Turbine bypass valves

A fast response, modulating-type valve, controlled by the steam bypass pressure regulator system, is used to perform three basic functions. The primary function is to reduce the rate of rise of reactor pressure when the turbine admission valves are moved rapidly in the closing direction. To perform this function, the bypass valve needs about the same speed of response as the turbine admission valves to prevent a pressure-induced reactor scram from high neutron flirx when the turbine load is suddenly reduced by partial or complete closure of the turbine admission valves. 

The second function of the bypass valve is to control reactor pressure during startup of the turbine. This allows the reactor power level to be held constant while the turbine steam flow is varied as the turbine is brought up to speed under the control of its speed governor. 

The third function of the bypass valve is to help control reactor pressure after the turbine has been tripped. It is used to discharge the decay heat to the condenser and to control the rate of cooling of the reactor system. 

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Turbine trip 1 Turbine trip with turbine bypass valve failure... 

The only application in which a throttling control valve must shut off tightly is when it is required to do double duty as a remote-controlled block valve. There are a number of instances where this is a legitimate requirement. For example, there are turbine bypass valves that need to conserve energy when closed, but open quickly and throttle when the turbine is tripped off the line. 

U-20b Turbine bypass valve fails open following reactor trip 5... 

ATWS Events Limiting (e.g., MSIV valve closine) Moderate Impact (e.g., loss of condenser vacuum) Minimum Impact (inadvertent opening of all turbine bypass valves)... 

Generator or turbine trips are less severe because the turbine bypass valves can be assumed to open and the condenser to be operative. Although the transient proceeds more slowly in these cases, the result still would be a high reactor coolant system pressure. 

C. The core outlet coolant flows to the water separator in the bypass line. The saturated steam from the separator goes to the condenser through the turbine bypass valves. The saturated water from the separator is led to additional heaters and the condenser. 

When the saturation temperature reaches 80 C, the reactor is temporarily kept subcritical. The MSIVs and the turbine bypass valves are opened. The condensers are put under vacuum and the reactor is deaerated. The purpose of this procedure is to avoid corrosion of the reactor internals and pipes. 

The plant transient analysis code SPRAT-DOWN, described in Sect. 4.2, is extended for the analyses of abnormal transients and accidents. The calculation model is shown in Fig. 6.12. The models of the equipment, i.e., the AFS, SRV, MSIVs, and turbine bypass valves, are added for safety analyses. A hot chaimel, where the linear heat generation late and maximum cladding surface temperature are the highest in the core, is modeled as well as the average channel in order to calculate the highest values of cladding temperature and pellet enthalpy. The flow chart is shown in Fig. 6.13. 

This is the typical transient where both RCPs trip. However, its sequence is different from a total loss of reactor coolant flow accident as described in Sect. 6.4. In the loss of offsite power, the motor-driven condensate pumps are assumed to trip instantaneously. The turbine control valves are quickly closed due to a turbine trip. The turbine bypass valves open immediately after that. A scram signal and AFS signal are released by detecting the loss of offsite power or turbine control valves quickly closed or condensate pump trip. Both RCPs are assumed to trip at 10 s... 

There are two peaks in the cladding temperature curve. The first one appears within 1 s and is caused by the turbine trip it is higher than the initial (steady-state) value by 20°C. The second one appears after the trip of the RCPs, and it is lower than the initial value. The increase in the pressure is only 0.6 MPa due to the successful opening of turbine bypass valves. The second peak height of the cladding temperature is sensitive to the delay of the pump trip, the delay of the AFS start, and the capacity of the AFS as shown in Table 6.12. 

The event tree of the transients with PCS available at the initial stage is shown in Fig. 6.76. Since the PCS is intact initially, which means that the steam can be discharged from the core to the condensers through the turbine control valves or the turbine bypass valves, the SRVs are not needed. However, the turbine-driven RCPs are assumed to be unavailable at all times because a turbine trip is taken into account. If both the RPS operation and initiation of the motor-driven RCPs are successful, it is assumed that the core can avoid core damage based on scenarios of BWRs [41]. Even if the motor-driven RCPs fail, core damage can be avoided by the successful function of the AFS and containment cooling. If the AFS fails, the... 

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