System modeling speeds clamshell unloader delivery
Enhanced dust control concepts and design studies found best method to ensure quick, safe clamshell unloader transport and assembly
By James W. Schuster and Adolf H. Zirkler, McNally Wellman Co., Svedala Industries, and Gene Duke, U.S. Generating Co., Logan Generating Station
A new facility, U.S. Generating Co.?s Logan Generating Station, was built in New Jersey, along the Delaware River and four miles from Chester, Pa. At the outset, concerns arose over possible unusual regulatory issues because the plant?s coal barge unloading system extends into the river where it falls under the jurisdiction of the State of Delaware.
However, the project contract with the equipment supplier avoided complications by calling for a turnkey project, including erection, start-up, commissioning and training. The supplier responded by using a modeling technique to ensure environmental compatibility.
The contract called for one stationary clamshell bucket grab unloader, complete with a dust control system, barge haul and barge breasting systems, and auxiliary cranes for handling the barge haul lines.
Bucket coal capacity is 10 tons at 50 pounds per cubic foot density. When operating on a 40-second duty cycle, the unloader is rated at 910 tons per hour free digging capacity. Under dry, high dust conditions, the duty cycle is extended to 50 seconds to allow for pause time after the bucket closes and while over the hopper prior to bucket discharge.
At the contract?s outset, U.S. Generating specified constraints (the unloader had to be erected within 14 months) that led to a construction scheme involving offsite preassembly and the entire loader?s transport to the site.
Protecting the environment
Environmental considerations were a key factor in the unloader?s design. The handling equipment design minimizes dusting to 10 percent opacity or less while it is operating in winds of up to 25 mph. Opacity tests, incorporated as part of the contract and conducted by the manufacturer?s engineers, meet EPA Method 9.
Dusting in clamshell grab bucket unloader operation occurs when the bucket payload discharges into the unloader?s hopper. Many unloaders have wet (water) dust suppression systems that activate when the bucket approaches the hopper and then turn off after the bucket leaves the hopper area.
A proposed plan for Logan involved collecting the dust in a dry state. The area above the hopper would be enclosed on three sides and would involve rubber skirts along the front of the hopper. Vertical mounted air curtains across the enclosure opening were to deflect unwanted air currents. Four rows of dust collectors were to entrap all fugitive particulates.
Restrictive opacity limits for this site prompted tests of the concept through modeling. The equipment manufacturer?s tests of models took place concurrently with the unloader design process in a Dyna Gen Inc. laboratory in Cambridge, Mass. Tests were carried out in a progressive sequence wherein changes were made to the model as dictated by step-by-step test results. The goal was to achieve zero emission bucket discharge without wind, then with wind, and with subsequent bucket withdrawal.
Test model objectives were to:
Y Confirm the effectiveness of in-sertable dust collectors and air curtain sealed hopper enclosure in achieving minimum dust emissions during bucket discharge and withdrawal;
Y Optimize dust collection;
Y Evaluate the effectiveness of hopper baffles;
Y Determine factors that may influence the system?s effectiveness in fugitive dust emission prevention; and
Y Optimize the required dust collector cubic feet per minute (CFM).
The unloader?s two sides, rear and hopper top are completely closed. The front is sheeted up to the trolley and bucket clearance lines. Areas beyond that are covered with slotted rubber to allow only that space essential to normal bucket passage open to the environment. Vertical curtains lay a jet stream of air across the enclosure?s open area to neutralize extraneous air currents. Insertable dust collectors with the filters exposed to the hopper return dust to the hopper during filter cleaning.
The hopper has a longitudinal oriented inverted vee center baffle to direct discharged material toward the hopper side walls. This baffle blocks the center portion of the hopper against expansion of potential dust plumes and directs them toward the enclosure sides and dust collectors. Filters in the lower row of dust collectors extend into the plenums? open areas on the bottom to enhance plume flow to the dust collectors and avoid dust plume expansion into the bucket. The hopper grizzly?s outer five spacer bars are slanted to direct rising dust plumes toward the lower row of dust collectors. An inclined baffle blocks the rise of a dust plume from the hopper?s front lip next to the air curtain.
The unloader model
The barge unloader model hopper and enclosure was scaled to 1/5.65th of actual size and built of clear plexiglas and plywood. Simulated wind, generated by a moveable 2 by 2 matrix of propeller fans, corresponded to a 25 mph and 30 mph wind respectively. A grid and a blower system with exhaust ports were added to provide uniformity of wind flow and to create an exhaust capacity of 873 CFM at the dust collectors. Insertable collectors were simulated in plexiglas.
