ASME STS-1:2006 pdf download Steel Stacks
Mechanical design includes sizing of the gas passage,both in diameter and height; and the drop in gas temper-ature as heat is transferred through the stack wall. Methods for calculating draft, draft losses, and heat lossesare given. Differential expansion of stack componentsis discussed. Design considerations for stack appurtenances are established.
1.2 General
The purpose of a stack is to vent process exhaust gasesto the atmosphere. The mechanical design of stacks isnow controlled in part by air pollution rules and regula-tions.Heights and diameters are set by a balancebetween structural stability and function, while at thesame time meeting the requirements for air pollutioncontrol dispersion of the gases to the atmosphere. Theheights of steel stacks have increased to satisfy ambientair quality,and stack inlet gas temperatures havedecreased as more heat energy is recovered, The impor-tance of attention to stack heat losses has thereforeincreased.Stack minimum metal temperature should beheld above the acid dew point of the vented gases, ifpossible. Stacks are being designed with many appurtenances to monitor the gases and make stack inspections.
1.3 Size Selection (Height, Diameter, and Shape)1.3.1 Height. Stack height may be set by one or morefactors
(a) Environmental Protection Agency (EPA) regula-tions may set the required stack height for downwashdue to local terrain or adjacent structures, or to dispersepollutants at a minimum height above the site. Referproposed stack location and purposes to the proper EPAauthorities for the minimum height requirement undercontrolling air pollution control regulations. See Federalregister part II EPA 40CFR, part 51,Stack Height Regulation ((uly 8，1985).
(b) The National Fire Protection Association (NFPAsets minimum height of high-temperature stacks abovebuilding roofs and structures for fire protection andhuman safety Local codes are often more stringent andmust be followed. A minimum of 8 ft of height abovea roof surface or roof mounted structure within 25 ft ofa stack emitting gases above 200°F(93C) should bemaintained.
(c) The draft requirement of the process to be ventedmay establish stack height. Formulas to calculate available draft are presented in subsequent paragraphs.(d) The effective height of a stack considering plumerise may be increased by installing a nozzle or truncatedcone at the top to increase the exit velocity of the gasesSeveral plume rise formulas are available but in actualpractice, plume rise can be essentially negated by highwind velocities, low temperatures, and site conditions
1.3.2 Diameters.The stack diameter may be set byone or more factors.
(a) Gas passage diameter is usually established bythe volume of process gas flowing and available draft(natural draft minus draft losses), Velocities in a roundstack between 2400 and 3600 ft/min are most commonStacks venting saturated gases sometimes limit maximum stack velocities between 1800 and 2400 ft/min toreduce entrained or condensed moisture from leavingthe stack exit. Tests by EPRI give different ranges foreach type of inner surface (see EPRI Wet Stack DesignGuide TR-107099-1996).
(b) Stack shell diameters may be controlled by transportation shipping limitations. Caution should be takento ensure that mechanical performance and structuralstability are maintained.
(c) Structural stability may control a stack shell diameter selection and therefore, any size selection based onmechanical criteria must be maintained as tentative untila structural analysis can confirm its acceptability.
(d) Future increases in stack gas volume should beconsidered as well as future changes in process gas tem-peratures and gas quality in the diameter selection.(e) EPA regulations may set stack exit diameterbecause of plume rise considerations. EPA requirementshave sometimes set stack diameters in the test zone toprovide optimum velocities for testing