AirPol of Parsippany, NJ, installed two Venturi scrubbers at a Georgia mill in order to meet new, more stringent air compliance regulations.The old system, installed in 1974, worked very well until th…
AirPol of Parsippany, NJ, installed two Venturi scrubbers at a Georgia mill in order to meet new, more stringent air compliance regulations.
The old system, installed in 1974, worked very well until the capacity of the two boilers was increased in the mid 1990’s. As part of an upgrade, the mill was required to comply with the new regulations for emissions from its No. 1 and No. 2 power boilers. These multi-fuel boilers burn an assortment of wood/bark, sludge, coal, tire derived fuel (TDF) and oil. The boilers produce approximately 500 000 lb/h of steam each, and operate 24 h/d, 350 d/y.
To handle process gas volume fluctuations, each Venturi was equipped with a double bladed large diameter adjustable throat Venturi scrubber, which ensures constant pressure drop to accommodate variations in flue gas volume. Also included was a cyclonic separator, exhaust stack and recycle tank. The project also included structural support towers, stairways, ductwork, instrumentation, demolition of existing equipment and installation, commissioning and start-up.
BEARINGS: Marathon Saves $20k/Y With Graphite
Since installing GRAPHALLOY, graphite metal alloy bushings in an unbleached brownstock tank agitator, Marathon Pulp Inc., Marathon, ON, is said to save up to $20 000 annually in normal maintenance costs. Unplanned shutdowns to fix broken bushings are also prevented, which cost about $300 000 in lost income. Stock tank agitators present an especially challenging bushing application because of the corrosive and abrasive nature of the stock. Marathon Pulp’s original bushings wore so much that they always had to be replaced at annual maintenance inspections. Often the entire bushing assembly had to be replaced as well. Occasionally bushings failed between annual inspections, causing a costly emergency shutdown. The company has now replaced the old bushings with GRAPHALLOY, a proprietary grade of self-lubricating graphite-metal alloy bushings. These experience much less wear in the harsh brownstock environment and do not need to be replaced yearly.
While the brownstock is in the various chests, it must be kept circulating. Many of the chests are equipped with Alexander Fleck 60-inch propeller agitators for this purpose. These are horizontal, side-mounted devices with the motor positioned outside the chest. The shaft coming off the motor goes through a right angle gearbox, through some packing at the wall of the stock chest, and continues inside the chest. There, the 12-foot shaft connects to a three-blade propeller through a spider arm assembly. The shaft is stainless steel, and because it is submerged, the use of a ball bearing isn’t possible. Instead, the spider assembly contains a housing containing a bushing that acts as a steady bearing. “The agitator is a critical piece of equipment,” says Rick Manuel, fibre unit maintenance leader at Marathon Pulp. “If it breaks, it affects consistency of the stock, and slows the flow to the rest of the plant.” The only problem is that the bushings wear out too quickly, and sometimes fail between annual inspections.
Replacing these parts meant shutting down the process upstream of the stock chest, draining the tank, and calling in a vacuum truck to remove the remainder of the stock that couldn’t be pumped out. Then the chest had to be ventilated to cool it down. And since it is a confined space, maintenance personnel must go through a special vessel entry procedure involving a great deal of testing to make sure it is safe to enter.
Manuel read about bushings made of graphite-metal alloys that take advantage of the special properties of graphite. A graphite structure can be compared to a deck of cards with individual layers able to easily slide off the deck. This phenomenon gives the material a self-lubricating ability, making external lubricants unnecessary. The graphite matrix can be filled with a variety of impregnants to enhance chemical, mechanical and tribological properties.
Graphite Metallizing recommended Grade GM105.3 babbit-impregnated GRAPHALLOY bushings. Manuel decided to purchase one and install it on his most problematic stock chest, the unscreened brownstock chest. “This stock is high in alkalinity and very abrasive, so its bushings wore out the fastest.” One GRAPHALLOY bushing was installed in May 1999. It ran well for a full year and then was inspected during the May 2000 scheduled maintenance. There was only minor wear. The success of the bushing in this, the worst application, led Manuel to install two additional Grade GM105.3 bushings on the agitators in the unscreened and screened bleach chests. He is also considering GRAPHALLOY throttle for use in the company’s high-density tower agitators during a scheduled maintenance planned for 2002.
The material works at temperatures up to 1000F (525C) where oil-based lubricants burn off or oxidize and plastics fail. It maintains its integrity even when submerged in hostile liquids such as acids, alkalies, hydrocarbons, black liquor and liquid gases. The material provides a constant, low coefficient of friction. GRAPHALLOY is a solid, uniform, self-lubricating material, rather than just a surface layer, helping to protect against catastrophic failure. Lubrication is maintained even during linear motion; lubricant is not drawn out and dust is not pulled in.
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