TECHNICAL FIELD
[0001] This invention generally relates to devices for cleaning heat exchanger vessels,
and is specifically concerned with an improved pressure pulse cleaning apparatus for
loosening and removing sludge and debris from the secondary side of a nuclear steam
generator.
BACKGROUND OF THE INVENTION
[0002] Pressure pulse cleaning devices for cleaning the interior of the secondary side of
a nuclear steam generator are known in the prior art, and have been disclosed in U.S.
Patent Nos. 4,655,846 and 4,699,665. Such devices generally comprise a gas-operated
pressure pulse generator having an outlet that is mountable in communication with
the interior of the secondary side of the steam generator. The purpose of these devices
is to loosen and remove sludge and debris which accumulates on the tubesheet, heat
exchanger tubes and support plates within the secondary side. In operation, the secondary
side of the generator is first filled with water. Next the outlet of the gas-operated
pressure pulse generator is placed into communication with the water, such as by a
nozzle which may be formed from either a straight section of conduit oriented horizontally
over the tubesheet of the generator, or a pipe having a 90 degree bend which is oriented
vertically with respect to the tubesheet. Finally, pulses of gas pressurized to between
50 and 5000 pounds per square inch are generated out of the nozzle of the pressure
pulse generator. The succession of pressure pulses create shock waves in the water
surrounding the tubesheet, the heat exchanger tubes and support plates within the
secondary side of the generator. These shock waves effectively loosen and remove sludge
deposits and other debris that accumulates within the secondary side over protracted
periods of time.
[0003] While the cleaning devices disclosed in these patents represent a major advance in
the state of the art, the applicants have found that there are limitations associated
with these devices which limit their usefulness in cleaning nuclear steam generators.
However, before these limitations may be fully appreciated, some general background
as to the structure, operation and maintenance of nuclear steam generators is necessary.
[0004] In the secondary side of such steam generators, the legs of the U-shaped heat exchanger
tubes extend through bores in a plurality of horizontally-oriented support plates
vertically spaced from one another, while the ends of these tubes are mounted within
bores located in the tubesheet. The relatively small, annular spaces between these
heat exchanger tubes and the bores in the support plates and the bores in the tubesheet
are known in the art as "crevice regions." Such crevice regions provide only a very
limited flow path for the feed water that circulates throughout the secondary side
of the steam generator. The consequent reduced flow of water through these regions
results in a phenomenon known as "dry boiling" wherein the feed water is apt to boil
so rapidly in the crevice regions between the heat exchanger tubes and the bores in
the support plate and tubesheet that these areas can actually dry out for brief periods
of time before they are again immersed by the surrounding feed water. This chronic
drying-out of the crevice regions due to dry boiling causes impurities dissolved in
the water to precipitate out in these regions. The precipitates ultimately create
sludge and other debris which can obstruct the flow of feed water in the secondary
side of the generator to an extent to where the steam output of the generator is seriously
compromised. Moreover, the presence of such sludges is known to promote stress corrosion
cracking in the heat exchanger tubes which, if not arrested, will ultimately allow
water from the primary side of the generator to radioactively contaminate the water
in the secondary side of the generator.
[0005] To remove this sludge, many other types of cleaning devices were used prior to the
advent of pressure pulse cleaning devices. Examples of such prior art cleaning devices
include ultrasonic wave generators for vibrating the water in the steam generator
to loosen such debris, and sludge lances that employ a high-powered jet of pressurized
water to flush such debris out. However, such devices were only partially successful
in achieving their goal due to the hardness of the magnetite deposits which form a
major component of such sludges, and the very limited accessibility of the crevice
regions of the steam generator.
[0006] Since its inception, pressure pulse cleaning has been a very promising way in which
to remove such stubborn deposits of sludges in such small spaces, since the shock
waves generated by the gas operated pressure pulse operators are capable of applying
a considerable loosening force to such sludges. However, the applicants have found
that the devices disclosed in both U.S. Patents 4,655,846 and 4,699,665 have fallen
short of fulfilling their promise in several material respects. For example, research
conducted by the applicants indicates that the orientation of the nozzle used to introduce
the pulses of gas into the secondary side significantly affects the peak stresses
applied to the tubes closest the nozzle, and that prior art nozzle geometries fell
far short of minimizing these stresses. Still another shortcoming observed by the
applicants was the lack of any means to remove dissolved ionic species from the water
during such prior art cleaning processes. Such ionic species, if not removed, are
capable of precipitating out in the form of new sludges after the termination of the
pressure pulse cleaning process if no provision is made to remove them. Additionally,
applicants observed that if no provision is made to remove fine particulate matter
from the water during the pressure pulse cleaning method, these fine particles of
sludge are capable of settling onto the tubesheet and densely depositing themselves
into the crevice regions between the tubesheet and the legs of the heat exchanger
tubes, thereby defeating one of the purposes of the cleaning method. Finally, the
applicants have observed that the relatively rapid pulse frequency taught in the prior
art does not give the nozzle and manifold of the pulse generator sufficient time to
fill back with water, and thus leaves pockets of shock-absorbing gas in the pulse
generator which limits the efficacy of later generated pulses in generating sludge-loosening
shock waves. Clearly, what is needed is an improved pressure pulse cleaning apparatus
which overcomes the limitations associated with prior art pressure pulse cleaning
devices and which is imminently practical for use in the secondary side of a nuclear
steam generator.
DISCLOSURE OF THE INVENTION
[0007] Generally speaking, the invention further is an apparatus for loosening and removing
sludge and other impurities from the interior of a heat exchanger vessel of the type
having one or more access openings that overcomes the limitations associated with
the prior art. The apparatus comprises a pulse distributing conduit having a first
end that is detachably mountable onto one of the access openings, which may be a sludge
lance port in the case of a nuclear steam generator, and a second end that extends
into the interior of the heat exchanger vessel and is canted at an angle with respect
to the horizontal surface of the tubesheet. The apparatus further comprises a pulse
generator having an outlet connected to the pulse distributing conduit for generating
a succession of pulses that are conducted out of the canted end of the conduit to
create shock waves in the surrounding water that impinge upon and loosen the sludge,
and a controller for controlling the power level of the pulses generated. A pulse
flattener is provided in the pulse generator for lowering the maximum amplitude of
the shock wave generated. In operation, the canted end of the pulse distributing conduit
is spaced as far as possible from the nearest heat exchanger tube so that the power
level controller of the pulse generator can be adjusted to create shock waves in the
water at the highest possible power level without generating pressures that would
jeopardize the integrity of the heat exchanger tubes. When the apparatus is used to
loosen and remove the sludge and other impurities from the secondary side of a steam
generator having a plurality of U-shaped heat exchanger tubes, the canted end of the
pulse distributing conduit is aligned within the centrally disposed main tube lane.
