Steam-plant control systems

In order to successfully apply controls to steam generation, a thorough familiarity with its thermodynamic properties is essential. The most important point to remember is that steam is valued principally for its 

Steam is also superheated by passing through an orifice or pressure-reducing valve, since in theory, no enthalpy is lost across a throttling device. Thus the pressure of the steam would drop while the temperature remained virtually constant. 

The mass flow of steam may be measured with an ordinary orifice meter, but the reading must be corrected, if pressure or temperature deviate from the conditions under which calibration was specified. In the case of saturated steam, pressure and temperature are not independent of one another, so either one is capable of indicating density. It so happens, however, that pressure is a linear function of density, with an intercept of 0 psig  

The density of superheated steam varies inversely with temperature and directly with pressure to make the mass flow calculation more complicated and less accurate. But if a steam flowmeter is used to indicate the actual delivery of thermal power, an interesting phenomenon appears temperature causes the enthalpy of superheated steam to vary in a way which offsets its effect upon density. Thus thermal power Q only varies with differential and pressure  

Coefficients H and Ho represent steam enthalpy at flowing and calibration 

---

Nuclear steam supply control system— Implements the operator s control decisions and automatically changes the plant to and maintains it at selected operating states. Reactor protection system—Protects the reactor core and the NSSS by monitoring operating parameters and initiating safeguard actions on the detection of abnormal conditions. 

When automatic-extraction steam turbines have a condensing exhaust, the control system can control pressure as well as power generation by varying flow to the condenser. The amount of power called for by the steam turbine unit control system can be continuously modulated by the plant control system. 

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 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]. 

Since the Super LWR does not use saturated steam, the main steam temperature changes with the power to flow rate ratio in the core. It needs to be kept constant in order to avoid too much thermal stress or thermal fatigue on the structures. Since the Super LWR has no superheaters that are utilized to control the main steam temperature as in FPPs, another method is needed. The analysis results described in Sect. 4.3.2 show that the main steam temperature is sensitive to the feedwater flow rate. Thus, the main steam temperature is controlled by regulating the feedwater flow rate. It is also suitable from the viewpoint of the safety principle of the Super LWR, i.e., keeping the core coolant flow rate (described in Sect. 6.2) because the feedwater flow rate indirectly follows the reactor power in this control method. The plant control system employed for the Super LWR is shown in Fig. 4.16. The plant control strategies of the Super LWR, PWRs, BWRs, and FPPs are compared in Table 4.3. 

The analyses in the previous section show that the Super FR qualitatively has the same basic plant dynamics as the Super LWR although the change in the main steam temperature is larger. Thus, the same plant control system as that of the Super LWR is designed and tuned here as the basis of improvements. 

Fig. 1. Pressurized water reactor (PWR) coolant system having U-tube steam generators typical of the 3—4 loops in nuclear power plants. PWR plants having once-through steam generators contain two reactor coolant pump-steam generator loops. CVCS = chemical and volume-control system.

Electrical. The plant electrical system is sometimes more important than the steam system. The electrical system consists of the utihty company s entry substation, any ia-plant generating equipment, primary distribution feeders, secondary substations and transformers, final distribution cables, and various items of switch-gear, protective relays, motor starters, motors, lighting control panels, and capacitors to adjust power factor. 

Environmental Factors These inchrde (I) eqrripment location, (2) available space, (3) ambient conditions, (4) availabuity of adeqrrate rrtilities (i.e., power, water, etc.) and ancillary-system facilities (i.e., waste treatment and disposal, etc.), (5) maximrrm aUowable emission (air polhrtion codes), (6) aesthetic considerations (i.e., visible steam or water-vapor phrme, etc.), (7) contribrrtions of the air-poUrrtion-control system to wastewater and land poUrrtion, and (8) contribrrtion of the air-poUrrtion-control system to plant noise levels. 

Water level control and the use of organic antifoam chemicals are essential in steam plants in order to break dowm the bubbles at the water surface in steam systems, which cause foaming. 

When hydrogen is burned in a combustion chamber instead of a conventional boiler, high-pressure superheated steam can be generated and fed directly into a turbine. This could cut the capital cost of a power plant by one half. While hydrogen is burned, there is essentially no pollution. Expensive pollution control systems, which can be almost one third of the capital costs of conventional fossil fuel power plants are not required. This should also allow plants to be located closer to residential and commercial loads, reducing power transmission costs and line losses. 

There are six primary in-plant control methods for removal of priority pollutants and pesticides in pesticide manufacturing plants. These methods include steam-stripping, activated carbon adsorption, chemical oxidation, resin adsorption, hydrolysis, and heavy metals separation. Steam-stripping can remove volatile organic compounds (VOCs) activated carbon can remove semi volatile organic compounds and many pesticides and resin adsorption, chemical oxidation, and hydrolysis can treat selected pesticides [7]. Heavy metals separation can reduce toxicity to downstream biological treatment systems. Discussion of each of these methods follows. 

The hot combustion gases rise to enter the boiler, producing superheated steam. Each incinerator has its own boiler, and they both feed steam to the same turbine generator. It produces 14 MW of power, yielding 100 million kilowatt hours each year under normal operations. The power plant includes a full pollution control system, with flue gas desulfurization, thermal de-NOx, and a fabric filter baghouse. 

At the next level are plant decisions about allocation among various plant operating units. These may be made daily or even more frequently depending on the plant automation and control system. For example, steam may be generated by a number of different sources, with different... 

In the case of "flash steam" power plants, the steam is either generated directly by the production wells or the wells produce hot water from which steam can be separated to drive conventional steam turbine generators. The size of these plants ranges from 100 kW to 150 mW. Figure 2.104 illustrates the optimizing control system for such a geothermal facility. 

The steam turbines require one-third the energy of An electric motor. Each refrigeration unit has a different horsepower/ton characteristic, which also depends upon ambient conditions. There are hard constraints on compressor loads and cost penalties (soft constraints) on electrical load. Steam, refrigeration, compressed air, and electrical loads to the plant vary continually. While the author does not suggest that the optimum operating and control strategy is known, he does imply that a computer control system is the only way to operate the plant in an optimum manner. 

---