StackAssessment-Finalreducedsize塔式壓力容器的風載計算.ppt

Stack Evaluation,Michael Guillot, Ph.D., P.E. Senior Staff Consultant Stress Engineering Services,Its Hurricane Season,What is the condition of your stack?,Problem,Corrosion reduces the wall thickness of stacks at the upper levels Inspection and repair of stacks at upper levels is expensive Wind loads are larger at the upper stack levels Collapse or damage of a corroded stack is a possibility during high winds occurring during events such as a hurricane,Corrosion Mechanisms,Sulfuric Acid Dew Point Corrosion Calculation procedure Determine SO3 Approximate Value SO3 = 0.03 * SO2 Determine dew point from curve for SO3 content See curve for materials to resist corrosion Sulfurous Acid Dew Point Corrosion 120 normal, 149F for scrubbed gas systems HCl Corrosion 130 140F,Dew Point in Stack Gases,Sulfuric Acid Saturation Curve,Where to Look,Critical Corrosion Locations,Locations with air leaks Locations with fin cooling such as spoiler, stiffening rings, flanges Support attachment points Downdraft areas near the top of the stack Presence of chlorides or fluorides greatly increases rate Halogen concentrations (min) unrelated to temperature HF at 0.025 w%, Cl at 0.1 w%, HCl at 0.1w%,Corrosion Solutions - Coating Systems,(1) Must be top coated,Other Approaches,Insulate the stack to keep metal temperatures above the dew point Install a corrosion resistant metal liner Maintain the flue gases at temperatures 50F above the calculated dew point Place the stiffening rings slanted down on the inside of the stack to minimize cooling effect,Stack Design,ASME STS-1 Steel Stacks Originally issued in 1988, revised in 2006 Covers mechanical design, structural design, materials, dynamic wind loads, fabrications, inspection and maintenance Includes single and multiple walled stacks Includes lined and unlined stacks Includes guyed stacks Supported by PVElite Structural design Dynamic wind loads design,Stack Design Structural Design,Applied loadings Dead Load Weight (full plate thk), coatings, internal liner, insulation, platforms, ladders, instrumentation, etc Live Load Minimum of 50 psf for platforms & ladders Need not be considered for wind or earthquake combinations Wind Load Velocity pressure qz = 0.000256 V2 (I) (Kzt) (Kz) Seismic Load Thermal Load Both static and transient thermal conditions Construction Load,Basic Wind Speeds - Current,Galveston = 130 mph,Basic Wind Speeds - 1972,Galveston = 110 mph,Load Combinations Structural Design,Factor of Safety Required,Stack Design - Structural Design,Based upon AISC procedures Load cases to be satisfied Longitudinal Compression Weight Longitudinal Compression and Bending Weight and bending due to wind Circumferential Stress External wind pressure between stiffeners Combined Longitudinal and Circumferential Compress Stress,Basic Required Dimensions,Minimum plate thickness and maximum stiffener spacing,(1) Minimum plate thickness does not include corrosion allowance.,Stack Design - Dynamic Wind Loads,Vortex Shedding Critical velocity calculated using hourly speed at 5/6 height above grade If critical wind speed for vortex shedding is 1.2 times the mean hourly wind speed at 5/6 stack height, vortex shedding is not an issue Interference between a line of stacks must be considered when the distance between stacks divided by diameter is less than 15,Stack Design - Dynamic Wind Loads,Ovalling Wind pressures and vortex forces can cause ovalling resonance Stiffening rings sized to prevent ovalling Not considered for lined stacks,Pressure distribution around a cylinder at high Reynolds number,Case Study,Client discovered significant localized corrosion during an outage just prior to hurricane season Stack is approximately 175 tall and located on US Gulf Coast Time not available to perform full inspection or repair Client requested a calculated minimum thickness for each segment of stack to be used in full inspection of stack,Stack Drawing,Thickness Measurements,Thickness Measurements,PVElite Stack Design,Analysis Procedure to Find Stack Minimum Wall Thickness,Build model of stack Place nodes at locations where thicknesses change Place nodes at stiffening rings Input wall thickness in model using as-built values Determine wind speed to be used Run model and check results Check weights and dimensions adjust as needed Iterate minimum wall thickness Select only one segment and reduce wall until either the structural or dynamic wind load criteria fail Note failure criteria and minimum wall thickness Restore segment to as-built thickness and repeat process with adjacent segment,Stack Model Geometry,Angle 14 Results follow which govern Section 5D wall thickness,Section 2D Results follow indicates wall thickness governed by combined longitudinal load,Stack Analysis PVElite Results,Stiffening Ring Results for Determining Min. Wall for Stack Section 5-D,Ovalling governs,Stack Analysis PVElite Results,Combined Longitudinal Results for Determining Min. Wall for Stack Section 2-D,Longitudinal load governs,Stack Analysis Results Summary,Minimum thickness must be 0.1875”,Stack Analysis Conclusion