Compensator (pipe)
The compensator is a flexible element to compensate for movements in pipelines , especially in the case of thermal changes in length , vibrations , wall penetrations or settlement phenomena . The compensation takes place mechanically via an elastic bellows . Depending on the medium, pressure, temperature and service life requirements, the expansion joint is made of different materials. Metal and soft material expansion joints are the most common in the power plant sector, but there are also applications in which expansion joints made of rubber, PTFE or other plastics are used.
history
- historical brochures
Metal hose expansion joints around 1930
Back of the brochure from 1930
As early as 1920 Emil Witzenmann applied for a patent for the first so-called metal hose compensator (German Reich patent no. 367 185 of July 29, 1920). From a purely technical point of view, this forerunner of today's expansion joints was a large, pressure-tight metal hose with defined restricted freedom of movement.
In the 1930s, the metal hose principle was replaced as a central functional element by the metal bellows. This construction principle - metal bellows with connection pieces - is the constructive basis of modern metal expansion joints to this day.
Structure of the compensator
Each expansion joint consists of a single or multi-layer elastic bellows and a pipe connection. With metal expansion joints, the bellows and connection usually form a unit, while soft material expansion joints always consist of a combination of a metal frame or tensioning straps and fabric bellows. The fabric is clamped in the frame or directly on the pipeline. Clamping takes place either with tension bands (only possible with round cables), screw connections or special clamps.
Compensator bellows
The bellows of the compensator can be made of metal, a composite of technical fabrics, plastics or rubber. Both a single-layer and a multi-layer structure in which the layers follow different design criteria (e.g. tightness, corrosion protection, etc.) are always possible. The elasticity of the bellows comes about through a mostly undulating material compression.
Duct connection
The expansion joint is connected to the duct either via a flange or directly, in each case by screwing or welding.
Baffle
In the case of gaseous flow media, guide tubes or guide plates are optionally used in order to minimize efficiency losses due to flow deflection and damage to the expansion joint. Turbulent eddies due to the enlargement of the flow cross-section in the area of the compensator can lead both to deposition of flow particles in the compensator and to damage due to abrasion. When designing an expansion joint with a baffle, it is essential that the maximum movements and the direction of flow are defined.
insulation
Soft expansion joints are additionally insulated outside or inside in hot gas pipes. Otherwise, due to excessive temperature differences (between the outside and inside of the duct or in the event of a sudden inflow of cold air from an emergency stop), there is a risk that material stresses lead to failure of the expansion joint.
cover
To protect against external influences (chemical interactions, temperature influences, e.g. due to heat radiation from other components, weather), expansion joints can also be provided with a cover. However, the insulating effect created by the cover must be taken into account.
Special structure of metal expansion joints (multi-layer and multi-walled)
In modern expansion joints, the metal bellows are often manufactured in what is known as a multilayer design. In order to increase flexibility and stability at the same time, several thin layers of metal are combined as the wall of the bellows. A distinction is made between two types of construction, the multi-layer and multi-walled bellows structure. The multi-layer structure consists of a pressure-tight longitudinally welded outer and inner cylinder made of stainless steel. In between there is an open spiral cylinder, which, depending on the design, forms several inner layers. In the multi-walled construction, there are several longitudinally welded cylinders that are plugged into one another. Each cylinder forms a pressure-tight and self-contained "wall".
The main advantages of the multi-walled bellows:
Solid bending beam
Bending beam divided
- Control of high and extremely high pressures
- large movement absorption
- small dimensions
- low adjustment forces
- optimal compensation in the smallest space
- Early leak detection (in the event of damage) via standard control bore
- complete safety against bursting
- Possibility of permanent leak monitoring with critical media
- economical use of high-quality, corrosion-resistant materials such as Inconel, Incoloy, Hastelloy, titanium and tantalum
- insulating against structure-borne noise up to 20 dB
This construction brings not only technical but also economic advantages.
Different materials can be used for the bellows structure. Such as B. high-alloy stainless steels for the medium-loaded inner and / or outer tube and low-alloy stainless steels for the intermediate layers.
