Stack design focuses on achieving a balance between environmental compliance, structural stability, and functional efficiency in venting process exhaust gases. For the fabrication, selecting appropriate materials and linings to withstand corrosive gases as well as fabrication challenges are critical for compliance and structural integrity.
The design and engineering of stacks play a crucial role in the effective venting of process exhaust gases into the atmosphere. This process is now subject to stringent air pollution regulations, requiring a careful balance between structural stability, functional efficiency, and adherence to environmental standards. Key considerations in the mechanical design of stacks include:
- Height and Diameter: Determined by a balance between structural stability and meeting air pollution control requirements. Steel stacks have seen an increase in height to comply with ambient air quality standards.
- Temperature Control: Stack inlet gas temperatures have decreased with the emphasis on heat energy recovery. Attention to stack heat losses has become more critical. Maintaining a minimum metal temperature above the acid dew point is advisable.
- Regulatory Compliance: Stacks are designed in accordance with air pollution rules. Compliance with regulations necessitates not only proper venting but also dispersion of gases to meet environmental standards.
- Monitoring and Inspection: Modern stack designs often incorporate various accessories for monitoring gases. Regular stack inspections are essential to ensure ongoing compliance and address any issues promptly.
The evolving landscape of air quality standards and environmental regulations continues to shape the mechanical design of stacks, emphasizing the need for efficiency, compliance, and environmental responsibility.
2. Size Selection
- Environmental Protection Agency (EPA) Regulations: Stack height requirements may be dictated by EPA regulations to address factors like downwash due to local terrain, adjacent structures, or pollutant dispersion. Compliance with air pollution control regulations is essential.
- National Fire Protection Association (NFPA): Sets minimum height requirements for high-temperature stacks above buildings for fire protection and human safety. Local codes, often more stringent, must also be followed.
- Draft Requirements: The process’s draft needs may influence stack height, with available draft calculated using specific formulas presented later.
- Effective Stack Height: Plume rise considerations may increase effective stack height, achieved by installing a nozzle or truncated cone to enhance gas exit velocity.
2. 2 Diameters
- Gas Passage Diameter: Typically determined by the volume of process gas and available draft. Common stack velocities range from 2,400 ft/min to 3,600 ft/min. Velocities for stacks venting saturated gases may be limited to reduce moisture entrainment.
- Stack Shell Diameters: Controlled by transportation shipping limitations, ensuring mechanical performance and structural stability are maintained.
- Structural Stability: May influence stack shell diameter selection, subject to confirmation through structural analysis.
- Future Considerations: Anticipate changes in stack gas volume, process gas temperatures, and gas quality in diameter selection.
- EPA Regulations: Stack exit diameter may be set by EPA regulations, considering plume rise for optimum velocities during testing.
2. 3 Shape
- Cylindrical Shapes: Common for efficiency in structural stability and fabrication economy. Diameter variations along the stack height are allowed at an angle not exceeding 30 degree from the vertical.
- Other Shapes: Geometrical shapes beyond cylindrical (octagonal, triangular) require special consideration for dynamic stability and adherence to engineering design standards, especially for aesthetic choices. Unusual shapes should follow basic engineering principles both structurally and mechanically.
3. Components and Accessories
- Access Doors: Appropriate-sized doors for inspecting the inside bottom base and other locations.
- False Bottoms: Recommended below the lower stack inlet for enhanced structural support.
- Drains: Installed in false bottoms and/or foundations to divert water away from the base and anchor bolts.
- Test and Instrument Ports: Sized and located for specific applications.
- Inspection Ports: Spaced appropriately over the stack’s height for thorough inspection.
- Access Ladder and Test Platforms: Selected based on job conditions, with specified platform width.
- Lighting: Compliance with national directives. Access platforms for corrosion-resistant construction.
- Rain Caps: Generally not required for full-time active stacks. If specified, diameter and clear height recommendations apply.
- Spark-Arresting Screens: Stainless steel material, minimum two stack diameters high, specified as needed.
- Lightning Protection: Required grounding at the base per national requirements.
- Internal Dampers: Special consideration for shutoff dampers and cap dampers if specified.
- Straightening Vanes: Specification as required for effective gas flow distribution during testing.
- Splitter Baffles: Sometimes used for stacks with two opposite inlets to reduce back pressure.
- Top-of-Stack Roofs: For multiple flue stacks and dual-wall stacks, providing weather protection and accommodating expected differential expansion. Consider ash build up on flat surfaces.
- Noise Pollution Control: Acoustical sound attenuators within the stack may be required.
