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.

API 671 Coupling Guarding Requirements

Guarding requirements for API 671 couplings are similar to all coupling guards as they must meet all applicable codes and regulations (such as OSHA 1910.219).  However, additional requirements are included in the API 671 Standard.  A few key features are:

• The ability to inspect the coupling without requiring any additional work beyond removing the guarding.
• Capability to withstand a 200 pound static load.
• A minimum of 1 inch radial clearance between the guard inner wall and coupling maximum outer diameter.
• Anti-swirl baffles installed as required to diminish the effects of windage.
• Guards that are directly installed on the machine base or bed plate should be removable as a single piece.
• No guarding should be constructed of woven wire, only expanded metal, perforated plate or plate is permitted.

API 671 Coupling Types - Disc, RM Disc, & Diaphragm

Multiple options exist when an API 671 (ISO 10441) coupling is required for an application: Disc, RM Disc and Diaphragm.  Each coupling type integrates features to meet the needs of a particular segment within the marketplace.

Disc Couplings

This coupling type is commonly used when API 671 adherence is required for auxiliary equipment such as a pump system.  A Disc type coupling provides a good balance between capability and cost as the coupling type is based on upgrading the standard API 610 design to meet the API 671 requirements.  While providing a high torque capacity which permits use in smaller turbine driven systems, this coupling type is limited in regards to maximum permitted rotational speed.

RM Disc

Specifically designed for API 671 applications, the Reduced Moment (RM) Disc coupling is used in applications that a standard disc coupling is unable to be specified (typically due to a rotational speed requirement).  More cost effective than the Diaphragm type, the RM Disc coupling utilizes similar features such as a larger diameter thin walled spacer while retaining the disc packs element as seen in standard Disc Couplings.  The RM Disc Coupling further integrates the guard rings and hubs into a single unit to reduce mass & inertia allowing the coupling to withstand higher rotational speeds.  A recent application example is a 47,000 HP turbine driven axial compressor system that had a trip speed of 6,600 rpm.

Diaphragm

Replacing the disc pack element with a contoured continuous disc, the Diaphragm Type Coupling offers increased torque and rotational speed capacity at the expense of misalignment capability.  As the contoured disc is complex to manufacture, the Diaphragm type is one of the highest cost API 671 couplings available.  This limits the usage of Diaphragm couplings to critical applications, such as the primary drive of a high powered mechanical drive turbine system.

Recommended Follow-On Reading: To learn more about API671 couplings, we recommend the following CouplingAnswers.com article: API 671 Coupling Standard.

API 610 Standard - Coupling Highlights

The API 610 standard (with ISO 13709 as the Identical European standard) was created by the American Petroleum Institute to outline regulations regarding centrifugal pumps for petroleum, petrochemical and natural gas applications.  The standard covers multiple components (housing castings, bearings, mechanical shaft seals, etc.) within the pump system in addition to the coupling connection.

For the coupling connection the standard defaults to an all-metal flexible element spacer type coupling, commonly known as a Disc Coupling.  In addition, the Disc coupling is required to be designed to retain the spacer if a flexible element (disc pack) fractures thereby preventing the spacer from impacting the coupling guarding and have a minimum balance of class 9 according to the AGMA 9000 standard. 

The standard allows other couplings to be used (with Gear coupling being the leading alternate) but still requires the coupling to have a balance of Class 9 to obtain the API 610 certification.

An important item to consider is that couplings operating above 3,800 rpm the API 610 standard states the coupling must be balanced to meet the API 671 (ISO 10441) standard.  While this will not provide a coupling that meets all the API 671 requirements, it will deliver the proper coupling balance needed for the application.