Duct work Structural and its Process

Process piping transports large volumes of hot, dusty air from processing equipment to mills, baghouses and other process equipment. Process piping can be round or rectangular.Although circular ductwork is more expensive to manufacture than rectangular ductwork, it requires fewer stiffeners and is preferred over rectangular ductwork in many applications.Air in process piping may be at ambient conditions or may be operating at temperatures as high as 900 °F (482 °C).Process piping systems can vary in size from 2' diameter to 20' diameter, or maybe a 20' x 40' rectangle.Large process piping systems can be filled with dust, depending on slope, up to 30% of the cross-section, weighing 2 to 4 tons per foot.Circular ductwork is prone to duct suction collapse and requires stiffeners to minimize this, but is more efficient in terms of materials than rectangular ductwork.There is no comprehensive design reference for process piping work design.The ASCE Reference to Piping Design for Power Plants provides some general guidance on piping design, but does not specifically provide designers with sufficient information to design process piping work.

Structural process ductwork

Structural process piping systems transport large volumes of hot, dusty air between process equipment.The design of this piping system requires an understanding of the interaction of metal heat softening, the potential effect of dust accumulation in large piping systems, and structural design principles. Structural process piping systems come in two basic shapes: rectangular and circular. Rectangular piping systems are covered in ASCE "Structural Design of Air and Gas Ducts for Process Power Stations and Industrial Applications."In the actual design of major circular structural process piping systems in the cement, lime, and lead industries, pipe sizes ranging from 18 inches (45 cm) to 30 feet (10 meters) are involved.Air temperature may vary from ambient to 1000 °F (515 °C). Process piping systems experience significant loads due to dust buildup, fan suction pressure, wind and seismic forces. As of 2009, a 30-foot diameter process piping system might cost $7,000 per ton.Failure to properly integrate design forces could result in catastrophic pipeline collapse.Overdesign of piping systems is costly.

Design of circular and rectangular duct structures

The structural design of the piping system plate is based on the buckling of the plate elements.Circular piping system plate design is based on the ratio of diameter to piping plate thickness and the allowable stresses are covered in several references such as US Steel Plate, ASME/ANSI STS-1, SMNACA,Tubular Steel Structures, and others. In fact, a curved circular piping system is approximately 30% stronger in bending than a similar shape in compression, but uses the same allowable stresses in bending as in compression.Circular ducts require typical stiffeners spaced at approximately 3 diameters, or approximately 20 feet O.C.for wind ellipse and fabrication and trucking requirements.Round pipe diameters greater than 6 ft 6 inches (1.98 m) (1/4" plate) require support ring stiffeners.Smaller diameter pipes may not require support ring stiffeners but can be designed with saddle supports.When reinforced Rings are required.They are traditionally based on the "Roark" design, although this reference is rather conservative.The allowable stress of circular pipe elbows is lower than that of straight pipes K factor = 1.65/(h 2/3power), where h=t(pipe)*R(elbow)/(r(pipe)*r (Pipeline).This equation, or a similar one, can be found in Section 9.9 for Steel Tube Construction.Rectangular piping system design properties are based on aspect ratio. This is usually simplified from corner elements or corner stiffeners to width = t/16, although in practice the entire duct top and side plates do participate, affecting the duct section properties to some degree.

Pipeline logic

Duct logic is the process of planning the thermal movement of ducts, combined with planning to minimize duct dust loss.Pipes move as the internal temperature changes.Assume the pipe is at the same temperature as the gas inside it, up to 900 °F. If internal pipe temperatures exceed 1000 °F, use a refractory lining to minimize pipe surface temperatures. At 1000°F, the pipe may grow about 5/8 inch per 10 feet of length.This movement must be carefully planned, using cloth (or metal) expansion joints at each equipment flange, and one joint for each straight section of each piping system.Ductwork sloped at or above the duct dust repose will minimize dust accumulation.As a result, many pipes carrying high dust loads are sloped 30 degrees or steeper.