Types of compensation
When designing expansion joints, a basic distinction is made between axial, lateral and angular movements. With rectangular expansion joints, not only the movement itself but also the spatial direction of the movement plays a role. Depending on the movement, different restoring forces develop . Restoring forces are spring forces that force the expansion joint back into its original position. Especially with metal expansion joints, the restoring forces increase sharply with increasing deformation.
Axially
With axial compensation, the thermal expansion of a straight line section between two fixed points is absorbed by an axial compensator. The distance between the two fixed points determines the line length to be compensated and thus the axial movement to be carried out by the compensator.
The following basic rules apply to axial compensation:
- The planar or spatial pipeline system is divided into individual, straight sections by fixed points so that they can each be compensated for by a single axial compensator.
- The fixed points are to be designed so that they can withstand the pressure and spring forces of the axial expansion joint, the friction forces of the guide bearings and the flow forces.
- Long pipelines between the fixed points must be secured against kinking with guide bearings.
- The axial expansion joint should be installed in the immediate vicinity of a fixed point and a guide bearing.
- Inadmissibly high fixed point loads can be avoided by means of pressure-relieved axial expansion joints.
Angular
At least two angular expansion joints are required for angular compensation of thermal expansion, and even three angular expansion joints for complete compensation. Angular expansion joints offer a multitude of possible combinations in so-called two- or three-joint systems.
The flat three-joint systems manage with one-sided angularly movable expansion joints, while the three-dimensional three-joint systems to absorb thermal expansion in three axial directions require at least two angularly movable universal joint expansion joints.
The following basic rules apply to angular compensation: At least two angular compensators are always required.
- The angular compensation is always associated with multiple flow deflections by 90 °.
- Since angular expansion joints, as joint expansion joints, absorb the pressure forces released by the bellows themselves, the pipe fixed points are only burdened by their adjustment forces and moments, as well as the frictional forces of the line guides and the flow forces.
- The angular compensation is particularly suitable for spatially complicated pipe runs.
Lateral
The lateral compensation is also associated with a deflection of the flow by 90 ° within the planar or three-dimensional line system. In most cases, the right-angled deflections in the system are used for the installation of lateral expansion joints.
The movement of a lateral expansion joint always consists of the desired lateral movement and a slight, unavoidable axial movement from the expansion joint itself. The simple lateral expansion joints for lateral movements in only one plane allow a significantly greater expansion absorption compared to the axial expansion joints. Lateral expansion joints, which can be moved on all sides, also enable expansion of two lines in different directions to be absorbed at the same time.
The following basic rules apply to lateral compensation:
- According to their type of movement, lateral expansion joints are always arranged perpendicular to the compensating pipeline. This means that lateral compensation is always associated with flow deflection.
- Relief of the fixed points from compressive forces as with angular expansion joints.
The unavoidable, small axial movement of lateral expansion joints is absorbed by an additional joint expansion joint during "complete compensation". However, it can often compensate for the pipeline by means of elastic bends. In this case, sufficient clearance must be provided in the line guides. Lateral expansion joints allow angular movement around the bolts or joint axes. This can be used to take up cable slack between the pipe supports. Lateral and angular expansion joints are often used in combination in three-joint systems.
Materials and designs
The choice of a suitable compensator is based on the principle of the most cost-effective function fulfillment. In a profitability analysis, not only the costs of the expansion joints but also those of the required fixed points, pipe supports and manhole structures must be taken into account. In addition, maintenance and repair costs in particular must be included in economic considerations. Repair and replacement of expansion joints usually exceed the investment costs many times over: in the event of damage, in addition to the new investment, the costs for system downtime, safety precautions to be taken and scaffolding in places that are sometimes difficult to access may also be added.
