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Pumps & Systems, February 2008

Non-metallic (rubber) expansion joints are a critical part of any pump piping system as they provide the flexibility necessary to absorb system dynamics associated with vibration and/or temperature and pressure changes within piping. Careful selection of expansion joint design and material for a given application, as well as properly engineered installation, are important factors for determining performance. When designed, applied and installed properly, it is not unusual to achieve useful service life for 15 to 20 years in most industrial applications.

Unlike metal expansion joints, the majority of failures related to rubber expansion joints are not catastrophic in nature. During routine inspections, signs of fatigue and/or premature failure can be easily detected in rubber expansion joints, long before the actual failure occurs. 

Common Signs of Rubber Expansion Joint Fatigue 

Cracking (see the image below)

Exterior surface cracking is most commonly the result of aging and/or elevated temperature. As rubber ages, it becomes hard and brittle and loses inherent flexibility and resilience. Cracking or crazing may not be serious if only the outer cover is involved and the fabric is not exposed. If necessary, repair onsite with rubber cement where cracks are minor. Carefully inspect cracks to determine if underlying fabric reinforcing plies are compromised. 

Cracking where the fabric is exposed and torn, indicates the expansion joint should be replaced. Such cracking is usually the result of excess extension, angular or lateral movements. Such cracking is identified by (1) a flattening of the arch, (2) cracks at the base of the arch and/or (3) cracks at the base of the flange. Cracking at the base of the flange/arch is more associated with movement and misalignment.

Note: As a corrective action, measure the face-to-face and lateral alignment prior to ordering a replacement and install control units to maintain acceptable movement limits of the expansion joint during operation.   

Blisters/Deformation/Ply Separation (see the image below)

 Some blisters or deformations-when on the external portions of an expansion joint-may not adversely affect the performance of the expansion joint. These blisters or deformations are cosmetic in nature and do not require repair.

If major blisters, deformations and/or ply separations or delamination exist in the tube, the expansion joint should be replaced as soon as possible. The root cause of delamination is either over-torqued bolts, chemical attack by the media, aged rubber or some other form of break in the inner tube layer. Corrective actions include replacing the aged product and ensuring the correct material selection for the application.

Ply separation at the flange outer diameter (O.D.) can sometimes be observed and is not a cause for replacement of the expansion joint.

Exposure of Metal Reinforcement

If the metal reinforcement of an expansion joint is visible through the cover, the expansion joint should be replaced as soon as possible.

Dimensions

Any inspections should verify that the installation is correct; no excessive misalignment between the flanges exists; and the installed face-to­-face dimension is correct. Check for over-elongation, over­ compression, lateral or angular misalignment. If incorrect installation has caused the expansion joint to fail, adjust the piping and order a new expansion joint to fit the existing installation.