The canted tip of the pulse distributing conduit and its orientation down the main
tube lane in combination with the pulse flattener all allow the pulse generator to
be operated at a maximum power level while exerting a minimum peak stress on the heat
exchanger tube closest to the conduit outlet. Moreover, the power level controller
allows the operator to easily adjust the power of the pulses to compensate for power
losses that result when the water level in the generator is raised.
[0008] The apparatus may further include a recirculation system having a pump for inducing
a flow in the water and a particulate filter for removing loosened particles of sludge
and debris from the water in the vessel. The recirculation system may further include
a demineralizer bed for removing ionic species from the liquid in the steam generator.
[0009] In the preferred embodiment, the apparatus also includes an array of inlet and suction
tubes for inducing a circumferential flow in the heat exchanger vessel to help keep
loosened particles of sludge in suspension so that they may be filtered out of the
water as it flows through the recirculation system. Such an array of inlet and suction
tubes includes an inlet conduit, a suction conduit and a suction-inlet conduit in
combination with a valve arrangement which allows the last named conduit either to
withdraw or to introduce water into the vessel. In operation, the suction conduit
and the suction-inlet conduit are circumferentially disposed around the interior wall
of the steam generator in order to induce a circumferential flow of water within
the vessel during recirculation.
[0010] Finally, the apparatus includes a portable conduit coupling station so that the recirculation
system may be easily connected to a second heat exchanger vessel when the operator
wishes to drain the water used to clean a first heat exchanger into the interior of
a second heat exchanger.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011]
Figure 1 is a perspective view of a Westinghouse-type nuclear steam generator with
portions of the exterior walls removed so that the interiors of both the primary and
secondary sides may be seen;
Figure 2 is a partial cross-sectional side view of the steam generator illustrated
in Figure 1 along the line 2-2:
Figure 3A is a cross-sectional plan view of the steam generator illustrated in Figure
2 along the line 3A-3A;
Figure 3B is an enlarged view of the area circled in Figure 3A;
Figure 3C is a cross-sectional side view of the portion of the support plate and heat
exchanger tubing illustrated in Figure 3B along the line 3C-3C;
Figure 4A is a plan view of a portion of a different type of support plate and tubing
wherein trifoil broaching is used in lieu of circular bores;
Figure 4B is a perspective view of the portion of the support plate and tubing illustrated
in Figure 4A;
Figure 5 is a cross-sectional side view of the steam generator illustrated in Figure
1 along the line 5-5 with the apparatus of the invention installed therein;
Figure 6A is an enlarged view of the circled portion of Figure 5 along with a schematized
representation of the pressurized gas source used to power the pressure pulse generator
of the apparatus;
Figure 6B is a cross-sectional side view of the air gun used in the pressure pulse
generator of the invention;
Figure 7 is a plan view of the steam generator illustrated in Figure 5 along the line
7-7;
Figure 8 is a schematic view of the recirculation system used to implement the method
of the invention;
Figure 9 is a graph illustrating the diminishment of the pressure of the gas within
the pressure pulse generator after the pulse generator is fired, and
Figure 10 is a graph illustrating the relationship between the maximum stress experienced
by the heat exchanger tubes in the steam generator, and the location of these tubes
with respect to the tubesheet.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
General Overview of the Apparatus of the Invention
[0012] With reference now to Figures 1 and 2, wherein like numerals designate like components
throughout all of the several figures, the apparatus and method of the invention are
both particularly adapted for removing sludge which accumulates within a nuclear steam
generator 1. But before the application of the invention can be fully appreciated,
some understanding of the general structure and maintenance problems associated with
such steam generators 1 is necessary.
[0013] Nuclear steam generators 1 generally include a primary side 3 and a secondary side
5 which are hydraulically isolated from one another by a tubesheet 7. The primary
side 3 is bowl-shaped, and is divided into two, hydraulically isolated halves by means
of a divider plate 8. One of the halves of the primary side 3 includes a water inlet
9 for receiving hot, radioactive water that has been circulated through the core barrel
of a nuclear reactor (not shown), while the other half includes a water outlet 13
for discharging this water back to the core barrel. This hot, radioactive water circulates
through the U-shaped heat exchanger tubes 22 contained within the secondary side 5
of the steam generator 1 from the inlet half of the primary side 3 to the outlet half
(see flow arrows). In the art, the water-receiving half of the primary side 3 is called
the inlet channel head 15, while the water-discharging half is called the outlet channel
head 17.
[0014] The secondary side 5 of the steam generator 1 includes an elongated tube bundle 20
formed from approximately 3500 U-shaped heat exchanger tubes 22. Each of the heat
exchanger tubes 22 includes a hot leg, a U-bend 26 at its top, and a cold leg 28.
The bottom end of the hot and cold legs 24, 28 of each heat exchanger tube 22 is securely
mounted within bores in the tubesheet 7, and each of these legs terminates in an open
end. The open ends of all the hot legs 24 communicate with the inlet channel head
15, while the open ends of all of the cold legs 28 communicate with the outlet channel
head 17. As will be better understood presently, heat from the water in the primary
side 3 circulating within the U-shaped heat exchanger tubes 22 is transferred to nonradioactive
feed water in the secondary side 5 of the generator 1 in order to generate nonradioactive
steam.
[0015] With reference now to Figures 2, 3A, 3B and 3C, support plates 30 are provided to
securely mount and uniformly space the heat exchanger tubes 22 within the secondary
side 5. Each of the support plates 30 includes a plurality of bores 32 which are only
slightly larger than the outer diameter of the heat exchanger tubes 22 extending therethrough.
