Pellet Transfer: Preventative Measures Employed to Reduce Streamer Formation
- Published: May 01, 2001, By Jeff P. Pro and Andrew G. Womack, Chevron Chemical Corp.
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Polymer pellets typically transfer through dilute phase pneumatic conveying systems. Pellets are entrained in streams of air and effectively “blown” from one location to another. A resin supplier location uses this method of transfer to move pellets from production areas into railcars. A final article manufacturer also uses the technique to move pellets from a railcar to holding silos and equipment hoppers. The transfer of pellets at both facilities can cause accumulation of resin imperfections and contamination. These can significantly influence processing.
As pellets blow through transfer pipes, they tend to slide along the walls of the piping and fittings. This sliding action produces frictional heat that can cause the pellet to melt and smear along the walls of the transfer pipe. The melted smear can release from the walls of the pipe creating a streamer. Other terms are angel hair and snake skins.
The movement of plastic pellets through transfer systems also generates polymer dust, fluff, and fine particles. The presence of streamers and dust can cause downstream processing problems and contamination issues when several different resins transfer through the same system.
As streamers move through the system, they tend to entangle into larger aggregates. The tangled streamers can plug transfer lines and restrict the movement of product throughout a manufacturing site. Since most transfer equipment is designed to move polymer pellets, the presence of streamers will negatively influence the function and effectiveness of certain equipment. Filters, feeders, and blowers are susceptible to decrease in performance when streamers occur in a system. Railcar loading and unloading systems will become less efficient with an abundance of larger streamers. Even silo level indicators will perform poorly when streamers hang up on level probes. Besides a decrease in transfer system efficiency, the presence of streamers, dust, fluff, and fines can have several negative effects on downstream extrusion.
Resin Imperfections
The finishing process for polyethylene pellets including transfer in and out of bulk shipping mechanisms can lead to several imperfections in the shape and form of a polymer. These imperfections can cause several downstream processing problems or introduce contamination. The following are different types of resin imperfections:
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Streamers, angel hair, or snake skins: These form as transferred plastic melts and smears along the walls of transfer pipe. With time, the melted plastic accumulates and forms large ribbons or streamers that flake off into the moving pellet stream.
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Tails: These are typically the result of ineffective pellet cut at the resin supplier. As molten polymer passes through the die plate, the pelletizing knives do not achieve an effective cut. Essentially, the pellet has an attached “tail.”
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Clusters: These are the fusion of two or more partially melted pellets. They are usually the result of a blower that is too hot. The heat causes entrained pellets to melt together.
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Fluff or fines: This term is a catch-all for other small particles and dust created in the transfer system. The term describes any small order streamers or snake skins, any tails freed from the pellets themselves, small polymer chips formed at rotary feeders, and any other polymer fluff or dust present in the transfer system.
Reducing the effects of streamers involves three techniques:
- Minimizing the potential to form streamers in the system
- Removal of streamers before downstream use
- Good resin handling practices.
Minimizing formation of streamers in a transfer system involves focus on the key components of the transfer system. The most basic transfer systems consist of holding vessels (silos), a blowing mechanism, transfer piping, and fittings. Each component can have a design and operate in a manner that reduces formation of streamers.
The design of piping systems for use in resin transfer requires special consideration. The design should use the shortest possible path with the least number of bends and fittings. Transfer pipe should always be completely horizontal or vertical. Diagonal piping layouts increase the sliding effect of polymer pellets and can cause formation of more streamers.
Transfer pipe fittings or elbows cause changes in the directional movement of plastic pellets through the transfer system. The movement of plastic pellets through elbows and other fittings increases the surface contact between the plastic and metal. This increased contact can cause more formation of streamers and can cause wear to the transfer system.
The typical elbow fitting design used in the transfer of plastic pellets is the long radius elbow. These fittings use a design and size involving the ratio of the bend radius to the pipe diameter. Long radius elbows allow a gradual change in direction of pellet motion and minimize pressure drop across the fitting. The long radius design actually promotes the sliding of pellets along the inside of the bend. This sliding motion not only leads to the formation of streamers but also results in significant wear of the fitting itself. Product transfer quality, short lifetime, and maintenance are significant issues with long radius elbows. Figure 1 shows several alternative elbow designs used in transfer systems.
The blower mechanism is a key component of any dilute phase transfer system. The blower produces the air stream necessary to entrain plastic pellets and effectively move them from location to location. Typical blower systems come in three forms — positive pressure, positive vacuum, or a combination of both. The type of blower system used depends on the desired path of the material transferred and the position of the blower relative to that path. Blowers can cause streamer formation by introducing more heat into the transfer system. Increased temperature will facilitate easier melting of plastic as it slides along piping walls and promote formation of streamers in the system.
The key design consideration for blowers is the capacity to deliver sufficient air pressure to move plastic pellets. Other features can be part of the blower system to minimize formation of streamers. Blower coolers can be effective means of maintaining ideal temperatures throughout the transfer system and eliminating the effects of hot blower air. These coolers are also effective in countering the effects of environmental heat from outdoors. The “air conditioning” effect of blower coolers can significantly minimize the potential to form streamers.
Good Resin Handling Practices
Design and operation of transfer systems are key factors in limiting the formation of streamers. Several simple guidelines can improve the efficiency and quality of product transfer. These are system design, system operation, and other practices.
Transfer piping should run horizontally and vertically only. Diagonally sloped lines can increase pellet slide and allow product to drop back in the system. The length of transfer pipe from the storage silo to the processing area should not exceed 300 ft. The number of directional changes in the layout should be minimal to reduce pressure drop. Plugged fittings and special bends should be used whenever possible to reduce the formation of streamers and fines.
For pellets to accelerate to conveying velocities, an initial section of straight piping is necessary. The general rule of thumb is that a straight section equal to 25 times the pipe diameter is necessary before the first vertical bend.
The piping and fittings of the transfer system require inspection on a regular basis to assess the amount of wear on the system. Heavily worn piping, elbows, and other fittings require replacement or servicing before continued operation. Straight sections of pipe can be rotated 90 or 180 degrees to extend their service life.
Flexible hoses used in the loading and unloading of railcars should undergo inspection to ensure that product flows in the direction of the coils or spirals. The flow of product in the opposite direction can create fines and dust.
Product silos require maintenance at higher levels whenever possible. Streamers and fines present in the resin collect at the top of the silo during loading and unloading cycles of the same product. Complete emptying of the silo can introduce a higher concentration of streamers and fines from the top of the silo into the pellet stream.
Conclusions
Transfer of polyethylene pellets is a key component of product quality for resin suppliers and resin consumers. The presence of resin imperfections such as streamers, fluff, and fines can be detrimental to the extrusion process used to produce films and coatings. Several measures are available to address the problem of streamers.
Using measures to reduce the formation of streamers in the transfer system is important. The use of equipment to filter and remove streamers from product in the pellet form and following good resin handling procedures will cooperatively work to eliminate problems associated with resin imperfections.
The transfer systems used in the polyethylene industry often receive consideration as the trivial means by which product moves from bulk transport to processing areas or vice versa. Careful consideration in the design and operation of these systems will greatly improve the quality output of any facility. Investment in maintenance and improvement of transfer systems is essential to the production of quality products.
