding machines to its production line to increase its production capacity. However, after adding the new equipment, the company wasn’t able to unload the PET fast enough to keep up with the increased production. To solve this problem, the company worked with a bulk materials handling systems supplier to install a new bulk railcar unloading system.
Amcor’s Mississauga plant receives the pelleted PET in 200,000-poundcapacity 65-foot-long bulk railcars that have four independent storage compartments, each with a bottom discharge hopper. The company pneumatically unloads the approximately 1⁄8-inch-diameter pellets from the railcar compartments and conveys them into six 290,000-pound-capacity silos. From the silos, the pellets are conveyed to dryers before being fed to the injection molding machines.
In the past, the company used a vacuum unloading system to unload the PET, conveying it in batches to a 5cubic-foot-capacity vacuum hopper located on a silo’s top deck. When the hopper was filled, the unloading system stopped moving pellets and the hopper discharged them into the silo. The unloading system then filled the hopper again. The company could empty a railcar’s four storage compartments in about 24 hours, but this wasn’t fast enough to keep up with its new production capacities.
Additionally, the unloading system caused angel hair to develop in the conveying lines and damaged some of the pellets during conveyance, breaking them into fines that created dust and made the PET unusable. The angel hair and fines also caused problems with the
The unloading system consists of two independent parallel unloading systems that each have (from left to right) a fines-removal system, an electric vacuum-pressure blower, a P/D pot, and various lengths of conveying line manifold.
silo hoppers and the drying operation’s filters that required shutting down the unloading system and production line for maintenance. These problems decreased the line’s production capacity and efficiency and increased maintenance and operating costs.
Moving in the right direction
To solve the unloading system problems, the company looked at several options, including expanding the existing system by adding more silos and blowers. The company discussed the various options with its dryer supplier to ensure that the drying operation would be properly integrated with an expanded or new unloading system. During this time, the dryer supplier recommended that the company contact ALL-CON World Systems, Seaford, Del., a supplier of automated and integrated bulk material handling systems, including feeding, weighing, and conveying systems for the chemical, plastics, and food industries.
The company met with the systems supplier’s reps and gave them operating specs for a new unloading system that would unload the pellets at a minimum of 25,000 lb/h, eliminate angel hair in the conveying lines, and reduce the amount of fines created during conveyance. Company members then traveled to a plant operated by one of the system supplier’s customers to witness a bulk railcar unloading system in action. Based on what they saw, the company decided to purchase a system for its plant.
The bulk railcar unloading system
In summer 2000, the systems supplier installed the bulk railcar unloading system at the company’s plant. The overall system consists of two independent parallel unloading systems (system 1 and system 2) that each use one blower to create both vacuum for unloading the material from the railcar to a hopper (also called a pressure differential pot or P/D pot) and pressure for moving the material from the P/D pot into the storage silos. The systems continuously move material through the conveying lines using more material per cfm of air than most dense-phase systems use so that the lines are always full of material during operation. Additionally, each system operates independently of the other so that one can be in vacuum mode while the other is in pressure mode.
System 1 and system 2 each have one 75-horsepower electric vacuum-pressure blower, one P/D pot, one fines-removal system, one inlet vent, one stainless steel connection hose, about 720 feet of 6-inch-diameter conveying line manifold, two 6-inch-diameter manifold Y-branches, 41 material pickup points, various valves and other components, and various lengths of 6inch-diameter piping to connect the systems to the silos. Both unloading systems are controlled by one centrally located Allen-Bradley PLC that allows an operator to view each unloading system’s operation in real-time.
Each system’s fines-removal system, electric vacuum-pressure blower, and P/D pot are located side by side inside the plant. A conveying line manifold connects to each P/D pot and extends through the plant’s wall to an outside unloading area where up to 10 railcars are positioned on two rail tracks. A pressure discharge line also extends from each P/D pot, passes through the exterior wall, and goes to a silo-quickchange station located near the silos.
The silo-quick-change station consolidates the six silo conveying lines into one location the operator can easily access. Each conveying line runs to a silo’s top deck where it connects to a target box. The target box is a large material-transition device with a material pouch located directly across from the conveying line inlet. As material enters the target box, it packs into the material pouch, providing material-on-material impact to reduce the incoming material’s velocity as it transitions 90 degrees from the horizontal conveying line into the silo.
The unloading operation
In the railcar unloading area, track 1 (the main line track) runs parallel with the plant’s exterior wall that the manifolds extend through. Track 2 (a siding track) also runs parallel to the plant’s exterior wall and connects with track 1 about 280 feet from where the manifolds extend through the wall. The system manifolds, which are stacked on top of each other, run parallel with each track.
Track 1 has six railcar unloading stations and track 2 has four. Each unloading station has six pickup points per manifold (except track 1, position 1, which has 5). The pickup points are located about 10 feet apart and are positioned to correspond to the railcar hopper discharge outlets so that each outlet can be connected to either one of two pickup points. Each manifold connects to a dedicated P/D pot, so material from one railcar compartment can’t end up in both P/D pots.
