Liquid heaters and vaporizers – Feed heaters – Boiler circuit
Reexamination Certificate
2003-05-05
2004-03-23
Lu, Jiping (Department: 3749)
Liquid heaters and vaporizers
Feed heaters
Boiler circuit
C122S398000, C122S403000, C122S415000
Reexamination Certificate
active
06708651
ABSTRACT:
FIELD AND BACKGROUND OF INVENTION
The present invention relates, in general, to steam generator pressure vessels and, in particular, to an apparatus for supplying relatively cool feedwater to a heated pressure vessel while moderating the thermal gradients therein and in the vessel.
The present invention is particularly suitable for the type of steam generators that are associated with nuclear power plants. In this regard, such steam generators may be viewed as comprising a vertically oriented and substantially closed vessel within which a primary fluid which has been heated by circulation through the reactor core and a vaporizable fluid, in the form of feedwater, are made to flow in indirect heat exchange relationship, such that heat is transferred from the heated fluid to the feedwater. Moreover, in accordance with conventional practice, the steam generator vessel contains a bundle of heat exchange tubes with the ends of each of the heat exchange tubes being suitably retained within a pair of tube sheets. The steam generator vessel is generally substantially cylindrical in configuration, and has a tube sheet suitably mounted therewithin, such as to be positioned adjacent but spaced from each of the ends of the steam generator vessel. Each of the heat exchange tubes in the bundle is in turn suitably supported from the steam generator vessel so as to extend longitudinally therewithin, with the respective ends thereof emplaced in a corresponding one of the aforesaid pair of tube sheets. A cylindrical baffle or shroud is disposed about the bundle of heat exchange tubes to divide the steam generator vessel interior into an annular down flow passageway and an axially disposed evaporator chamber containing the bundle of heat exchange tubes. A plurality of feedwater inlet nozzles communicates with the annular down flow passageway. The feedwater inlet nozzles are generally formed as an integral part of the steam generator vessel, and are spaced at a common elevation around the steam generator vessel.
The heated primary fluid enters the steam generator vessel through a primary fluid inlet and is made to flow through the heat exchange tubes of the bundle, and thence discharged out of the steam generator vessel through a primary fluid outlet, to be conveyed through the remainder of the reactor coolant system. The feedwater is introduced through the feedwater inlet nozzles, and is made to flow down the annular passageway until the tube sheet near the bottom of the annular passageway causes the feedwater to reverse direction, passing in heat transfer relationship with the outside of the heat exchange tubes while flowing upwardly through the inside of the shroud. While the feedwater is circulating in heat transfer relationship with the heat exchange tubes of the bundle, heat is transferred from the heated primary fluid in the tubes to the feedwater surrounding the tubes causing a portion of the feedwater to be converted to steam. The steam then rises and is discharged from the steam generator vessel through one or more steam outlets for circulation through typical generating equipment to produce electricity in a manner well known in the art.
The feedwater inlet nozzle is fed by a supply conduit which is connected thereto for discharge into a thermal sleeve that extends within and through the feedwater inlet nozzle and has one end generally formed with or connected to a sparger, the latter distributes the feedwater downwardly through the annular passageway. The thermal sleeve acts as a shield to reduce the temperature gradients between the relatively cool feedwater flowing therethrough, as compared to the heated feedwater inlet nozzle and steam generator vessel.
The relatively large temperature gradients extending through the feedwater inlet nozzle from the warm steam generator vessel to the relatively cool feedwater tend to produce thermal stresses. Thermal gradients, and the thermal stresses resulting therefrom, are particularly aggravated as a result of changes in the feedwater flow through the inlet nozzle of this type steam generator, under certain operating conditions such as during the reactor start-up as well as during changes in the reactor power output. It is during these changes in feedwater flow that there occurs thermal cycling of the feedwater inlet nozzle and the thermal sleeve. Such thermal cycling may induce fatigue failure in the dissimilar metal weld which fixedly secures the thermal sleeve, through a transition ring, to the feedwater inlet nozzle. In fact, due to restricted access to this thermal sleeve weld region, it is difficult to detect and eliminate weld flaws. Moreover, since the nozzle is usually made of low alloy steel, it corrodes much faster than the thermal sleeve which is made of corrosion-resistant material. Thus, the feedwater inlet nozzle side of this dissimilar metal weld will be severely thinned. Obviously, this corrosion problem could be eliminated if the feedwater inlet nozzle were made of the same expensive corrosion-resistant material as that of the thermal sleeve. However, the material cost of such a modification would be high because of the heavy section size of the feedwater inlet nozzle. When the cantilever thermal sleeve and sparger unit is subjected to a bending moment by feedwater injection and pressure difference or the occurrence of an earthquake, significant bending and axial stresses will occur at the thinned cross section on the feedwater inlet nozzle side of the dissimilar metal weld. As a result, the thermal sleeve may develop fatigue cracks, and the ensuing leaks of feedwater may flow around the outer surface of the thermal sleeve, and come in direct contact with the feedwater inlet nozzle and hence cause undesirable cooling which may lead to thermal stresses in the area of the feedwater inlet nozzle and the surrounding wall portion of the steam generator vessel. The thermal stresses imposed on the feedwater inlet nozzle and the surrounding wall portion of the steam generator vessel will reduce the life expectancy of this equipment, if the undesirable cooling is not eliminated. Therefore, repair of the thermal sleeve is required whenever such leaks occur. However, the repair of the thermal sleeve has proven to be a difficult task, because of the restricted access to the dissimilar metal weld which is used to secure the thermal sleeve to the feedwater inlet nozzle.
Accordingly, this prior art feedwater inlet nozzle, thermal sleeve and sparger assembly has encountered limitations as to, the operating conditions of the feedwater system with respect to reactor start-ups and changes in reactor power output, and also with respect to feedwater flow-induced vibration and fretting of the thermal sleeve, and further with respect to the repair of the thermal sleeve. Thus, there is a need to provide industry with solutions to these problems.
SUMMARY OF INVENTION
These difficulties are overcome, to a large extent, through the practice of the present invention which provides an improved apparatus for supplying feedwater to a nuclear type steam generator pressure vessel. The apparatus is generally comprised of a feedwater inlet nozzle, a thermal sleeve and a sparger, and is structured to supply relatively cool feedwater as compared to its heated self and the heated pressure vessel, while moderating the thermal gradients across the feedwater inlet nozzle and the surrounding wall portion of the pressure vessel; reducing the feedwater flow-induced vibration and fretting of the thermal sleeve; improving the structural support of the thermal sleeve and sparger; and facilitating the repair of the thermal sleeve.
Accordingly, there is provided a feedwater source including a conduit to supply the feedwater to the thermal sleeve which extends through the bore of the feedwater inlet nozzle and through an inlet in the steam pressure vessel wall. The thermal sleeve, which is fixedly supported by the feedwater nozzle, conveys the feedwater to the sparger located in the steam pressure vessel. The underside of the sparger includes a plurality spray holes which inj
Babcock & Wilcox Canada Ltd.
Grant Kathryn W.
Lu Jiping
Marich Eric
LandOfFree
Feedwater apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Feedwater apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Feedwater apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3208560