The materials recommended below are proposed for utilization due to their capacity to fulfil the physical, mechanical, chemical, and environmental criteria of a particular application. Approval of a material for a specific use should rely on its performance history or confirmation of its appropriateness through independent verification.
4.1 Shell and Base Plates
Shell and base plates are typically fabricated from a variety of structural materials, including:
- Carbon steels that meet ASTM A36, A283, or A529 specifications.
- High-strength, low-alloy steels that adhere to ASTM A242, A572, or A588 specifications.
- Stainless steels in accordance with ASTM A666 specifications. ASTM A666 304 L Stainless steel may also be used for lining and cladding sheet.
- Stainless chromium-nickel steel clad plates (ASTM A264) and nickel-base alloy clad steel (ASTM A265), which can be considered for shell plates.
Pressure vessel quality carbon steels like ASTM A285, A515, and A516; alloy steels such as ASTM A387; and stainless steels like ASTM A240 can be substituted for structural quality materials as needed.
For specific temperature conditions:
- Carbon steels (e.g., ASTM A516, Grades 55 through 70) and low-alloy steels (e.g., ASTM A517, Grades A through T, and ASTM A537) are typically specified for service temperatures as low as −50°F (−46°C).
- Nickel-containing alloy steels like ASTM A203, Grades A and B, are used for temperatures as low as −75°F (−59°C), and Grades D, E, and F are employed for temperatures as low as −150°F (−101°C). Even lower temperatures may require nickel-containing alloy steels and nickel stainless steels.
Suppliers of structural quality steels can provide data on notch toughness when specified.
Additionally, protection against corrosion and/or oxidation on interior and/or exterior surfaces may be necessary based on the materials and conditions.
4.2 Stiffeners and Structural Braces and/or Framework
Stiffeners and structural braces and/or framework may be of the following materials:
- Carbon steels conforming to the ASTM A36, A283, or A529 Specifications.
- High-strength, low-alloy steels conforming to the ASTM A242, A572, or A588 Specifications.
- Stainless steels conforming to the ASTM A240 or A666 Specifications or nickel-containing alloys with compositions similar to those of the shell plate.
- Purpose of Linings:
- Corrosion Resistance: Linings may be necessary to resist corrosive gases, vapors
- Heat Resistance: Linings can provide resistance to high temperatures.
- Temperature Maintenance: Linings help maintain stack surface temperatures to prevent condensate corrosion.
- Analysis for Lining Decision:
- A comprehensive thermal analysis of the entire system, considering the heat source to stack outlet, is essential.
- Primary focus should be on determining the stack surface temperature.
- Perform a thorough chemical and physical analysis of the flue gas to identify chemically corrosive constituents.
- Assess the characteristics of particulate loading in the flue gas.
The decision to use a lining should be based on the results of these analyses. If corrosive constituents are present or if there is a risk of condensate corrosion due to temperature variations, linings become crucial. The type of lining material and its specifications should align with the specific requirements identified through these analyses.
Consideration should be given to the use of metallic linings and cladding when there is a need for resistance to corrosion and/or elevated temperatures. High-performance metals and alloys, such as stainless steels, nickel-based alloys, and titanium, can be employed as linings or as cladding on carbon steel plates to address these concerns.
6.1 General Considerations
- Hole Enlargement:
- Drifting, if required, should not enlarge holes or distort members.
- Holes needing enlargement should be reamed.
- Bolt Tightening:
- Bolts should be tightened using one of the following methods:
- Turn-of-the-nut method.
- Load-indicating washers.
- Calibrated wrenches.
- Other approved methods.
- Any required straightening of material should be done using procedures that minimize residual stress.
- Straightening or bending of anchor bolts by heating is prohibited.
- All vertical shop and field plate or panel butt weld seams should be staggered a minimum of 20 degrees.
- Welded cylindrical sections joined by circumferential welds should have vertical seams staggered by a minimum of 20 degrees.
- Calculation and Comparison of Dimensions:
- Dimensions and weights of stack sections should be accurately calculated and compared with crane capabilities at working radii during erection.
- Crane capacities and working radii must not be exceeded.
- Removal of Temporary Items:
- Lifting clips, lugs, dogs, brackets, and other items welded to stack sections for erection or fit-up purposes, if not left in place, should be removed without damaging the base material.
- Any remaining weld on the internal surface of the stack subjected to flue gas should be made flush and ground smooth.
- If backing is used for welding, it need not be removed.
