Crack the Code

When specifying dust collectors, it may not be necessary to follow ASME code to the letter. Ask the right questions and ask them early

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Dust collection systems are integral parts of most chemical processing plants, allowing companies to prevent process contamination or recover valuable products. However, working with dust, particularly at high pressures, poses critical safety problems. Codes and standards for high-pressure dust-collection equipment, developed by the American Society of Mechanical Engineers (ASME), ensure worker and facility safety, and protect users from liability. Many of the standards are routinely adapted to design and build low- and negative-pressure collector units as well. You may want to comply with Code standards to indemnify your facility and provide peace of mind. However, there are occasions when you may not need to follow the Code to the letter, and you won't compromise safety. Only an experienced vendor or designer will be able to tell you, if you ask the right questions and ask them early in the design stage.

Since most chemical engineer users are not all that familiar with the Code, this article summarizes what the process engineer or manager needs to know to work more effectively with designers and manufacturers. This basic knowledge will prevent costly mistakes, and cut the time and capital required for any dust collector design project.

"U" must understand

The first place to look for critical design data is in the "U Code" section of the ASME Code, Section VIII Division 1, Rules for construction of pressure vessels, which focuses on materials, design, fabrication, examination, inspection, testing, certification and pressure relief.

Specific issues are addressed by the Code (Table, p.47) and annually through individual appendices that provide guidelines for optimizing vessel configuration, considering wall thicknesses, maximum allowable stress values, accessories and construction materials. The formulas used will depend on operating parameters including operating pressure and temperature, materials being processed, throughput rates, and corrosive characteristics. Individual appendices also define specific design functions, including configuring dish heads, doors, flanges, bolting patterns, reinforcing rings, and pipe connections.

Familiarize yourself with the most relevant Appendices as early as you can in any dust-collection vessel design project. Remember that each issue may be independent, but many are interrelated. Designers generally cannot alter any one of these variables without changing others down the line. Having a firm grasp of what will be required before the project starts can help reduce rework, and prevent unnecessary "overengineering" and waste.

Conditions drive design

In any dust collector design and construction project, process engineering work generally takes place first to determine such parameters as design pressure, process temperature and materials of construction. Once these issues have been resolved, mechanical design begins, and required strength, wall thicknesses, nozzle fitting reinforcements, flange connections, gaskets, valves, joints, fittings and other accessories are determined. Fabrication then begins.

After reviewing performance requirements with the end user, the vessel manufacturer develops preliminary calculations, based upon customer needs and relevant Code standards. Critical issues such as reinforcement pads, wall thicknesses, accessories and operating conditions must be resolved early in the project. End-user specifications are reviewed and compared with applicable Code standards at this stage.

Account for corrossion

The material of construction selected will depend on operating pressure, temperature, and the process or material for which the vessel is being used. Consider your process. Will corrosion be a factor? How about high-temperature process throughput? Both of these cases will require specific materials and maybe even additional protective features.

Typical plate materials of construction include 316 stainless steel, Hastelloy, Monel, Inconel, and Code quality carbon steel; (e.g., SA-516-70 or SA-285C). Whatever material is selected, Code vessels will require welding, a topic that is covered in detail by Section UW, which describes acceptable materials and procedures, and the certification/ testing methods associated with them.

Some Code vessel users may require 100 percent X-ray for approval, while others may require only spot X-rays. Still others may forego weld X-rays.

In addition to radiography, ultrasonic testing (UT), liquid penetrative inspection (LPI) and magnetic particle (MP) inspection also can be used to determine weld integrity.

Quantifiers are used in design formulas to determine "weld efficiency factors." These factors are used to calculate the allowable stresses to be placed on the vessel or any of its components. Maximum allowable stress as it relates to weld efficiency is known as "SE" (where S represents maximum allowable stress and E represents efficiency). This formula states that maximum allowable stress is multiplied by the efficiency. For example, if there is a weld efficiency factor of 0.7 for double-welded butt joints without radiography examination; the allowable stress on the vessel must be reduced by 30 percent as a conservative measure.

In addition to the familiar "U" stamp applied to the Code vessel by its manufacturer, ASME also governs the "R" stamp, authorizing repairs. If your plant needs a new nozzle for a Code vessel to accommodate a new or different application, your engineers or maintenance people can't simply open the vessel and weld or bolt on new hardware. Nor is an ordinary outside maintenance agency allowed to make that upgrade, which must be done by a licensed ASME Code repair shop.

Although repairing is inevitable, modifying a Code vessel generally is not recommended, since cutting into a dust collector vessel's shell places that vessel under the same testing and documentation requirements as a brand new Code vessel.

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