POLYFLURON® Expansion Joints / Bellows

POLYFLURON® bellows have a unique combination of outstanding properties and are suitable for very diverse applications. Because of their near-universal corrosion resistance, they are used particularly as components in chemical plants.                  

POLYFLURON® bellows are installed in steel and PTFE-lined steel pipelines, and especially, thanks to their large adjustment range, in plastic and FRP pipelines. Because of their flexibility and low spring rates, they are also used for sensitive plant components with enameled finishes, graphite process equipment, and graphite heat exchangers. Therefore they compensate thermal expansion of the pipeline and thus prevent stresses, so offering a compact alternative to material-intensive horseshoe bends.

POLYFLURON® bellows are also used to absorb oscillations and vibrations, for example those generated by pumps. Here the quite special properties of POLYFLURON® are exploited, i.e. its low tendency to material fatigue and complete absence of brittle fracture as compared with other plastics and elastomers or certain metals.

Characteristics of POLYFLURON® bellows

POLYFLURON® bellows produced by SGL Group are characterized by exceptional safety and long service life. Their operational reliability, in particular, is the result of our special manufacturing process, careful material selection, and targeted optimization through extensive practical trials and FEM simulations. Based on this, we are able to supply POLYFLURON® bellows that perform impressively, even under extreme operating conditions.

To optimize bellows geometry, FEM calculations were carried out, and they clearly show the positions of the maximum stresses: in the case of axial loading, the inside of the convolution crest, and in the case of tangential loading, the outside of the convolution trough in the area of the supporting ring (yellow-red picture areas).

FEM simulation of a bellows convolution under stress. Top: axial loading with maximum stress on the inside of the convolution crest (red area). Bottom: tangential loading with maximum stress on the outside of the convolution trough in the area of the supporting ring (orange-red area).

The simulation confirms the result of practical burst pressure tests, in which the bellows cracks start predominantly from the convolution crest. With a manufacturing method developed by us, we have succeeded in significantly reducing the stress gradient in the convolution crest. This is evident particularly in the uniform liner wall thickness of the POLYFLURON® bellows and has been confirmed by an independent test institute. 

The material we use exclusively for the bellow is POLYFLURON® with a very low void content (low stretch-void index). In this way, we minimize possible notch effects generated by ultra-small pores under flexural fatigue stress. We also achieve a far higher number of load cycles to failure, as evidenced in comparative tests with competition products. The result is that POLYFLURON® bellows increase plant service life and ensure safe, reliable operation.

POLYFLURON® bellows are not only suitable for compensating axial, lateral, and angular movements individually but can also be used when several of these types of movement occur at the same time. To help you take this operating condition into account in pipe planning, we can supply you on request with more detailed information on allowable combined movements, including temperature-dependent spring rates. 

It is, however, a general rule that the more convolutions a bellows has, the greater the allowable movements but the lower the temperature-dependent pressure rating. Conversely, bellows with fewer convolutions are more pressure-resistant with rising temperature but absorb less movement than bellows with more convolutions.

FEM simulation of the maximum operating pressure without plastic deformation

Pressure/temperature loading limits are normally determined in burst pressure tests. Since temperature and operating time influence burst pressure, creep rupture tests must also be included in the assessment.

To enable you to rely on safe long-term operation, we have conducted extensive creep rupture tests on our POLYFLURON® bellows at 150°C (based on DIN EN ISO 9080 and DVS guideline 2205‑1 sheet 21). The pressure/temperature loading limits quoted in our product brochures are based on these. FEM pressure simulations carried out by us, which take into account the actual temperature-dependent stress-strain behavior of POLYFLURON®, confirm these data and also, in particular, provide guidance in the case of very large diameters for which burst pressure tests cannot be carried out. 

POLYFLURON® vacuum bellows (type: FLUROFLEX® FX-0)

Pipelines are subject to highly variable stresses, particularly in chemical plants. These often include vacuum loads that may be process-related or due to emptying or shut-down operations in the plant. As a pipeline component, a bellows must withstand these often considerable pressure variations.

Up to a nominal diameter of DN 80, POLYFLURON® bellows with three convolutions (FX-N3) are fully vacuum-resistant at 200°C. For larger nominal diameters and lengths or for a larger number of convolutions, internal supporting rings (optional) ensure vacuum resistance. Additional possibilities are offered by our special FX-0-type vacuum bellows. These are designed for large nominal diameters and continuous operation under vacuum at high temperatures.

When it comes to load limits, the question always arises as to the limits for the number of load cycles to failure. Here, POLYFLURON® differs fundamentally from metals.

Pressure-less measurements with POLYFLURON® bellows found the number of load cycles to failure exceeded 2 million (full cycle and frequency of 0.1 to about 0.3 Hz). The end of these tests was determined by time, not by malfunction of the bellows. The simultaneously measured adjusting force showed scarcely any change (< 3%). From this, it may be concluded that POLYFLURON® does not fatigue and has excellent long-term stability

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Contact


Dr. Schnabel GmbH
Offheimer Weg 21
65549 Limburg/Germany

polyfluron
@sglgroup.com

Tel: +49 6431 9106-0