Nuclear Steam Generator Tube Orifice for Primary Temperature Reduction

1.Field of the Invention

The invention relates in general to the maintenance of nuclear steam generator tubes, and more specifically to an orifice insert for placement into nuclear steam generator tubes for primary temperature reduction.

2. Description of the Prior Art

A nuclear steam generator 10 of the type found in the art is shown in FIG. 1 of the attached drawings as comprising an array of a large number of vertically oriented U-shaped tubes 12. The tubes 12 are disposed in a cylindrical portion 14 ot the steam generator 10 whose bottom end is associated with a radiation confining housing or channel head 16, typically having a bottom portion or bowl 18 of a hemispherical configuration. The channel head 16 is divided by a divider plate 20 into a first half 22 typically known as the hot leg plenum, and a second half 24 typically known as the cold leg plenum. An inlet pipe (not shown) supplies hot water to the hot leg plenum 22, while a return pipe 26 is coupled to the cold leg plenum 24. The hot water entering the hot leg plenum 22 passes into the exposed openings of the plurality of U-shaped tubes 12, then through tubes 12 to be introduced into the cold leg plenum 24. A circular tube sheet 28 is disposed at the bottom of cylindrical portion 14. Tube sheet 28 is divided into a first semi-circularly shaped half 30 and a second semi-circularly-shaped half 32 by divider plate 20. As shown in FIG. 1, the water entry openings of the tubes 12 are supported within openings in first tube sheet half 30, while the water exit openings of the tubes 12 are supported within openings in second tube sheet half 32.

Corrosion of the U-shaped tubes 12 is a concern in certain steam generators. Corrosion is usually the result of chemical attack and can occur on the outside surface, which is exposed to boiler steam and on the inside surface, which is exposed to water. Water chemistry treatment has been used in the past to protect the U-shaped tubes 12, as well as other system components, from corrosion. However, chemical treatment will not be successful unless all factors contributing to corrosion are detected and appreciated. Once corrosion has occurred, maintenance of the tubes 12 is typically effected by removing from service a defective tube by "plugging" each end. "Plugging" is carried out by entering a first portion of the channel head 16 to seal first one end of a defective tube 12 and then entering the second portion of the channel head 12 to seal the other end of the tube 12. This procedure is complicated and time consuming.

Briefly, the present invention is a new orifice insert for insertion into the openings of the water tubes which permits achieving a target temperature required to minimize corrosion or cracking while maintaining the maximum possible heat load for the steam tube. The orifice insert comprises a sleeve having a top end, a bottom end located in use in the vicinity of the opening of the steam generator tube, and a longitudinal axis. An orifice is disposed in and extends the entire length of the sleeve. At least a portion of the outer diameter of the sleeve is substantially the same as the inner diameter of the steam generator tube.

In one aspect of the invention, the orifice comprises a lower and an upper cylindrical portion which are coaxial with the longitudinal axis of the sleeve, wherein the diameter of the upper cylindrical portion is smaller than the diameter of the lower cylindrical portion.

In another aspect of the invention, the orifice comprises a lower cylindrical portion, first, second, third, and fourth intermediate cylindrical portions, and an upper cylindrical portion, wherein the diameter of the lower cylindrical portion is greater than the diameter of the upper cylindrical portion, the diameters of the second and fourth intermediate cylindrical portions are equal to the diameter of the lower cylindrical portion, and the diameters of the first and third intermediate cylindrical portions are equal to the diameter of the upper cylindrical portion. The lower cylindrical portion and the second and fourth intermediate cylindrical portions are coaxial with the longitudinal axis of the sleeve. The upper cylindrical portion and the first and third intermediate cylindrical portions can also be coaxial with the longitudinal axis of the sleeve, or they can be staggered, with the axes of the first and third intermediate cylindrical portions being disposed at diametrically opposed locations with respect to the lower cylindrical portion, and the upper cylindrical portion being coaxial with the first intermediate cylindrical portion.

