Temperature changes cause the pipeline to constantly expand and contract, creating loads on its elements. To prevent these natural movements from leading to deformations, depressurization, or damage to equipment, bellows compensators are installed in the system.
What tasks does a bellows compensator solve in a pipeline?
When the pipeline heats up, its length increases; when it cools down, it decreases. If the movement is rigidly blocked, additional forces arise in the pipe, supports, flanges and equipment nozzles. The bellows compensator accepts the calculated movement thanks to the flexible corrugated element and helps not to transfer all the temperature deformation to the system.
In addition to thermal expansion, a specially designed assembly can compensate for mounting or operating displacements and partially isolate vibration. It is important to understand that the temperature and power diagram must be known before selecting a product.
The principle of operation of a bellows compensator
Bellows is a thin-walled, hermetically sealed shell with annular corrugations. When the ends of the assembly come together, the corrugations bend and the compensator compresses, when they move apart, it stretches. The geometry, number of layers, material, and number of corrugations determine flexibility, pressure resistance, and estimated cycle life.
Under internal pressure, a force acts on the end, which must be perceived by anchors or special structural rods. The bellows itself does not replace fixed supports. If this is forgotten, the pipeline may shift, and the corrugation may lose stability. Therefore, the compensator is considered together with supports, guides and permissible loads on the equipment.
Axial, shear and angular expansion joints: what is the difference?
The axial compensator works by compression or extension along the axis of the pipe. It is compact and common in straight sections, but requires correctly located fixed and guide supports that do not allow lateral displacement. This is the design most often meant when looking for a bellows compensator for a heat network or steam pipeline.
The sliding, or lateral, compensates for transverse movement, in which its ends remain parallel. Usually, two bellows with an intermediate insert are used for this. The angular one allows the ends to rotate relative to each other and often works in a system of two or three nodes. Torsion around the longitudinal axis is dangerous for a metal bellows.
Design: bellows, nozzles, travel stops
The main element is made of thin stainless steel and welded to the pipes or connecting fittings. The pipe transfers the load to the pipeline, and the flange or welded end provides the mounting. The inner guide sleeve can protect the corrugation from abrasive flow, high velocity and turbulence, and the outer casing from mechanical damage.
Materials for manufacturing a bellows compensator
Corrosion-resistant steels are used for the bellows, capable of being repeatedly deformed within a given range. The pipes and flanges can be made of stainless or carbon steel. The specific grade is chosen according to the following characteristics:
- working environment;
- temperature;
- corrosion and compatibility with the pipeline.
At high temperatures, the permissible pressure and mechanical properties of the metal change. For chemically active environments, not only general corrosion is checked, but also the risk of pitting, intergranular and stress corrosion. The final limits are taken from the product passport and the design standard.
Where are bellows expansion joints used?
They are installed in heat networks, heating systems, on steam pipelines, process lines, near pumps, heat exchangers, boilers and turbines - where the calculation shows displacements or unacceptable loads. Some versions work in gas channels, cryogenic or vacuum systems.
The assembly can reduce vibration transmission, but the vibration problem requires data on frequency, amplitude, and number of cycles. A compensator selected based on thermal runout alone should not automatically be considered a vibration insert.
How to choose the right compensator
The output data includes:
- DN and pipe wall thickness;
- environment;
- calculated pressure and temperature;
- installation temperature;
- type and amount of displacement;
- number of cycles;
- flow rate;
- corrosion allowance;
- vacuum if available;
- support scheme.
The manufacturer also needs external loads if they can act on the node.
Compensatory ability and course
The thermal elongation of a straight section is preliminarily estimated as the product of the coefficient of linear expansion of the material, the length of the section and the temperature change. But the required stroke should not be mechanically divided between a random number of nodes: pre-tension or compression, installation tolerances, simultaneous displacements and cycle life are taken into account. The greater the deformation of each corrugation, the higher the fatigue stresses.
Working pressure and temperature
Compare not only the nominal PN, but also the permissible pressure and temperature combination for the materials and design. Check the test pressure, vacuum resistance, expansion force, bellows resistance and permissible anchor loads. The operating parameters must remain within the rating range, taking into account transient conditions.
Expansion joint connection type
A flanged assembly is easier to dismantle, but requires space, compatible flanges, gaskets and controlled tightening. A welded assembly creates a compact, hermetic connection, but requires skilled welding and protection of the thin bellows from splashes and overheating.
For small diameters, threaded versions are available if system parameters allow. End fittings must match the pipe in size and material.
Installation and common errors when installing compensators
Before installation, check the construction length, alignment, flow direction if an inner sleeve is present, and the design position of the supports. Do not stretch the assembly to tighten a poorly installed pipe, correct angular misalignment if it is axial, or allow torsion during tightening. Transport ties are removed only in the order specified by the manufacturer.
Welding spatter, tool impacts, and chloride-containing wet insulation can damage the thin sheath. After starting, check to see if the pipeline is moving in the intended direction, and if there are no protruding corrugations, leaks, or the casing is touching moving parts. Any deviation is a reason to stop and inspect the assembly.
For the project, it is worth comparing the drawings, permissible stroke, pressure, temperature and requirements for supports. View available bellows compensators can be used as a starting point, but the final decision must be confirmed by the pipeline calculation and the manufacturer's technical documentation.
