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
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
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
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.
Once a Code vessel has been designed and built, and its welds tested, hydrotesting is used to determine its overall integrity. In order to receive a "U" stamp by a Code inspector, the vessel is filled with water and pressurized to 1.5 times its design pressure. While under pressure, construction materials, welds, accessories and other hardware associated with the vessel must be rigorously checked before approval.
Adapting the code
Some processes handled by Code dust collector vessels may be changed from positive to negative pressure, using special ASME formulas. As a typical example, most vessels under negative pressure require a stiffening ring around their housings because of the compressive forces encountered during operation. These rings generally are not required on positive pressure vessels, since they are not subject to the same stresses. In addition, vessel wall thickness ," a key issue for negative-pressure applications ," is less critical in most positive-pressure applications.
Doors or access ports, built in to permit inspection, cleaning or filter bag replacement generally are considered structural weak points, and must be reinforced by an approved Code method to maintain integrity. Generally, reinforcement is accomplished by placing a reinforcing ring around the nozzle opening. However, the reinforcing ring may not be needed on some vessels because structural reinforcement also can be accomplished by increasing the thickness of the vessel's shell. For example, if the calculated thickness requirement of a Code vessel's shell is 3/16 in., its designers might specify a reinforcement of 1/4 in. instead of 3/16 in. That 1/16 in. additional thickness can contribute to the vessel's reinforcement and often is strong enough to accommodate an access port without the need for a separate reinforcement ring. In this instance, costs are lowered without affecting vessel performance or integrity.
Generally, vessel housings with filter bags at the bottom require access doors, while other vessels may incorporate liftoff heads. For practical purposes, only round, elliptical or oblong doors can be used on Code vessels to eliminate stress cracking. Other safety considerations include davit or crane-like assemblies that might be required on doors too heavy to be lifted by workers. Code vessels with walk-in plenums would typically fit this description.
Design differences
Keep in mind, however, that there may be situations where it's better to err on the side of safety.
A Code vessel's configuration, materials and accessories, must be scrutinized closely for applications in which explosive dust is being processed. However, even in these applications, there are ways to design and build safe dust collectors without a Code stamp.
In certain situations, a properly designed and built Code vessel rated for operation at 14.9 psig positive pressure might be able to handle 25 psig operation in complete safety. Such a vessel might be designed for negative-pressure operation, yet handle up to 25 psig positive pressure.
However, safety is critical when a dust collector is located inside a building. Containment vessels designed for explosion-proof operation above 50 psig must be substantially overbuilt, exceeding Code requirements, since explosive dust cannot be vented safely. Safety concerns and fears of liability drive many companies to request Code vessels even when they don't need them," even for systems with operating pressures well below 15 psig.
When dust collector vessels are subjected to variable process pressures (including those less than 15 psig), it may be prudent to build them to Code standards. As mentioned in the Code, Section 8, Division 2, Appendix 5, the cyclic nature of pressure loading, rather than the actual operating pressures, places significant stress on equipment and must be accounted for in overall design of shells, lugs and supports.
While these Code vessels may require certain types of header configurations for elbows or ducting, a number of standard couplings can operate to 150 psig. There are also quick connect couplings that can operate up to 500 psig ," yet are not mentioned specifically in the Code Standards.
Mike Grobstein is a design engineer at the Flex-Kleen Division of Met-Pro Corp., Itasca, Ill. He can be reached at [email protected]. Jim Becker is regional sales manager at Flex-Kleen, and can be reached at [email protected].
