Steam generator pressure, reduction

The original steam generators were simple pressure vessels that were prone to caiasirophic failures and loss of life. Due to better boiler design, tube-fired boilers, and boiler inspections, the incidence of catastrophic failure is now to a rare event (about once every 100,000 vessel-years). In Great Britain in 1866, there were 74 steam boiler explosions causing 77 deaths. This was reduced to 17 explo.sions and 8 deaths in 1900 as a result of inspections performed by the Manchester Steam User Association. In the United States, the American Society of Mechanical Engineers established the ASME Pressure Ves.sel Codes with comparable reductions. 

With steam generated at or close to the boiler design pressure it is inevitable that some of the steam-using equipment will have to be supplied at a lower pressure. In some cases the plant items themselves have only been designed to withstand a relatively low pressure. Sometimes a reaction will only proceed when the steam is at a temperature below a certain level or an unwanted reaction will occur above a certain level. For these and similar reasons, steam often is distributed at a relatively high pressure which must then be lowered, close to the point of use. Pressure-reduction stations incorporating pressure-reducing valves are fitted to perform this function. 

A design of turbine in which a partial reduction in steam pressure takes place in fixed nozzles (vanes) and a further steam pressure reduction takes place in nozzles created by moving rotor blades. A reactive force is generated that results in the rotation of the turbine shaft. 

Power generation using steam or gas turbines is now well established, however power recovery by the pressure reduction of process fluids is more difficult and less common. In general, the equipment is not considered to be particularly reliable. Rankine cycle heat engines have been developed/adapted to use relatively low-grade waste-heat sources (particularly from organic fluids) to generate power in the form of electricity or direct drives. They tend to be used when the heat source... 

All parameter changes were within the operational permissible limits, and they were easily compensated by the automatic control systems without intervention of the operators. Personnel support was only required on the water-steam circuit in order to bring parameters of No.6 turbine generator back to the normal values after all the transients were over (pressure regulator of TG No.6 was opened for pressure reduction in the controlled stage chamber of the turbine). 

The water leak rate increases as the tubes break, but is limited by the feedwater pump capacity. After the water leak rate reaches the peak value, the main steam pressure decreases because the turbine trips due to the reduction of the main steam flow rate, and the feed water pump trips due to the reduction of its rotational firequency. Due to these quasipassive features, the water leak rate decreases and adverse consequences can be avoided by providing two pressure relief pipings with rupture disks at Ae bottom of the steam generator which operate at a pressure of 11 kg/cm. ... 

For Steam Header Subsystem Steam enters a header (/) via boiler steam, steam from PRV (pressure reduction valve), turbine extraction, process waste heat boiler steam, and recovered steam from a flash dmm. Process steam generation and use depend on feed rate and process conditions. Steam leaves a header to go to steam turbines, processes, PRV as well as steam losses due to leaks, and trap losses. For the balance closure, steam input to a header must be equal to the output ... 

Replace a pressure reduction station (on process fluid or steam) by a turbine which drives an equipment item or generates power, saving more in discounted operating cost than the capital cost. 

In Fig. 6.67 a regenerative-cycle gas turbine is followed by a low-pressure process steam generator. One of the consequences of the low fuel consumption of the regenerative-cycle gas turbine is a reduction of the regenerator exhaust gas temperature to approximately 600°F (316°C).This arrangement should be selected when only a relatively small amount of process steam is required. 

Feedwater system malfunctions causing a reduction in feedwater temperature were not modelled as described above rather, the transient was analysed by calculating conditions at the feedwater pump inlet following the removal of a low-pressure feedwater heater train from service. The feedwater conditions were then used to recalculate a heat balance through the high-pressure heaters. This heat balance gives the new feedwater conditions at the steam generator inlet. The decrease in feedwater temperature transient so calculated was less severe than (and therefore... 

For small power reductions (less than a 10% change in load), the turbine bypass system is not actuated. Instead, it is accommodated by the reactor power control, the pressuriser level control, the pressuriser pressure control and the steam generator level control systems. 

A considerable reduction of the thickness of pressurized components (reactor vessel, steam generator (SG), etc.) ... 

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