Metal bellows seals are problem solvers for many industries, often providing solutions to common challenges such as harsh applications, high heat or pressure situations, and installation issues.
There are two main classifications of metal bellow seals: edge welded metal bellows and formed bellows. A variety of elements must be considered when deciding which bellow seal to use, but the most critical factors to consider are suitability and reliability.
The right metal bellows seal should be one that addresses the application and its operating requirements as well as meets the key conditions of seal balance, materials selection, operating lengths, and spring rates.
Choosing the best metal bellows seal design for a specific situation can be daunting but with a clear understanding of the advantages and limitations of the two main bellows seal classifications, a facility can curb cost inefficiencies and be confident that their metal bellows seal is the appropriate solution for their sealing challenge.
To clear up any confusion, here is a comparison guide to choosing the right type of bellow seals for your sealing application.
Two Classifications of Bellows
Bellows seals are engineered, flexible, metal components which act as a leak-tight barrier to successfully separate two environments from one another. Here is a more detailed breakdown of the two main classifications of metal bellows seals.
Edge Welded Metal Bellows
Edge welded metal bellows are formed by microscopically welding the interior diameters of individual, stamped metal diaphragms into two-piece convolutions. These convolutions are then welded together on the outer diameter to form a welded bellows core.
Edge welded metal bellows can be used in a variety of industries due to their high level of precision, increased durability, and ability to reduce risk of catastrophic equipment or operational failure.
- Oil & Gas
Formed bellows have several subclassifications including cold-formed, hydroformed, and electroformed bellows. Cold-formed and hydroformed bellows are typically manufactured from either welded or seamless tubing and utilize extensive automation for mass production. Electroformed bellows are created by electroplating metal onto an aluminum mandrel.
The process in which formed bellows are manufactured offers some cost benefits, however, this is not without drawbacks. Historically the limits of mechanical strength and operating temperatures are much narrower than edge welded metal bellows. These limitations restrict the usage of formed bellows in high temperature or harsh application environments typically found in the oil and gas, cryogenic, and power generation industries.
A Side by Side Comparison of Bellows
In order to choose the right metal bellows seal for an application, it is imperative to address several key points: materials of construction, operating characteristics, and cost. Using these points as the basis of the comparison, the overwhelming benefits of edge welded metal bellows become very clear.
Materials of Construction
When considering the best metal bellows seal design for your application, the materials of seal construction are crucial. These materials determine the overall successful operation of the seal.
Edge welded metal bellows can be produced from a wide range of materials including corrosion-resistant alloys and specialty metals suitable for operating parameters as high as 800°F (425°C) to -350°F (-212°C for cryogenic applications), allowing for diverse applications across different industries. Edge welded metal bellows also offer additional options to increase sealing viability. These processes include:
- Double Ply Bellows: Creates strength and flexibility without thickness. Double-ply bellows are typically utilized in high-pressure situations and in services where more start-up torque strength may be necessary.
- Heat treating : Conducted after a bellows core has been welded. This process modifies the material’s characteristics to provide higher tensile strength, increased spring rates, and pressure ratings.
Formed bellows, unlike edge welded metal bellows, are typically limited to metals with high elongation properties such as brass or bronze. The malleability and integrity of these types of metals can be challenged when installed in high pressure and/or corrosive applications.
A breakdown of the operating characteristics of metal bellows seals provides guidance when choosing the correct solution. These defining characteristics, positive and negative, impact the seal’s overall suitability and efficacy.
Edge welded metal bellows seals are defined by their flexibility. This trait is the result of a thin individual metal diaphragm which is measured in mils (thousandths of an inch). This characteristic allows for dramatically greater flexibility, range of motion, and space allowance. In terms of spring rate, edge welded metal bellows can be characterized in one word: precision. Edge welded metal bellows seals allow for the addition or subtraction of individual convolutions, enabling a precise spring rate. Achieving the correct spring rate lightens the force affecting the faces which in turn generates less heat and leads to a longer seal life cycle.
Formed bellows, conversely, are defined by their rigidity which is the result of thick, formed walls with inherent stiffness. The process of forcing material into form can cause inconsistent wall thickness on both the OD and ID of the bellows unit which create vulnerabilities and hotspots for failures. In terms of spring rate, formed bellows have higher spring rates and larger deviations than edge welded metal bellows. The typical thicker convolution structure of formed bellows has a detrimental impact on precision loading.
Edge welded metal bellows are typically less expensive than formed bellows for small volume runs as they are produced with a lower initial tooling cost. Actual costs are variable due to quantity of manufacture, materials of construction, and size of the unit.
Formed bellows tend to be less expensive per unit than edge welded metal bellows when produced in high quantity. This manufacturing process may offer some cost benefits, however these benefits must be calculated in with the associated higher tooling and set up costs.
While price is often a factor when determining which metal bellows seal design is the best fit for an application, it shouldn’t be the primary factor in selection. By ensuring the correct construction materials and operating parameters are selected, potential seal failure (and the associated costs of product loss, equipment repair, and even compromised personnel safety) can be avoided.
The Right Bellow for The Right Application
Not all metal bellows technologies are created equal. The two most important criteria for choosing the best bellows seal – suitability and reliability – quickly prove that edge welded metal bellows are the superior choice over less effective counterparts.
Different manufacturing methods and construction materials can profoundly affect how a metal bellows seal will perform in terms of stroke, pressure capabilities, spring rate, and temperature response. These characteristics also impact how long a metal bellows seal will last in the field: the wrong bellows may operate for the short term but fail to achieve the expected long term results.
Edge welded metal bellows have earned their place in today’s fluid sealing marketplace because of their ability to deliver the desired results consistently and across a wide range of industries and sealing environments. Equipped with the knowledge presented here engineers and technicians can navigate the available options, confident they are choosing the right metal bellows seals for their specific needs and requirements.