Startup bypass system

The plant system of the constant pressure supercritical fossil-fired boiler is shown in Fig. 5.1. The startup bypass system includes a flash tank, pressure-reducing valves, and bypass valves. First, a minimum feedwater flow is established in the furnace prior to the firing of the boiler to prevent overheating of the tube walls. During the cold cleanup mode of operation, the flow is bypassed from the inlet of the primary superheater to the flash tank, until the water chemistry is brought to a predetermined level and the boiler firing starts. 

The redesigned system is schematically described in Fig. 5.69. The startup bypass system in the original design is replaced by the separate recirculation system that consists of a steam drum, a heat exchanger, a circulation pump, and pipes. Neither the inlet nor outlet of the recirculation system are connected to the main lines, rather they are directly connected to the reactor vessel in order to form a closed space for pressurization like the recirculation system of BWRs. 

In this chapter, the plant startup and stability were introduced. Both constant pressure and sliding pressure startup schemes are designed by referring to FPPs. The constant pressure startup system requires a startup bypass system consisting of a flash tank and pressure-reducing valves. The sliding pressure startup scheme... 

Main valve is installed in the main steam line Simplified startup bypass system Shift operation of startup valves in necessary Shift operation of startup valves is not Operation of drain valves and vent valves is necessary... 

Warming of startup b5q)ass system Warming of startup bypass system... 

Circuit driers are used in the magnox reactors (52) to keep the moisture level at about 2 ppm. After startup, the level of residual gases in the CO2 increases to an asymptotic level of about 1.5 vol-% in about 100 days, 85 to 95 % of the residual gas being carbon monoxide. Particulate matter is removed in a bypass system having cyclone filters. 

The exhaust from the low pressure turbine cylinders flows to the main turbine condenser which has three shells, located under the exhaust hoods of the low pressure turbine cylinders. The condenser is designed to accept also the steam flow from the main steam bypass system on startup, hot standby and turbine trip. During normal power operation, the steam flow to the condenser amounts to about 60% of the total steam flow, but the condenser system is designed to accommodate the full steam flow for a limited time period the steam flow shall be reduced to 60% within 20 seconds to avoid a reactor trip due to too high condenser pressure. 

The turbine exhaust flows to a condenser which has three shells, located under the low pressure turbine exhaust hoods. The condenser also accepts the exhaust flow from the feed pump turbines and, on startup, hot standby and turbine trip, flow from the main steam and bypass system. 

The HPCS system can operate independently of normal auxiliary AC power, plant service air, or the emergency cooling water system. Operation of the system is automatically initiated from independent redundant signals indicating low reactor vessel water level or high pressure in the primary containment. The system also provides for remote-manual startup, operation, and shutdown. A testable check valve in the discharge line prevents backflow from the reactor pressure vessel when the reactor vessel pressure exceeds the HPCS system pressure such as may occur during initial activation of the system. A low flow bypass system is placed into operation until pump head exceeds the nuclear system pressure and permits flow into the reactor vessel. 

This boiler is designed to maintain a minimum flow inside the furnace water wall tubes to prevent tube overheating during all operating conditions. This flow must be established before startup of the boiler. A bypass system, integral with the boiler, turbine, condensate, and feedwater system, is provided. 

For positive displacement pumps, a bypass-type control valve should be furnished to set the primary lube system pressure. The valve should be able to maintain system pressure during pump startup and pump transfers, which includes relieving the capacity of one pump, while both are running. The valve should provide stable, constant pressure during these transients. Flow turndown of 8 to 1 is not unusual. Multiple valves in parallel should be used if a single valve is not suitable. The valve should be sized to operate between 10 and 90% of the flow coefficient (Cv). Additional pressure control valves should be furnished as required to pro ide any of the intermediate pressure levels. 

Most state laws and safe practice require a safety relief valve ahead of the first stop valve in every positive displacement compressed air system. It is set to release at 1.25 times the normal discharge pressure of the compressor or at the maximum working pressure of the system, whichever is lower. The relief valve piping system sometimes includes a manual vent valve and/or a bypass valve to the suction to facilitate startup and shutdown operations. Quick line sizing equations are (1) line connection, (i/1.75 (2) bypass, ii/4.5 (3) vent, dl63 and (4) relief valve port, cU9. 

Caustic, similarly, should be flowing under control around the catholyte system, through the cell room headers and the electrolyzer bypass. Again, the flow should be appropriate to a current density of 2-3 kA m , and the caustic temperature at the electrolyzers should be about 50°C. A concentration of 30-32% is usually satisfactory in the case of NaOH. The caustic should meet all specifications, and its concentration and iron content should be checked just before startup. 

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