Miscellaneous Mechanical ComponentsDOE-HDBK-1018/2-93PRESSURIZERSOperationThe level of water within a pressurizer is directly dependant upon the temperature, and thus thedensity, of the water in the system to which the pressurizer is connected. An increase in systemtemperature causes the density of the water to decrease. This decreased density causes the waterto expand, causing the level of water to increase in the vessel. The increased level of water ina pressurizer is referred to as an insurge. An insurge compresses the vapor space, which in turncauses the system pressure to rise. This results in slightly superheated steam in contact with thesubcooled pressurizer liquid. The superheated steam transfers heat to the liquid and to thepressurizer walls. This re-establishes and maintains the saturated condition.A decrease in system temperature causes the density to increase which causes the system watervolume to contract. The contraction (drop) in pressurizer water level and increase in vapor spaceis referred to as an outsurge. The increase in vapor space causes the pressure to drop, flashingthe heated water volume and creating more steam. The increased amount of steam re-establishesthe saturated state. Flashing continues until the decrease in water level ceases and saturatedconditions are restored at a somewhat lower pressure.In each case, the final conditions place the pressurizer level at a new value. The system pressureremains at approximately its previous value, with relatively small pressure variations during thelevel 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 pressurizercan accommodate for example, additional control is obtained by activating the spray. This spraycauses the steam to condense more rapidly, thereby controlling the magnitude of the pressurerise.When a large outsurge occurs, the level can drop rapidly and the water cannot flash to steam fastenough. This results in a pressure drop. The installed heaters add energy to the water and causeit to flash to steam faster, thereby reducing the pressure drop. The heaters can also be left onto re-establish the original saturation temperature and pressure. In certain designs, pressurizerheaters 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 surgevolume. The surge volume is the volume that accommodates the expansion and contraction ofthe system, and is designed to be typical of normal pressurizer performance. Plant transients mayresult in larger than normal insurges and outsurges. When the surge volume is exceeded, thepressurizer may fail to maintain pressure within normal operating pressures. Rev. 0ME-05Page 33
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