epriRepair_0Replace[1].pap
Content
HEAT EXCHANGER TUBE SIDE MAINTENANCE –
REPAIR vs. REPLACEMENT
Bruce W Schafer
Framatome ANP, Inc.
155 Mill Ridge Road
Lynchburg, VA 24502
(434) 832-3360
[email protected]
HEAT EXCHANGER TUBE SIDE MAINTENANCE –
REPAIR vs. Replacement
Bruce W Schafer
Framatome ANP, Inc.
155 Mill Ridge Road
Lynchburg, VA 24502
(434) 832-3360
[email protected]
Abstract
The traditional method of repairing degraded tubes in shell-and-tube heat exchangers is to
remove the effected tubes from service by plugging. Since heat exchangers are designed with
excess heat transfer capability, approximately 10% of tubes can be plugged before performance
is affected. When the number of plugged tubes becomes excessive, heat exchanger efficiency is
lost, resulting in reduced power output, high system pressure drop, further heat exchanger
damage, or abnormal loads placed on other plant heat exchangers.
As an option to component retubing or replacement, repair methods, including tube sleeving and
tube expansion, have proven to be an effective method to repair defective tubes and keep the
existing heat exchanger in service. For the sleeving process, a new tube section is installed
inside the existing tube to bridge across the degraded area. Tube expansion is used to close off a
gap between the tube and the tubesheet or end plate (to eliminate a leak path) or between the
tube and tube support (to minimize vibration). While not all heat exchangers can be returned to
their original design condition by performing tube repairs, in some instances it may be possible
to get many more years of useful life out of a heat exchanger at a fraction the cost of
replacement.
This paper presents options which the Plant Maintenance Engineer should consider in making
the repair versus replacement decision. This includes the repair options (sleeving and tube
expansion), other conditions within the heat exchanger, and the effect of tube repair on heat
exchanger performance.
Introduction
Traditionally, when maintenance is performed on shell-and-tube heat exchangers, the only
options considered when tube defects are found are to plug tubes and, when the number of plugs
became too great, replace the heat exchanger. The decision to replace the heat exchanger was
based on a number of factors. These included: the number of tubes plugged, the number of
forced outages due to tube damage (and the cost associated with replacing lost power and
repairing the damaged tubes), the impact that the plugged heat exchanger is having on the plant
(due to lost flow or heat transfer surface area), the rate at which tube plugging is occurring, the
availability of funds to replace the heat exchanger, and the expected life of the unit (how much
longer will the unit operate before retirement).
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From a sampling of industry data, tube failures have been shown to cause between 31% to 87%
(depending on the data source) of the events related to feedwater heaters (1). Since so many of
the failures were related to the tubing, the replacement of an entire heat exchanger due to
damage in one area is an expensive as well as a schedule and manpower intensive option.
The typical means for major heat exchanger repair included complete replacement, rebundling,
and retubing, as described below.
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For the replacement option, the entire heat exchanger shell and tube bundle are replaced with
a new unit.
For rebundling, the shell is temporarily removed from the heat exchanger and the old tube
bundle, including, at a minimum, tubes, tube supports, and tubesheet, are removed. A new
tube bundle is inserted and the shell is welded back in place.
For retubing, either the shell (u-tube design) or tube side access cover (straight tubes) is
removed from the heat exchanger and the old tubes are removed from the bundle. New tubes
are then inserted and re-attached to the tubesheet (typically by either mechanical expansion,
welding, or both). In many instances, the existing shell side hardware is used as-is, although
some modifications may be made. Retubing is typically performed on straight tube heat
exchangers, such as condensers and coolers.
Since the 1970’s, tube sleeving has been used to allow damaged tubes to remain in service. The
sleeves are installed by various means (roll, explosive, or hydraulic expansion, explosively
welded, or press-fit or epoxied in place) over the defective area of the tube. Through the use of
sleeving, which is a low-cost option to retubing, rebundling, or replacement, the useful life of a
heat exchanger can be economically extended. The decision to perform sleeving also can be
made with short notice as opposed to replacement (2-6 weeks compared with 18 months),
possibly allowing repairs to be performed the same outage that the damage is noted.
Tube expansion also can be performed to minimize or eliminate leakage within heat exchangers.
