In this blog article we are going to take a closer look at the API Standard 671 which specifies the requirements for special purpose couplings used in the petroleum, petrochemical and natural gas industries. This Standard has been developed by the American Petroleum Institute (API), which is not to be confused with the identical abbreviation used in the IT field (Application Programming Interface). Let's begin with a few details about the institute that gave the name to the API Standard.
American Petroleum Institute
Established on March 20, 1919, and based in Washington D.C., the American Petroleum Institute (API) is the largest national trade association that represents all aspects of the American oil and gas industry, including the petrochemical industry. These account for 8 % of the American economy and cover 10.3 million jobs in the USA. Among the 625 current members of API, are large company groups and corporations in the fields of exploration and production, refining, marketing, pipelines, marine businesses as well as service and supply firms.
Standardization is one of the important tasks within their scope of services. To date, API has developed nearly 700 standards and recommended practices, which cover all areas of the oil and gas industry, and have also been partly adopted by the International Organization for Standardization (ISO).
The API Standard 671
As already mentioned above, couplings in compliance with the API Standard 671 are used for critical applications in the petroleum, gas and chemical industries. Generally, these couplings are required to operate continuously for longer time periods. The Standard 671 also specifies that couplings must be designed to accommodate angular misalignments and axial displacements of the shafts.
Disc couplings and diaphragm couplings are mainly recommended for API 671 applications. The operating speed, the nominal torque and the peak torques are the decisive criterions for determining which type and size of the mentioned couplings are best suited for the specific application. Gear couplings, quill shaft couplings and elastomeric couplings are also permitted according to the API Standard 671, but not commonly used.
According to the standard, the coupling shall be designed for a minimum service life of 5 years for torsionally rigid disc couplings and 3 years for gear couplings as well as for torsional damping and flexible couplings. This means that the coupling must be capable of continuous operation throughout the stated periods. A short explanation of the five most important sections of the API Standard 671 is given below.
1. Basic Design
This section describes the basic design of a flexible coupling with respect to its rating and the factors of safety to be considered in order to define the minimum and maximum loads as well as true running capacities. These factors notably include:
- Coupling size,
- Torque capacity,
- Quality of pilot fits,
- Bending natural frequency of the spacers,
- Temperatures and oscillatory torques.
Specific operation conditions also need to be considered, although sufficient dimensioning of the coupling is obtained by using the equations and calculation formulas (factors of safety) in accordance with the API Standard 671.
The requirements set forth in the API Standard can therefore also serve as a checklist to correctly design a coupling. This standard also ensures that suppliers offer – e.g. during a tender process - couplings on the basis of uniform specifications which will allow the purchaser to compare the different proposals.
The API Standard 671 pays particular attention to the balancing and the balance quality of a coupling and provides three optional balancing methods including the required degree of balance:
1. Component balance
The individual balancing of the main components is required for all couplings operating at 1,800 rpm or less. For other components like bolts etc. it is sufficient to check the weight tolerance.
2. Component balance with assembly check
This applies to applications operating above 1,800 rpm. After the main components have been individually balanced, the coupling is assembled and a check balance of the complete coupling is performed prior to its shipment. In the event that the residual unbalance exceeds a certain value, the balance quality of the individual components must be checked and corrected, where necessary.
3. Component balance with assembly balance
The main components are individually balanced as in method 1. The coupling is then assembled completely and balanced as an entity. The disadvantage of this method is that particular care must be taken to proper positioning when re-mounting a coupling after it has been disassembled.
3. Materials of Construction
There is a wide range of possible materials for couplings. The seller and the purchaser shall agree on the materials of construction, including the elastomers. The purchaser is required to specify all the relevant operating and environmental conditions, such as temperatures, and whether corrosive atmospheres or abrasives are present in the environment. In addition, the purchaser has to inform the supplier or vendor about any environment-related constraints, which may limit the material selection.
According to the API Standard 671, hubs, sleeves and spacers should preferably be forged. In exceptional cases, welded spacers and ductile iron are allowed. Materials like grey cast iron should principally not be used here.
The Standard addresses quality criterions in a limited manner only, and covers more or less general subjects, e.g. the location of identification markings or stampings, and that quality records have to be archived for at least 5 years.
In addition, the API Standard 671 specifies that repairs to the coupling are only allowed after prior consultation with the manufacturer.
5. Vendor Data
The last section of the Standard requires the manufacturer or vendor to provide the buyer with details regarding the relationship between the permissible continuous torque and the nominal speed of the coupling. Apart from typical data such as:
- operating speed,
- maximum continuous angular misalignment of the coupling,
- maximum continuous axial displacement as a function of temperature,
- oscillatory torque and its frequency,
details about the bending and torsional natural frequency of the coupling components are required. The methods to calculate these natural frequencies are also described in detail in this Standard.
For couplings with non-linear torsional stiffness, the manufacturer has to furnish a curve or diagram showing the deflection versus torque and the values of damping.
The API Standard 671 aims to ensure that high-quality couplings are furnished for critical applications, and that replacement components will still be available after many years. As Royce N. Brown states in his publication "API 671 Couplings – What is the impact on the User", the API Standard provides clarity for operators and buyers regarding the specifications of couplings. Since the publication and subsequent revisions of this Standard, engineers and users have been able to approach the design of couplings for critical applications with more certainty than this had been the case prior to the development of this Standard.