February 25, 2020 | 5 minute read
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 disc-like 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 is then stacked and welded at the outside diameters to form the bellows capsule. Suitable end-fittings complete the assembly.
Figure 1: High-temperature welded metal bellows assembly with flexible graphite secondary seals.
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, face-angle, etc., all impact product effectiveness.
Operators should understand these differences in order 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 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 (Figure 2) and it prevents a phenomenon known as oil-canning, where in/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 allow the seal to be designed with a short axial space.
Figure 2: Three-sweep design optimizes stress distribution, thus increasing resistance to fatigue fracture. The tilted bellows axis distributes bending stresses away from the welds and through the sweep radius.
For bellows with straight flat segments, the variability of the micro-structure 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 stress at the welds and heat-affected zones. The analysis indicates that the stresses at the welds are predominantly bending stresses, but increasing tilt angles lower these bending stresses. 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° tilt angle, bending stresses are directed away from the weld heat-affected zone. This results in plate rigidity, which adds reliability and reduces fatigue.
- Weld integrity
It is important that state-of-the-art manufacturing processes are used to create the bellows to ensure weld integrity by preventing excessive root gap with bead geometry, bead thickness and roll-over control (Figure 3). Be sure that the bellows units are 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 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.
Figure 3: Weld bead integrity is ensured by preventing excessive root gap with bead geometry, bead thickness and roll-over control.
- Plate thickness (thin plates)
Thin plates provide lower spring rates, which result in lower face loads, less unit loading, less heat generation and longer seal life. 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 life. Increasing the plate material thickness, though easier to weld, increases its stiffness and the spring rate of the bellows significantly. A high spring rate is undesirable because of the significant changes in sealing face loading with only slight changes in seal operating length. This causes excessive closing force, which in turn causes loss of the lubricating film between the sealing faces, excessive face heat and, eventually, seal failure. This is especially critical in high-temperature and poor-lubricating environments. Bellows with high spring rates are also less capable of compensating for installation problems, shaft movements, impeller adjustments, pump end play, shaft growth due to heat and gradual wearing of the sealing faces. Thinner plates are more difficult to manufacture, which is why many manufacturers are forced to use thicker plates.
- Controlling vibrations
Not all bellows in the industry are fitted with vibration dampeners, which help prevent any potential damage from harmonic vibration caused by episodes of dry-running. However, a vibration dampener is ideal. In certain seal designs, the vibration dampener pad is a built-in design feature that allows protection, when needed, against potentially damaging vibrations.
- Double-ply bellows
Double ply bellows create strength and flexibility without thickness. Double-ply bellows are typically utilized in higher-pressure applications and often used in services in which the fluid is thermo-sensitive or tends to set up and solidify on the seal faces where more start-up torque strength may be required (Figure 4). The double-ply design principle is like the leaf springs used in light automotive trucks and trailers. A spring, comprised of a single, thick, metal member with the strength necessary to support the load would result in much too stiff a spring. But, when the strength is obtained by using a “stack” of separate, individually flexing thin leaf elements, the spring rate is well within desired limits. Similarly, the spring rates of two-ply bellows proved to be significantly lower than those of single-ply bellows with twice the plate thickness.
Figure 4: A laminated metal bellows offers a significantly lower spring rate compared with single-ply plates of 2X the thickness. This bellows type is used in higher pressure applications.
- Pressure and seal balance
Before you specify a metal bellows seal, you need to determine the proper pressure rating required by your application in relation to temperature, speed and sealed fluid lubricity, as well as determine if the seal is capable of handling reverse pressure, especially in dual-pressurized seal arrangements. Most standard bellows seals are typically balanced to approximately a 70/30 ratio. This means that 70% of the face contact area is above the effective diameter. More recently, 50% balanced bellows seals have been designed to handle both inner diameter (ID) and outer diameter (OD) pressure. Fifty-fifty balanced seals are able to handle reverse pressure which is an upset condition that may occur in dual seal operation.
Additional welded-metal bellows design features:
- Plate span: Narrower plate spans typically provide greater stability under pressure than wider spans.
- Face width: Narrow face width typically results in less heat generation at the seal faces. Less heat improves face stability and narrow faces are less susceptible to coking than wider faces.
- Multitude of available metallurgies and face materials: Be sure your manufacturing offers an extensive array of materials such as AM-350, hastelloy-C, inconel 718, alloy 20, monel and titanium.
Design characteristics: Key to consider is not just the bellows seal head assemblies, but also the cartridge seal designs. Compare design features such as inside or outside mounted, reverse pressure capability, API 682 desig