In the tubesheet, tubes can be re-expanded to strengthen the original tube-to-tubesheet joint,
reducing or eliminating leakage and prolonging the life of the heat exchanger. Expansions also
can be made deep within the tube to expand the tube into tube support plates and end plates.
These expansion can reduce tube-to-plate clearance for vibration control or, at end plates, to
minimize steam flow from the high to low pressure side of the plate.
Repair vs. Replace – Factors To Consider
There are numerous factors to consider when deciding whether to repair the tubes in a heat
exchanger or to perform a larger repair scope and rebundle or replace the component. The
following factors should be considered when making the repair vs. replace decision.
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The budget available for repair or replacement needs to be determined. Typically, the cost of
performing a substantial heat exchanger repair (consisting of plug removal, tube inspection,
tube expansion, and sleeving) is less than 10% of the cost of replacing the unit. Because of
the lower cost, the payback time on the repair option is much shorter than for replacement.
If the heat exchanger is critical to plant operation (either from a safety, efficiency, or power
production standpoint) or is resulting in costly forced outages, it may be possible to justify a
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repair to the unit in the near-term and a scheduled replacement when a longer outage can be
planned.
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If there are a large number of tube plugs to remove, or if they are difficult to remove
(explosive or welded), then the cost to repair the heat exchanger will increase, and the
scheduled time needed on-site may not fit within the outage window. If it appears that tube
repair may be possible, it may be worthwhile to plug tubes, using removable plugs, until a
certain quantity of tubes are removed from service. At that point the plugs would be
removed and sleeves installed, thereby minimizing the overall maintenance cost.
The location and quantity of the tube defects need to be examined to decide if tube repair is
an option. Tube repair may be appropriate if the damage is limited to a certain area of the
tube, which would allow the use of a short repair sleeve. If the damage is over a significant
portion of the tube, it is possible to install a longer sleeve (up to the full length of the tube) to
ensure that all tube defects are repaired. However, if the u-bend region of the tube is
damaged then tube repair is not possible. Also, it would not be possible to install a sleeve if
a large portion of the tube had damage but there was inadequate clearance for a long sleeve
at the tube end.
One of the more important items to consider when deciding whether a heat exchanger can be
repaired is the condition of the remainder of the heat exchanger. The condition of the shell
side components, such as the impingement plates, tube supports, end plates, and other
structural members, should be in good shape if a long term repair is being planned. An
evaluation also should be made of the shell thickness in areas that are prone to shell
erosion/corrosion. If the tube repair is only a short-term fix, to allow component operation
until a replacement heat exchanger can be installed, the condition of the shell side is not as
critical.
The life expectancy of the power plant needs to be factored into the decision to repair or
replace a heat exchanger. If the only problem with the heat exchanger is in one section of the
tube, and the expected run time on the unit is relatively short, it would be advantageous to
repair rather than replace the heat exchanger since it will be very difficult to pay back the
cost for replacement over the remaining plant life.
The outage time required to repair a heat exchanger, even when tube and shell side
inspections are performed, is typically much less than for replacement. In addition, very few,
if any, plant modifications need to be made to make the repairs. This allows other work to be
performed in the vicinity of the heat exchanger.
Along with the shorter outage duration, the site support required for repair is much less.
Usually, there are no shell or head modifications required since all work can usually be
performed through the manways and pass partition plates. Less repair equipment is required,
resulting in less space being needed in the area of the heat exchanger for setup and storage.
In addition, the time required to prepare for tube repair is much less than for replacement (26 weeks compared with 18 months), allowing a decision on repair to be made just before, or
even during, an outage.
At nuclear plants, the added cost for the disposal of radioactively contaminated heat
exchangers must be taken into account. Before disposal, there is the cost of surveying the
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heat exchangers for release and, if contamination is found, they must either be
decontaminated or disposed of as radioactive waste. Tube repairs can eliminate these costs.
If the heat exchanger is being replaced to eliminate detrimental materials in the cooling
system (i.e. copper in the condensate/feedwater system) then tube sleeving will not be
beneficial. The only practical solution would be to retube/rebundle/replace to change out the
tube material.
