In a steam power plant, steam from the steam turbine is condensed in a water cooled or air cooled condenser that operates under vacuum. The condensing capacity of the condenser is impaired by the presence of air. Air leaks into the condenser from flanged connections, turbine seals, valves and other equipment connected to the condenser. The air tends to collect in the condenser and impact its condensing capability. This trapped air has to be continuously evacuated to preserve the condensing capacity of the condenser.

The air can be evacuated from the condenser using the steam jet air ejector system or the vacuum pump system. The vacuum pump is driven by an electrical motor. The steam jet air ejector is driven by motive steam. In certain low suction pressure applications, a hybrid system consisting of a steam jet air ejector and vacuum pump or a three stage ejector system is used for evacuating the air from the condenser.

The evacuation packages perform two distinct services namely hogging and holding. During startup, or hogging operation, the evacuation system removes air from the steam space of the condenser and steam turbine. The pressure in the steam space is reduced from atmospheric pressure to typically 10.0” HgA or a pressure specified by the steam turbine supplier in about 30 minutes. During normal or “holding operation” with the steam turbine in service, the evacuation system removes a specified amount of air from the condenser.

The suitability of vacuum pump or steam jet air ejector systems for a given application depends on the availability of motive steam during startup, the costs associated with the usage of steam or electricity and the preferences of the end user. Each system has its advantages and disadvantages. In each case, there are numerous design details that must be addressed to ensure that the selected system meets the evacuation needs for the condenser.

This paper examines the various configurations of evacuation systems and discusses their suitability for the wide ranging operating scenarios in the condenser. A broad background on the design and operation of the evacuation system is included to assist the end user in making the proper selection of the evacuation system.

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