Material Handling, Policy & Regulation

Economical Ductwork Design and Engineering

Issue 10 and Volume 112.

By Kerry Pennington, PE, WorleyParsons

Careful planning in designing ductwork for emission control structures goes a long way toward reducing overall plant construction costs.

Increasing restrictions on power plant emissions are causing owners and utilities to look for cheaper and faster ways to upgrade their plants to conform. Steel prices and skilled labor shortages increase daily and more than ever engineering firms and contractors have to be more creative to minimize cost while providing a quality product with minimal impact to the project facility and power generation.

A significant way to do so is minimizing construction costs of erecting flue gas ductwork. That’s just what happened for an air quality retrofit project at one of biggest coal plants in the United States.

The project involved a complete mercury bag house retrofit for Southern Company’s 3,500 MW Scherer plant in Juliette, Ga. BE&K Construction was the contractor for the bag house retrofit projects and Worley Parsons partnered with the firm to provide services for the detailed engineering and balance of plant for all four boiler units.

Unit 3 was the first of the four bag house retrofits because of space limitations created by plans for future selective catalytic reduction and flue gas desulfurization retrofit systems. Construction began in October 2007 with the tie in outage for the system scheduled for late September 2008.

A separate system or series of systems is required for each of the emissions that have to be controlled, including SO2, NOX and mercury. As the name suggests, a mercury bag house removes only mercury from the exhaust flue gas. First, powder activated carbon is injected into the flue gas to absorb the mercury. Then the bag house filters mercury out of the exhaust flue gas by forcing the air through a series of fabric filter bags which collect the powder activated carbon. The mercury contaminated fly ash can then be collected and discarded or reused.

Many challenges had to be overcome during the preliminary concept engineering and bidding stage of the project. The successful bidder had to be able to offer economical solutions to these complex challenges while designing a retrofit system that minimized the impact to the plant’s daily operation.

Where to Put It?

The first major challenge was to decide the best location for the mercury bag house. The mercury bag house is about 190 ft. x 165 ft. and covers around an acre, with road access and support equipment. The only site large enough to accommodate the bag house was more than 1,000 feet north of the existing power house. That location was selected to allow ongoing truck traffic access for activities at the on-site fly ash storage and loading facility owned and operated by an outside company.

One challenge was to find a location for the mercury bag house. Photos courtesy WorleyParsons.
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Next to be determined was routing the ductwork that would transport the 350 F flue gas to the bag house for mercury removal and back to the existing exhaust stack. The ductwork had to be arranged to allow for constant truck traffic, to accommodate dozer access into the coal yard and to cross Unit 4’s coal conveyor. To maintain necessary access road clearance, the ductwork’s bottom was elevated to more than 25 feet above grade. Due to limited space between the ash loading facility and the coal yard, the ductwork was stacked with the return train from the bag house on top of the supply ductwork (see photo on page XX).

This arrangement served two purposes. The overall ductwork footprint was narrower and accommodated the erection lifting process. And the cross-sectional area of the 24×43-foot-wide ductwork was designed to accommodate the stacked arrangement and maintain clearance and access for construction equipment on both sides of the ductwork during construction.

The locations of the ash loading facility and the coal yard also prevented the use of large cranes to construct the ductwork. Therefore, the ductwork sections were designed to be fully assembled on the ground beneath the permanent support structures, then lifted onto their supports by chain fall pneumatic hoists (see photos on pages 78 and 79). To achieve this, the support structures had to be designed as both lifting gantries and permanent support. Because the large ductwork sections were to be hoisted from the ground beneath and within the structural steel, large moment frames would be used to provide stability for the structure and allow the clearance required for the lift. The support structures consisted of four W36x330 columns and two levels of W36x361 main support beams which created two story moment frames.

Each ductwork section was designed to perform like a bridge girder spanning more than 100 feet between their support structures to minimize structural steel, foundations, underground interferences and time of construction. The completed duct section lifted weighed more than 250 tons. Lifting was done by four 100-ton pneumatic chain fall hoists on each corner of the ductwork section.

Each duct section was assembled at grade resting on its permanent support beams, which would also serve as lifting beams to hoist the duct section to its permanent elevation at more than 90 feet. Each air hoist was independently controlled by a pendant operator on the ground at each of the four corners of the duct. Four teams of spotters in man lifts monitored the clearance between the support beam end plate and the column flange to ensure that the required 5/16-inch clearance gap was maintained during the lift to prevent the steel from binding in place. The permanent support beam was then bolted into its final position using finger shims to fill the 5/16-inch gap between the column and the moment resisting extended end plate connection.

The first of 10 duct section lifts took two hours to complete. Subsequent lifts averaged about 30 minutes each. Ground assembly of the ductwork also allowed contractors to install insulation using scaffolding from the ground and personal man lifts, thus speeding installation and enhancing safety.

Because the plant is not located on a river, it cannot take advantage of barge delivery for large shop fabricated ductwork sections. To minimize field welding at the site, the ductwork was fabricated in large panels with stiffeners, corner angles, corner channels and internal truss cords that could be shipped by truck. The preassembled panels were shipped in 16×43-foot-long sections that could be stacked on a flatbed truck trailer.

On site, a panel for each of the top, bottom and two sides were bolted together in the corners forming a duct sub-assembly section that was almost 16 feet long. Internal truss work diagonal struts were then bolted to shop welded gusset plates along the center cord members located on the top and bottom panels to form the center support truss. A series of 16-foot sub-assemblies were then bolted together in series to form the long spanning duct sections of more than 100 feet.