- Erection and Scaffolding:
- Erection and scaffolding, including ladders, should comply with the latest applicable and/or specified codes.
All welding provisions, workmanship, techniques, welder and inspector qualifications, and welding tests shall adhere to the guidelines outlined in the American Welding Society Structural Welding Code ANSI/AWS D1.1 (latest edition) or ASME BPVC, Section IX. Specifically, all structural butt welds are required to be full penetration welds.
- Minimum Weld Inspection Requirements:
- Visual Inspection:
- Continuous visual inspections during and after welding to ensure thorough fusion between adjacent layers and base metals.
- Removal of slag after completion of welding.
- Cleaning of welds and adjacent metal surfaces.
- Particular attention to surface issues such as cracking, porosity, slag inclusion, undercut, overlap, and gas pockets.
- Correction of defective welding per ASME or AWS Code requirements.
- Radiographic Inspection:
- A minimum of one radiograph for every three shop circumferential seams on the stack structural shell, preferably at vertical weld intersections.
- Inner or outer shell considered structural when designed to resist controlling wind or seismic load.
- Visual Inspection for Field Welds:
- All structural full penetration field welds undergo visual inspection.
- Radiographs not usually feasible for shell or flue field splice welds due to design constraints.
- Types of Welding Inspection:
- Radiographic Inspection: Conducted in the shop on full penetration butt welds.
- Visual Inspection: Applied to all shop and field welds.
- Magnetic Particle Inspection: Applicable to all ferromagnetic material welds.
- Ultrasonic Inspection: Used on all shop butt welds with a thickness of ≥5⁄16 inch.
- Dye Penetrant Inspection: Utilized as required to supplement visual inspection. Standard methods per ANSI/AWS D1.1 (latest edition). Acceptance criteria according to ASME or AWS Codes.
6.3 Common Fabrication Problems
Various potential issues should be considered regarding the condition of stacks, including:
- Atmospheric corrosion and weathering on the exterior surface.
- Corrosion resulting from acid condensation in flue gases on internal surfaces.
- Accumulation of fly ash or particulates at the base, false bottom, or roof cap of the stack.
- Moisture condensate at the base of the stack.
- Infiltration of acid/moisture into insulation.
- Deformation due to thermal or other loading.
- Corrosion of anchor bolts.
- Fatigue cracks.
- Loss or deterioration of insulation, coating, or linings.
- Loosening of anchor bolts.
- Loosening of splice/flanged bolts.
- Formation of “hot spots” on the shell of stacks with internal lining.
- Separation of ladder supports from the stack shell.
6.4.1 Exterior Inspection
- Shell Thickness:
- Ultrasonic devices for non-destructive thickness testing or core samples and drill tests may be utilized to measure the shell thickness.
- It is recommended to take one shell thickness reading for each portion of the stack height equal to the stack diameter.
- Maintain a record of the results to monitor corrosion of the steel shell.
- Inspect the exterior finish for damage, wear, and any discontinuity.
- Record all deficiencies identified during the inspection.
- Access System:
- Inspect all components of the access system, including ladders, ladder anchors, cages, safety climb devices, and platforms, to ensure integrity and safety.
- Lightning Protection System:
- Inspect all components of the lightning protection system, including the grounding connection, for electrical continuity.
- Support System:
- Check any braces, guy wire anchors, guy cables, guy fittings, and similar items.
- Note and analyse any deficiencies identified.
- Inspect all bolts, including anchor bolts.
- Electrical System:
- Note the presence of any moisture condensation inside conduits and fittings.
- Inspect for corrosion of fittings and conduits.
- Replace any burned-out lamps.
- Recognize the potential for soaking of insulation due to acid infiltration in insulated stacks.
- Wet and acid-saturated insulation can accelerate corrosion of the shell, leading to significant structural damage.
6.4.2 Interior Inspection
- Utilize ultrasonic devices for non-destructive thickness testing to measure the shell thickness.
- Take one shell thickness reading for each portion of the stack height equal to the stack diameter.
- Maintain a record of the results for monitoring corrosion of the steel.
- The lining is critical in terms of wear, cracks, spalls, and other deficiencies.
- Exercise care to detect deficiencies that may be hidden by overlaying particulate deposits.
- It is recommended to take pH readings using litmus paper, reagents, or chemical analysis of representative samples from lining surfaces.
- Check for the accumulation of particulates such as combustion residue, fly ash, etc., on the stack wall and at the base of the stack.
- Accumulation provides a matrix for acid condensate.
Novelty Structures offers contract fabrication services for stack fabrications.