To facilitate a vertically-oriented circulation of the nonradioactive water within
the secondary side 5, a plurality of circulation ports 34 is also provided in each
of the support plates 30. Small annular spaces or crevices 37 exist between the outer
surface of the heat exchanger tubes 22, and the inner surface of the bores 32. Although
not specifically shown in any of the several figures, similar annular crevices 37
exist between the lower ends of both the hot and cold legs 24 and 28 of each of the
heat exchanger tubes 22, and the bores of the tubesheet 7 in which they are mounted.
In some types of nuclear steam generators, the openings in the support plates 30 are
not circular, but instead are trifoil or quatrefoil-shaped as is illustrated in Figures
4A and 4B. In such support plates 30, the heat exchanger tubes 22 are supported along
either three or four equidistally spaced points around their circumferences. Because
such broached openings 38 leave relatively large gaps 40 at some points between the
heat exchanger tubes 22 and the support plate 30, there is no need for separate circulation
ports 34.
[0016] With reference back to Figures 1 and 2, the top portion of the secondary side 5 of
the steam generator 1 includes a steam drying assembly 44 for extracting the water
out of the wet steam produced when the heat exchanger tubes 22 boil the nonradioactive
water within the secondary side 5. The steam drying assembly 44 includes a primary
separator bank 46 formed from a battery of swirl vane separators, as well as a secondary
separator bank 48 that includes a configuration of vanes that define a tortuous path
for moisture-laden steam to pass through. A steam outlet 49 is provided over the steam
drying assembly 44 for conducting dried steam to the blades of a turbine (not shown)
coupled to an electrical generator (not shown). In the middle of the lower portions
of the secondary side 5, a tube wrapper 52 is provided between the tube bundle 22
and the outer shell of the steam generator 1 in order to provide a down comer path
for water extracted from the wet steam that rises through the steam drying assembly
44.
[0017] At the lower portion of the secondary side 5, a pair of opposing sludge lance ports
53a, 53b are provided in some models of steam generators to provide access for high
pressure hoses that wash away much of the sludge which accumulates over the top of
the tubesheets 7 during the operation of the generator 1. These opposing sludge lance
ports 53a, 53b are typically centrally aligned between the hot and cold legs 24 and
28 of each of the heat exchanger tubes 22. It should be noted that in some steam generators,
the sludge lance ports are not oppositely disposed 180 degrees from one another,
but are only 90 degrees apart. Moreover, in other steam generators, only one such
sludge lance port is provided. In the steam generator arts, the elongated areas between
rows of tubes 22 on the tubesheet 7 are known as tube lanes 54, while the relatively
wider, elongated area between the hot and cold legs of the most centrally-disposed
heat exchanger tubes 22 is known as the central tube lane 55. These tube lanes 54
are typically an inch or two wide in steam generators whose tubes 22 are arranged
in a square pitch, such as that shown in Figures 3A, 3B, and 3C. Narrower tube lanes
54 are present in steam generators whose heat exchanger tubes 22 are arranged in a
denser, triangular pitch such as shown in Figures 4A and 4B.
[0018] During the operation of such steam generators 1, it has been observed that the inability
of secondary-side water to circulate as freely in the narrow crevices 37 or gaps 40
between the heat exchanger tubes 22, and the support plates 30 and tubesheets 7 can
cause the non-radioactive water in these regions to boil completely out of these
small spaces, a phenomenon which is known as "dry boiling." When such dry boiling
occurs, any impurities in the secondary side water are deposited in these narrow crevices
37 or gaps 40. Such solid deposits tend to impede the already limited circulation
of secondary side water through these crevices 37 and gaps 40 even more, thereby promoting
even more dry boiling. This generates even more deposits in these regions and is one
of the primary mechanisms for the generation of sludge which accumulates over the
top of the tubesheet 7. Often the deposits created by such dry boiling are formed
from relatively hard compounds of limited solubility, such as magnetite, which tends
to stubbornly lock itself in such small crevices 37 and gaps 40. These deposits have
been known to wedge themselves so tightly in the crevices 37 or gaps 40 between the
heat exchanger tubes 22 and the bores 32 of the support plates 30 that the tube 22
can actually become dented at this region.
[0019] The instant invention is both an apparatus and a method for dislodging and loosening
such deposits, sludge and debris and removing them from the secondary side 5 of a
steam generator 1.
Apparatus of the Invention
[0020] With reference now to Figures 5, 6A, 6B, 7 and 8, the apparatus of the invention
generally comprises a pair of pressure pulse generator assemblies 60a, 60b mounted
in the two sludge lance ports 53a, 53b, in combination with a recirculation system
114. Because both of these generator assemblies 60a, 60b are identical in all respects,
the following description will be confined to generator assembly 60b in order to avoid
unnecessary prolixity.
[0021] With specific reference to. Figures 6A and 6B, pulse generator assembly 60b includes
an air gun 62 for instantaneously releasing a volume of pressurized gas, and a single
port manifold 92 for directing this pressurized gas into a generally tubular nozzle
111 which is aligned along the central tube lane 55 of the steam generator 1. The
air gun 62 includes a firing cylinder 64 that contains a pulse flattener 65 which
together are dimensioned to store about 1442 cubic centimeters of pressurized gas.
Air gun 62 further includes a trigger cylinder 66 which stores approximately 164 cubic
centimeters of pressurized gas, and a plunger assembly 68 having an upper piston 70
and a lower piston 72 interconnected by means of a common connecting rod 74. The upper
piston 70 can selectively open and close the firing cylinder 64, and the lower piston
72 is reciprocally movable within the trigger cylinder 66 as is indicated in phantom.