The bolted corners, truss diagonals and sub-assembly sections permitted faster field assembly of the ductwork. When all sub-assemblies were bolted together, welders and welding machines were used to complete the seal welds required on the inside of the duct to make the duct air tight. The bolt and welding details that were used reduced the overall field welding requirements more than 50 percent. This allowed the constructor to assemble the ductwork with fewer skilled welding laborers, significantly reducing construction costs and construction schedule.

The foundation system used to support the structures were auger cast pile supported caps battered to accommodate large lateral loads and minimize deflection. Auger cast piles were used because they are faster and cheaper to install than drilled caisson piles. Moment resisting foundations were designed to minimize lateral deflections and prevent racking of the support frame during the construction sequence lifting. Limiting deflection of the frame was essential to ensuring that the duct section did not bind with the steel during the lift.

The ductwork was stacked due to limited space between the ash plant and the power plant’s coal yard.
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Ductwork is engineered to withstand sustained forces created from dead loads as well as continuous system operating loads and temporary environmental loads such as wind and earthquake forces. The duct system is composed of structural steel plate that serves as the skin of the duct that contains the air-tight boundary—for air or gas being conveyed. Wide flange structural shapes are often used on uniform spacing to resist system operating loads such as static pressure and ash. Internal pipe struts and trusses provide overall stability of the duct structure and reduce the span of external stiffeners to save weight. Insulation and lagging are used to provide protection from outside elements and prevent injury and heat loss for higher temperature applications. The duct plate and stiffener system is designed to sustain loads up to as much as 350 psf.

Special structural details are required to enable the duct section to support these heavy loads over long distances without structural support. Proper design of field splice connections is required to ensure load transfer through joints and to maintain overall stability of the ductwork.

Large Box Girder

The ductwork superstructure is designed like a large scale box girder. The relatively thin steel side walls act as the box girder web able to transmit large shear loads between the external stiffeners, which provide stability and strength like a compression strut of a warren truss or a stiffened girder web. The top and bottom duct plates and corner angle serve as the stabilizing flange for the side walls to prevent local buckling from the large in plane axial loads created from bending.

The long spanning ductwork had to be specially designed to resist large bending and shear stresses created by dead weight and design ash loading. The corner angle and top plate connection play an essential role in the overall stability and strength of the duct section. Additional steps were therefore taken to ensure longevity of the corner strength, even under harsh operating conditions such as fly ash abrasion and corrosion. The corner system utilizes an external channel parallel to the duct at the top and bottom connections that is protected from the harsh internal environment of the flue gas.

This arrangement allowed for easy bolting/welding of the adjacent panels and the channel served as the stability flange for the side walls normally performed by the internal corner angle. The internal corner angle was used to provide the air tight seal required to complete the corner construction.

Designing ductwork to span long distances between support structures minimized material and construction time for structural steel and foundations. By requiring fewer foundations, the contractor was able to complete pile installation earlier to allow construction of foundations and structural steel to begin sooner. Fewer piles and foundations also minimized interferences and possible expensive rework of underground utilities. This helped a great deal because information on existing underground locations was nonexistent or inaccurate.

Platforms for ductwork maintenance door access were supported along the ductwork by stiffeners that were intermittently extended outside the insulation and lagging at the bottom of the duct. Because the platform steel is outside of the insulation, special connections were needed to allow the ambient platform support steel to accommodate thermal expansion of the ductwork while maintaining support during lateral loading conditions. The platform steel also supported the electrical cable trays and piping required to operate the bag house equipment.

Operators closely monitored column clearance during each duct lift.
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Due to the length of the duct system, special provisions had to be taken to reduce the effect that internal components had in increasing pressure drop. Internal trusses, arranged perpendicular to the gas flow, are customarily used to transfer lateral wind and seismic forces to the ductwork supports and provide overall stability. This type of arrangement created a large pressure drop that increased at every transverse truss.

Therefore, external stiffener moment frames were used, at the supports, to provide stability for the ductwork. A center truss arranged parallel to the gas flow provided internal support of the long spanning top and bottom stiffeners to reduce their overall depth and weight. The center truss struts were closely spaced to create a center wall affect, splitting the gas flow and reducing pressure drop. A vertical moment resisting wide flange beam was used as the center strut in the moment frames to provide additional lateral stability. A pipe was then split in half and welded to both sides of the wide flange post to increase its aerodynamics and decrease pressure drop as well as minimize fly ash abrasion.

Resource Constraints

The American Welding Society projects that by 2010 there will be 200,000 fewer welders than were available just a few years ago. This year alone, 50,000 skilled welders will retire while only 25,000 will complete their education and begin their trade. From preparation to inspection, the time and resources needed to complete the process consume construction schedules and budgets. Reducing the number of welders a contractor needs by providing workable bolted connections is an economical solution to the manpower shortages and expedited schedules that are the norm for the power industry today. Careful consideration of constructability and modularization can greatly lower overall project costs by reducing construction time and expensive field welding.

The price for steel has doubled in the last three years. On average, quotes for steel prices are only good for three months. Therefore, bidders have to provide contingency in estimates to account for fluctuating steel prices between the bid stages of a project and final procurement. Due to uncertainty of material price escalation, these contingencies increase bids and can prevent a contractor from getting a project. Long lead times on mill rolls are increasing lead times for steel up to 10 months. This creates longer wait times for fabricators, increases project schedules, and decreases time for engineering and detailing, which can lead to errors and rework.

Engineering service costs are small in comparison to overall project costs. Yet, economical engineering on such major elements as emission control structures can improve overall costs and provide the bidder a major advantage in securing the contract.

Author: Kerry Pennington, PE, supervising structural engineer for WorleyParsons, has spent 10 years in the power industry. His experience includes structural engineering and designing retrofit ductwork systems and support structures for air quality control projects of all kinds.