API 671 Coupling Standard

API 671 couplings are typically specified in critical applications that operate for extended periods of time, are often non-spared, have high torque and/or speed requirements.  

Metallic flexible-element couplings are the primary type permitted for API 671 (ISO 10441) applications.  Only disc couplings and diaphragm couplings can be used in API 671 applications.  The application speed & torque are the primary determining factors as to which type of metallic coupling is best suited for the application.  Gear and quill shaft type couplings are permitted in the API 671 standard but are not common. 

While the coupling meets the API 610 requirements; additional steps such as component serialization, precision balance, record retention of 20+ years and an assembly print containing application information are just some of the additional requirements needed to meet the API 671 specification.  This is to ensure that a high quality coupling was provided for a critical application and that spare components can be produced decades into the future. 

One feature of the API 671 specification are the various methods allowed to achieve and measure the residual imbalance.

• Method 1 only requires the major components to undergo balance verification but is limited to applications at or below 1,800 rpm.
• Method 2 is the standard for applications operating above 1,800 rpm.  While the major components are balanced as with Method 1, the coupling is fully assembled and the balance of the entire coupling is verified prior to shipment. 
• Method 3 is similar to Method 1 where the major components are individually balanced.  The coupling is then fully assembled and the coupling is balanced as a single entity (where Method 2 only checked the balance of the coupling assembly). One limiting item is that method 3 balancing removes the ability to interchange coupling components (with the exception of disc pack units) as the coupling assembly was balanced as a cohesive unit.

Recommended Follow-On Reading: To learn about API610 Couplings, we recommend: API 610 Standard - Coupling Highlights

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.