The area of the lower piston 72 that is acted on by pressurized gas in trigger cylinder
66 is greater than the area of the upper piston 70 acted on by pressurized gas in
the cylinder 64. The connecting rod 74 of the plunger 68 includes a centrally disposed
bore 76 for conducting pressurized gas admitted into the trigger cylinder 66 into
the firing cylinder 64. The pulse flattener 65 also includes a gas conducting bore
77 that is about 12.70 millimeters in diameter. Pressurized gas is admitted into the
trigger cylinder 66 by means of a coupling 78 of a gas line 80 that is connected to
a pressurized tank of nitrogen 84 by way of a commercially available pressure regulator
82. Gas conducting bores 86a and 86b are further provided in the walls of the trigger
cylinder 66 between a solenoid operated valve 88 and the interior of the cylinder
66. The actuation of the solenoid operated valve 88 is controlled by means of an electronic
firing circuit 90.
[0022] In operation, pressurized gas of anywhere between approximately 1 and 11 megapascals
is admitted into the trigger cylinder 66 by way of gas line 80. The pressure that
this gas applies to the face of the lower piston 72 of the plunger 68 causes the plunger
68 to assume the position illustrated in Figure 6B, wherein the upper piston 70 sealingly
engages the bottom edge of the firing cylinder 64. The sealing engagement between
the piston 70 and firing cylinder 64 allows the firing cylinder 64 to be charged with
pressurized gas that is conducted from the trigger cylinder 66 by way of bore 76 in
the connecting rod 74, which in turn flows through the gas-conducting bore 77 in the
pulse flattener 65. Such sealing engagement between the upper piston 70 and the firing
cylinder 64 will be maintained throughout the entire charging period since the area
of the lower piston 72 is larger than the area of the upper piston 70. After the firing
cylinder 64 has been completely charged with pressurized gas between 1 and 11 megapascals,
the pressure pulse generator 60b is actuated by firing circuit 90, which opens solenoid
valve 88 and exposes gas passages 86a and 86b to the ambient atmosphere. The resulting
escape of pressurized gas from the trigger cylinder 66 creates a disequilibrium in
the pressures acting upon the lower and upper pistons 70, 72 of the plunger 68, causing
it to assume the position illustrated in phantom in less than a millisecond. When
the air gun 62 is thus fired, 164 cubic centimeters of pressurized gas are emitted
around the 360 degree gap 91 between the lower edge of the firing cylinder 64 and
the upper edge of the trigger cylinder 66, while the remaining 1262 cubic centimeters
follows 2 or 3 milliseconds later through the gas conducting bore 77 of the pulse
flattener 65. The two-stage emission of pressurized air out of firing cylinder 64
lowers the peak amplitude of the resulting shock wave in the secondary side, thereby
advantageously lowering the peak stress experienced by the heat exchanger tubes 22
in the vicinity of the nozzle 111. In the preferred embodiment, air gun 62 is a PAR
600B air gun manufactured by Bolt Technology, Inc., located in Norwalk Connecticut,
U.S.A. and firing circuit 90 is a Model FC100 controller manufactured by the same
corporate entity.
[0023] The single port manifold 92 completely encloses the circumferential gap 91 of the
air gun 62 that vents the pressurized gas from the firing cylinder 64. Upper and lower
mounting flanges 94a, 94b are provided which are sealingly bolted to upper and lower
mounting flanges 96a, 96b that circumscribe the cylinders 64, 66 of the air gun 62.
The manifold 92 has a single outlet port 98 for directing the pulse of pressurized
gas generated by the air gun 62 into the nozzle 111. This port 98 terminates in a
mounting flange 100 which is bolted onto one of the annular shoulders 102 of a tubular
spool piece 104. The other annular shoulder 107 of the spool piece 104 is bolted around
a circular port (not shown) of a mounting flange 109. The spool piece 104 and outlet
port 98 are sufficiently long so that the body of the air gun 62 is spaced completely
out of contact with the shell of the steam generator 1. This is important, as such
spacing prevents the hard outer shell of the air gun 62 from vibrating against the
shell of the generator 1 when it is fired. In the preferred embodiment, both the single
port manifold 92 and spool piece 104 are formed from stainless steel approximately
12.70 millimeters thick to insure adequate strength. The mounting flange 109 is also
preferably formed from 12.70 millimeters thick stainless steel, and has a series of
bolt holes uniformly spaced around its circumference which register with bolt receiving
holes (not shown) normally present around the sludge lance port 52b of the steam generator
1. Hence, the pulse generator assembly 62b can be mounted onto the secondary side
5 of the steam generator without the need for boring special holes in the generator
shell.
[0024] The nozzle 111 of the pressure pulse generator assembly 60b includes a tubular body
112. One end of the tubular body 112 is circumferentially welded around the port (not
shown) of the mounting flange 109 so that all of the compressed air emitted through
the outlet port 98 of the single port manifold 92 is directed through the nozzle 111.
A complete-penetration weld is used to insure adequate strength. The other end of
the tubular body 112 is welded onto a tip portion 113 which is canted 30 degrees with
respect to the upper surface of the tubesheet 7. Because the 30 degree orientation
of the tip portion 113 induces an upwardly directed movement along the nozzle 111
when the pulse generator 60b is fired, a gusset 113.5 is provided between the tubular
body 112 of the nozzle and mounting flange 109. In the preferred embodiment, the body
112 of the nozzle 111 is formed from stainless steel about 12.70 millimeters thick,
having inner and outer diameters of 50.8 and 63.5 millimeters, respectively. The nozzle
111 is preferably between 508 and 610 millimeters long, depending on the model of
steam generator 1. In all cases, the tip portion 113 should extend beyond the tube
wrapper 52. Finally, two vent holes 113.9 that are 6.35 millimeters in diameter and
25.4 millimeters apart are provided on the upper side of the tubular body 112 of the
nozzle 111 to expedite the refilling of the nozzle 111 with water after each firing
of the air gun 62 (as shown in Figure 7). The provision of such vent holes 113.9 does
not divert any significant portion of the air and water blast from the air gun 62
upwardly.