Heat Exchanger Repair Options
There have always been options available to either repair or replace heat exchanger tubes in the
event that tube leakage or degradation is present. The initial option, after the problem tubes have
been located (either through non-destructive examinations, such as eddy current testing, visual
inspections, or leak tests) is to plug the tube. Depending on the type of service and operating
pressures of the heat exchanger, various types of plugs are employed. These include tapered
fiber and metal pin plugs, rubber compression plugs, two piece ring and pin plugs, two piece
serrated ring and pin plugs (installed with a hydraulic cylinder), welded plugs, and explosively
welded plugs. In addition to the tube end plug, there also may be a stabilizer rod or cable that is
inserted into the tube to minimize future tube vibration damage.
At the beginning of the life of a heat exchanger, inserting a few plugs into damaged tubes has
little effect on the performance of the heat exchanger. However, if heat exchanger problems
continue, and the number of plugs increases significantly, it is possible that the heat exchanger
will eventually reach a point that it will not handle the full load that is placed on it. This is due
to a combination of loss of heat transfer area and the increased pressure drop. In addition, as the
number of plugged tubes increases, abnormal temperature conditions (either hot or cold spots)
may be set up in the heat exchanger. These conditions can result in an acceleration of tube
damage, creating a faster demise of the heat exchanger.
Once the number of plugs reaches a unacceptable level, the heat exchanger will need to be
repaired, replaced, or bypassed. However, bypassing the unit is usually not recommended, at
least for a long time period, since it will result in a loss of efficiency and heat transfer area.
Also, the heat load from the bypassed heat exchanger will be transferred to another heat
exchanger in the string, resulting in greater than normal operating flow rates and higher
degradation in that heater.
The following sections show the options that can be used to replace or repair the entire heat
exchanger or just the tubes.
Retubing
If the unit has straight tubes, good access, and the remaining components (shell, tube supports,
internal structural pieces) of the heat exchanger are in good shape, the tubes can be replaced.
The old tubes are removed from the unit and new ones, typically manufactured from an
improved material, are inserted, and then expanded, into place. Insertion of the new tubes is
shown in Figure 1. In addition to performing retubing to replace damaged tubes, retubing has
been performed to eliminate detrimental materials (such as copper from condenser tubes) to
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minimize damage to other equipment within the plant (nuclear steam generators or fossil
boilers).
Figure 1
Condenser Retubing
Rebundling
Some heat exchangers are designed to be rebundled rather than replaced. For these units the
entire tube bundle, including tubes, tubesheet, and tube supports are replaced, as shown in
Figure 2. The original shell and any other internal structural pieces would be reused (although
any necessary internal repairs could be made when the shell was removed). The new tube
bundle can be manufactured to ensure that original design problems with the existing unit are
corrected. However, the same basic design must be maintained since the new bundle must fit
within the existing heat exchanger shell. Rebundling costs about 15-25% more than retubing (1).
Figure 2
Heat Exchanger Rebundling
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Replacement
A third and typically widely used option is to replace the entire heat exchanger, as shown in
Figure 3. Full replacement allows alternate tube materials, changes in heat transfer area, and
structural changes to be employed, including added clearances in areas where erosion or other
problems may be occurring, to ensure that the current heat exchanger problems do not re-occur
in the future. However, the cost associated with a full replacement is the greatest of the three
options, about 5% more than for rebundling (1). In addition, there are no guarantees that the new
heat exchanger design will not have new, unanticipated problems.
Figure 3
Heat Exchanger Replacement
Sleeving
An alternate approach to retubing, rebundling, or replacement of a heat exchanger is to install
sleeves over the defective portions of the tubes The sleeve consists of a smaller diameter piece
of tubing that is inserted into the parent tube and positioned over the tube defects. After
insertion, each end of the sleeve is expanded into the parent tube material. These expansions
serve the dual function of structurally anchoring the sleeve into the tube and providing a leak
limiting path, allowing the sleeve to become the new pressure boundary for the tube. This means
that a sleeved tube can have a 100% through-wall indication and still remain in-service, since the
sleeve is now the new structural and pressure boundary. The installation of the sleeve into the
tube will allow the majority of the tube’s heat transfer area and flow to be maintained.
If heat exchanger repair by sleeving is a possibility then a strategy needs to be used to prepare
for future repair. It may be cost effective to plug a quantity of tubes, per the non-destructive
examination results, each outage using a removable plug. When the quantity of plugged tubes
reaches a certain level the plugs can be removed and sleeves installed. Using this approach will
minimize the cost and time during each inspection outage while allowing the maximum tube
repair later in the heat exchanger’s life.
