Turbine control valve

Calculate the flow through the turbine control valve using the methods of Chapters 6 and 10. Enthalpy is unchanged by the throttling process, and so the specific enthalpy at the inlet to the first stage will be equal to the upstream specific enthalpy ... 

The main turbine and turbine control valves, were analyzed to identify capability to mange the new steam generation rate from the nuclear reactor, control capability of the turbine control valves, considering that the pressure in the reactor vessel was unchanged related to the original design pressure. For such purposes tests were successfully performed to show the capability of the control system. 

The plant control scheme is based on the "reactor follows plant loads". A grid fluctuation can be compensated for through turbine control valves in case of a fi equency drop. A decrease in pressure at the turbine would require an increase in reactor power. 

Reactor pressure increase Several events may cause this e.g., inadvertent closure of one turbine control valve, pressure regulator downscale failure, generator load rejection, turbine trip MSIV closure, loss of condenser vacuum, loss of nonemergency AC power to station auxiliaries, loss of feedwater etc. All these have been analysed. Features are included in the instrumentation and control systems or redundancies to maintain reactor pressure through a combination of component automatic responses or operator actions, depending on the identified cause. 

Turbine stop valve closure and turbine control valve fast closure Turbine stop valve closure and fast closure of the turbine control valve will cause a reactor trip. 

Turbine stop-valve or turbine control-valve closure due to pressure transient or generator load rejection... 

In previous years, there have been some other ruptures of minor pipes belonging to the "balance of plant", such as drains of turbine control valves housings, etc. that led to repairs requiring power reductions. 

The flow of main steam entering the high-pressure turbine is controlled by four control valves. The turbine control valves are adjusted automatically by electrohydraulic servo actuators. These actuators control the turbine speed when it is starting up, and for load control after the turbine-generator unit is synchronised. In series with each control valve is a stop valve, whose function is to shut off and isolate the steam flow to the turbine when required. 

The rate of change of drum pressure at any time Is, to a first approximation, proportional to the difference between the rate at which steam Is being carried away and the rate at which heat Is being delivered from the reactor core, and the constant of proportionality is itself approximately Inversely proportional to the thermal storage. When the turbine control valve (TCV) Is opened while reactor power Is held constant, drum pressure falls and the delivery of steam to the turbine is represented in Fig. 5 by the curve which asymptotically approaches the time axis. 

The plant control system has been designed in a similar way to that of BWRs [36-39]. It is shown in Fig. 1.14. The plant transient analysis code SPRAT-DOWN was developed and used in the design work. The node-junction model, shown in Fig. 1.15, contains the RPV, the control rods (CRs), the main feedwater pumps, the turbine control valves, the main feedwater lines, and the main steam lines. The characteristics of the turbine control valves and the changes of the feedwater flow rate according to the core pressure are given in the calculation. 

Decrease in the main steam flow rate by 5% resulting from closure of the turbine control valves. 

According to the calculated step responses, the pressure is sensitive to the turbine control valve opening and the feedwater flow rate. The main steam temperature is sensitive to the control rod position and the feedwater flow rate. Therefore the turbine inlet pressure is controlled by the turbine control valves. The main steam temperature is controlled by the feedwater pumps. The core power is controlled by the control rods. 

Super LWR Turbine following reactor Reactor power Turbine control valves Control rods... 

PWR Reactor following turbine Turbine control valves Reactor power Control rods... 

FPP Boiler turbine coordinated Turbine control valves, boiler input ... 

Fig. 4.4 Change of steam flow rate with turbine control valve stroke. (Taken from [6] and used with permission from Atomic Energy Society of Japan)...

Equations (4.2), (4.3), and (4.4) are for the fuel channel, water rod channel, and outside core, respectively. The governing equations are discretized using the upwind difference scheme and the full implicit scheme. The boundary conditions are the feedwater flow rate, the feedwater temperature, and the mrbine inlet flow rate. The characteristic of the turbine control valve, expressed as the change of steam flow rate, is shown in Fig. 4.4 [6]. The feedwater flow rate changes with the core pressure as shown in Fig. 4.5 [6]. 

