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Figure 15   Basic Pressurizer
Pressurizer  Summary

Mechanical Science Volume 2 of 2
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Miscellaneous Mechanical Components DOE-HDBK-1018/2-93 PRESSURIZERS Operation The level of water within a pressurizer is directly dependant upon the temperature, and thus the density, of the water in the system to which the pressurizer is connected.  An increase in system temperature causes the density of the water to decrease.  This decreased density causes the water to expand, causing the level of water to increase in the vessel.   The increased level of water in a pressurizer is referred to as an insurge.  An insurge compresses the vapor space, which in turn causes the system pressure to rise.  This results in slightly superheated steam in contact with the subcooled  pressurizer  liquid.    The  superheated  steam  transfers  heat  to  the  liquid  and  to  the pressurizer walls.   This re-establishes and maintains the saturated condition. A decrease in system temperature causes the density to increase which causes the system water volume to contract.  The contraction (drop) in pressurizer water level and increase in vapor space is referred to as an outsurge.   The increase in vapor space causes the pressure to drop, flashing the heated water volume and creating more steam.  The increased amount of steam re-establishes the  saturated  state.   Flashing  continues  until  the  decrease  in  water  level  ceases  and  saturated conditions are restored at a somewhat lower pressure. In each case, the final conditions place the pressurizer level at a new value.  The system pressure remains at approximately its previous value, with relatively small pressure variations during the level change, provided that the level changes are not too extreme. In actual application, relying on saturation to handle all variations in pressure is not practical. In conditions where the system water is surging into the pressurizer faster than the pressurizer can accommodate for example, additional control is obtained by activating the spray.  This spray causes  the steam to condense more rapidly, thereby controlling the magnitude of the pressure rise. When a large outsurge occurs, the level can drop rapidly and the water cannot flash to steam fast enough.  This results in a pressure drop.  The installed heaters add energy to the water and cause it to flash to steam faster, thereby reducing the pressure drop.   The heaters can also be left on to re-establish the original saturation temperature and pressure.   In certain designs, pressurizer heaters are energized continuously to make up for heat losses to the environment. The pressurizer's heater and spray capabilities are designed to compensate for the expected surge volume.   The surge volume is the volume that accommodates the expansion and contraction of the system, and is designed to be typical of normal pressurizer performance.  Plant transients may result  in  larger than  normal  insurges  and outsurges.  When  the surge  volume  is  exceeded, the pressurizer may fail to maintain pressure within normal operating pressures. Rev. 0 ME-05 Page 33







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