Question 324Steam Plants - 1st Asst/Chief

The correct answer is A) seal the casing during periods of low casing pressure. When turbine rotor shafts extend through the casing, an external source of sealing steam is used in conjunction with labyrinth packing to maintain a positive pressure in the casing during periods when the internal casing pressure is low. This prevents air from being drawn into the casing, which could lead to oxidation and corrosion of the turbine components. The other options are incorrect because: B) maintaining rotor journal temperature is not the primary purpose of the sealing steam, C) sealing during high casing pressure is not necessary, and D) providing a constant flow to the gland leak off condenser is not the main function of the sealing steam in this case.