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There are three types of sleeves that are installed into heat exchanger tubes. These are full
length, partial length structural, and partial length barrier sleeves. The three types are discussed
below. Figure 4 shows the sleeve layout.
Figure 4
Heat Exchanger Sleeve Designs
Full Length Sleeve
These sleeves are installed from one end of the tube to the other in straight tubed heat
exchangers. After insertion, the full length of the sleeve is expanded into the parent tube. This
step serves the dual purpose of maintaining heat transfer as high as possible (typically 75%-90%)
while minimizing flow pressure drop through the tube. After the full length expansion step,
shown in Figure 5, the sleeve ends are trimmed flush with the existing tube ends and the sleeve
is roll expanded into the tubesheet.
The full length sleeve is typically used in a condenser or cooling water heat exchanger when the
tubes have multiple defects along their length. Full length sleeving is an attractive option when a
relatively small percentage of the tubes require repair. Through sleeving, the majority of the
tube heat transfer area can be left in service, resulting in a heat exchanger that is close to its asdesigned condition.
Full length sleeving is comparable in many ways to retubing in the methods employed to install
the sleeves. However, since removal of the existing tube is not required, and the typical number
of tubes that will be full length sleeved are below the number that would be retubed, the cost for
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material and manhours are much less than for retubing, making sleeving a cost-effective option
to return and keep tubes in service.
Figure 5
Full Length Sleeve Expansion
Partial Length Structural Sleeve
This type of sleeve is used to repair shorter defects in the tube. The sleeve can be installed
anywhere along the straight length of the tube. Various methods are used to expand the sleeve in
place. These include roll expansion (both in the tubesheet and in the freespan portion of the
tube), hydraulic expansion in the freespan portion of the tube, and full length expansion. These
expansion types are discussed below. The installation of a hydraulically expanded sleeve is
shown in Figure 6.
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If one end of the sleeve is in the tubesheet, a torque-controlled roll expansion will be made.
This expansion is similar to the original tube-to-tubesheet roll. Freespan roll expansions are
made to either a torque controlled setting or to a diameter controlled hardstop setting.
Usually, freespan roll expansions are only used when the sleeve length is relatively short,
since it can be difficult to insert a roll expander deep into the tube. Both the tubesheet and
freespan roll expansion parameters are set so that they can provide both the structural and
leakage requirements for the sleeve.
For sleeves installed deep within the tube, a hydraulic expansion device is used to connect
the sleeve to the tube. The expander consists of multiple plastic bladders that are filled with
high pressure water. As the water pressure increases, the bladders expanded against the
inside of the sleeve, pushing the sleeve into the tube. The expansion process, which is
computer controlled, continues until either a preset volume of water or a preset pressure is
reached. At this point the sleeve is properly expanded and the bladders are depressurized.
Hydraulic expansions can be made anywhere along the tube length since the expander is
connected to flexible high pressure tubing and is not restricted by tube end access. The
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expansion parameters are qualified to meet the proper structural and leakage requirements for
the sleeve.
Full length expansions are not usually used for structural or leak limiting purposes but
instead are used to improve heat transfer and flow through the sleeve and to close the annulus
between the sleeve and tube. The full length expansion is made by placing a tool, with seals
on each end, into the sleeve. The inside of the sleeve is filled and then pressurized with
water to a preset pressure setting, expanding the sleeve into tight contact with the tube. After
the full length expansion is made, the ends of the sleeve are typically either roll or
hydraulically expanded to form the structural and leak limiting sleeve-to-tube joint.
Many times, the partial length structural sleeves are used to repair indications at one particular
area of the tube, such as wear damage at tube support locations, cracking in roll transitions, or
pitting indications at one discreet location along the tube length. Longer versions of these
sleeves also have been used to repair an entire damaged section of a heat exchanger, such as a
desuperheater or drain cooler section of a feedwater heater. Because of the wide variety of uses,
the sleeve length can range from as short as 1 foot to over 12 feet in length.
Qualification testing is performed on the structural sleeves to ensure that they can withstand the
design temperature and pressure conditions imposed on them. The test results must show that
the sleeve will be the new pressure boundary even with a 100% through-wall indication in the
parent tube. Sleeves of this type, using mechanical expansions (roll and hydraulic), have reliably
been in-service for more than 15 years.
