Which of the following statements is true regarding mechanical seals?

Correct Answer: Option D — They are normally lubricated and cooled by the fluid being pumped.

Mechanical seals are designed to operate with a thin film of the pumped fluid between the seal faces, which provides both lubrication and cooling. This fluid film is critical for preventing excessive heat generation and wear. Consequently, the seal's longevity is directly dependent on the properties and presence of the pumped liquid.