The bellow is a unique component of any piping system. It is a flexible unit of an expansion joint. It consists of one or more convolutions, having end tangents and is made up of one or more plies. Unlike other piping components, Bellow is made up of relatively thin sheet of metal. In addition to this, it has to withstand the system pressure/temperature condition, while remaining flexible enough to absorb thermal and/or mechanical movements of the system for required life cycle with minimum resistive force. The designing of the bellow element of an expansion joint is the most critical as it operates under same operation condition as the rest of the piping system, but is made up of relatively thin gauge material. The designing of the bellow is also critical as there are circumferential stresses acting on convolutions and tangents and meridional stresses acting on the bellows.
- Pressure thrust:
The pressure thrust is a force exerted due to internal pressure times the bellow effective area. It is of main concern because the bellow is manufactured from thin sheets of metal and it has a tendency to expand. Without any pressure bearing accessories like limit rods, control rods, hinge pins or gimbal rings, the bellow has a tendency to elongate as the internal pressure is increased. The only resistive force acting at this moment is the spring force of the bellows, which is generally less than the pressure thrust force. When the spring rate of the bellow is significantly less than the pressure of the system, the piping system should be anchored properly for proper functioning of the bellow. In straight pipe configurations, the pressure bearing accessories like hinges, limit rods, tie rods or gimbal rings should be designed to withstand the pressure thrust.
- Axial Movement:
- Angular movement:
- Lateral movement:
- Torsional movement:
The twisting of one of the ends of expansion joint, with respect to other, about the longitudinal axis is called torsional movement. Extreme care has to be taken before design and manufacturing of this type of expansion joint because of extremely high shear stress induced.
- Concurrent movement:
- Centre Pipe:
the pipe which connects the two bellow element in a universal type expansion joint is called centre pipe. Usually, it is made up of same material and thickness as that of weldends, but can be changed on specific requirements.
- Internal Liner/Sleeve