A compensator is a flexible part in the pipeline and by absorption allows movements in the pipeline. According to its usage the compensator absorbs axial-, lateral- or angular movements. The absorption of thermal expansions in a pipeline can be achieved be several methods, and when absorbing the pipeline's movements the compensators start moving in the direction it is designed to. Most pipeline engineers prefer compensators because they take up less space in the pipe system. Compensators are an economically advantageous choice, and they absorb several kinds of movement than their alternatives; pipe bends and loop expansion joints. Due to the wide scope the pipe layout the costs for design, calculation and construction are minimized. Moreover operation costs are reduced because of no maintenance and minimized heat- and friction losses. When a product with following characteristics is needed, a steel compensator is the best solution:

• Resistance towards pressure
• Resistance towards temperatures
• Resistance towards corrosion
• Safe and reliable
• Relatively free of maintenance
• Balancing flexibility and resistance towards pressure
• Flexible
• Economically advantageous

The history of the compensator
Since the first compensator has been invented and constructed by the German Heinrich Witzenmann, in the 20th century, a lot has happened.
In the beginning bellows were manufactured as single-layer bellows with a thickness of up to 15 mm, either made of alloyed steel or non-alloyed steel. While being heavy and space-consuming, the bellows were practically only deformable in axial direction, while the reactive forces were enormous.
With increased knowledge and expertise a state-of-the-art bellow is made of one or more thin layers of stainless steel this is rolled into a bellow. It appeared that the thicker the steel of the bellow gets, the shorter its service life becomes. To overcome this conflict, today several layers are used. By manufacturing bellows from several thin layers both thickness of material and flexibility are achieved. The number of bulges and the thickness of the layers depends and is based on the movements, the compensator is designed to absorb and the reactive forces it has to resist.

Compensator design
Basically a compensator is an assembly kit. The bellow and the additional components are chosen according to the operating conditions. In that way it can be ensured that the optimum compensator is chosen.
Belman designs and manufactures the compensator needed for the application: 

Kinds of bellows:
• Single bellow
• Double bellow 1
• Double bellow 2

Additional components:
• Connection ends; flanges and welding ends
• Collar ring
• Casing
• Insulation
• Gimbal  
• Tie rod
• Pantgym
• Hinges
• Inner sleeve

 
Basis of calculation
To be able to design and manufacture the optimum compensator Belman needs the following specifications:

1. Operational demands for the pipeline and the design of it
It is important to have as much information as possible about the pipe layout. These informations are needed when calculating and evaluating which kind of compensator must be chosen and where in the pipeline it is to be placed. In this way the best positioning of the compensator, fixed points and achors is achieved and optimum absorption is ensured.   When evaluating this, the pipeline is often separated into sections and according to the best way of absorbing movements one or more compensators are needed in each section. For each section it is important that Belman are informed about the temperature, pressure, movements etc. Belman also needs to know the size of the pipes (the actual outer diameter of the pipe and the actual thickness) as the size influences the compensator's ability to absorb the given movements.

2. Design pressure
The design pressure must be taken into account when the thickness of the bellow and the connection ends are calculated. The higher the pressure is, the thicker the material has to be. This conflict is solved if instead of a few thick layers several thin layers of bellow material will be used. In this way a greater flexibility is also ensured with a higher thickness of the bellow.
The expected design pressure in the pipeline under test and operation needs to be precisely determined. A better solution is achieved by calculating a larger design pressure than the current pressure, an oversized thickness of material can result in exhaustion.

3. Vibrations 
Vibrations are essential information, because they can significantly shorten the service life of the compensator, if the compensator is not designed for vibrations. Vibrations are defined from their frequency and the extent of the variation.

4. Movements
Movements the pipeline occur during standard operation. The movements determine both the kind of compensator and the accessories needed.  Furthermore the movements are important for many other parameters such as the built-in length of the compensator.

5. Medium
The medium, which is running through the pipeline, influences the choice of material the bellows are made of. The chosen quality must be resistant to the media. If the media will settle in the bellow, it is necessary to include further design arrangements to avoid this. Such settlements inhibit the bellow to work as intended to. An inner sleeve could be the solution.

6. Design temperature
It is important to know the exact maximum- and minimum temperature under operation as well as the temperature at installation. Additionally the operating pressure and the expansion the compensator is going to absorb must be known. Likewise knowledge about the temperature is important for the choice of material because the material has to be resistant towards a given temperature.

7. Material
The choice of materials is depending on the temperatures, the movements and the media that is continuously running through the pipeline. Another important factor for the choice of material is the environment the compensator and the pipeline operates in.

