Metal bellows are a bit of a strange design. They have to be both rigid and strong and also flexible at the same time. They have to be strong enough to withstand intense pressure across their entire design. At the same time, they need to be able to bend. Very little else in manufacturing is like this, so how are metal bellows designed?
Metal bellows and custom expansion joints must be designed with a focus on 12 different, yet all vital, attributes. The geometric constraints of their placement are important. Also key are stress modes and the pressure differential, exposure, and temperature extremes to which the metal bellows will be exposed. The assembly method is vital, as is vibration rates, and the end configuration. Finally, design must also consider rigid stops, flexing, life cycles, and the spring rate. Multi-ply bellows normally have two or three tubes, but they can have as many as five for certain special applications.
What are Metal Bellows Made From?
Choosing the right materials when making metal bellows is also very important. Metal bellows must be made of durable materials that are resistant exposure and corrosion. Iron ore, for example, does not melt until 2,750 degrees Fahrenheit. Stainless steel bellows have a slightly lower melting point, of 2,500 degrees; however stainless steel has greater resistance to corrosion. Marines Grade Stainless Steel is an alloy that contains molybdenum and is specially designed to resist salt and other corrosives that typically appear in a marine environment. Molybdenum has a melting point of 4,748 degrees.
What Special Materials Exist for Metal Bellows?
When metal bellows must resist extremely high temperatures, Hastelloy X is the metal of choice. This alloy is particularly valued for aerospace applications but is occasionally used even in commercial applications where temperatures regularly exceed 1,600 degrees Fahrenheit. Researchers at Brown University found in 2015 that a mixture of nitrogen, carbon, and hafnium had the highest melting point of any substance on Earth, at 7,460 degrees.
How is Pressure Factored For?
Metal Bellows must often operate in conditions of extremely high pressure. Forged steel 150# flanges are capable of withstanding pressures up to 285 psig even at 100 degrees Fahrenheit. Other materials can withstand pressures even higher, and as the temperature rises allowable pressure must drop. For example, at 850 degrees Fahrenheit, the allowable pressure for forged steel becomes 65 psig.
Metal bellows are essential to the oil and gas industry, marine and aerospace applications, and a wide variety of other industrial, military, and commercial applications. It is therefore of great importance that metal bellows be made of the proper materials, taking into account all 12 essential design elements.