What type of pumps have an external motor connected to the armature with a mechanical shaft seal?

The correct choice reflects the design of semi-hermetic pumps, which feature an external motor coupled to the armature through a mechanical shaft seal. This design provides a level of separation between the motor and the pumped medium, allowing for easier maintenance and servicing of the motor without needing to disassemble the entire pump. Semi-hermetic pumps operate effectively in applications where a sealed environment is critical, yet maintenance access is needed.

In contrast, closed hermetic pumps are completely sealed and typically do not allow for any external maintenance without replacing the entire unit. Open hermetic pumps can have their motors exposed to the process fluid, which limits maintenance options and can lead to potential environmental hazards if leaks occur. Submersible pumps, on the other hand, are designed to operate completely submerged in the fluid, which differs significantly from the structure of a semi-hermetic pump.