As a separate issue - Rubber bellows are often installed in the belief that they will accept axial compression without anchors or guides. Pressure thrust forces and bellows spring rate forces are two separate load factors.Īll unrestrained bellows – metal and rubber – will exert an axial force when pressurised.Thrust forces in restrained bellows that could damage the pipeline are contained within the bellows assembly by the hinges or gimbals when the assembly is positioned at 90° to the axial expansion.When a bellows with external hardware ( DHEB, DGEB or similar) is correctly selected for the pipeline, the bellows and system will function properly.Without proper pipe guiding and anchoring, failure is a high probability with high stresses being a certainty.When a pressurised pipe is guided and anchored correctly unrestrained bellows (EG SEB type) will function properly.Working closely with Aflex on the bellows recommendations results in significant savings and the avoidance of potential failures and rectification costs. Often a much more suitable bellows can be installed that requires less anchoring, less guiding, fewer bellows, and overall a much cheaper solution. End users, re-sellers or consultants are commonly unaware of pressure thrust and its effects. At Aflex, we have observed over the last 30 years approximately 70% to 80% of bellows failures in pressurised pipelines are due to pressure thrust. Pressure thrust is the least understood principle in bellows application and selection. The higher the pressure, the higher the force. All unrestrained bellows installed in a pipeline without correct anchors or guides can cause instability of the system when pressurised.
The pressure thrust’s axial force can potentially cause bellows failure and make the pipeline yield or deform. Pressure thrust is the single most important factor for bellows in pipelines.
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