Conclusiones sobre la peligrosidad y el riesgo de inundación de las

The need for cyclic operation of base load plants during the past two decades has become more critical for several reasons. Forecasts of electric power demand for the previous decade greatly exceeded actual demand, so many utilities had excess capacity. Also, many nuclear plants that were under construction were completed and are being used to meet base load requirements. Finally, fluctuations in fuel prices for oil and gas fired units, the traditional choices for cyclic or peak load plants, may make these units less cost effective. These three factors make it necessary to have dependable, efficient variable output from plants which were originally designed for base load operation.9BĆ

In a turbineĆgenerator, the electrical power output is dependent on the pressure of steam entering the turbine. First, the boiler is fired and brought up to a constant discharge pressure. The turbine is equipped with several valves, known as turbine control valves, which regulate the turbine inlet pressure. As load decreases, the valves move closed to reduce turbine inlet pressure. All valves may move closed equal amounts in unison (full arc admission) or they may close sequentially (partial arc admission). This is known as constant

pressure operation, since the inlet pressure to the turbine control valves is essentially constant. Constant pressure operation has two adverse effects when large load changes occur. First, the turbine will experience temperature fluctuations that cause fatigue and reduce its life. Second, the net thermal efficiency or heat rate of the turbine drops at lower loads.

Sliding pressure operation is designed to eliminate these effects. In general, it consists of adding pressureĆreducing valves upstream of the turbine control valves. These valves reduce the discharge pressure from the boiler and allow the turbine control valves to remain fully open. Temperature

Figure 9BĆ1. OnceĆthrough main steam system ĆĆ constant pressure control

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Figure 9BĆ2. OnceĆthrough main steam system ĆĆ sliding pressure control

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variations are reduced and the net thermal efficiency is improved.

Figure 9BĆ1 shows the typical main steam system of a supercritical, onceĆthrough fossil unit from the boiler to the turbine. In order to cycle the plant, the inlet pressure to the turbine must be varied. In constant pressure operation, turbine throttle valves control the inlet pressure to the turbine, which is in proportion to plant load.

For units with sliding pressure control, a large valve or series of large valves are installed between the primary superheater and the secondary superheater (Figure 9BĆ2). Although the turbine throttle valves are still in the system,

Figure 9BĆ3. Constant pressure control

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Figure 9BĆ4. 100% sliding pressure control

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they are held wide open, and the sliding pressure control valve varies plant load (turbine pressure.) Figures 9BĆ3 and 9BĆ4 show the relationship between boiler pressure and secondary

superheater inlet pressure for constant pressure control and sliding pressure control respectively. The combustion control system is set up to bring the boiler to a preĆestablished discharge header pressure, where it is maintained throughout operation. Under constant pressure operation, a set of small startup valves controls the secondary superheater inlet pressure and therefore, turbine

Figure 9BĆ5. 70% sliding pressure control

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pressure, up to 25% of maximum plant load. Above 25% load, the turbine throttle valves control the turbine pressure. 100% sliding pressure control (Figure 9BĆ4) means that the sliding pressure control valve is sized to control the plant load from a minimum load through 100% plant load. If the sliding pressure control valve is sized for 70% of the plant load, the unit is said to have 70% sliding pressure control (Figure 9BĆ5). From 70 to 100% plant load, the turbine throttle valves are used to control plant load and the benefits of sliding pressure control are lost. In summary, the sizing of the sliding pressure control valve

determines the amount of sliding pressure control that a plant has available.

The thermodynamics of a control valve must be reviewed in order to understand the benefits of sliding pressure control. Thermodynamic laws state that for a system which does no work, the enthalpy of the system is constant. Since no work is done in a control valve, the enthalpy upstream of a valve must equal the enthalpy downstream of the valve. In equation form:

h1 = h2

h = enthalpy, BTU/lb

Since enthalpy is a function of pressure and temperature, steam tables can be used to show that as steam pressure is reduced, steam temperature is also reduced.

In constant pressure operation, the pressure reduction and consequently, temperature reduction, occur at the inlet to the turbine. This

9B-3 temperature loss reduces the efficiency of the

turbine. Additionally, this uncontrolled inlet steam temperature places increased stress and potential wet steam erosion problems in the latter stages of the turbine.

Under sliding pressure control, the pressure drop and corresponding temperature drop occurs upstream of the secondary superheater. The steam then travels into the secondary superheater where the temperature is elevated to its full load limit of around 1005 degrees F. As a result, sliding pressure control allows constant temperature steam to enter the turbine at all plant loads. This increases turbine efficiency and reduces thermal cycle effects on the turbine.

Sliding Pressure Operation also provides the following benefits:

DĄFull ARC admission of steam to the turbine. DĄLower turbine thermal stresses.

DĄFaster load changes because of reduced temperature differentials.

DĄImproved overall plant heat rate. DĄLower minimum load capabilities.

Sliding pressure control is generally associated with larger supercritical pressure units, although drum style subcritical units can operate under sliding pressure control. The main reasons that supercritical units are better candidates for sliding pressure control than drum style units can be obtained from looking at the inherent differences between the two boiler types. Supercritical, onceĆthrough units are typically larger, harder to startĆup, and more difficult to cycle than drum style units. Therefore, the benefits of quick load

changes are of greater magnitude in a supercritical unit. The efficiency that sliding pressure control provides is of far greater concern to the utility for the larger supercritical units than the smaller drum units. Therefore, sliding pressure control will be discussed only as it concerns supercritical boilers in the upcoming sections. In order to look at the control valves required for sliding pressure control, a basic understanding of the onceĆthrough startĆup systems for various boiler manufacturers must be obtained. StartĆup systems for Babcock & Wilcox, Combustion Engineering, and Foster Wheeler boilers will be discussed in the following sections. Although the benefits of sliding pressure control versus

constant pressure control are many, the function of the valves in the startĆup system are very similar. Generally, only the sizing of the valves is different between the two modes of operation. The supercritical boiler is required to have a minimum flow inside the furnace waterwalls to prevent overheating of the boiler tubes. This flow must be established before firing of the boiler. A bypass system that is integral with the main steam, condensate, and feedwater systems is required so that the minimum design flow can be maintained at startĆup and at times when the required minimum flow exceeds the turbine steam demand.

The bypass system performs the following additional functions:

DĄReduces the pressure and temperature of the steam leaving the boiler during startĆup to conditions that are suitable for the flash tank, condenser, and auxiliary equipment.

DĄProvides a way of recovering heat from the feedwater that flows to the bypass system by utilizing the feedwater heaters.

DĄProvides a means for conditioning the water during startĆup without delaying boiler and turbine warming operations.

DĄProtects the secondary superheater against thermal shock from water during startĆup.

DĄProvides a means for relieving excess pressure in the boiler during a load trip.

As previously mentioned, bypass arrangements differ considerably in detail, but are similar in their general design. The slight differences are required to accommodate the needs of the turbine as well as the supercritical boiler. The following sections describe each type of bypass system and sliding pressure control system in detail.