Improving Piping Flexibility and Performance with Rubber Joints

As detailed in Pumps & Systems’ article “Rubber Expansion Joints Provide Piping Flexibility,” rubber expansion joints play a crucial role in piping installations by compensating for thermal growth, relieving stress, and reducing vibration and noise from rotating equipment. These flexible joints are valuable for accommodating pipeline misalignments, but they come with specific installation and operational considerations. To maximize their effectiveness, it is essential to follow standardized piping practices and maintain an installation tolerance of less than 1/8 inch.  

Rubber expansion joints are designed to manage various movements in piping systems, including compression, elongation, lateral (transverse), angular, and torsional movements. The concurrent movement formula, established by the Fluid Sealing Association’s (FSA) Rubber Expansion Piping Division Edition 7.31, helps designers calculate the allowable offset and movement compensation for these joints. This formula combines the effects of different movements to ensure that the joint operates within its design limits.

Concurrent Movement Calculation
Concurrent movement is the combined effect of compression, elongation, lateral, and torsional movements in an expansion joint. It is crucial to keep this combined value below a certain threshold, as exceeding it means the joint is operating outside its intended design, potentially leading to failure. The sum of all these movements, except for angular movement, should be less than one. Angular movement is accounted for by compression and elongation within the calculation.

Example Calculation:
For a typical calculation:

• Compression change (ΔC) is 2 inches with a rated compression (RC) of 4 inches.
• Elongation change (ΔE) is 0 inches with a rated elongation (RE) of 2 inches.
• Lateral change (ΔL) is 0.75 inches with a rated lateral (RL) of 1 inch.
• Torsional change (ΔT) is 0 degrees with a rated torsional (RT) of 5 degrees.

Calculating these movements:
1 ≥ (2 in. / 4 in.) + (0 in. / 2 in.) + (0.75 in. / 1 in.) + (0°/5°)
1 ≥ 0.5 + 0 + 0.75 + 0
1 ≥ 1.25

In this example, the concurrent movement exceeds the allowable value, indicating the joint is outside its design limits.

Custom Fabrication for Misalignments
In cases where piping misalignments exceed standard tolerances due to space, cost, or time constraints, custom rubber expansion joints can be fabricated to specific field dimensions. While custom solutions can maintain the joint’s full movement-absorbing capability, they should be considered as a last resort. Users should keep a spare custom part on hand to avoid future disruptions. Correct dimensions, including parallel, angle, and hole misalignments, are essential for the supplier to fabricate an effective custom joint.

For installations where standard tolerances cannot be met, using the concurrent movement formula or designing built-in offsets ensures the joint stays within acceptable operating limits, thus enhancing the reliability and longevity of the piping system.

Click here to learn more about Meccom Industrial Products’ range of rubber expansion joints.

Article with all rights reserved, courtesy of Pumps & Systems.