Startup bypass operation

After this firing is initiated, and the temperature of the steam leaving the primary superheater reaches about 300°C, the steam from the flash tank is led to the condenser. When the flash tank pressure becomes high, the saturated steam from the flash tank can be used to roll the turbine. After this, the turbine is ramped to about 7% load. At about 20% unit load, the startup bypass operation is shifted to once-through operation. When the temperatures become stabilized, the feedwater flow and the turbine load are increased slowly to 100% of full load value. 

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. 

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... 

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. 

We consider a plant designed to convert a feed stream rich in compound A (of molar concentration cao) into compound B in a high-temperature, mildly exothermic, first-order reaction carried out in an adiabatic reactor (Figure 6.8). For improved operability, the plant features a heater that is used at full capacity in startup mode and as a trim heater during operation, as well as a bypass stream that is used to regulate heat recovery in the FEHE. 

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 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. 

Ammonia feed is shut off by a low temperature switch when the operating flue gas temperature drops below the minimum recommended value. This prevents deactivation of the catalyst from ammonium bisulfate deposition. This control feature is also applied during system startup and shutdown. An economizer bypass is used to maintain the flue gas temperature above the minimum recommended SCR operating temperature and an SCR bypass is shown (though not often provided). The SCR bypass is used to protect the SCR catalyst during startup and shutdown when the flue gas temperature can be below its dew point. Economizer bypasses are used at the Chambers, Indiantown, and Keystone power plants (Franklin, 1993). 

The covering of all major abnormal transients by these proposed models are confirmed by comparing the results obtained by them with results obtained from detailed fuel rod analyses modeling each abnormal transient event. The following eight abnormal transient events are analyzed for confirmation inadvertent startup of the auxiliary feedwater system (AFS) loss of feedwater heating loss of load without turbine bypass withdrawal of control rods at normal operation main coolant flow control system failure pressure control system failure partial loss of reactor coolant flow and loss of offsite power. 

Inadvertent startup of AFS 2 Loss of feedwater heating 3 Loss of turbine load without turbine bypass 4 Uncontrolled CR withdrawal at normal operation 5 Reactor coolant flow control system failure 6 Pressure control system failure 8 Partial loss of reactor coolant flow 9 Loss of offsite power... 

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. 

Relay K3 may next be energized in the startup sequence, provided upper limit switches on all three safety rods are closed. A manually operated key switch completes the interlock circuit for Relay K3, allowing the dump valve to be closed by operation of the control switch. The "dump valve" key switch is bypassed by a contact of Relay K3 and may then be removed. 

The plant arrangement assumed for the two Brayton system heat balance is illustrated in Figure 5-27. This system has two converter loops each with one turboaltemator, recuperator, and gas cooler. No cross-strapping of converter loop components is assumed. Each turboaltemator Is sized to produce the entire 200 kWe required for full power. The recuperator and gas cooler are appropriately sized to meet this power requirement. One converter loop is normally operating while the other is an idle spare A check valve is located at the outlet of each compressor to prevent backflow through the idle loop to maximize system efficiency, minimize bypass cooling flow around the reactor, and prevent potential damage to turbomachinery due to reverse rotation. An isolation valve installed the outlet of each compressor enables loop shutdown and startup evolutions and provides Brayton overspeed protection for certain electric plant casualties. 

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