The invention may be better understood, and further advantages and uses thereof are readily apparent, when considered in view of the following detailed description of explanatory embodiments, taken with the accommpanying drawings, in which:

FIG. 1 is a cut-away perspective view of a nuclear steam generator;

FIG. 2 is a perspective, partially blown-apart view showing the position of orifice inserts according to the invention with respect to the steam entry openings of the steam generator tubes of the nuclear steam generator of FIG. 1;

FIG. 3a is a partially cut-away perspective view of a first embodiment of an orifice insert according to the invention;

FIG. 3b is a cross-sectional view of the orifice insert shown in FIG. 3a, taken along line 3b--3b, installed in a steam generator tube;

FIG. 4a is a partially cut-away, perspective view of a second embodiment of an orifice insert according to the invention;

FIG. 4b is a cross-sectional view of the orifice insert shown in FIG. 4a, taken along line 4b--4b, installed in a steam generator tube;

FIG. 5a is a blown-apart perspective view of a third embodiment of an orifice insert according to the invention;

FIG. 5b is an assembled perspective view of the orifice insert shown in FIG. 5a;

FIG. 5c is a cross-sectional view of the orifice insert shown in FIG. 5b, taken along line 5c--5c;

FIG. 6a is a perspective view of a fourth embodiment of an orifice insert according to the invention;

FIG. 6b is a cross-sectional view of the orifice insert shown in FIG. 6a, taken along line 6b--6b, installed in a steam generator tube;

FIG. 7a is a perspective view of a fifth embodiment of an orifice insert according to the invention;

FIG. 7b is a cross-sectional view of the orifice insert shown in FIG. 7a, taken along line 7b--7b, installed in a steam generator tube;

FIG. 8a is a perspective view of a sixth embodiment of an orifice insert according to the invention; and

FIG. 8b is a cross-sectional view of the orifice insert shown in FIG. 8a, taken along line 8b--8b, installed in a steam generator tube.

Referring now to FIG. 2, there are illustrated a number of orifice inserts 34 according to the invention which are adapted for affixation in tube 12 by welding or similar means. Orifice inserts 34 are inserted into the steam entry openings 36 of steam generator tubes 12. In general, each orifice insert 34 comprises a cylindrical sleeve 38 whose outer diameter is substantially the same as the inner diameter of steam generator tubes 12, to permit a tight fit. Orifice inserts 34 can be permanently affixed within tubes 12 by any conventional means, such as by welding, or as will be described in greater detail hereinafter, by mechanical expansion. They can also be temporarily affixed within tubes 12 by any conventional means.

It should be understood that all directional designations such as top, bottom, upper, lower, and longitudinal are made with reference to the orientation of an orifice insert which has been inserted in a steam generator tube.

Each cylindrical sleeve has a bottom end 38a, a top end 38b, and a longitudinal axis A. Top end 38b is inserted into a tube 12 so that bottom end 38a is located in the vicinity of the opening of the tube 12. An orifice 40, whose shape can vary, extends the entire length of orifice insert 34. Orifice inserts 34 thus effectively reduce the inner diameter of steam generator tubes 12, thereby reducing the primary flow and the U-bend primary temperatures within steam generator tubes 12.

When an orifice insert is placed in a steam generator tube 12, the flow through the tube 12 is reduced due to the greater hydraulic resistance. If orifice inserts are placed in a small number of steam generator tubes (for example , Row 1), the pressure drop from the tube inlets to the tube outlets does not significantly change. The velocity V of the flow through the tubes having orifice inserts can be calculated as: ##EQU1## wherein g=the gravitational constant,

P=the pressure drop,

.rho.=the flow density,

K=the hydraulic resistance without an orifice insert,

K.sub.o =the hydraulic resistance with an orifice insert,

f=the friction factor,

L=the total tube length, and

d=the tube inner diameter, and

where K can be calculated from available data such as that found in I.E. Idel'chick, "Handbook of Hydraulic Resistance," AECTR 6630 (1060) or Crane, "Flow at Fluids through Valves, Fittings and Pipe," Crane Company Technical Paper No. 410 (1969).

The mass flow m in the tubes having orifice. inserts is:

where

Using this mass flow rate, the temperature distribution along the length of the tubes is: ##EQU2## where T.sub.x =the temperature at a given distance from the tube outlet,

T.sub.s =the saturation temperture of the secondary side,

T.sub.h =the temperature at the tube inlet,

U=the overall heat transfer coefficient for a tube

D=the tube outside diameter

X=the distance from the tube inlet

m=the mass flow rate inside the tube, and

C.sub.p =the specific heat.

