ASME STS-1–2006 pdf download.Steel Stacks.
1 MECHANICAL DESIGN 1.1 Scope Mechanical design includes sizing of the gas passage, both indiameter and height; and the drop in gas temper- ature as heat is transferred through the stack wall. Meth- ods for calculating draft, draft losses, and heat losses are given. Differential expansion of stack components is discussed. Design considerations for stack appurte- nances are established. 1.2 General The purpose ofa stack is to vent process exhaust gases to the atmosphere. The mechanical design of stacks is now controlled in part by air pollution rules and regula- tions. Heights and diameters are set by a balance between structural stability and function, while at the same time meeting the requirements for air pollution control dispersion of the gases to the atmosphere. The heights of steel stacks have increased to satisfy ambient air quality, and stack inlet gas temperatures have decreased as more heat energy is recovered. The impor- tance of attention to stack heat losses has therefore increased. Stack minimum metal temperature should be held above the acid dew point of the vented gases, if possible. Stacks are being designed with many appurte- nances 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 more factors. (a) Environmental Protection Agency (EPA) regula- tions may set the required stack height for downwash due to local terrain or adjacent structures, or to disperse pollutants at a minimum height above the site. Refer proposed stack location and purposes to the proper EPA authorities for the minimum height requirement under controlling air pollution control regulations. See Federal register partII EPA 40CFR, part 51, StackHeight Regula- tion (July 8, 1985). (b) The National Fire Protection Association (NFPA) sets minimum height of high-temperature stacks above building roofs and structures for fire protection and human safety.
(c) The draft requirement of the process to be vented may establish stack height. Formulas to calculate avail- able draft are presented in subsequent paragraphs. (d) The effective height of a stack considering plume rise may be increased by installing a nozzle or truncated cone at the top to increase the exit velocity of the gases. Several plume rise formulas are available but in actual practice, plume rise can be essentially negated by high wind velocities, low temperatures, and site conditions. 1.3.2 Diameters. The stack diameter may be set by one or more factors. (a) Gas passage diameter is usually established by the volume of process gas flowing and available draft (natural draft minus draft losses). Velocities in a round stack between 2400 and 3600 ft/min are most common. Stacks venting saturated gases sometimes limit maxi- mum stack velocities between 1800 and 2400 ft/min to reduce entrained or condensed moisture from leaving the stack exit. Tests by EPRI give different ranges for each type of inner surface (see EPRI Wet Stack Design Guide TR-107099-1996). (b) Stack shell diameters may be controlled by trans- portation shipping limitations. Caution should be taken to ensure that mechanical performance and structural stability are maintained. (c) Structural stability may control a stack shell diam- eter selection and therefore, any size selection based on mechanicalcriteria mustbe maintained as tentative until a structural analysis can confirm its acceptability. (d) Future increases in stack gas volume should be considered as well as future changes in process gas tem- peratures and gas quality in the diameter selection. (e) EPA regulations may set stack exit diameter because of plume rise considerations. EPA requirements have sometimes set stack diameters in the test zone to provide optimum velocities for testing. 1.3.3 Shape. The shape of the stack varies with designers’ preferences.