[0025] It has been found that a 30 degree downward inclination of the tip portion 113 is
significantly more effective than either a straight, pipe-like nozzle configuration
that is horizontal with respect to the tubesheet 7, or an elbow-like configuration
where the tip 113 is vertically disposed over the tubesheet 7. Applicant believes
that the greater efficiency associated with the 30 degree orientation of the nozzle
tip 113 results from the fact that the blast of water and pressurized air emitted
through the nozzle 111 obliquely hits a broad, near-center section of the tubesheet
7, which in turn advantageously reflects the shock wave upwardly toward the support
plates 30 and over a broad cross-section of the secondary side. This effect seems
to be complemented by the simultaneous, symmetrical blast of air and water from the
pulse generator 60a located 180 degrees opposite from pulse generator 60b. The symmetrical
and centrally oriented impingement of the two shock waves seems to create a uniform
displacement of water in the upper portion of the secondary side 5, as may be best
understood with reference to Figure 5. This is an important advantage, as one of the
primary cleaning mechanisms at work in the upper regions of the secondary side 5 of
the steam generator seems to be the near instantaneous and uniform vertical displacement
of the water. Still another important advantage associated with the oblique orientation
of the blast of air and water is that the peak stress on the heat exchanger tubes
22 in the vicinity of the tip 113 is lowered. By contrast, if the nozzle tip 113 were
directed completely horizontally, no part of the blast would be widely reflected upwardly,
and the force of the air and water blast would act orthogonally on the nearest tube
22. Similarly, if the blast were directed completely vertically toward the tubesheet
7, the impact area of the blast against the tubesheet would be narrower, and peak
tube stresses would again be higher as the blast would be more concentrated.
[0026] With reference now to Figures 6A, 7 and 8, the apparatus of the invention further
includes a recirculation system 114 that is interconnected with the pressure pulse
generator assembly 60b by inlet hose 115, a suction-inlet hose 121a, and a suction
hose 121b. As is best seen in Figure 6A, inlet hose 115 extends through the circular
mounting flange 109 of the pressure pulse generator assembly 60b by way of a fitting
117. At its distal end, the inlet hose 115 is aligned along the main tube lane 55
above nozzle 111 as is best seen in Figure 7. At its proximal end, the inlet hose
115 is connected to an inlet conduit 119b that is part of the recirculation system
114. Suction-inlet hose 121a and suction hose 121b likewise extend through the mounting
flange 109 by way of fittings 123a, 123b. Inlet hose 115 is provided with a diverter
valve 126a connected thereto by a T-joint 126.1 for diverting incoming water into
suction-inlet hose 121a as shown. Suction-inlet hose 121a includes an isolation valve
126b as shown just below T-joint 126.2. When suction-inlet hose 121a is used as a
suction hose, valves 126a and 126b are closed and opened, respectively. When suction-inlet
hose 121b is used as an inlet hose, valves 126a and 126b are opened and closed, respectively.
[0027] The distal ends of the hoses 121a, 121b lie on top of the tubesheet 7, and are aligned
along the circumference of the tubesheet 7 in opposite directions, as may best be
seen in Figure 7. Such an alignment of the inlet hose 115 and hoses 121a, 121b helps
induce a circumferential flow of water around the tubesheet 7 when hose 121a is used
as an inlet hose by shutting valve 126b and opening valve 126a. As will be discussed
later, such a circumferential flow advantageously helps to maintain loosened sludge
in suspension while the water in the secondary side is being recirculated through
the particulate filters 145 and 147 of the recirculation system 114. The proximal
ends of each of the hoses 121a, 121b are connected to the inlet ends of a T-joint
125. The outlet end of the T-joint 125 is in turn connected to the inlet of a diaphragm
pump 127 by way of conduit 125.5b. The use of a diaphragm-type pump 127 is preferred
at this point in the recirculation system 114 since the water withdrawn through the
hoses 121a, 121b may have large particles of suspended sludge which, while easily
handled by a diaphragm-type pump, could damage or even destroy a rotary or positive
displacement-type pump.
[0028] Figure 8 schematically illustrates the balance of the recirculation system 114. The
suction-inlet hose 121a and suction hose 121b of each of the pressure pulse generator
assemblies 60a, 60b are ultimately connected to the input of diaphragm pump 127. The
output of the diaphragm pump 127 is in turn serially connected to first a tranquilizer
129 and then a flow meter 131. The tranquilizer 129 "evens out" the pulsations of
water created by the diaphragm pump 127 and thus allows the flow meter 131 to display
the average rate of the water flow out of the diaphragm pump 127. The output of the
flow meter 131 is connected to the inlet of a surge tank 135 via conduit 133. In the
preferred embodiment, the surge tank 135 has an approximately 1 cubic meter capacity.
The outlet of the surge tank 135 is connected to the inlet of a flow pump 137 by way
of a single conduit 139, while the output of the pump 137 is connected to the inlet
of a cyclone separator 141 via conduit 143. In operation, the surge tank accumulates
the flow of water generated by the diaphragm pump 127 and smoothly delivers this water
to the inlet of the pump 137. The pump 137 in turn generates a sufficient pressure
head in the recirculating water so that a substantial portion of the sludge suspended
in the water will be centrifugally flung out of the water as it flows through the
cyclone separator 141.
[0029] Located downstream of the cyclone separator 141 is a one to three micron bag filter
145 that is serially connected to a one micron cartridge filter 147. These filters
145 and 147 remove any small particulate matter which still might be suspended in
the water after it passes through the cyclone separator 141. Downstream of the filters
145 and 147 is a 2 cubic meter supply tank 151. Supply tank 151 includes an outlet
conduit 153 that leads to the inlet of another flow pump 155. The outlet of the flow
pump 155 is in turn connected to the inlet of an ionic remover or demineralizer bed
157. The purpose of the flow pump 155 is to establish enough pressure in the water
so that it flows through the serially connected ion exchange columns (not shown) in
the demineralizer bed 157 at an acceptably rapid flow rate. The purpose of the demineralizer
bed 157 is to remove all ionic species from the water so that they will have no opportunity
to reenter the secondary side 5 of the generator 1 and create new sludge deposits.
