Miscellaneous Mechanical Components
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
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.