Patent Number: 055419690
Section: description

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a partial cross-sectional view of the midloop water level monitor 10 connected to the main pipe 12 of the hot water leg of a nuclear power plant. Main pipe 12 is the hot water leg. Chamber 18 is placed at substantially the same elevation as the main pipe 12. An upper connecting pipe 14 forms a fluid connection from the top region of pipe 12 to the upper region of tank 18. A lower connecting pipe 16 fluidly connects the bottom region of pipe 12 to the lower region of tank 18. A fluid 20, such as water, is shown partially filling the main pipe 12 and the chamber 18. A fluid level indicator 22 is located in chamber 18. In the preferred embodiment of the invention, the water level indicator 22 is comprised of closely spaced heated junction thermocouples (HJTCs) that indicate by temperature gradient the level of water in the chamber 18 and therefore in the pipe 12. The signal from the HJTCs is transmitted from the chamber to a remote location along line 24. HJTCs are preferred because they are fully qualified for operation at system pressures and temperatures during plant power production. In the preferred embodiment of the invention, the water level monitor 10 further comprises two isolation valves 26, 28 that can be used to selectively isolate the chamber 18 from the pipe 12. These selective isolation valves 26, 28 allow maintenance to be performed on chamber 18 while the power plant is running and pressurized hot water is running through pipe 12. It should be recognized that a plurality of valves may be used to isolate the chamber 18 from the pipe 12. It is recognized that chamber 18 may be located at any remote location from the pipe, as long as that location is on substantially the same elevation as the hot water pipe 12. Therefore, the chamber 18 can even be located in a completely separate room from the hot water pipe. The preferable location of the connecting upper pipe 14 and the lower connecting pipe 16 is onto the main pipe 12 at substantially the longitudinal middle of the hot water pipe. This allows the best indication of the water level in the pipe and will allow the monitoring of the water level so that work areas in the steam generator are not flooded during maintenance, and that loss of cooling at the core does not occur. FIG. 2 is a partial cross section of a variation of the water level monitor of FIG. 1 wherein the water level monitor is not directly connected to the hot water pipe 12, but is instead connected into secondary pipes 30 and 32 that connect to the hot water pipe 12. The lower connecting pipe 16 may be directly connected to any lower secondary pipe coming from the bottom of pipe 12, for example, the shut down cooling line. Similarly, the upper connecting pipe may be connected to any secondary upper pipe 32 that connects to the top of the main pipe 12, for example, the surge line. The secondary and connecting pipes are of sufficient size to ensure equalization of the air pressure between the chamber and the hot leg pipe, and therefore the water levels in the chamber and the hot leg pipe. Again, it is recognized that tank 18 must be at substantially the same elevation as pipe 12. The result is that a fluid 20, such as water, in the hot water main pipe 12 will be at the same fluid level in the pipe and in the chamber. Isolation valves 26 and 28 may isolate the chamber from the rest of the hot water system comprising the main pipe and any secondary or connecting pipes, such as pipe 32. FIG. 3 is a partial view of a nuclear power plant. The midloop water level monitor 10 comprising the tank 18 and upper and lower connecting pipes 14, 16 is connected substantially midway on the hot water pipe 12. The hot water pipe 12 connects the reactor vessel 40 to steam generator 42. Water is heated in the core within vessel 40, travels through the hot pipe 12 to the steam generator 42 where steam is generated in the secondary water loop (not shown). The water, thus cooled, then leaves the steam generator 42 by line 44, travels through a pump (not shown) and returns to the reactor core 40 by inlet 46. This path of the water comprises a hot water loop. It is necessary during down times of the plant operation to perform maintenance inside the steam generator 42, particularly in the lower head 45. Because of the orientation of the reactor 40 and the steam generator 42, the hot water pipe 12 is in a substantially horizontal position at an elevation such that the upper region of pipe 12 aligns with a portion of head 45. The result is that even small changes in the level of water in the pipe 12 can result in the flooding of the bottom of the heat exchanger 42 if the level is too high, or insufficient water in the core to perform critical cooling functions if the water level is too low. In addition, if the level of water in the hot leg falls too low, the shut down cooling suction pipe (not shown) will form a vortex of cooling water while drawing the water from the hot leg pipe. This vortex reduces cooling flow of the water. As a result, the core appears to be adequately covered with water, but the reduced flow because of the vortex allows a core overheating condition to occur. Because of this critical cooling function, it is important that operators in the control room 48 be able to monitor the level of water in the pipe even during shut down periods. The signal indicative of the water level from the midloop water level monitor is transmitted along line 24 to control panel 50 where the information is displayed. This allows an operator in the control room 48 to completely monitor all information about the hot water loop system even during a shut down period. The signal can also be transmitted to a computer for integration into a plant safety monitoring and display system.