Process duct work
Large process ductwork may fill with dust, depending on slope, to up to 30% of cross section, which can weigh 2 to 4 tons per linear foot.
Rectangular ductwork is covered by the ASCE "The Structural Design of Air & Gas Ducts for Process Power Stations and Industrial Applications".
In the practical design of primarily round structural process ductwork in the cement, lime and lead industries, the duct size involved ranges from 18 inches (45 cm) to 30 feet (10 m).
Process ductwork is subject to large loads due to dust buildup, fan suction pressure, wind, and earthquake forces.
Failure to properly integrate design forces may lead to catastrophic duct collapse.
Round ducts, larger than 6 feet 6 inches (1.98 m) in diameter (1/4" plate) require support ring stiffeners.
When stiffener rings are required they are traditionally designed based on "Roark", although this reference is quite conservative.
Because of the potential for high dust loading, most process ductwork is run at a 30 to 45 degree slope.
(this is based on personal experience, and my vary with loads, duct size, and industry standards) Large, round process ductwork is usually fabricated from 1/4-inch (6 mm) mild steel plate, with ovaling stiffening rings at 15 to 20 ft (5 to 6 M) on center, regardless of diameter.
SMACNA, (2ND Edition) chapter 4 has many useful formulas for round ducts, allowable stresses, ring spacing, effect of dust, ice, and live loads.
See API 560 for design of wind ovaling stiffeners See Tubular Steel structures, chapter 2, 9 & 12 for the allowable stresses for thin, round ducts, their allowable stresses, elbows, elbow softening coefficients, and some procedures for the design of duct support rings.
Round ductwork allowable compressive stress is = 662 /(d/t) +339 * Fy (tubular steel structures, chapter 2).
most ductwork, non-equipment pressure drop occurs at transitions and changes of directions (elbows).
Round duct support rings are traditionally designed from the formula's found in Roark & Young.
Therefore, these formulars can be shown with Ram, or other analysis methods to have conservatism factor of roughly 2 above the stresses given In Roark.
Tubular steel structures, Section 9.9 gives the (Beskin) reduction factor of K= 1.65/(h (2/3 power)) where h= t (plate) *R(elbow)/ r (duct) (where suction pressures are smaller).
Round duct rings are fabricated from rolled tees, angles, or plates, welded into the shape required.
Typical intermediate ring factor of safety should be 1.6, because there is ample evidence in various codes, (API 360, etc.)
that intermediate rings designed for wind ovaling and suction pressure combinations are safe.
Typical main support ring factor of safety, if designed by "Roark" formulas should be 1.6, (If constructed to the Roark normal 1% out of round standard tolerance) because it can be shown by various methods that these formulas are at least a factor of two, above three D duct ring analysis results etc..
On one such project My firm added L8x8x3/4 at the top 45 degree location to stiffen the plate near the point of maximum stress for tubes (as per Timoshenko, and others).
Rectangular cement plant ductwork is often 1/4" (6 mm) duct plate, with stiffeners spaced at about 2'-6", depending on suction pressure and temperature.
Power plant ductwork can be 5/16" thick duct plate, with "fixed end" W stiffeners at roughly 2'-5" spacing.
Power plant, and other larger ductwork, usually goes thru the expense of creating "fixed End" corner moment.
Duct stiffener temperatures are assumed to drop about 100 °F per inch of depth (when uninsulated) (no reference available).
As reducing the loss of heat at plants has changed over the years, ductwork now connects more pieces of equipment than ever before.
Metal joints prefer axial movements, and provide significant lateral loads onto duct segments.
Ducts are shipped from fabricating facility to job sites on trucks, by rail, or on barges in lengths accommodating the mode of transport, often in 20 foot sections.
This is a smaller duct, with light loads, so that the bottom flange was slightly modified by support clearance requirements.
Duct Wear: High temperature ductwork often carries large volumes of hot abrasive dust.
