Bellow convolution is the structural core and the most typical appearance feature of the bellows.
This appearance feature distinguishes it from standard stainless steel straight pipes.
Changes in the parameters and numbers of convolution will significantly impact the bellows’ performance.
Their influence on the performance of the bellows includes the compressive strength, fatigue life, stability and stiffness of the bellows.
Any adjustment of the bellows parameters needs to be recalculated and verified according to the EJMA standard.
It is prevalent to not meet the ideal bellows design parameters during procurement or production.
The final bellows manufacturer is only sometimes the one specified by the designer.
Or the customer has to find a new bellows supplier to replace a small number of bellows expansion joints.
The bellows are often redesigned according to a specific manufacturer’s existing bellows forming mould to obtain an optimal parameter close to the ideal state.
Redesigning the bellows is more cost economical and helps shorten the production cycle.
Convolution Parameters
Height: Increasing the height can make the bellows softer and improve the compensation ability of the bellows.
But at the same time, it will also lead to a decrease in the compressive strength of the bellows, increasing the risk of plane instability.
According to production experience, the ratio of the peak’s outer diameter to the trough’s outer diameter (expansion coefficient k) is generally controlled at 1.1-1.3.
When the expansion coefficient k≥2, hydroforming bellows becomes quite difficult. At this time, edge welded metal bellows is a better choice.
Pitch: The pitch is also an essential factor affecting the performance of the bellows. As the pitch increases, the bellows’ compensation capability and fatigue life will increase; simultaneously, the pressure resistance and single-convolution stiffness of the bellows will decrease.
The pitch increases as the inner diameter of the bellows increases.
The pitch is the most challenging parameter to control in the production of bellows because the spring-back of the stainless steel sheet will cause certain manufacturing deviations.
The height is usually between 0.65 and 1.25 times the pitch.
Wall thickness: The wall thickness of the bellows is determined by the design pressure.
Increasing the wall thickness can improve the bellows’ compressive strength and plane instability resistance.
However, if the wall thickness is too thick, the compensation ability of the bellows will be reduced, the flexibility will be reduced, and the fatigue life will be shortened.
Furthermore, the wall thickness increase will increase the bellow’s weight, and the corresponding purchase cost of stainless steel materials will increase.
Layers: When the total thickness is constant, increasing the number of layers will improve the bellows’ compensation ability and fatigue life.
The single-convolution stiffness of the bellows will also be significantly reduced, and the flexibility will increase.
Under the same stress state, the total thickness of the multi-ply corrugated pipe is thicker than that of the single-ply corrugated pipe.
The ratio of the entire wall thickness of the multi-ply bellows to the inner diameter of the bellows is generally 0.05.
The multi-ply bellows is prone to plane instability in the compressed state and external pressure circumferential instability in the stretched state.
Bellow Effective Length
The effective length of the bellows refers to the total length of the corrugated part, excluding cuffs at both ends and other connecting accessories.
To ensure the reliable stability of the bellows when working, the ratio between the effective length and the outer diameter should be less than or equal to 1.3.
Bellow Convolution Types
Bellows are classified according to the shape of the axial section, and can be divided into U-shaped bellows, Ω-shaped bellows, S-shaped bellows, V-shaped bellows, etc.
Currently, most bellows have a U-shaped corrugated structure, but each type has its own advantages and disadvantages.
According to the use conditions of different applications, choose the appropriate bellows to ensure the regular operation of the pipeline system.
U-shaped bellows: the forming is relatively simple, and the compensation capacity is rather large.
Ω-shaped bellows: higher pressure resistance, but minor compensation ability.
S-shaped bellows: similar to Ω-shaped bellows, but the compensation ability has been improved.
V-shaped bellows: It has a large compensation capacity and is suitable for low-pressure environments. The working pressure should exceed 0.4MPa.
How Does The Bellow Convolution Work?