The temperature at the U-bend can be determined by substituting the following into equation (3):

The mass flow in a tube without an orifice insert can be determined using equations (1) and (2) and setting K.sub.o =0. The reduction in temperature is calculated by subtracting the values calculated from equation (3) using flow with and without an orifice insert.

Referring now to FIGS. 3a and 3b, there is shown a first embodiment of the invention, in which orifice insert 34 includes a simple orifice 42. Orifice 42 comprises lower and upper cylindrical portions 44 and 46, which are coaxial with longitudinal axis A of sleeve 38. Lower cylindrical portion 44 includes a bottom end 44a opening into bottom end 38a of sleeve 38 and a top end 44b. Upper cylindrical portion 46 includes a bottom end 46a, which opens into top end 44b of lower cylindrical portion 44, and a top end 46b which opens into top end 38b of sleeve 38. The diameter of upper cylindrical portion 46 is smaller than the diameter of lower cylindrical portion 44.

Referring now to FIGS. 4a and 4b, there is shown a second embodiment of the invention, in which an orifice insert 34' includes a multiple orifice 48. Multiple orifice 48 comprises a lower cylindrical portion 50, a first intermediate cylindrical portion 52, a second intermediate cylindrical portion 54, a third intermediate cylindrical portion 56, a fourth intermediate cylindrical portion 58, and an upper cylindrical portion 60. Lower cylindrical portion 50, intermediate cylindrical portions 52, 54, 56, and 58, and upper cylindrical portion 60 each have respective bottom ends 50a, 52a, 54a, 56a, 58a, and 60a and respective top ends 50b, 52b, 54b, 56b, 58b, and 60b. Bottom end 50a of lower cylindrical portion 50 opens into bottom end 38a of sleeve 38; bottom end 52a of first intermediate cylindrical portion 52 opens into top end 50b of lower cylindrical portion 50; bottom end 54a of second intermediate cylindrical portion 54 opens into top end 52b of first intermediate cylindrical portion 52; bottom end 56a of third intermediate cylindrical portion 56 opens into top end 54b of second intermediate cylindrical portion 54; bottom end 58a of fourth intermediate cylindrical portion 58 opens into top end 56b of third intermediate cylindrical portion 56; bottom end 60a of upper cylindrical portion 60 opens into top end 58b of fourth intermediate cylindrical portion 58; and top end 60b of upper cylindrical portion 60 opens into top end 38b of sleeve 38. The diameter of upper cylindrical portion 60 is smaller than the diameter of lower cylindrical portion 50, and the diameters of first and third intermediate cylindrical portions 52 and 56 are equal to the diameter of upper cylindrical portion 60, while the diameters of second and fourth intermediate cylindrical portions 54 and 58 are equal to the diameter of lower cylindrical portion 50. Lower cylindrical portion 50, intermediate cylindrical portions 52, 54, 56, and 58, and upper cylindrical portion 60 are all coaxial with longitudinal axis A of sleeve 38.

Referring now to FIGS. 5a-5c there is shown a third embodiment of the invention in which orifice insert 34" includes a staggered multiple orifice 62. Staggered multiple orifice 62 comprises a lower cylindrical portion 64, a first intermediate cylindrical portion 66, a second intermediate cylindrical portion 68, a third intermediate cylindrical portion 70, a fourth intermediate cylindrical portion 72, and an upper cylindrical portion 74. Lower cylindrical portion 64, intermediate cylindrical portions 66, 68, 70, and 72, and upper cylindrical portion 74 each have respective bottom ends 64a, 66a, 68a, 70a, 72a, and 74a, and respective top ends 64b, 66b, 68b, 70b, 72b, and 74b. Bottom end 64a of lower cylindrical portion 64 opens into bottom end 38a of sleeve 38; bottom 66a of first intermediate cylindrical portion 66 opens into top end 64b of lower cylindrical portion 64; bottom end 68a of second intermediate cylindrical portion 68 opens into top end 66b of first intermediate cylindrical portion 66; bottom end 70a of third intermediate cylindrical portion 70 opens into top end 68b of second intermediate cylindrical portion 68; bottom end 72a of fourth intermediate cylindrical portion 72 opens into top end 70b of third intermediate cylindrical portion 70; bottom end 74a of upper cylindrical portion 74 opens into top end 72b of fourth intermediate cylindrical portion 72; and top end 74b of upper cylindrical portion 74 opens into top end 38b of sleeve 38. The diameter of upper cylindrical portion 74 is smaller than the diameter of lower cylindrical portion 64, and the diameters of first and third intermediate cylindrical portions 66 and 70 are equal to the diameter of upper cylindrical portion 74, while the diameters of second and fourth intermediate cylindrical portions 68 and 72 are equal to the diameter of lower cylindrical portion 64. Lower cylindrical portion 64, and second and fourth intermediate cylindrical portions 68 and 72 are coaxial with longitudinal axis A of sleeve 38. The axes of first and third intermediate cylindrical portions 66 and 70 are disposed at diametrically opposed locations with respect to lower cylindrical portion 64, and upper cylindrical portion 74 is coaxial with first intermediate cylindrical portion 66.

