Steam-water separator

There are various factors that affect the quality of steam/ water separation, such as ... 

The wet steam passes through steam-water separators within the top drum. Separated water is returned to the inlet of the external downcomer, while the moist-to-dry steam may pass through a secondary steam-water separator to remove any residual moisture before passing into the steam header part of the steam delivery system. 

An alternative way to use flash steam is to provide several, small self-contained FSHR systems, keeping the system pipework to a minimum and lagging the pipes to avoid unnecessary heat losses. Flash steam forms at the point where the pressure drops, which is at the BD valve or the valve seat of the steam trap. From this point the flash steam and condensate travel together until the flash vessel is reached. The vessel then acts as a steam-water separator. 

Corrosion may be associated with fouling. For example, oxygen corrosion of a steam-water separator typically results in corrosion debris that builds up and fouls the separator device, thus preventing the effective separation of steam from BW. 

Oxygen in a large operating boiler may corrode steam-water separators and boiler surface components such as the top drum (especially at the waterline) and tubes. Oxygen corrosion also may occur in superheater and reheater tubes, especially in places where moisture can collect, such as in bends and sagging tubes. 

Any action or operating condition that reduces effective steam-water separation leads to some level of BW carryover and a consequential reduction in steam purity and increase in the water content. 

Cause-and-effect analysis reveals that steam purity and quality are both reduced by the degree of carryover taking place in a boiler, and carryover is itself a function of the effectiveness of steam-water separation. In turn, the mechanics of separation are a function of three areas, each with its own variables ... 

Where very effective steam-water separation is required, primary functional control lies in the efficiency and operating condition of the mechanical devices employed. As pressure increases, the density differences between water and steam decrease, so in order to ensure good separation, large HP boilers may employ several types and designs of both primary and secondary mechanical separation devices. 

The Effect of Boiler Operating Variables on Steam-Water Separation... 

Certain operating variables may seriously affect steam-water separation either independently or more often in combination with adverse... 

Steam load swings. Optimum steam-water separation comes at constant steaming rates rather than wide swings in demand and load. 

Poor steam quality Inadequate steam/water separation results in wet steam with reduced heat content. 

A steam-water separation device that functions by the use of centrifugal force and changes in direction. 

A steam-water separation device consisting of a horizontal closed pipe, perforated at the top and with drain holes at the bottom. 

Given the perturbations caused by steam-water separation in existing geothermal production wells, what is the best technique for accurately determining just how close to chemical equilibrium these geothermal wells are ... 

The secondary circuit is composed of two loops, each one includes one secondary pump, one steam evaporator, one steam-water separator, one superheater, one expansion tank, valves and draining tank and emergency draining tanks. The steam generators will produce 96.2t/h dry steam with die temperature 480 C and the pressure 14 MPa for a 25MWe turbine generator. The heat in the condenser will dissipate to the air by a cooling tower. 

The schematic overview of the plant system concept is shown in Fig. X-1. In RMWR, natural circulation core cooling is introduced and hence, circulation pumps and the related power supply can be eliminated. This results in a simplified and economic core cooling system. In this system, gravitational steam/water separation is expected due to the low steam velocity from the core and, hence, the steam/water separator and the steam dryer can also be eliminated. 

In the reactor system design, a natural circulation core cooling system and a gravitational steam/water separation without mechanical separation devices are adopted, taking into account the characteristics of the RMWR core and the low thermal power. The absence of large capacity pumps and valves used in conventional large capacity BWRs reduces the number of systems on-line, construction costs, and periodic inspection loads. 

For more efficient steam-water separation in high-pressure boilers, mechanical devices have been introduced in steam drums. One such device is a cyclone separator (Fig. 11.6). A separator... 

The water coolant is pressurized to the supercritical pressure by the main coolant pumps. They drive the coolant through the core to the turbines. A comparison of plant systems of BWRs, PWRs, and supercritical FPPs is made in Fig. 1.6. The coolant cycle of the Super Light Water Reactor (Super LWR) and Super Fast Reactor (Super FR) is a once-through direct cycle as the supercritical FPPs. The steam-water separators, dryers, and recirculation system of BWRs and the... 

The control rod drives are mounted on the top of the RPV since there is no need for the steam-water separators and dryers. The position of the RPV in the containment vessel (CV) is lowered due to the top-mounted control rod drives. No space below RPV is necessary for the withdrawal and maintenance of the control blades. 

There are two types of supercritical FPPs. One is the constant pressure FPP that starts heating and operates at partial load at the supercritical pressure. The other is the sliding pressure FPP that starts heating at a subcritical pressure, and operates at subcritical pressure at partial load. A steam-water separator and a drain tank are needed for the startup of the sliding pressure FPP. The sliding pressure FPP operates with better thermal efficiency at subcritical pressure at partial load than the constant pressure FPP. In Japan, nuclear power plants are used for base load, and the FPPs are used for daily load following. Minimum partial load is 30% for the constant pressure FPP and 25% for the sliding pressure one [41,42]. 

The sliding pressure startup systems of the Super LWR and a supercritical FPP are shown in Fig. 1.17 [41]. A steam-water separator is installed on the bypass line for the Super LWR, while it is installed on the main steam line for the supercritical FPP. The Super LWR has an additional heater installed to recover heat from the drain of the steam-water separator. When the enthalpy is low, the drain is dumped into the condenser directly. A boiler circulation pump can be used instead of the additional heater the same as in the sliding pressure FPP. 

Compact plant system due to elimination of steam-water separators, steam dryer, recirculation system, and steam generators... 

The plant system of the sliding pressure supercritical fossil-fired boiler is shown in Fig. 5.2. It requires a steam-water separator, a separator drain tank, drain valves, and recirculation pumps. A minimum flow rate is maintained through the furnace walls by using a recirculation pump to add a recirculating flow to that provided by a boiler feedwater pump. The water leaving the furnace is passed to the steam-water separators. The water from the separators is collected in the drain tank and routed back to the economizer inlet via the boiler recirculation pump. 

The constant pressure startup is proposed with reference to that of FPPs. Nuclear heating starts at supercritical pressure, and the pressure is kept constant during load change. Because the reactor operates at a constant supercritical pressure, the coolant in the fuel channels is single phase and steam-water separation is not necessary. The constant pressure startup system for the Super LWR is shown in Fig. 5.3 [2]. It is required to establish a sufficient flow rate to prevent the... 

During subcritical pressure operation in the sliding pressure startup of the Super LWR, a steam-water separator is required to separate the steam and water such that the water can be recirculated to the reactor inlet by recirculation pumps or by additional heaters, in order to maintain adequate core cooling. The size and weight of the steam-water separator are determined by referring to those of sliding pressure supercritical FFPs. The characteristics of the reference 700 MW supercritical boiler and the properties of its steam-water separators are given in Table 5.3. 

Table 5.3 Characteristics of the steam-water separator of the reference supercritical boiler (taken from ref. [3] and used with permission from Atomic Energy Society of Japan)...

It is desirable to pressurize the reactor with low flow rate and low power in order to minimize the size of the steam-water separator. The minimum flow rate required... 

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