Fabric compensator
Soft material or fabric expansion joints are pipe and duct components made from technical fabrics or elastomers. They are used both to compensate for mechanical misalignment and to reduce noise and vibration. The soft material is either fastened to a steel frame or - if the load is low - connected to the pipeline with straps. Depending on the intended use, fabric expansion joints are produced in single or multi-layer designs. Due to the multi-layer structure, different functions, e.g. B. insulation, tightness, pressure surge behavior etc. can be combined according to the specific requirements. Compared to metal expansion joints, soft material expansion joints are characterized by their low weight and low reaction forces (also known as restoring forces ). Therefore, fabric expansion joints are particularly suitable for large dimensions at low operating pressures and gaseous media (air, exhaust gas, flue gas). There is no standardization for the design, but a collection of technical information from the quality association for soft material compensators in the RAL association , which describes the state of the art.
Metal expansion joint
Metal expansion joints are preferably used when the pipe systems are pressurized, when the temperature is high or when aggressive media have to be conveyed. They consist of one or more metal bellows and connecting parts at both ends. Depending on the area of application and the movements to be compensated, they also have joint anchors. According to the three basic types of movement, a distinction is made between axial, angular and lateral expansion joints. In addition to low-alloy stainless steels, nickel-based alloys or pure metals are also used as materials for the manufacture of compensators. For example, in areas of application in chemistry or in the field of seawater or subsea Hastelloy, Inconel, Incoloy or even titanium and tatal are often used materials. In the field of steel production, brick-lined expansion joints made of high-alloy stainless steels are used, which have a temperature resistance of well over 1000 ° C. Round stainless steel expansion joints are limited in their diameter due to the manufacturing capabilities. Rectangular metal expansion joints can be manufactured in any size and their dimensions are only limited by the transport options.
Axial expansion joint
The axial compensator is used to absorb movement in the axial direction. Standard connection parts of the axial expansion joint are welding ends, fixed and loose flanges. Axial expansion joints are often equipped with a guide tube on the inside of the metal bellows. This reduces the flow resistance and prevents damage that can result from direct contact with the flowing medium. Axial expansion joints that can absorb large movements often consist of two metal bellows and an inner or outer guide tube that protects against buckling due to internal pressure. In the case of small nominal widths, protective tubes are used to prevent mechanical damage during assembly and operation. Axial expansion joints are suitable for internal and external overpressure.
If the metal bellows of axial expansion joints are subjected to external pressure, the expansion joints allow very large axial movements in a pipeline under internal pressure. Since there is no risk of buckling in the event of external overpressure, very long metal bellows or combinations of bellows with very high movement absorption can be used here.
Angular expansion joint
The angular expansion joint absorbs bends or angular movements. Like the simple axial expansion joint, it consists of a metal bellows and the connecting parts on both sides. But he also has one
- articulated anchoring of these connecting parts for angular movements in a plane or
- a cardanic anchorage for all-round angular movements
Consequently, the anchoring determines the type of movement absorption.
Lateral expansion joint
The lateral expansion joint absorbs transverse or lateral movements. It consists of
- one or two metal bellows with an intermediate tube
- the connecting parts on both sides and an articulated anchoring of these connecting parts for lateral movement in one plane or for all-round lateral movements.
Normally, the anchoring consists of round anchors on ball bearings. Flat anchors with pin or universal joints are used for high axial compressive forces. The size of the lateral movement increases with the bending angle of the two metal bellows and with the length of the intermediate tube.
Universal expansion joint
The universal expansion joint can absorb not only axial, but also angular and lateral movements combined. It consists of two metal bellows with an intermediate pipe and connecting parts on both sides. As a special form of the axial compensator, the universal compensator has only a low compressive strength for reasons of stability and also loads the subsequent line supports with the axial compressive force resulting from the internal pressure. It is mostly used when large axial and lateral movements have to be compensated for with low pressure.
In contrast to the non-anchored axial and universal expansion joints, joint expansion joints do not load the connecting cable supports with the axial compressive force from internal pressure, as this is absorbed by the joint anchoring.
