What would be the most likely indication that a plate type jacket water cooler had an internal leak permitting mixing of the two fluids?

Correct Answer: Option D — Dropping level and frequent need to top off jacket water expansion tank.

A leak in a plate-type cooler allows the higher-pressure jacket water to escape into the lower-pressure seawater side. This results in a loss of jacket water volume, requiring frequent topping off of the expansion tank. This is a classic symptom of internal plate gasket or plate failure.