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.
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 firstname.lastname@example.org. Jim Becker is regional sales manager at Flex-Kleen, and can be reached at email@example.com.