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Chapter 3 Systems of PWR Nuclear Power Plants

  1. Turbine protection system

A turbine protection system is provided for the turbine-generator unit, independent of the turbine governor system. The function of the turbine protection system is to shut off the steam flow to the unit in the following cases.

©Turbine over-speed

(2) Internal failure of generator

© Excessive thrust bearing wear

(4) Drop of condenser vacuum

© Low bearing oil pressure

© Excessive vibration of turbine shaft

  1. Main Steam System

The lines from the steam generators to the main steam stop valves are called the main steam system. Besides transporting steam from the steam generators to the high pressure turbine, the main steam system distributes steam flows to the steam reheater and to the turbine gland sealing system. It also supplies steam to heat the feedwater deaerator and steam converters and to drive a turbine-driven feedwater pump at the plant low load operation. The

flow diagram of main systems of a typical PWR plant turbine system including the main steam system is shown in Figure 3.5.4.

  1. Main steam safety and relief valves

The total discharge capacity of the main steam safety valves is designed equal to the steam flow rate at the rated full power operation of the plant, so that the main steam system pressure is kept under 1.1 times the maximum working pressure, setting the lifting pressure of the valves at around the maximum working pressure, in accordance with the requirements of the ordinance of the Ministry of International Trade and Industry. The main steam relief valves, normally together with the turbine bypass system, and when the turbine bypass system is not available by themselves, discharge steam to the atmosphere, relieve the pressure in the main steam system to maintain the plant at hot shutdown conditions and subsequently to cool down the plant at a given cooling rate. The opening pressure of the main steam relief valves is set lower than the lifting pressure of the safety valves to prevent the main

[Source] Mitsubishi Heavy Industries Catalog

Figure 3.5.4 Turbine system (example)

3-53

NSRA, Japan