Tests proceeded with insertable dust collectors in conjunction with vertical arranged air curtains at the hopper enclosure. However, this concept did not provide zero emission bucket discharge or clear the enclosure of dust in the allotted time.
The initial design configuration was first modified by bringing the dust plenums down to the top of the grizzly. Then the upper and lower rows of collectors were combined into a single row of insertable collectors to better use the available exhaust capacity and this collector concept.
Tests carried out with the modified configuration showed no visible emission during the bucket discharge for a 25-mph wind. Tests also showed that no dust is withdrawn in the wake of an exiting empty bucket under the same wind conditions.
Tests demonstrated that the optimum arrangement for preventing fugitive dust emission for a clamshell bucket discharge is to capture dust below the hopper grizzly. No practical amount of exhaust can overpower the flow patterns set up by air curtains and wind and it is thus more practical to keep the dust out of the influence of these flow patterns.
Design and construction
Tests of the model dust collection concept revealed that modifications were needed after much of the overall unloader design was complete and the 14-month contract schedule was threatened by the change after two and one-half months of work. For this reason, McNally Wellman contracted with F.W. Hake Inc., Eddystone, Pa., to fabricate the unloader structural steel as well as perform the structural/mechanical and electrical erection. Hake?s engineers preassembled the unloader off site and transported it to the site in three large sections. This scheme offered better control and the preassembly was done on land rather than on floating rigs. Therefore, the unloader was designed for a large piece erection sequence and lifting lugs were built into major subassemblies.
The preassembled unloader was moved by traveling gantry cranes from the Hake facilities onto barges for transport to the Logan dock. The sections were the lower tower, the combined 80-plus ton electrical/machinery house and the upper tower, complete with apron and hopper.
On Friday, October 29, 1993, the lower tower section was loaded aboard a barge and a 300-ton floating crane off-loaded the unloader sections at Logan?s dock in final position. The lower tower section was set on its anchor bolts that same day.
The second lift at site was the electrical/machinery house which was moved to site and assembled to the lower tower structure. The fit of this piece was critical. When fully erected the weight of the apron would offset the moment of the electrical/machinery house. However, during the erection sequence, the entire moment of the electrical/machinery house had to be resisted by the lower tower and, in turn, carried into the dock foundations. It worked to perfection.
The upper tower/apron was set in place on the lower tower and the unloader was erected. The electrical/machinery house was then turned over to the prime contractor on November 18, 1993, eight days ahead of the required date. Over the next several weeks, the electrical connection between the major pieces was completed. The first barge was unloaded at the power plant in April 1994. Training also was conducted in April. END
Gene Duke is maintenance superintendent at U.S. Generating Co.?s Logan Generating Plant. Duke has more than 25 years experience in the power industry in various management and supervisory positions.
James W. Schuster is the operations manager for McNally Wellman. He received a bachelor?s degree in mechanical engineering from Drexel University, Philadelphia, Pa., and a master?s in mechanical engineering degree from Carnegie Mellon University, Pittsburgh, Pa. He is a registered professional engineer in Pennsylvania. Schuster has more than 25 years experience in engineering management and mechanical design of bulk materials handling systems and equipment.
Adolf Zirkler is a senior mechanical engineer for McNally Wellman?s engineering department. He earned a bachelor?s degree in engineering from the University of Alabama and is a registered professional engineer in Pennsylvania. Zirkler has more than 25 years experience in mechanical design, design coordination, field engineering, proposal engineering and project engineering of bulk materials handling systems and equipment including ship and barge loaders and unloaders, stackers, bucketwheel stacker/reclaimers and belt conveyors.
Operational coal barge unloader at Logan dock
Logan Generating Station, a 208-MW pulverized coal-fired cogeneration facility in N.J., was developed and managed by U.S. Generating Co. and constructed by Bechtel Power Corp. McNally Wellman supplied the coal handling turnkey system. Steam is supplied to Monsanto Chemical Co. Electrical output satisfies in-house loads and the balance is sold to Monsanto and Atlantic Electric Co. Coal is transported to the cogeneration facility by 6,000-ton to 10,000-ton river barges. Three 10,000-ton capacity barges or four 7,500-ton capacity barges deliver coal to the facility every two weeks.
(Left) Barge unloader model
(Right) Model operating clamshell bucket