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Understanding the design features to get the most effective solution 7 Things to know before selecting welded metal bellows seals for your application Welded metal bellow seals are a core sealing technology whose installation has grown dramatically, as use expands to a variety of innovative sealing technologies such as high temperature-, non-contacting gas lubricated- and corrosion-resistant seals. This is especially important in the oil and gas and chemical industries, where pumping liquid from one area to another is complicated by great temperature extremes. Different factors and product characteristics will impact overall seal effectiveness, and only with an understanding of these differences can plant operators select the most appropriate solution for the application. A welded metal bellows seal is made through a process of stamping disclike plates in specific contoured shapes and welding them in pairs at their inside diameter to form the individual convolutions of the bellows. A series of convolutions are then stacked and welded at the outside diameters to form the bellows capsule. Suitable end-fittings complete the assembly. The welded metal bellows assembly (Figure 1) performs several functions: • Acts as a spring to keep the primary sealing faces together • Acts as a dynamic seal • Transmits torque from the set screw collar in the seal’s rotating face • Welded bellows have specific advantages, including: • High strength with the ability to withstand high pressures • Wide operating temperature range • Precise design and performance characteristics • Low spring rate (the amount of force required to compress the bellows a given distance) • Low stress in critical areas • Allow for optimal plate shape design―nesting ripple • Only one moving part • Static secondary seal Deciding that an edge-welded metal bellows seal is optimal for your application is not the end of the selection process. Differentiating bellows features include plate shape and thickness, vibration attributes, double- or single-ply, faceangle, etc., all impact product effectiveness. Operators should understand these differences to select the most suitable sealing solution and their influence on seal reliability, mean time between repair, standardized inventory, fugitive emissions control and water conservation. The following are seven key distinctions and features of


Pipes, Pumps and Valves Africa - Mar/Apr 2020

Figure 1: High-temperature welded metal bellows assembly with flexible graphite secondary seals. welded metal bellows.

Plate shape The plate shape influences flexing, stroke and operating length. In a nesting-ripple configuration, all the plates in the bellows are identical and contoured to permit nesting when compressed. Contouring also improves the ability to withstand high pressure. The nesting-ripple plate shape is also more effective in achieving maximum flexing, long (axial motion) stroke with short operating lengths and a low spring rate. The sweep radius is optimized at 20-25% of span and it prevents a phenomenon known as oil-canning, wherein/ out bulging of the plate occurs like on the bottom of an oil can when it is pressed. Each convolution is made up of a male and female plate that allows the seal to be designed with short axial space.

Angle For bellows with straight flat segments, the variability of the microstructure in the heat-affected zones results in less reliable weld joints. By theoretical analysis using

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Pipes, Pumps & Valves Africa Mar-Apr 2020  

Pipes, Pumps & Valves Africa Mar-Apr 2020