Referring now to FIGS. 6a and 6b there is illustrated an orifice insert 62 which has been modified for affixation in tubes 12 by mechanical expansion. Orifice insert 62 comprises a sleeve 64. A simple orifice 66 extends the entire length of sleeve 64.

Sleeve 64 has a bottom end 64a and a top end 64b,and comprises an upper cylindrical portion 68 having a bevelled top 70, an intermediate cylindrical portion 72, and a lower cylindrical portion 74 having a bevelled top 76. Top end 64b is inserted into a tube 12 so that bottom end 64a is substantially flush with the opening of the tube 12. The outer diameter of intermediate cylindrical portion 72 is substantially the same as the inner diameter of steam generator tubes 12, the outer diameter of lower cylindrical portion 74 is smaller than the outer diameter of intermediate cylindrical portion 72, and the outer diameter of upper cylindrical portion 68 is smaller than the outer diameter of lower cylindrical portion 74.

Simple orifice 66 comprises lower and upper cylindrical portions 78 and 80, similar to lower and upper cylindrical portions 44 and 46 of simple orifice 42 shown in FIGS. 3a and 3b, except that uper cylindrical portion 80 of orifice 66 is shorter than upper cylindrical portion 46 of orifice 42. Simple orifice 66 further comprises a necked-in portion 82 interposed between the bottom of lower cylindrical portion 78 and bottom end 64a of sleeve 64. The diameter of necked-in portion 82 at all points along its length is smaller that the diameter of lower cylindrical portion 78.

Necked-in portion 82 of simple orifice 66 extends lengthwise above bevelled top 76 of lower cylindrical portion 74 of sleeve 64. Lower cylindrical portion 74, including bevelled top 76, and necked-in portion 82 are adapted for outward expansion upon application of an expansion tool (not shown) to necked-in portion 82, whereby lower cylindrical portion 74 is held in mating engagement with the inside of tube 12 along a portion of its length.

Referring now to FIGS. 7a and 7b there is illustrated another orifice insert 84 which has been modified for affixation in tubes 12 by mechancial expansion. Orifice insert 84 comprises a sleeve 86. A multiple orifice 88 extends the entire length of sleeve 86.

Sleeve 86 has a bottom end 86a and a top end 86b, and comprises an upper cylindrical portion 90 having a bevelled top 92, an intermediate cylindrical portion 94, and a lower cylindrical portion 96 having a bevelled top 98. Sleeve 86 is thus similar in configuration to sleeve 64 shown in FIGS. 6a and 6b, except that upper cylindrical portion 90 of sleeve 86 is longer than upper cylindrical portion 68 of sleeve 64, in order to accommodate multiple orifice 88.

Top end 86b of sleeve 86 is inserted into a tube 12 so that bottom end 86a is located in the vicinity of the opening of the tube 12. The outer diameter of intermediate cylindrical portion 94 is substantially the same as the inner diameter of steam generator tubes 12, the outer diameter of lower cylindrical portion 96 is smaller than the outer diameter of intermediate cylindrical portion 94, and the outer diameter of upper cylindrical portion 90 is smaller than the outer diameter of lower cylindrical portion 96.

