Feedwater flow rate

Figure 15.54b is a schematic of a feedforward controller applied for steam drum level control. If the flow rate of the makeup feedwater is equal to the steam usage, the drum level remains constant. One is tempted to conclude that the feedforward controller is aU that is needed for this application. Unfortunately, the measurements of the steam usage and the feedwater flow rate are not perfectly accurate. Even small errors in measured flow rates add up over time, leading to one of two undesirable extremes. The drum can till with water and put water into the steam system, or the liquid level can drop, exposing the boiler tubes, which can damage them. As a result, neither feedback nor feedforward are effective by themselves for this case. In general, feedforward-only controllers are susceptible to measurement errors and umneasured disturbances, and, as a result, some type of feedback correction is typically required. 

Notes Feedwater flow rate = 45 rn /h. Reiect/brine osmotic pressure range = 3.1 (I)—7 (VI) bar. RO pump w = 80%. RO pump motor w = 90%. Calculated specific energy for the BWRO unit only. Source [8]. 

Feedwater flow rate Corresponding to the power Corresponding to the power... 

The feedwater lines enter the containment via two lines, each with inner and outer isolation valves, splitting up into four lines adjacent to the RPV for connection to four nozzles, at "mid-height" of the vessel. The nozzles and the internal removable feedwater distributers are of a special ABB Atom design that ensures a "thermal sleeve" protection against the "cold" feedwater for the RPV wall, and efficient distribution into the downcomer. The feedwater flow rate is adjusted to match the steam flow rate from the vessel, to keep the water level within close limits, by speed control of the feedwater pumps at high power operation, but valve arrangements enable flow rate control also at low reactor power levels in these situations the feedwater flow is routed via smaller nozzles that can easier withstand thermal transients. 

Reactor power can be controlled automatically by the feedwater flow rate,... 

The fundamental concept of the 4S is that of continuous monitoring rather than active operation . The reactor operates using a system of pre-programmed movable reflectors and the power control is executed from the outside, through feedwater flow rate changes in the power circuit. The plant and component conditions and/or unauthorized access could be continuously monitored from outside the site, e.g. by satellite systems. 

Fig2- The effect of the variation of feedwater flow rate upon water/steam temperature. 

Primary coolant flow rate Reactor operating pressure Coolant inlet temperature, at core inlet Coolant outlet temperature, at riser outlet Mean temperature rise across core Primary circuit volume, including pressurizer Number of coolant loops Steam flow rate at nominal conditions Feedwater flow rate at nominal conditions Steam temperature/pressure Feedwater temperature... 

At the next step, water was purged from SHS channels. The transioit processes took place in the second loop, while constant pressure and boiling-free cooling of BWs were provided in the primary loop. Reactor power was rqjidly reduced to 2% of its nominal level and feedwater flow rate was reduced to provide water level in the SGs to purge SHS channels. The water—steam mixture from evaporators and steam from the steam loop were directed to the bubbler and then to the deaerator and the turbine condenser. 

The mass feedwater flow rate is normally exactly equal to the mass steam flow rate. If, due to imperfect equilibration, the water level in the reactor should rise or fall outside predetermined limits, the water level indicator will override the steam flow to demand more or less feedwater to correct the water level in the reactor. 

At about 4 00 am, maintenance personnel were trying to unclog resin lines used to deminerahze the feedwater sent to the TMI Unit 2 steam generators (Kemeny, 1979). Power plant operators rely on carefully cleaned water infused with special additives to keep corrosion at bay in the plant systems. These deminerahzers were part of that water cleanup system. The resin blockage had produced a dramatic drop in the feedwater flow rate. As a result, the feedwater pump tripped, and went offline due to low flow rates. The loss of feedwater flow to the steam generator produced a commensurate drop in steam pressure and flow to the steam turbine. This in turn... 

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. 

The core power of the Super LWR was found not to be sensitive to the feedwater flow rate due to the existence of many water rods. 

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. 

FPPs adopt turbine-boiler-coordination control. The ratio of the boiler (fuel) input and the feedwater flow rate is used for the control parameter of the feedwater pumps. The plant control strategies of BWRs, PWRs, FPPs, and the Super LWR are compared in Table 1.5. 

The turbine-boiler coordination control using the power to feedwater flow rate ratio was studied for the control of the Super FR and good performance was predicted to be obtained [40],... 

After setting the feedwater flow rate at 35%, nuclear heating starts at a subcriti-cal pressure. When the pressure of the core reaches an adequate value, saturated steam from the separator flows to the turbines. After startup of the turbines, the core is pressurized to a supercritical pressure with a core power at 20%. Startup operation ends and the plant is switched to the normal operation mode. The reactor power increases with the feedwater flow rate. 

Fig. 1.20 Maximum cdlowable power and minimum required power during pressurization phase with feedwater flow rate of 35% and feedwater temperature of 280°C...

The plant and safety systems of the Super FR are the same as that of the Super LWR. The safety and stability analyses of the Super FR have been reported [97-100]. Improvement of the plant control system was studied for the Super FR. The power to flow rate ratio was taken for the control parameter of the feedwater pumps in order to suppress a fluctuation of the main steam temperature. This is the same as in supercritical FPPs. It showed better convergence than taking only the feedwater flow rate as the control parameter [101]. 

In supercritical FPPs, high thermal efficiency is favorable because the fuel cost occupies a large fraction in the total power cost. Fuel cost fraction, however, is a relatively small part of the NPP cost. Lowering the feedwater temperature reduces the number of feedwater heaters. This decreases the size of the BOP. The decrease in the coolant flow rate per electric output will be more effective in decreasing the capital cost than increasing the thermal efficiency in the Super LWR. The flow rate of the Super LWR with the outlet temperature around 500°C decreases by 31% when the feedwater temperature is 210 0 and by 35% when it is 150°C, compared with that of the ABWR [7]. The size of the turbines and the capacities of the pumps will decrease with the feedwater flow rate. 

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]. 

Fig. 4.5 Change of feedwater flow rate with core pressure. (Taken from [6] and used with...

Feedwater flow rate (kg/s) (= main steam line flow rate) 1,190... 

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. 

Fig. 4.11 Response to stepwise decrease in feedwater flow rate (1)...

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... 

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