It is necessary to analyze the responses of the plant against stepwise perturbations in the components used for plant control. In BWRs, these components are the control rods, the recirculation pumps, the feedwater pumps and the turbine control valves. 

Since the Super LWR is a plant with a direct-steam cycle like BWRs, those components are selected by referring to BWRs. The control rods, the feedwater pumps and the turbine control valves are to be used for plant control of the Super LWR. It is noted that the Super LWR has no recirculation pumps. 

A positive reactivity of 0.1 is inserted stepwise as a reactivity perturbation. The feedwater flow rate and the turbine control valve opening are kept constant. The results are shown in Figs. 4.9 and 4.10. The power quickly increases to 111% of the initial value. It is consistent with the analytical solution of prompt jump. Then, the power decreases due to reactivity feedbacks from Doppler and coolant density. The main steam temperature changes by following the power. The main steam pressure and the core pressure increase due to increases in the temperature and hence the volume flow rate of the main steam. The fuel channel inlet flow rate changes with the core pressure due to the relation between the feedwater flow rate and the core pressure shown in Fig. 4.4. The plant almost reaches a new steady state in 40 s. 

The turbine control valve opening decreases stepwise to 95% of the initial value. The control rod position and the feedwater flow rate are kept constant. The results... 

Fig. 4.14 Response to stepwise decrease in turbine control valve opening (1)...

The pressure at the turbine inlet is kept constant by regulating the opening of the turbine control valves. The same logic as used in BWRs is adopted. It is shown in Fig. 4.17. The opening is proportional to the deviation of the pressure from the setpoint with lead-lag compensation. The turbine control valve opening ratio is calculated from the following equations. 

The setpoint of the main steam pressure increases stepwise from 24.5 to 24.75 MPa. The results are shown in Figs. 4.25 and 4.26. The turbine control valves are rapidly closed by the pressure control system. At the beginning, the feedwater flow rate decreases because of the increase in the core pressure. Thus, the main steam... 

The power setpoint decreases stepwise from 100 to 90%. The results are shown in Figs. 4.29 and 4.30. The control rods are inserted so as to decrease the power. The power reaches the new setpoint without oscillation. The main steam temperature decreases with the power. The feedwater flow rate is gradually decreased to 90% of the initial value so as to keep the main steam temperature 500°C. The main steam pressure is kept crmstant by the turbine control valves. The pressure loss in the main steam lines decreases because of the decrease in the main steam flow rate. As a result, the core pressure decreases by about 0.1 MPa. After 80 s, the plant is settled at a new steady state. The variation of the main steam temperature is around yC. 

Main steam stop valve Turbine control valve... 

Turbine control valve quickly closed Main stop valve closure Earthquake acceleration large Loss of offsite power... 

Since the function of the AFS is to keep the main coolant flow rate in the event of the unavailability of the RCPs, its actuation signals should be released by detecting an abnormality in the RCPs or a decrease in the main coolant flow rate. Reactor coolant pump trip and main coolant flow rate low are taken as the AFS signals. Loss of offsite power, condensate pump trip, turbine control valves quickly closed, main stop valves closure, and MSIV closure are also taken as the AFS signals because these abnormalities cause a trip of the RCPs. 

Loss of supply of coolant to the deaerator would also cause a trip of the RCPs because the inlet pressure of the RCPs decreases with the water level in the deaerator. This abnormality is represented by the loss of offsite power transient where the motor-driven condensate pumps stop. Since there is a large amount of water in the deaerator, the RCPs are expected not to stop for some period after the trip of the condensate pumps. The capacity of the deaerator has not yet been determined. If it is 140 m, which corresponds with the typical design of 1,000 MWe class FPPs, the water level in the deaerator would decrease by only 7% in 10 s after the trip of the condensate pumps. In the safety analysis, it is conservatively assumed that the trip of the RCPs occurs 10 s after the condensate pump trip [5]. This transient is less severe than a total loss of reactor coolant flow accident because a reactor scram is possible before the trip of the RCPs. In the safety analysis, the reactor scram by the signal of loss of offsite power, condensate pump trip, or turbine control valves quickly closed is credited. 

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