Heat exchanger tube support bar

Details Heat exchanger tube support bar Heat exchanger tube support bar

2002-04-23

2004-08-10

Bennett, Henry (Department: 3743)

Heat exchange

Casing or tank enclosed conduit assembly

With support in casing

C165S178000, C165S069000, C165S076000

Reexamination Certificate

active

06772832

ABSTRACT:

FIELD AND BACKGROUND OF INVENTION
The present invention relates generally to the field of heat exchanger tube supports, and in particular to a new and useful tube support bar for restraining and positioning the U-bends of water tubes within a nuclear steam generator.
In a pressurized water nuclear power station, steam generators, which are large heat exchangers, transfer heat, produced via nuclear reactions in the reactor core, from a primary water coolant to a secondary water coolant that drives the steam turbine. The primary coolant is pressurized, which allows the primary water coolant to be heated in the reactor core with little or no boiling. For example, in a light water reactor, the primary coolant is pressurized to about 2250 psia and heated to about 600 deg F. in the reactor core. From the reactor, the primary water coolant flows to a steam generator, where it transfers heat to the secondary coolant. In a U-tube, or recirculating steam generator, the primary coolant enters at the bottom of the steam generator, flows through tubes having an inverted U-shape transferring heat to the secondary coolant, and then exits at the bottom of the steam generator. The secondary coolant is pressurized only to a pressure below that of the primary side, and boils as it flows along the outside of the tubes, thereby producing the steam needed to drive the turbine. Nuclear steam generators must be capable of handling large quantities of two-phase secondary coolant moving at high flow rates, and are therefore very large structures. For example, a nuclear U-tube steam generator can weigh more than 450 tons, with a diameter exceeding 12 feet and an overall length of greater than 70 feet. It may contain as many as 9,000 or more of the long, small diameter, thin-walled U-shaped tubes. For a general description of the characteristics of nuclear steam generators, the reader is referred to Chapters 47 and 52 of
Steam/Its Generation and Use
, 40th Edition, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., ©1992, the text of which is hereby incorporated by reference as though fully set forth herein.
Heat exchangers such as nuclear steam generators require tube restraints or supports, to position the tubes and to restrain the tubes against flow induced vibration forces. In the U-bend region of a nuclear steam generator, a large flow of steam and water mixture passes upwards through the tube array, in a general direction which locally is normal to the axis of the individual U-bend tubes. This large two phase flow is able to cause excitation of the U-bend tubes via the turbulent and other flow forces imparted by the flow. As a result, the tubes tend to vibrate in both the out-of-plane and in-plane directions relative to the U-bend plane. Typically this restraint function is provided by an array of flat U-bend support bars. While such flat bars provide positive restraint in the U-bend out-of-plane direction, they provide restraint only by friction in the in-plane direction.
One known type of nuclear steam generator U-bend support, depicted in
FIG. 1
, and in greater detail in
FIG. 2
, is manufactured by Babcock & Wilcox Canada Ltd.
FIG. 1
shows a nuclear steam generator
100
having a plurality of U-bend tubes
110
. The U-bend tubes
110
are arranged in layers, with each layer having multiple tubes all positioned within the plane of their respective U-bends. Each layer incorporates a set of tubes of successively larger radius which are nested to create the layer of tubes in the particular plane. For purposes of illustration, however, FIG.
1
shows only a limited number of U-bend tubes
110
, and
FIG. 2
shows only the outermost tubes of the center U-bend layers. The straight leg portions of the U-bend tubes
110
are supported at several locations by vertically spaced apart tube support plates
200
, as shown in FIG.
1
.
As shown in
FIG. 1
, and in greater detail in
FIG. 2
, the U-bend tubes
110
are positioned and restrained in the U-bend region by a U-bend support assembly
120
which includes a number of U-bend support bar arrays
180
. Each U-bend support bar array
180
is comprised of flat U-bend support bars
160
, which are positioned in sets between layers of tubes within the U-bend region of the steam generator. The flat U-bend support bars
160
fan out from the center of the U-bend such that individual bar sets are assembled into a U-bend support bar array
180
, or “fan” bar array, in which the lower ends of the individual bars are interconnected. As shown in
FIG. 2
, the inner ends of the flat U-bend support bars
160
of a particular U-bend support bar array
180
, are interconnected by a mechanical or welded joint
210
. Each U-bend support bar array
180
incorporates about 4 to 12 of the flat U-bend support bars
160
. The flat U-bend support bars
160
are positioned so as to provide support to the U-bend tubes
110
at certain points along the arc of each U-bend tube in the array. The angular separation of the flat U-bend support bars
160
depends upon the U-bend size and flow conditions, and the flat U-bend support bars
160
are located to minimize unsupported tube lengths. The individual flat U-bend support bars
160
are typically made of stainless steel, and are about 1″ to 1.5″ wide and about 0.1″ to 0.2″ thick. A U-bend support assembly
120
may incorporate 100 to 200 of the fan-shaped U-bend support bar arrays
180
, with one such array located between each plane of U-bend tubes. The outer ends of the flat U-bend support bars
160
are collected, restrained and supported by arch bar support structures
170
located adjacent the steam generator U-bend. Each arch bar support structure
170
positions the flat U-bend support bars
160
of a U-bend support bar array
180
, carrying the weight of the bars and redistributing the weight of the U-bend support assembly
120
back to the peripheral layer of U-bend tubes.
The U-bend support bar arrays
180
position the planes of U-bend tubes
110
in space, and most importantly, restrain the individual U-bend tubes against flow induced vibration. Restraint against out-of-plane motion is provided by the physical presence of the flat U-bend support bars
160
, which are situated immediately adjacent to the U-bend tubes
110
. The bar-to-tube clearance is purposely quite small, with the bar-to-tube diametral clearance varying from about 0 to 0.010″ or more. The flat U-bend support bars
160
, with their small bar-to-tube clearances, thus prevent significant motion of the tubes in the out-of-plane direction
140
. In the in-plane direction
130
, however, the U-bend tubes
110
are not positively restrained, but instead depend solely upon friction between the U-bend tubes
110
and the flat U-bend support bars
160
to restrict and dampen the flow induced motion of the tubes in their in-plane direction. Depending on the design details and flow conditions, the effect of the friction in providing in-plane restraint may not be fully adequate in providing in-plane restraint.
SUMMARY OF INVENTION
The present invention is drawn to an improved heat exchanger tube support bar which is particularly suited for the U-bend region of a U-tube nuclear steam generator. The bar is configured so that it has scalloped pockets on opposite surfaces of the bar, which can be positioned to engage and restrain the tubes during steam generator operation. Each pocket is arranged so that it provides a support surface for a tube. As the tube is supported on each side by one of these pockets with close clearance, the tube is positively constrained in its out-of-plane direction by virtue of the small tube-to-bar clearance, and positively supported in the in-plane direction by the contour of the pocket as it engages with the U-tube.
The support bar has a thin profile width section whereby the bar may be positioned within an existing tube array in two different orientations, i.e. with either the diagonal or vertical cross sectional axes of the bar positioned parallel to the plane of the tubes. In the diagonal orienta

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