Sliding pressure operation

The present analysis is based on the heat transfer correlations for smooth tubes. When turbulence is promoted, the cladding temperature rise at dryout will be suppressed. The maximum allowable power between 20 and 22 MPa will increase. Ribbed or rifled tubes and spiral tapes are used in supercritical FPPs to suppress the boiling transition during the sliding pressure operation and the sliding pressure... 

Faster ramp rates are possible in sliding pressure operation, as the thermal stress in the turbine during the load change is reduced. Plant thermal efficiency is much improved in partial load operations. Turbine life expenditure per cycle can also be reduced by ramping pressure with load. 

Once-Through Boilers (Benson Boilers for Sliding Pressure Operation)... 

The sliding pressure operation is a control system in which the main steam is controlled by sliding pressure in proporticm to the generation output as shown in Fig. A.4 [1]. Steam quality at the turbine inlet can be changed at constant volume flows while keeping the turbine governing valve open. 

For a supercritical sliding pressure operation boiler, flow stability through tubes and pipes against various changes in flow characteristics between supercritical and subcritical pressure are important factors. 

Fig. A.4 Features of coal firing supercritical sliding pressure operation boiler (Taken from ref [1])...

J. Matsuda and K. Saito Low grade coal firing super critical sliding pressure operation boiler, Proc. 2nd Int. Sym. on Clean Coal Technology, November 8-10, 1999... 

The sliding-pressure Venturi scrubber developed by Siemens/KWU and described by Eckardt (1988) allows operation at the pressure level prevailing inside the containment down to an overpressure of about 0.1 MPa, without need of a throttle valve in the venting line the water droplets generated in the liquid phase of the scrubber during operation of the system are retained by a steel fiber filter located inside the scrubber tank. The scrubber can be designed in a manner to reduce water losses due to evaporation caused by the decay heat of the absorbed radionuclides to an insignificant level. 

A supercritical fossil-fired power plant with a once-through steam cycle is usually operated with a sliding pressure the turbine governor valve is kept open in the upper load range and the boiler outlet temperature is kept constant such that the boiler outlet pressure increases proportionally with the steam mass flow and thus with load. Consequently, the boiler is operated at subcritical pressure below approximately 80—90% load. However, such control is not permitted for the SCWR because dryout... 

The modified sliding pressure staitup as proposed by Yi et al. (2005) can be adapted to the proposed operating conditions in the Canadian SCWR concept. To provide a starting point for future analysis of critical performance characteristics (eg, fuel cladding temperatures and thermal-hydraulic and neutron stabilities), reference operating conditions (eg, flow rates, reactor power levels, and mechanical equipment configurations) have been selected. 

In Japan, the first supercritical FPP, Anegasaki No.l started operation in 1967 with a rated power of 600 MWe. The supercritical FPP technologies have been improved constantly in Japan because of the high fossil fuel prices. Since fuel cost is the major part of the power generation cost in FPPs, improvement of the thermal efficiency would reduce the power cost. The sliding pressure plant Hirono No. 1 was deployed in 1980. It operates at subcritical pressure at partial load. Japanese... 

FPPs need to be operated in the daily load-follow mode. Frequent startups and shutdowns are necessary. Sliding pressure plants meet these needs. 

Since sliding pressure plants are operated at subcritical pressure at partial load, they achieve higher thermal efficiency than constant pressure operation at supercritical pressure. To improve the thermal efficiency at rated power, the high pressure plant, Kawagoe No. 2 started operation with conditions of 31 MPa and 566°C in 1989. This was followed by the high temperature plant, Tachibanawan No. 1, with conditions of 25 MPa and 610 C. 

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]. 

There are two kinds of startup schemes currently used in FPPs [1]. One is the constant pressure startup scheme, in which the boiler operates at constant supercritical pressure after the coolant is pressurized to this point. The other is the sliding pressure startup scheme, in which the boiler operates with variable pressures and the pressure increases with the generation output. 

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. 

