3.1 STEAM DRUM WATER-LEVEL CONTROL

Grid-connected power stations are operated in either decoupled or coupled mode [80,117], which describes the form of their response to grid frequency variations1 from changes in network demand. These alternatives are illustrated in Figures 3.1 and 3.2 where C1 (s) and C2 (s) represent controller transfer functions. By creating strict limits on steam plant temperature and pressure variations, decoupled control promotes steam turbine efficiency and boiler longevity (e. g., by reducing the exfoliation of tube magnetite deposits). Consequently, decoupled con­trol is generally the economic choice for large base-load stations having a high thermal efficiency and capital investment. With a coupled control scheme, a fossil or nuclear heat source is modulated to sustain boiler pressure when changes in Grid frequency operate the turbine control valve. Because Grid frequency fluctuations widely outpace heat-source dynamics, boiler-pressure changes can be offset only by exploiting the thermal energy stored in the rest of the plant. Such spontaneous changes of stored energy in some plant components are readily accommodated and their responses help to relieve thermally induced stresses in other less robust items [141].

1 See Section 3.3.

Nuclear Electric Power: Safety, Operation, and Control Aspects, First Edition. J. Brian Knowles.

© 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.

The evaporator section of the La Mont boiler system [117] in Figure 3.3 produces a low steam quality (< 10%) flow into a large steam drum which separates the saturated steam for superheating. Feed water to match steam generation is usually injected downwards from a sparge pipe near the base of a drum. A Richardson number analysis [117] establishes that its mixing with less dense saturated liquid from the evaporator is thereby largely suppressed,[35] so the drum’s liquid content remains markedly stratified.

When the turbine control valve opens under coupled control, the increased mass flow rate of steam to the turbine reduces boiler pressure, so some of the upper layer of saturated water in the drum flashes rapidly into steam to preserve thermodynamic equilibrium. This extra steam partially supports system pressure, and there is a corresponding fall in drum water level. Because a large enough reduction would cause the drawdown of steam into the recirculation pump [117], drum water-level control is necessary to prevent cavitation damage to its impeller. On the other hand, too high a water level would impair steam separation leading to damaging thermal shocks to the superheater tubing.

Figure 3.3 depicts the drum water-level control scheme for a 250MW(e) fast reactor nuclear power plant. The intermediate heat exchangers (IHXs) provide an additional safety barrier to obstruct an explosive ingress of water into the sodium-cooled reactor circuit. Because improvements in station efficiency of as little as 0.1% are financially material, the feed pump was driven from the same high-pressure steam supply as the main turbine to exploit this opportunity. Under coupled control a reduction in Grid frequency opens the turbine control valve via ytg and the drum pressure falls as a result of the increased steam flow. Its saturated surface-water then flashes very rapidly into steam, so a

correspondingly fast increase in feed-water flow is necessary to prevent the drawdown of steam. However, because the driving pressure to the feedpump turbine has reduced, the feed-flow is reduced just when it’s needed. Hence, the main turbine and feedpump turbine control-valve settings ytg and yyv are strongly interactive making the plant’s transfer function matrix far from diagonally dominant. It can be readily appre­ciated therefore that SISO control system design techniques proved unsatisfactory. By consummate skill, Hughes [81] devised a broadly satisfactory MIMO control system design using a 3 x 3 matrix controller with proportional plus integral diagonal elements. Nevertheless the incipient deployment of an independent electrically driven feedpump would have circumvented the problem to allow largely independent SISO control schemes and a more transparent design of ad hoc accident management strategies. This example highlights the importance of engineering insight and awareness of plant operating conditions: thereby demonstrating the industry specific nature of control engineering. Finally, it should be noted that the relatively high initial capital and low fuel costs of nuclear stations usually favours their top merit order placings with operation at maximum power under decoupled control.