[0030] Located downstream of the demineralizer bed 157 is a first T-joint 159 whose inlet
is connected to conduit 161 as shown. An isolation valve 160a and a drain valve 160b
are located downstream of the two outlets of the T-joint 159 as shown to allow the
water used in the cleaning method to be drained into the decontamination facility
of the utility. Located downstream of the T-joint 159 is another T-joint 163 whose
inlet is also connected to conduit 161 as shown. Diverter valves 165a and 165b are
located downstream of the outlet of T-joint 163 as indicated. Normally valve 165a
is open and valve 165b is closed. However, if one desires to fill a second steam generator
1 with the filtered and polished water drained from a first steam generator in order
to expedite the pressure pulse cleaning method, valves 165a and 165b can be partially
closed and partially opened, respectively. Flowmeters 167a, 167b are located downstream
of the valves 165a and 165b so that an appropriate bifurcation of the flow from conduit
161 can be had to effect such a simultaneous drain-fill step. Additionally, the conduit
that valve 165b and flowmeter 167b are mounted in terminates in a quick connect coupling
167.5. To expedite such a simultaneous drain-fill step, valves 165a and 165b are mounted
on a wheeled cart (not shown) and conduit 161 is formed from a flexible hose to form
a portable coupling station 168. Downstream of the portable coupling station 168,
inlet conduit 161 terminates in the inlet of a T-joint 169 that bifurcates the inlet
flow of water between inlet conduits 119a and 119b.
[0031] Water is supplied through the recirculation system 114 through deionized water supply
170, which may be the deionized water reservoir of the utility being serviced. Water
supply 170 includes an outlet conduit 172 connected to the inlet of another flow pump
174. The outlet of the flow pump 174 is connected to another conduit 176 whose outlet
is in turn connected to the supply tank 151. A check valve 178 is provided in conduit
176 to insure that water from the supply tank 151 cannot back up into the deionized
water reservoir 170.
Method of the Invention
[0032] With reference now to Figures 5, 6A and 6B, the method of the invention is generally
implemented by the previously described pressure pulse generator assemblies 60a, 60b
in combination with the recirculation system 114. However, before these components
of the apparatus of the invention are installed in and operated in a steam generator
1, several preliminary steps are carried out. In the first of these steps, the relative
condition of the heat exchanger tubes 22 is preferably ascertained by an eddy current
or ultrasonic inspection of a type well known in the art. Such an inspection will
give the system operators information which they can use to infer the maximum amount
of momentary pressures that the tubes 22 of a particular steam generator can safely
withstand without any danger of yielding or without undergoing significant metal fatigue.
In this regard, applicants have observed that heat exchanger tubes 22 in moderately
good condition can withstand momentary pressures of up to approximately 131 megapascals
without yielding or without incurring significant amounts of metal fatigue. By contrast,
it is anticipated that relatively old heat exchanger tubes 22 whose walls have been
significantly weakened by corrosion and fretting may only be able to withstand only
103 megapascals, while relatively new tubes which are relatively free of the adverse
affects of corrosion or fretting may be able to withstand up to 207 megapascals without
any adverse mechanical effects.
[0033] After the tubes 22 have been inspected by an eddy current or ultrasonic probe to
the extent necessary to ascertain the maximum amount of momentary pressure they can
safely withstand, the secondary side 5 of the steam generator 1 is drained and all
loose sludge that accumulates on top of the tube sheet 7 is removed by known methods,
such as flushing or by sludge lancing. In the preferred embodiment, sludge lancing
techniques such as those disclosed and claimed in U.S. Patents 4,079,701 and 4,676,201
are used, each of which is owned by the Westinghouse Electric Corporation. Generally
speaking, such sludge lancing techniques involve the installation of a movable water
nozzle in the sludge lance ports 53a, 53b in the secondary side 5 which washes the
loose sludge out of the generator 1 by directing a high velocity stream of water down
the tube lanes 54.
[0034] After all of the loose sludge on top of the tubesheet 7 has thus been removed, the
pressure pulse generator assemblies 60a, 60b are installed in the sludge lance ports
53a, 53b in the positions illustrated in the Figures 6A and 7. Specifically, the tubular
body 112 of the nozzle 111 of each of the generator assemblies 60a, 60b is centrally
aligned along the main tube lane 55 in a horizontal position as shown so that the
canted nozzle tip 113 assumes a 30 degree orientation with respect to the flat, horizontal
upper surface of the tubesheet 7. Next the recirculation system 114 is connected to
each of the pulse generator assemblies 60a, 60b by coupling the inlet hose 115 of
each to the flexible inlet conduits 119a and 119b, and the suction-inlet hose 121a
and suction hose 121b of each to flexible suction conduits 125.5a, 125.5b via the
T-joint 125 of each assembly 60a, 60b. Next, the recirculation system 114 is connected
via conduit 172 to the supply 170 of deionized water from the utility, as is best
seen in Figure 8. The flow pump 174 is then actuated in order to fill supply tank
151 approximately one-half full, which will occur when tank 151 receives about 250
gallons of water.
[0035] Once supply tank 151 is at least one-half full, flow pump 155 is actuated to commence
the fill cycle. In the preferred method of the invention, pump 155 generates a flow
of purified water of approximately 0.454 cubic meter per minute which is bifurcated
to two 0.227 cubic meter per minute flows at T-joint 169 between inlet hose 119a and
119b on opposing sides of the generator 1 in order to fill the secondary side 5 of
the steam generator 1. During the time that the secondary side 5 is being filled via
pump 153, valves 165a and 165b are opened and closed so that the entire flow of water
from pump 153 enters the generator 1. Additionally, valves 126a, 126b are opened and
closed in each of the generator assemblies 60a, 60b in order to further bifurcate
the 0.227 cubic meter per minute flow from inlet conduit 119a, 119b between the inlet
hose 115 and the suction-inlet hose 121a of each of the generator assemblies 60a,
60b. As soon as the water level on the secondary side 5 becomes great enough to submerge
both hoses 121a, 121b diaphragm pump 127 is actuated and adjusted to withdraw 0.189
cubic meter per minute a piece out of the secondary side 5. Since the flow pump 155
introduces 0.454 cubic meter per minute, while the diaphragm pump 127 withdraws 0.189
cubic meter per minute, the secondary side 5 is filled at a net flow rate of 0.265
cubic meter per minute. Additionally, since the suction-inlet hose 121b of each of
the generator assemblies 60a, 60b is used at this time as a fill hose, whose output
is circumferentially directed toward an opposing suction hose 121a, a peripheral flow
of water is created around the circumference of the secondary side as is best seen
in Figure 7. Such a peripheral flow of water is believed to help keep in suspension
the relatively large amounts of sludge and debris that are initially dislodged from
the interior of the secondary side 5 when the generator assemblies 60a, 60b are actuated
which in turn allows the recirculation system 114 to remove the maximum amount of
dislodged sludge and debris during the fill cycle of the method.