To unload the railcars, an operator first positions them so that one is at each unloading station and their hopper discharge outlets line up with the corresponding pickup points. The operator chooses which hopper discharge outlet to unload material from and attaches the inlet vent to a port on the opposite side of the hopper from the discharge outlet. The inlet vent allows air into the system so that the blower can efficiently pull the material from the railcar compartment. The operator attaches an adapter with a quick-change coupler to the hopper discharge outlet and connects the stainless steel hose to it. The operator then connects the hose to the corresponding pickup point on one of the manifolds. The operator also connects the system’s pressure discharge line to the appropriate silo conveying line at the silo-quick-change station.
To simultaneously unload a second railcar compartment from the same railcar or a different one at another unloading station, the operator connects the other inlet vent and adapter to an open hopper discharge outlet, the stainless steel hose to the corresponding pickup point on the other manifold, and the second discharge line to a silo conveying line. (The company can unload two railcar compartments at a time — one with each manifold.)
An air line with a valve assembly is attached to the P/D pot’s top and functions as both a vacuum and pressure line, depending on how the valve assembly is set. After the operator accesses the controller and hits the start button, the controller activates the vacuum-pressure blowers and switches the various valves in the top air lines to create a vacuum in the system. In vacuum mode, a blower pulls air and material from a railcar compartment and through the manifold, the P/D pot, and the fines-removal system. The pellets are pulled into the pot in 2,500-pound batches through an inlet located about one-third the way down from the pot’s top, filling the pot in about 60 seconds. The supplier set the vacuum pressure so that the pellets fall to the pot’s bottom and the fines are pulled through the top conveying line to the fines-removal system.
Discharging to the silo
When the P/D pot reaches capacity, a level detector mounted inside the pot just below the inlet signals the controller to switch the unloading system from vacuum to pressure mode, and the blower forces air into the P/D pot through the top air line, pressurizing the pot. At the same time, the blower forces air through a pressure line that branches off from the top air line just above the valve assembly. This line runs down behind the P/D pot and connects with the pot’s bottom discharge and the pressure discharge line that connects to the silos’ conveying lines. Air from this line fluidizes the material inside the pot to aid material discharge and provides pressure in the discharge line to move the material to the silos.
When the P/D pot is empty, the controller senses low pressure and switches the unloading system back to vacuum mode, and the unloading process continues until the railcar compartment is empty. Because the unloading system doesn’t use scales to keep track of the material amount unloaded from the railcars and the controller doesn’t keep track of the number of batches sent to the P/D pot, if the P/D pot level indicator isn’t tripped after about 21⁄2minutes during vacuum mode the controller automatically switches to pressure mode to discharge any material in the pot. At the same time, an alarm sounds, alerting the operator that the railcar compartment is empty. The operator then moves the inlet vent and stainless steel hose to another hopper discharge outlet and corresponding pickup point on the manifold and restarts the unloading system.
The fines removal system
In vacuum mode, the fines-removal system’s bag filters capture the fines and any angel hair that may have been present in the PET, allowing dust-free air to be pulled to the blower and exhausted to the plant. A reverse air-pulse system cleans the filters, dropping the material to the fines-removal system’s bottom hopper where a rotary airlock meters the material to a drum. The rotary airlock, which only operates during vacuum mode, maintains the air pressure in the unloading system. A level detector in the drum sounds an alarm that alerts the operator when the drum is full, and the controller prevents the system from going into vacuum mode until the operator empties the drum and hits a reset button. During pressure mode, the valves stop the airflow to the fines-removal system so that it isn’t affected by the pressurized air.
Unloading system proves successful
Since installing the new bulk railcar unloading system, a railcar can be unloaded in as little as 6 hours, which has enabled the company to increase its production capacity by 50 percent. “The unloading system also exceeded our original specifications for unloading the pellets,” says Mark Smyth, AMCOR’s group injection molding manager. “From the furthest point, which is more than three hundred ninety feet from the P/D pot, the unloading system unloads the pellets at more than thirty thousand pounds per hour, and, from the closest point it unloads them at more than fifty thousand pounds per hour.”
Additionally, the unloading system allowed the company to remove the silo hoppers and replace them with target boxes, which reduce material degradation and require very little maintenance. And because the unloading system uses air and fluidization techniques rather than mechanical devices such as rotary airlocks to convey material, fines have been drastically reduced and angel hair has been eliminated during conveyance. This has decreased the amount of time the company spends maintaining the drying operation’s filters and has improved the operation’s production capacity and efficiency. PBE Note: To find other articles on this topic, go to www.powderbulk.com, click on “Article Index,” and look under the subject headings “Loading, unloading” and “Pneumatic conveying,” or see Powder and Bulk Engineering’s comprehensive “Index to Articles” in the December 2003 issue .
ALL-CON World Systems, Seaford, DE