Patent Number: 047160144
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention will be described as applied to the steam generator of a pressurized water reactor (PWR) nuclear power plant, however, it will be realized by those skilled in the art that it will also have application to systems for measuring fluid level in other types of steam generators. As shown in FIG. 1, a steam generator 1 is provided with a lower tap 3 below the level 5 of the water 7 and an upper tap 9 in the region 11 of the steam phase. The lower tap 3 is located below the lowest water level to be measured and the upper tap is located above the highest level expected. In this manner, the lower tap is always in communication with liquid phase and the upper tap is always exposed to the steam phase within the steam generator 1. A differential pressure measuring device 13 is connected to the lower tap 3 by a lower connecting line 15 which is provided with an isolation valve 17. Typically, this line is tubing having, for example, an inside diameter of about 3/4 of an inch. A reference leg 19 in the form of a tube extends upward from the measuring device 13 to a point above the level of the upper tap 9. A condensation pot 21 is mounted on the top of the reference leg 19. An upper connecting line 23 has a first section 23' extending from the upper tap 9 and a second section 23" connected to the condensation pot 21. The two sections 23' and 23" of the line 23 are connected to a separator pot 25. An isolation valve 17 is provided in the line section 23'. As seen in FIG. 2, the separator pot 25 has a housing 27 which defines an expansion chamber 29. A baffle 31 depending from the top wall 33 of the housing divides the upper portion of the expansion chamber on one side of the baffle into an inlet section 35, and, on the other side, an outlet section 37. Steam from the steam generator 1 passes through the first section 23' of the connecting line, the separator pot 25 and the second section 23" of the connecting line to the condensation pot 21 where it condenses to keep the reference leg filled with water 39. Excess condensation 41 flows back down the connecting line 23" into the separator pot 25 which is below the level of the condensation pot 21. The velocity of steam through the first section 23' of the connecting line, combined with capillary effects due to the small diameter of line 23', tends to carry liquid slugs 43 with it. The volume of the chamber 29 slows the steam and entrained liquid down which aides in separation, with the liquid accumulating at the lower end 45 of the chamber 29. The diameter of the chamber 29 must be much larger than that of the connecting line 23'; the sizing must allow a free downflow of liquid against steam upflow. According to Zukoski (Influence of Viscosity, Surface Tension and Inclination Angle on Motion of Long Bubbles in Closed Tubes--J. Fluid Mechanics (1966) vol 25, part 4 pp 821-837) the governing parameter for this phenomenon is: EQU .SIGMA.=.sigma./.DELTA..rho.ga.sup.2 where .sigma.=superficial tension of water PA1 .DELTA..rho.=density difference between liquid and steam PA1 a=tube (chamber) diameter For .SIGMA.=1.2 the separation rate of liqid and steam is close to zero; below .SIGMA.=0.1, the separation rate no longer increases with a. For PWR applications, a chamber inside diameter of 2 inches is sufficient for an efficient separation. The baffle 31 directs the steam and liquid, which enters the chamber 29 horizontally through line section 23', downward in the inlet section 35. The liquid and steam separate from each other in chamber 29 with the steam flowing through the outlet section 37 toward the line section 23" while liquid is left at the bottom of the chamber. The baffle height is greater than the inside line diameter 23" to prevent liquid slugs delivered by line 23' from penetrating the inlet of line 23". Thus, all of the entrained slugs 43 of liquid have been removed from the steam by the time that it enters line section 23" and flows countercurrent to the excess condensate 41 returning through this same line section to the separator pot 25. Both sections of the connecting line 23 and the separator pot 25 are covered with insulation 47 to prevent condensation of the steam within these components, which would cause an unnecessary increase of the condensate flow. The condensation pot 21 is of course uninsulated. the liquid which accumulates in the lower portion 45 of the expansion chamber 29 in the separator pot 25 is returned to the steam generator through a drain line 49 connected to the lower connecting line 15 between the isolation valve 17 and measuring device 13. As indicated in FIG. 1, the separator pot 25 is above the highest level 5 of water in the steam generator to be measured so that the liquid can drain from the separator pot through this drain line. In the preferred arrangement, the first section 23' of the upper connecting line is horizontal to avoid having to consider a level measurement error caused by the two phase mixture density multiplied by the elevation difference between the steam generator upper tap 9 and the separation pot entrance. Moreover, as no credit is taken for steam liquid separation in this section of the line 23, there is no requirment to have it sloped in one direction or another. Since the drain line 49 forms one leg of a u-tube with the steam generator 1 forming the other leg, consideration must be taken of the effects of level oscillation which are always present in a steam generator. For a given frequency, level oscillation in the drain line 49 can be smaller or larger than those in the steam generator 1, with the ratio of oscillation amplitude between drain line and steam generator being the amplification factor. The resonant frequency, which is the frequency of natural oscillation of the water column in the drain line, is defined by the equation. EQU F=0.1592.sub.Ho (go)0.5 where go equals the acceleration of gravity and Ho equals water column height. FIG. 3, which is a plot of the amplification factor on the ordinate and the steam generator level oscillation frequency/resonance frequency on the abscissa (dimensionless), shows by the curve 51 that with a 1/4 inch inside diameter drain line 49, the level measurement will not be disturbed by excessive oscillation in the drain line. The curve 53 shows that with a 3/4 inch inside diameter drain line, unacceptable effects on level measurement would be encountered. While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.