[0036] After the water level in the secondary side 5 of the generator 1 rises to a level
of at least 152 millimeters over the nozzles 111 of each of the pressure pulse generator
assemblies 60a, 60b, the firing of the air gun 62 of each of the assemblies 60a, 60b
commences. If the prior eddy current and ultrasonic testing indicates that the heat
exchanger tubes 22 can withstand momentary pressures of approximately 131 megapascals
without any deleterious affects, the gas pressure regulators 82 of each of the generator
assemblies 60a, 60b is adjusted so that gas of a pressure of about 3 megapascals is
initially admitted into the firing cylinders 64 of the air gun 62 of each. Such a
gas pressure applies a peak stress to the tubes 22 which is safely below the 131 megapascals
limit, as will be discussed in more detail hereinafter. The firing circuit 90 is then
adjusted to fire the solenoid operated valve 88 of the trigger cylinder 66 every seven
to ten seconds. The firing of the air gun 62 at seven to ten second intervals continues
during the entire fill, recirculation and drain cycles of the method. While the generator
assemblies 60a, 60b are capable of firing at shorter time intervals, a pulse firing
frequency of seven to ten seconds is preferred because it gives the nitrogen gas emitted
by the nozzle 111 sufficient time to clear the nozzle 111 and manifold 92 before the
next pulse. If pockets of gas remain in the pulse generator 60b during subsequent
air gun firings, then a significant amount of the shock to the water within the secondary
side 5 would be absorbed by such bubbles, thereby interfering with the cleaning action.
[0037] It is important to note that the gas pressure initially selected for use with the
pressure pulse generator assembly 60a, 60b induces momentary pressures that are well
below the maximum safe amount of momentary forces that the tubes 22 can actually withstand,
for two reasons. First, as will be discussed in more detail hereinafter, the pressure
of the gas used in the generator assembly 60a, 60b is slowly raised in proportion
with the extent to which the secondary side 5 of the steam generator 1 is filled until
it is approximately twice as great as the initially chosen value for gas pressure.
Hence, when the initial gas pressure used when the water level is just above the nozzles
111 is approximately 3 megapascals, the final pressure of the gas used in the pressure
pulse generator assembly 60a, 60b will be approximately 5.52 to 6.21 megapascals.
Secondly, the gas pressure is chosen so that the maximum pressure used will induce
momentary forces in the tubes 22 which are at least 30 and preferably 40 percent below
the maximum megapascals indicated by the previously mentioned eddy current and ultrasonic
inspection to provide a wide margin of safety. In making the selection of which gas
pressure to use, applicants have discovered that there is a surprising, non-linear
relationship between the pressure of the gas used in the air gun 62 of each pulse
generator assembly 60a, 60b and the resulting peak stress on the tubes 22, as is evident
from the following test results:
Approximate Gas Pressure |
Approximate Peak Tube Stress |
3 megapascals |
38 megapascals |
6 megapascals |
83 megapascals |
11 megapascals |
212 megapascals |
[0038] In most circumstances, the firing of the air gun 62 of both the pulse generators
will be synchronous in order to uniformly displace the water throughout the entire
cross-section of the secondary side 5 of the generator 1. However, there may be instances
where an asynchronous firing of the air guns 62 of the different assemblies may be
desirable, such as in a steam generator where the sludge lance ports 53a, 53b are
only 90 degrees apart from one another. In such a case, the asynchronous firing of
the air guns 62 could possibly help to compensate for the non-opposing arrangement
of the pulse generators 60a, 60b in the secondary side 5 imposed by the location of
the 90 degree apart sludge lance ports 53a, 53b.
[0039] Figure 9 illustrates how the pressure of the gas within the 1442 cubic centimeter
firing cylinder 64 of the air gun 62 diminishes over time, and Figure 10 indicates
the peak stress experienced by the column of tubes closest to the nozzle 111. Specifically,
when the pressure of the gas within the firing cylinder 64 is 6 megapascals, and a
164 cubic centimeter pulse flattener 65 having a gas-conducting bore 13 millimeters
in diameter is used, the gas leaves the cylinder 62 over a time period of approximately
five milliseconds. Figure 10 shows that the peak stress experienced by the column
of tubes 22 closest to the tip portion 113 of the nozzle 111 is between 83 and 90
megapascals, which again is safely below the 131 megapascals limit. If no pulse flattener
65 were used, the closest column of heat exchanger tubes 22 in the secondary side
5 to the tip portion 113 of the nozzle 111 would be considerably higher, as the gas
would escape from the air gun in a considerably shorter time than 5 milliseconds.
[0040] The filling of the secondary side 5 at a net rate of about 0.265 cubic meters per
minute continues until the uppermost support plate 30 is immersed with water. In a
typical Westinghouse Model 51 steam generator, about 64 cubic meters of water must
be introduced into the secondary side 5 before the water reaches such a level. At
a net fill rate of about 0.265 cubic meters per minute, the fill cycle takes about
four hours. During the fill cycle, the pressure of the gas introduced into the firing
cylinder 64 of each air gun 62 is raised from approximately 3 megapascals to approximately
5.52 to 6.21 megapascals in direct proportion with the water level in the secondary
side 5. The proportional increase in the pressure of the gas used in the air guns
62 substantially offsets the diminishment in the power of the pulses created thereby
caused by the increasing static water pressure around the tip portion 113 of the nozzle
111 of each.
