Wednesday, February 11, 2015

API 671 Coupling Cold vs Hot Installed Length

A common issue with API 671 couplings is the potential variation in length from the as installed (cold) to the steady state operating condition (hot).  This variation in coupling length can be of sufficient deviation to require the coupling to operate outside of the allowable axial displacement permitted.  

In order to compensate for machinery movement due to thermal and dynamic transformations, the coupling can be designed with the intention to have shims inserted.  The shims are specifically designed to allow insertion at a bolted joint within the coupling eliminating the need to remove the coupling hub(s) from the equipment (as the hubs are installed with an interference fit). This allows tuning of the installed length in small increments and gives a straightforward method of altering the installed length as the equipment ages.

Note: If shimming the coupling causes the coupling to operate outside of the manufactures recommended limits, it is advised that the coupling manufacturer (which we certainly hope is Lovejoy) be contacted as an engineering analysis might be required to ensure that the coupling will have no operational issues.

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