Welded metal bellows seals continue to have success as a core sealing technology and have gained popularity recently in new innovative sealing technologies such as high-temperature non-contacting gas lubricated seals and high-temperature corrosion resistant seals. This is especially important in the oil, gas and chemical industries where pumping liquid from one area to another is complicated by great temperature extremes.
Unfortunately, not all seals are created equal. Differing features, materials and a host of other factors can impact the overall effectiveness of the seal. Only with an understanding of these differences can plant operators select the most effective product for the application.
A welded metal bellows seal is made through a process of stamping disc-like plates in specific contoured shapes and welding them in pairs at the inside diameters to form individual convolutions of the bellows. A series of convolutions is then stacked and welded at the outside diameters to form the bellows capsule. Suitable end-fittings complete the assembly.
The welded metal bellows assembly acts as a spring to keep the primary sealing faces together, acts as a dynamic seal and transmits torque from the set screw collar to the seal's rotating face.
Welded bellows have specific advantages, including:
- Higher strength with the ability to withstand greater pressures
- Wider operating temperature range
- Ability to be given precise design characteristics
- Lower spring rate (the amount of force required to compress it a given distance)
- Lower stress in critical areas
- Welded bellows allow for the use of optimal plate shapes such as the Nesting Ripple design
- Static secondary seals
- Only one moving part. . . the bellows
If you've discerned that an edge-welded metal bellows seal is optimal for your application, consider that not all seals are created equal. Differentiating bellows features-the plate shape and thickness, vibration attributes, the impact of double-ply, face angle and more-all impact the effectiveness of the welded metal bellows seal. Operators should understand these differences in order to pick the most effective product for their application as well as extend mean-time between repair, standardize inventory, increase reliability and improve fugitive emission control and water conservation. The following are some key distinctions and features of today's welded metal bellows:
The plate shape influences flexing, stroke and operating length. In the nesting ripple configuration, all plates in the bellows are identical and contoured to permit nesting when compressed. Contouring also improves the bellows' ability to withstand high pressure. The nesting ripple plate shape is 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 percent of span and it prevents a phenomenon known as oil-canning-the inversion of the plate geometry that results in a bulging in and out of the plate, similar to that on the bottom of an oil can when it is pressed. Each convolution is made up of a male and female plate, which allows the seal to be designed with a short axial space.
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 linear, thin-shell theory, it has been shown that tilting the bellows axis drastically reduces stresses at the welds and heat-affected zones. The analysis indicates that the stresses at the welds were predominantly bending stresses. With increasing tilt angles, the bending stresses are lowered. This design principle also has been thoroughly documented in both theoretical and empirical studies conducted by an independent government-sponsored agency and verified experimentally. With a 45-deg tilt angle, bending stresses are directed away from the heat-affected zone of the weld. This results in plate rigidity, which adds reliability and reduces fatigue.
State-of-the-art manufacturing processes ensure integrity of the weld by preventing excessive root gap with bead geometry, bead thickness and roll-over control. Bellows units should be checked for leak tight performance with helium mass spectrometry and vacuum-tested to 10-6 TORR. With helium mass spectrometry, the seal is evacuated internally and blanketed in helium. Traces of the gas, which then penetrate through either a break in the weld or a material flaw, are immediately picked up by the sensing probe and the seal is rejected.
Plate Thickness (Thin Plates)
Thin plates provide lower spring rates, which result in lower face loads, less unit loading, less heat generation and longer life than thicker plates. Thicker bellows plates have higher spring rates and are more susceptible to metal fatigue. Repeated plastic deformation of the plates (beyond their plastic limit) during deflection can result in fatigue and greatly reduce seal cycle life.