[0041] As soon as the water level in the secondary side 5 is high enough to completely submerge
the highest support plate 30, the recirculation cycle commences. If desired, valves
126a, 126b may be closed and opened, respectively, in order to convert the function
of suction-fill hose 121a into a suction hose. Moreover, the flow rate of fill pump
155 is lowered from 0.454 cubic meters per minute to only 0.189 cubic meters per minute,
while the withdrawal rate of the diaphragm type suction pump 127 is maintained at
0.189 cubic meter per minute. The net result of these adjustments is that water is
recirculated through the secondary side 5 of the steam generator 1 at a rate of approximately
0.189 per minute. This circulation rate is maintained for approximately 12-48 hours
while the air guns 62 of each of the generator assemblies 60a, 60b are fired at a
pressure of 6 megapascals every seven to ten seconds.
[0042] After the termination of the recirculation cycle, the drain cycle of the method commences.
This step is implemented by doubling the flow rate of the diaphragm-type suction
pump 127 so that each of the hoses 121a, 121b of each pulse generator 60a, 60b will
withdraw approximately 0.085 cubic meters per minute. Since the fill pump 155 continues
to fill the secondary side 5 at a total rate of approximately 0.189 cubic meter per
minute, the net drain rate is approximately 0.151 cubic meter per minute. As the secondary
side 5 has about 64 cubic meters of water in it at the end of the recirculation cycle,
the drain cycle takes about seven hours. During this period of time, it should be
noted that the pressure of the gas introduced into the firing cylinders 64 of the
air guns 62 of the generator assembly 60a, 60b is lowered from 5.52 megapascals to
2.76 megapascals in proportion with the level of the water in the secondary side 5.
[0043] To expedite the cleaning method in a utility where two or more steam generators are
to be cleaned, a second steam generator (not shown) may be filled with the filtered
and polished water that flows out of the demineralizer 157 of the recirculation system
114 during the drain cycle of a first steam generator. This may be accomplished by
wheeling the portable coupling station 168 over to a second generator where other
pulse generator assemblies 60a, 60b have been installed, and coupling the outlet of
flowmeter 167b to the inlet conduits 119a, 119b of the second generator. Next, diverter
valves 165a and 165b are adjusted so that part of the filtered and polished water
leaving the demineralizer 157 is shunted to the inlet conduits 119a, 119b of the second
generator. In order to maintain the seven hour time period of the drain cycle for
the first steam generator, the flow rate of the pump 155 is increased to approximately
0.644 cubic meters per minute. The valve 165a is adjusted so that the flow rate as
indicated by flowmeter 167a remains approximately 0.189 cubic meters per minute. The
balance of the 0.454 cubic meters per minute flow is shunted through valve 165b to
the secondary side 5 of the second steam generator. The implementation of this additional
step not only lowers the total amount of time required to clean a plurality of steam
generators by as much as 50 percent, but further considerably reduces the amount of
deionized and purified water that the utility must supply from source 170 to implement
the cleaning method of the invention. As it requires approximately 64 cubic meters
or 65,318 kilograms of water to clean a single steam generator 1, the savings in water
alone are clearly significant. Moreover, by reducing the overall amount of time required
to clean two generators, the amount of time that the operating personnel are exposed
to potentially harmful radiation is considerably reduced. The portability of the valves
165a, 165b afforded by the portable conduit coupling station 168 plus the use of a
flexible hose for conduit 161 greatly facilitates the implementation of such a combined
drain-fill step in the method of the invention.
1. A pressure pulse cleaning apparatus for loosening and removing sludge and other
impurities from the interior of a heat exchanger vessel (5) of the type having one
or more access openings (53a, 53b), and an interior having a bottom portion (7) that
houses a plurality of parallel heat exchanger tubes (22), and which is at least partially
filled with liquid characterized by:
a. a pulse generator (60a, 60b) having an opening (91) for generating a succession
of pulses to create shock waves in the liquid that impinge upon and loosen said sludge
and impurities, and
b. a pulse distributing conduit (111) having a first end that is connected to the
opening (91) of the pulse generator (60a, 60b) and a second end (113) that extends
into the interior of the heat exchanger (5) vessel and communicates with the liquid
therein, wherein said second end (113) is oriented to direct said pulses toward the
bottom (7) of the heat exchanger vessel (5) and at an oblique angle with respect to
the parallel heat exchanger tubes (22) to minimize the peak stress that said pulses
apply to said tubes (22).
2. The apparatus defined in claim 1, further including a mounting flange (109) for
detachably mounting said pulse distributing conduit (111) in one of said access openings
(53a, 53b) in the heat exchanger vessel (5).
3. The apparatus defined in claim 1, wherein said heat exchanger tubes (22) define
open tube lanes (54) within the vessel (5), and wherein the longitudinal axis of the
pulse distributing conduit (111) is aligned with one of said open tube lanes (54)
to maximize the distance between the second end (113) of the pulse distributing conduit
(111) and the closest heat exchanger tubes (22) thereto.
4. The apparatus defined in claim 1, further including a recirculation system (114)
including a pump (127, 137, 155) for inducing a flow in said liquid and a filter (145,
147) for removing loosened particles of sludge and debris from the liquid in the vessel.
5. The apparatus defined in claim 4, wherein said recirculation system (114) includes
a suction tube (121b) in communication with the interior of the vessel (5) for conducting
the liquid outside of the vessel and into the filtration (145, 147).
6. The apparatus defined in claim 5, wherein said recirculation system (114) includes
an ionic remover (157) for removing dissolved ionic species in the liquid.
7. The apparatus defined in claim 6, wherein said ionic remover (157) includes at
least one demineralizer bed (157).
8. The apparatus defined in claim 5, wherein the recirculation system (114) includes
an inlet tube (121a) in communication with the interior of the vessel (5) for conducting
liquid into the vessel (5).
9. The apparatus defined in claim 8, wherein said inlet tube (121a) is oriented substantially
parallel to the inner periphery of the vessel (5) in order to induce a circumferential
flow of liquid around the interior of the vessel (5) that helps maintain particles
of sludge and debris in suspension in the liquid.
10. The apparatus defined in claim 1, wherein the bottom portion of the vessel (5)
is defined by a tubesheet (7), and the second end of the pulse distributing conduit
(111) is oriented 30 degrees with respect to said tubesheet (7).