Abstract: To test a shutdown steam condenser for tube leakage, the pressure in the water boxes is reduced below atmospheric pressure while the steam casing is open to atmospheric pressure, and leakage of air into the boxes is used as an indication of the tube leakage. The leakage is preferably indicated with a soap bubble test by a workman inside the box. Although the entrance manholes to the water box are substantially sealed during testing, for safety protection one can be sealed with a U-tube water seal so that it will be broken automatically by an excessive underpressure.