Figure 6
Partial Length – Hydraulically Expanded Structural Sleeve Installation
Partial Length Barrier Sleeve
These sleeves, also known as shields, are used at the ends of the tubes to act as a barrier to tube
end erosion. These sleeves are usually very thing, are not designed to act as a pressure boundary
or structural repair, and are installed in areas of high turbulence. The materials for these sleeves
are compatible with the existing tube material and may include plastic inserts. The sleeves are
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either roll or hydraulic expanded or pressed or epoxied in place. If tube end erosion is occurring,
or is expected to occur, the use of these tube end sleeves will protect and prolong the life of the
parent tube, although over time tube erosion may begin to occur at the end of the sleeve. Many
heat exchanger tube ends have been protected with shields, significantly prolonging the life of
the tubes.
Items to Consider for Tube Repair
Prior to choosing to perform tube sleeving, the following factors should be considered.
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The length, location, and quantity of tube defects that would require sleeving need to be
determined. If the defects are in one or a few short areas then either a single or a couple of
partial length sleeves could be used. However, if the defects are spaced throughout the
length of the tube, then the only option would be a full length sleeve.
The parent tube in the area where the sleeve will be expanded is to be defect free. This will
insure the highest sleeve-to-tube joint integrity. Also, the tube support designations must be
clearly identified to insure that the sleeve is installed at the correct location along the tube
length. This is especially true in areas where there may be skipped baffles and the tube only
touches every other support plate.
The condition of the remainder of the tube away from the sleevable defects needs to be
known. If there are u-bend defects that may require plugging then the tube should not be
sleeved. Sleeving is an option if the remainder of the tube is in good shape.
The space available at the tube end to insert a sleeve and its installation tooling needs to be
known, as shown in Figure 7. If a short, partial length sleeve is being used, the amount of
space required is not as critical, although there can still be access issues around the tubesheet
periphery for hemi-head channel covers and at pass partition plates. However, if a full length
sleeve is required, there will need to be a significant amount of clearance from the tubesheet
face.
Figure 7
Required Clearance for Sleeve Installation
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Inspection records need to be reviewed to determine if there are any tube inside diameter
(ID) restrictions that would block the sleeve from being inserted to the target location. The
size of the eddy current probe used for the inspection, plus any other hardware that has been
inserted into the tube, can be used to help determine the tube ID access issues.
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The post-sleeving tube inspection requirements need to be considered. Typically, the ability
to inspect the tube beyond a sleeve is not a significant issue. While the presence of the
sleeve reduces the inside diameter of the tube, which will result in the need for a smaller
inspection probe, the probe will remain large enough to detect pluggable tube indications
(usually greater than 40%), however small indications may go undetected.
As part of the post-sleeve inspection, the sleeve and its attachment to the tube should be
examined. There is no need to inspect the section of the parent tube between the sleeve
expansions since this is no longer part of the pressure boundary.
If tube cleaning is to be performed in the heat exchanger, then the type of sleeve to be
installed needs to be evaluated. If on-line cleaning is performed, the sleeve size cannot
restrict the passage of the balls or brushes. For off-line cleaning, the projectiles need to pass
through the sleeve without becoming stuck. Many sleeves that are installed in tubes that
require cleaning are full length expanded to ensure the best results for the cleaning
equipment.
If it appears that tube sleeving is possible, then information will be needed to ensure that the heat
exchanger is properly repaired. The following information is used when planning for sleeving.
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Tube sleeving will need to be coordinated with eddy current inspection and plug removal.
If it is expected that sleeving may be performed, then it is important that the proper sleeve
material be purchased in advance of the job.
The sleeve material needs to be compatible with the heat exchanger parent tubing and with
the water chemistry within the heat exchanger. The galvanic corrosion potential between the
sleeve and tube needs to be determined. Also, effects of crevice corrosion between the
sleeve and tube, in the heat exchanger water chemistry, need to be considered to determine if
sleeving is a viable repair option.
The sleeve dimensions need to fit the heat exchanger operating and design conditions plus
any restrictions within the tube ID. The sleeve outside diameter (OD) is to be designed to fit
into the tube but must be long enough to limit the amount of sleeve expansion. The sleeve
wall thickness needs to be sized for the heat exchanger operating parameters, including any
ASME Code minimum wall thickness calculations, if needed. The sleeve length must be
long enough to span the expected tube defects but needs to be sized to fit any tube end
clearance restrictions.
