When securing a main propulsion turbine equipped with carbon packing glands, the vacuum should always be broken before securing the gland seal steam because __________.

Correct Answer: Option C — cold air rapidly entering the gland may result in damage to the carbon segments and sealing surfaces

Breaking vacuum before securing gland seal steam allows ambient cold air to rush across hot carbon packing rings, inducing thermal shock that fractures brittle carbon segments and scoring sealing surfaces.