8. Spring force
The spring force is necessary for the compensator to absorb the intended movements. The spring force contributes resistance into the system and function as a spring that is compressed or deflects. To moderate this spring force and to prevent it from damaging the compensator, the force on the anchors is limited.
The extent of spring force is determined by the compensator's spring force and by the amount of movements the compensator is exposed to.

9. Reactive forces
Reactive forces are the kind of forces that are the basis for the dimension of the fixed points, guides etc. in the pipeline. In that way the extent of the pipe's retaining is determined.

10. Insulation 
If the compensator must be insulated it is important to know.
There are two ways of insulating a compensator. Both are depended of the positioning of the inner sleeve.  The compensator is either insulated between the bellows and the inner sleeve or between the compensator and the outer casing. The best solution is chosen according to the design of the pipeline.


Materials
Belman manufacture compensators in many different materials and stock among others:

AISI 304 (W.1.4301)
The common stainless steel quality. This quality is resistant to organic chemicals, dry stuff and various inorganic chemicals. It is resistant to nitric acid in moderated concentrations and at moderate temperatures.

AISI 304L (W.1.4306 and W.1.4307)
Compared to AISI 304 this type has a lower content of carbon, as AISI 304L has a content of maximum 0,03% (AISI 304 has a content of 0,08%). This low content prevents the deposition of the chrome carbamide deposition. Furthermore this quality is resistant to intergranular corrosion in a higher extent. AISI 304L has better welding characteristicss. For the utilization of nitric acid this steel type is preferred.

AISI 316 (W.1.4401)
This quality contains more nickel than AISI 304. Compared to AISI 304 there are 2-3% of molybdenum extra added which increases the material's ability to resist corrosion. This is especially important in surroundings that inhibit chlorides, because this normally leads to pitting corrosion and can be avoided by choosing this steel type.

AISI 316L (W.1.4404, W.1.4432 and W.1.4435)
AISI 316L with a low content of carbon (maximum of 0,03%), is suited for applications, where integranular corrosion can be a danger to operation. The low content of carbon has also positive effects on the welding characteristics. In general the welding characteristics are better, the lower the carbon content is.

AISI 316Ti (W.1.4571)
Besides from nickel, chrome and molybdenum this type also contains titanium.

AISI 321 (W.1.4541)
Besides chrome and nickel this steel type also contains titanium, which acts as a stabilizing element. It avoids carbide depositions when the material is heated and cooled down at temperatures in the area 425ºC to approximately 800ºC. AISI 321 is heat resistant.

W.1.4828
This type of steel is also named ”high-temperature steel” because it is resistant to high temperatures and aggressive environments. It is made of 0,04% carbon, 20% chrome, 12% nickel and 2,0% silicium. W.1.4828 has high yield strength, a high resistance to corrosion and oxidation caused by high temperatures. Characteristic for this steel type is also the stability of the internal microstructure.  

Inconel 600
This nickel-chrome alloyed steel type is well suited for the application in corrosive environments and at high temperatures because it is resistant to oxidation, chloride ion stress-corrosion cracking, caustic corrosion and/or corrosion caused by high-purity water.

Inconel 625
This nickel-chrome-molybdenum alloyed steel type has additionally niobium, which makes it especially resistant to wind and weather, high temperature and corrosion. Moreover, it is resistant to a wide range of severely corrosive environments as well as eventual crevice corrosion and pitting corrosion. Together with molybdenum niobium stiffens the alloy's matrix, which in that way provides high strength to the material without the use of strengthening heat treatments.

Incoloy 825
This nickel-iron-chrome alloyed steel type has additionally molybdenum and cobber. This quality makes it especially well suited for high temperatures, reducing and oxidizing acids, sulphuric and phosphoric acids, stress corrosion cracking, pitting corrosion and crevice corrosion.

About stainless steel
The first stainless steel was produced at August 13th 1913. In the period of 1871-1913 the stainless steel had been in a continuous development and analysis process.  The stainless steel types that were produced at that time are some of the types we still use today. Stainless steel is defined as a series of alloyed products that all contain 12% chromium at the minimum. By generating a thin passive film of insoluble oxides on the surface this quantity of chromium makes the material resistant to corrosion.
The acid proof quality has the same chemical composition as the general stainless steel type, but further molybdenum is added. Because of this, this stainless steel type can be used in more aggressive and polluted environments.

Advantages of stainless steel:

• Resistance to corrosion
• Resistance to heat
• Ductile
• Formable
• Weldable
• Easy to clean and therefore hygienic
• Environmentally benign
• Long-lasting
• Optically neat