Multiple orifice 88 comprises a lower cylindrical portion 100, a first intermediate cylindrical portion 102, a second intermediate cylindrical portion 104, a third intermediate cylindrical portion 106, a fourth intermediate cylindrical portion 108, and an upper cylindrical portion 110, similar respectively to lower cylindrical portion 50, first intermediate cylindrical portion 52, second intermediate cylindrical portion 54, third intermediate cylindrical portion 56, fourth intermediate cylindrical portion 58, and upper cylindrical portion 60 of orifice insert 34 shown in FIGS. 4a and 4b. Multiple orifice 88 further comprises a necked-in portion 112 interposed between the bottom of lower cylindrical portion 100 and bottom end 86a of sleeve 86. The diameter of necked-in portion 112 at all points along its length is smaller than the diameter of lower cylindrical portion 100.

Necked-in portion 112 of muliple orifice 88 extends lengthwise above belleved top 98 of lower clylindrical portion 96 of sleeve 86, in the same manner so bevelled top 76 of lower cylindrical portion 74 of sleeve 64 shown in FIGS. 6a and 6b. Lower cylindrical portion 96, including bevelled top 98, and necked-in portion 112, are also adapted for outward expansion upon application of an expansion tool (not shown) to necked-in portion 112.

Referring now to FIGS. 8a and 8b there is illustrated a third orifice insert 114 which has been modified for affixation in tubes 12 by mechanical expansion. Orifice insert 114 comprises a sleeve 116. A staggered multiple orifice 118 extends the entire length of sleeve 116.

Sleeve 116 has a bottom end 116a and a top end 116b, and comprises a upper cylindrical portion 120, an intermediate cylindrical portion 122, and a lower cylindrical portion 124 having a bevelled top 126. Sleeve 116 is similar in configuration and dimensions to sleeve 86 shown in FIGS. 7a and 7b, except that upper cylindrical portion 120 does not have a bevelled top.

Top end 116b of sleeve 116 is inserted into a tube 12 so that bottom end 116a is substantially flush with the opening of the tube 12. The outer diameter of intermediate cylindrical portion 122 is substantially the same as the inner diameter of steam generator tubes 12, the outer diameter of lower cylindrical portion 124 is smaller than the outer diameter of intermediate cylindrical portion 122, and the outer diameter of upper cylindrical portion 120 is smaller than the outer diameter of lower cylindrical portion 124.

Staggered multiple orifice 118 comprises a lower cylindrical portion 128, a first intermediate cylindrical portion 130, a second intermediate cylindrical portion 132, a third intermediate cylindrical portion 134, a fourth intermediate cylindrical portion 136, and an upper cylindrical portion 138, similar respectively to lower cylindrical portion 64, first intermediate cylindrical portion 66, second intermediate cylindrical portion 68, third intermediate cylindrical portion 70, fourth intermediate cylindrical portion 72, and upper cylindrical portion 74 of orifice insert 34 shown in FIGS. 5a-5c. Staggered multiple orifice 118 further comprises a necked-in portion 140 interposed between the bottom of lower cylindrical portion 128 and bottom end 116a of sleeve 116. The diameter of necked-in portion 140 at all points along its length is smaller than the diameter of lower cylindrical portion 128.

Necked-in portion 140 of staggered multiple orifice 118 extends lengthwise above bevelled top 126 of lower cylindrical portion 124 of sleeve 116, in the same manner as bevelled top 76 of lower cylindrical portion 74 of sleeve 64 shown in FIGS. 6a and 6b. Lower cylindrical portion 124, including bevelled top 126, and necked-in portion 140 are also adapted for outward expansion upon application of an expansion tool (not shown) to necked-in portion 140.

Preferably, orifice inserts 34, 34', 64, and 84, which have simple or multiple orifices are bored from a single piece of material using a variable bet. Orifice inserts 34" and 114, which have staggered multiple orifices, cannot be bored from a single piece of material. Each portion of the staggered multiple orifice is bored separately in its own plate, as shown in FIG. 5a, and the individual plates are then assembled and welded together, as shown in FIG. 5b.

Thus, it will be seen that all embodiments of the present invention provide a unique method of controlling corrosion in steam generator tubes without the need for plugging. Moveover, the present invention is easy to install, so as to render use of all embodiments convenient to users. While preferred embodiments of the invention have been disclosed, it should be understood that the spirit and scope of the invention are to be limited solely by the appended claims, since numerous modifications of the disclosed embodiments will undoubtedly occur to those of skill in the art.