Rubber compensator
The rubber expansion joint consists of vulcanized synthetic rubber layers that can be selected depending on the medium and the desired resistance. Various support inserts such as nylon cord, aramid cord and steel cord are available for pressure and temperature resistance. Rubber expansion joints are characterized by high flexibility in axial, lateral and angular directions, as well as high expansion absorption with extremely short lengths. In addition to absorbing expansion, they also have an excellent sound and vibration-reducing effect.
Arch constructions ("natural" compensation)
Arch constructions, usually U-arches and Z-arches, are only suitable for the natural compensation of thermally induced changes in length. These arch structures are also called lyre arches , but are not special components, but are made from tubular components. For rigid pipes, four 90 ° bends are required for a U-bend and two 90 ° bends for a Z-bend, between which shorter pipe sections are inserted. In the case of flexible pipelines, the pipes themselves can alternatively be bent accordingly, whereby the permissible minimum bending radii must not be undercut during installation and operation. The use of bends, the angle of which deviates significantly from 90 °, is avoided, since very large forces can act in unfavorable directions in such structures. The great advantage of arched structures is that they are made of the same material as the rest of the pipeline. As a result, there are no restrictions on the operating parameters due to the compensation elements. In the case of metallic pipelines, the possibility of electrochemical corrosion at material transitions in the area of the already heavily loaded compensation elements is also eliminated. The disadvantage is the large amount of space required for the arches. With the Z-bend, there is also an offset of the pipeline, which must be taken into account when planning, but B. in district heating house connections can be quite advantageous.
Otherwise, non-flexible materials such as glass are also used for “natural” expansion joints . Such a glass component usually consists of particularly pressure- and temperature-resistant technical special glass, so that corrosion resistance is guaranteed. The material glass gives a particularly smooth and hard surface, which minimizes abrasion in the sheet. In addition, by varying the bending radius, an optimal flow can be achieved, similar to that of arched structures. Another advantage is the transparency, through which the substances flowing through can be optimally monitored, as well as the chemical resistance of special glasses such as borosilicate glasses in corrosive environments.
Lens compensator
A lens compensator consists of at least two S-shaped half-shells that are welded around the circumference. The wall thickness is significantly stronger than a corrugated pipe expansion joint. The lens compensator is often used in heat exchangers to compensate for the change in length between the shell side and the tube side.
Long sleeve expansion joint
Long sleeve expansion joints (also stuffing box expansion joints or expansion stuffing box ) consist of two telescopically displaceable pipes that are sealed by means of a sliding seal. The change in length corresponds to the insertion depth of the pipes.
Failure of expansion joints
Compensator failure can be traced back to a wide variety of causes. The most common causes of damage are:
- Transport damage, incorrect storage and handling
- Incorrect installation or insufficient protection during assembly or operation
- Inadequate anchoring, guidance and storage
- Suspension failure
- Corrosion: Due to the increase in the cross-section of the duct, the temperature difference between the outside and inside of the expansion joint can result in gaseous media falling below the dew point and thus the accumulation of condensate. This can damage or destroy the compensator and must therefore be minimized or discharged in a controlled manner by guiding devices.
- System overpressure
- Impermissible deflections of the bellows
- torsion
- Bellows support
Many risks of compensator failure can be minimized by properly designing the compensator. Most expansion joint manufacturers have extensive experience and can suggest suitable compensation solutions in coordination with the operating parameters. Damage that cannot be traced back to the design can be minimized through proper handling and storage in accordance with the manufacturer's instructions, compliance with safety and inspection regulations and regular maintenance.
Check after installation:
- whether the expansion joint was damaged during installation,
- whether the expansion joint is in the correct position,
- whether the direction of flow is correct according to the specifications.
Web links
literature
- Witzenmann Group : company archive
- Company history of Witzenmann GmbH by Gregor Mühlthaler
- Reinhard Gropp: Flexible metallic cables. In: The library of technology 188. Verlag modern industry, Landsberg / Lech 1999, ISBN 3-478-93216-5 .
- The expansion joints manual. Witzenmann, Pforzheim 2010.