Bulk material handling operators are seeking engineered systems that not only minimize spillage and dust excursions but those that do so while increasing the duration of major maintenance intervals. While at times these demands are in conflict with one another, Acensium’s experience has shown that diligent inspection and soft goods maintenance, combined with robust initial design, can produce the results these operators seek.
Transfer Point Retrofit Criteria
Coal Chute Design
Introduction
Transfer Point Replacement
Most transfer points we inspect appear to have received the minimal amount of care to maintain effective material transfer performance. At times their shells have been replaced in kind, liners have been barely maintained and significant spillage appears to be caused by chute failure. Furthermore, air control through these transfer does not appear to have been addressed during design or retrofit through the proper head-chute and load-zone sealing approaches.
Transfer point technology has advanced significantly since most plants were constructed. Transfer point features and materials now exist that allow better material control, while extending maintenance duration. The following discussion highlights the key criteria we have learned that if included, will result in a successful transfer chute retrofit project.
#1 Stainless Steel Exterior Shell Construction
Coal handling system engineers began to specify stainless steel construction for their chute work in the late 1990’s. The initial surge in the use of this material was correlated to the excessive corrosion these operators were experiencing after the conversion to low sulfur, western coals.
While more difficult to work within the fabrication shop and even more so in the field, stainless steel offered the advantage of providing a shell that could last as long as the new materials that were being used to line impact and sliding areas inside the chute.
Arguments have been made that coating the shell with an epoxy-based coating will have equal protection with less overall cost. Acensium’s experience has shown that coatings are typically damaged during installation, damaged in during the years of operation, never properly repaired and never properly recoated at any time during the service life of the equipment.
The result is a system that does not fulfill its projected service life, requires premature replacement and does not meet its required return on investment as originally proposed during funding requests.
#2 Impact and Sliding Wear Liner Application
Transfer point design should seek to minimize impact within the chute. Physical impact is the major cause of liner degradation through direct material fracture (ceramic) or “washing” of the metal matrix (overlay products). In addition, impact leads to further fracturing of the coal, creating more dust with the potential for liberation from the system. Transfer point design to minimize impact will be discussed below in more detail.
Acensium’s experience with wear liner application has demonstrated the best wear performance per dollar spent to be with ceramics. These materials are designed into the transfer point, detailed on the drawing, produced at the factory and are easily replaced should a catastrophic event occur, such as the passing of tramp iron through the chute.
Acensium has viewed these liners in properly configured chutes that show little to no wear after eight years of operation, passing over twenty million tons of coal.
#3 Hood and Spoon Arrangements
In the mid 1980’s Alan Huth first conceived a hood and spoon arrangement for material handling transfers. The concept of the hood was employed to consolidate the material flow as it left the delivery belt with a curved hood arrangement. The hood was constructed with curved or angled panels that closely followed the projected trajectory of the material.
By following the trajectory of the material, the impact could be reduced, or even eliminated. In addition, the hood had the effect of “squeezing” the air from the material, leaving a very dense material stream to pass through the chute, with little to no entrained air.
Located just above the receiving belt, at the bottom of the chute, the spoon delivers a similar impacting function. As the dense material stream from the hood travels to the receiving belt, the spoon presents a series of curved surfaces that ultimately discharges the material in the direction of the received belt.
Some spoon designs claim to discharge the material at not only the direction of the receiving belt but also at the same velocity. When this is achieved, material flow on the receiving belt appears very settled, absent of the usual turbulence that tends to liberate dust on the receiving belt.
#4 Belt Support in the Load Zone
While the spoon arrangement in the load zone minimizes impact onto the receiving belt, proper belt support is still needed in this area. Keeping a straight belt line in the load zone eliminates any “trampoline” effect caused by coal loading.
The belt support design in this area should allow no droop between support idlers. As the belt leaves the load zone, support can be achieved with standard idlers on two-foot spacing, returning to standard spacing at the end of the dust enclosure.
#5 Impact and Sliding Wear Liner Application
Skirting in the load zone shall be of an airtight design and construction. The skirting shall begin with a tail box immediately before the load-spoon, allowing tight sealing of a typically pressurized area. The skirt walls should continue forward past the spoon for a distance and at a height to effectively slow the air flow generated by the transfer chute. The walls should be joined with a top cover. Dust curtains shall be placed, per the design, to interrupt air flow moving through the enclosure.
Wear-liners of a ceramic construction shall be placed along the skirt walls and located with very close tolerance to the belt top cover. The wear-liner’s function is to contain material on the belt. Too often this function is left to the rubber skirting to achieve. Soft goods are not effective at containing material and subsequently wear through quickly. It is imperative that the soft goods are only designed to contain dust-laden airflow inside the skirting enclosure.
Skirt walls and covers, or the skirt enclosure, should be fabricated from stainless steel, along with the balance of the transfer point. While this material choice adds costs, it shares the same advantages outlined in Key Criteria #1 above.
#6 Head Chute Sealing
It is imperative to seal all head chute openings – between the enclosure and the material load, between the carrying and return side of the belt and below the return side of the belt. All shaft penetrations need to be sealed, along with all belt cleaner openings. Doors need to positively close against gaskets.
#7 Belt Cleaning
An often overlooked source of dust liberated in tripper and conveyor galleries is directly attributable to carryback on the conveyor. Dust clings to the belt due to static, resides in cracks and crevices of worn top covers and adheres due to environmental conditions such as rain or ice. This dust is liberated as the conveyor travels over the head pulley, snub pulley or takeup/bend pulleys. This dust is usually below 10 microns in size and becomes airborne or float dust.
No belt cleaning method will remove all of the dust from the top cover, except a wash-box arrangement, which is not typically practical in a power generating, coal handling environment. Acensium recommends no less than two belt cleaners per transfer point and where space allows, a third cleaner should be installed.
Utilize tungsten carbide (TC) tips on the cleaning edge of all cleaners, including those in the peeling or primary arrangement. The TC tips will extend the life of the cleaner; with proper installation of the cleaner there will be no adverse effect on the life of the belt top cover.
Belt cleaners demand frequent inspection to ensure proper cleaning and to prevent damage to the belt. Proper selection of the belt cleaner arrangement will lead to increased maintenance intervals and reduced expense on replacement blades. Acensium has experience in specifying mine grade cleaners with long service lives.
Material Handling Dust Control
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