Since the Super LWR plant system does not have a superheater, the main steam conditions need to be adjusted during startup and low power operations. The enthalpy of the core outlet coolant must be high enough to provide the required turbine inlet steam enthalpy. At subcritical pressure operation in the sliding pressure startup scheme, boiling and dry out in the descending moderator water rods are undesirable and should be prevented because they affect the inlet subcooling. 

There is no difference between the pressurization phases of the constant pressure startup and sliding pressure startup schemes because the pressurization phase appears after the line switching to the once-through mode. It is assumed that the core inlet and outlet temperatures are kept equal to their respective values in the normal operating condition. While the reactor core power is increased, the feed-water flow rate is also increased proportionally. MCSTs are calculated for various core powers from 30 to 100% at intervals of 10%, and the calculated results are shown in Fig. 5.11. It is found that MCST satisfies the criterion of 620°C throughout the power increase phase. 

The startup curve for the sliding pressure startup scheme that is designed based on only the thermal considerations (Fig. 5.23) is redrawn taking the stability considerations into account as well. The constant pressure startup is not discussed here because the partial power operating conditions in the constant pressure startup are covered by the temperature increase phase and power increase phase of the sliding pressure startup. 

As described in Chap. 5, the sliding pressure startup is one of the candidate startup schemes. It is necessary to understand the reactor behavior in case of abnormal transients and accidents during the pressurization phase. To do that, SPRAT-DOWN is extended to the SPRAT-DOWN-SUB which can be applied to the transients and accidents during subcritical pressure operation [12]. 

The Super LWR and Super FR may be operated at subcritical pressure during startup if the sliding pressure startup scheme is chosen. Also, the pressure decreases from supercritical to subcritical at depressurization events such as a LOCA. The CHF and the post boiling transition (post-BT) heat transfer are important, especially just below the critical pressure because the CHF, the enthalpy of the CHF conditicMi and the post-BT heat transfer coefficient are all substantially smaller than those at a... 

Operating pressure Subcritical (constant or sliding) Subcritical or supercritical (constant pressure) Subcritical to supercritical (sliding pressure)... 

By the sliding pressure, thermal efficiency of the turbine is improved in partial operating loads though with decreasing thermodynamic efficiency, as follows, in comparison with constant pressure operation. 

In comparison to constant pressure operations, a sliding type enables much Improvement in plant efficiency under partial load operations. 

Sliding-Vane Type This type is illustrated in Fig. 10-81. These units are offered for operating pressures up to 0.86 MPa (125 IbFin") and in capacities up to 3.4 X 10 mVh (2000 ftVmin). Generally, pressure ratios per stage are limited to 4 1. Lubrication of the vanes is required, and the air or gas stream therefore contains lubricating oil. 

The slide is located in the compressor casting below the rotors, allowing internal gas recirculation without compression. Slide valve is operated by a piston located in a hydraulic cylinder and actuated by high-pressure oil from both sides. When the compressor is started, the slide valve is fuUy open and the compressor is unloaded. To increase capacity, a solenoid valve on the hydraulic hne opens, moving the piston in the direction of increasing capacity. In order to increase partload efficiency, the slide valve is designed to consist of two parts, one traditional shde valve for capacity regulation and other for built-in volume adjustment. 

In die original system eonfiguration, die hot flue gas leaving die regenerator was expanded in die double slide valve and orifiee ehamber to atmospherie pressure, and dien passed via die waste heat boiler to die main staek. This mode of operation remains possible following die expander retrofit. 

Controlled catalyst circulation is one of the most important prerequisites for trouble-free operation of the FCC unit. Uniform circulation is ensured by controlling the differential pressure between the reactor and regenerator. The differential pressure in the existing plant is controlled by a differential pressure governor adjusting the position of the double slide valve upstream of the orifice chamber. 

Under normal operations, the existing differential pressure governor is switehed to manual and the double slide valve is wide open. This valve must be suffieiently opened so that, even in the event of an emergeney expander trip, the entire flue gas flow ean pass through the double slide valve without the regenerator diseharge pressure inereasing to nonpermissible levels. 

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