Bellows are vital components in piping systems, designed to absorb movement and vibration. However, under certain conditions, metallic bellows can experience “squirm” or column instability, a critical issue in which the bellows become unstable under internal pressure.
Preventing Bellows Squirm: Maximizing Lifespan and Safety
Column instability (bellows squirm) is when the bellows develop a lateral bowing of the bellows centerline under internal pressure.
In-plane deformation is when tilting, warping, or deformation affects one or more individual convolutions.
Both scenarios compromise the integrity of your system, leading to increased stresses and potential catastrophic failure. These are errors that can risk the lives of those working at the forefront of a plant. Thus, regular inspections and adherence to industry standards play a crucial role in mitigating the risk of instability and ensuring the long-term integrity of the system.
Critical Squirm Pressure: Causes and Prevention
Critical squirm pressure is directly influenced by the diameter of the bellows and the spring rate. It’s inversely affected by length (the longer the bellows, the lower the squirm pressure).
If the bellows are bent or angulated, the centerline can begin to move away from the center of curvature. (see photo illustration)
Smaller-diameter bellows with many convolutions are more vulnerable.
When internal pressure exceeds the critical squirm pressure, the metallic bellows can bow out, causing instability and increased hoop stresses that may result in sudden catastrophic failure.
To avoid bellows instability:
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- Verify critical squirm pressure rating
- Request hydrostatic testing before service
- Verify critical squirm pressure rating
- Avoid unnecessarily long metallic bellows
- Maintain the correct diameter-to-length ratio
- Ensure proper EJMA-calculated design
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At US Bellows, we prioritize the safety and longevity of your metallic bellows. That’s why we conduct rigorous pressure tests on all our bellows to ensure they will not experience column instability (metallic bellows squirm) while in service.
While testing is not mandatory by EJMA standards, these tests are crucial for ensuring the reliability and performance of your expansion joints. If you are concerned that your metallic bellows may experience column instability or bellows squirm, we highly recommend performing a hydrostatic test prior to service.
Click on a section link below:
- General Installation
- Post-Installation Inspection
- Inspection During and Immediately After System Pressure Tests
- Periodic In-Service Inspection
- Typical Causes of Failure
- Installation Misalignment
- System Operations
- Squirm or Instability
- Installation Instructions (Summary)
Experiencing bellows instability or unsure about squirm? Schedule a diagnostic review with our expansion joint engineer today.
Explore our solutions: See our full range of engineered bellows, download a catalog today.
Did you know that US Bellows is a Piping Technology Company?
We are proud to be a one-stop solution from expansion joints to pipe supports and engineering services. We work hard to simplify your supply chain, and ensure system reliability with quality assurance.