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API 682 has created definitions for many of the common features and attributes of mechanical seals and systems. When new concepts are introduced or options are added to the standard, they must be captured in the definitions.
Type A is a balanced, cartridge mounted seal which utilized elastomeric secondary seals. Type B is a cartridge mounted seal which utilizes the flexible metal bellows and elastomeric secondary seals. The Type C Seal is a cartridge mounted high temperature bellows seals which utilizes flexible graphite secondary seals. Other requirements such as face materials and elastomers are tied to these definitions.
Seal Types describe the basic design features of the seal. These definitions are carried over the previous editions.
This article contains excerpts from the paper, "Advancements in mechanical sealing -- API 682 Fourth Edition" at the 2013 International Pump Users Symposium held at Houston, Texas.
The Fourth Edition of API 682 expands on these definitions slightly. Type A and B seals have historically been defined as having flexible rotating elements. This means that the springs or bellows assembly will rotate with the shaft. This was selected as the default design in the First Edition due to the high population of these designs in the refinery industry. Type C seals have historically defaulted to stationary flexible elements.
There are many cases, however, when it is necessary and beneficial to change from the default designs. For this reason, Type A and B seals are no longer defined as having rotating flexible elements and Type C is not defined as having a stationary flexible element. Instead, both rotating and flexible elements are allowed on all seal types and are considered to be technically equivalent. Any seal which is outside of the scope of the standard (by design or operating window) is defined as Engineered Seal.
An Engineered Seal is not a seal Type but rather identification that special design features may be required to meet the application conditions. The seal OEM is free to deviate from any or all of the requirements of the standard in order to design an appropriate seal. There are no special qualification testing requirements for an Engineered Seal.
In industry, there is sometimes a need to provide a seal which challenges the operating window for any one seal type. In these cases, seal OEMs can provide a mix of Seal Types within the same seal cartridge. For example, an Arrangement 3 (dual pressurized seal) could be configured with a Type B inner seal for improved solids handling and a Type A outer seal for high-pressure capability. This design flexibility is specifically allowed in the Fourth Edition.