Before installing sleeves into heat exchanger tubes, testing needs to be performed to set the
installation parameters. Depending on the type of sleeve being used, these tests may include
setting the rolling torque, hydraulic expansion constants, and full length expansion pressure.
In addition, depending on the application for the sleeve, there may be a need to do
qualification testing, which would consist of hydrostatic leak and pressure tests and
temperature and pressure cycling. These tests would verify that the expansion parameters
were set correctly for the sleeve application.
If a large quantity of sleeves are being installed, it may be necessary to calculate the heat
transfer and flow loss due to sleeving. These calculations will give a sleeve-to-plug ratio that
can be used to determine the expected improvement in heat exchanger performance after
sleeving is complete (and tubes have been returned to service, if applicable).
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The sleeve may need to be full-length expanded based on the heat exchanger operating
environment. However, the production rates for sleeve installation are lower when full
length expansions are performed. While full length expansion is typically not needed in
many applications, such as most feedwater heaters, it should be considered for the following.
− if tube ID cleaning needs to routinely be performed
− if a long sleeve is being inserted that would severely restrict the tube’s heat transfer or
flow
− if the tube-to-sleeve crevice needs to be eliminated in a hostile water chemistry
environment
− if there are large eddy current probe fill factor restrictions
Heat Exchanger Tube Expansion Repair
In addition to sleeving, it is possible to expand the tube to improve the heat exchanger
performance. These tube repairs can minimize further tube damage and maximize the useful life
of the heat exchanger. Two methods of tube expansion can be performed. One is to expand
deep within the tube to close off a leak path between the tube and the end plate. The other is to
re-expand the tube into the tubesheet to minimize tube-to-shell side leakage.
Tube-to-End Plate Expansion
In some heat exchangers, typically feedwater heaters, there are internal plates which separate one
zone of the heat exchanger from another (usually condensing [steam] from drain cooler [liquid]).
Due to the pressure differential across the plate, and the different temperatures and phases
between the two sections, it is important that leakage not occur through the plate. However, in
some feedwater heaters, the plate design is too thin, resulting in leakage of steam from the
condensing to the drain cooler zones, as shown in Figure 8. When this occurs there is erosion of
the end plate and tube vibration due to the high steam velocities and the steam condensing to
liquid in the drain cooler region. The vibration causes wear at the tube supports which can lead
to tube failure. The leakage of steam also increases the drain cooler temperature, resulting in a
less efficient heat exchanger.
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Figure 8
End Plate Leakage in a Feedwater Heater
Expanding the tube can reduce the gap between the tube and the end plate. The expansion can
be performed using either a roll or hydraulic expander. Once the expander is in position the tube
is expanded until it contacts the end plate. An accurate expansion, which does not over-expand
the tube into the plate (the tube needs to be able to slide in the plate after expansion so that it
does not buckle during heatup/cooldown), needs to be performed. This can be achieved by using
a computer controlled hydraulic expansion that automatically shuts off the pressurization system
when it detects that the tube has contacted the plate.
After the tubes are expanded into the end plate, the steam flow is minimized or eliminated,
reducing the drain cooler temperatures and increases plant efficiency. Further tube damage, in
the form of tube wear and adjacent tubes impacting on one another, will be reduced to nearly
zero and the vibration operating stresses will be reduced significantly. The life of the heat
exchanger will be increased at a minimal cost as compared with replacement.
Tube-to-Tubesheet Expansion
In some heat exchanger designs, with a certain combination of materials, leaks develop between
the tube and tubesheet. In many low pressure units, the tube is only expanded into the tubesheet,
with no subsequent weld. Many of the leaks that occur in these units are the result of a
fabrication error and can be corrected by re-expanding the joint to the correct expansion size.
However, leakage occasionally occurs in high pressure heat exchangers, typically feedwater
heaters, even when the tubes have been welded to the tubesheet. The two prime causes of this
leakage are in areas where the original tube-to-tubesheet weld has either cracked or eroded due
to flow (in the case of soft materials, such as carbon steel) or where there is a crack in a tube-totubesheet expansion transition.
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For the first case it may be possible to re-expand the tube using a qualified roll expansion
process. The expansion would increase the contact pressure between the tube and tubesheet,
increasing the resistance to flow and decreasing or eliminating leakage. This process could
be performed on existing leaking tubes or preventatively on all tubes in the tubesheet.
If cracking is occurring at the original tube expansion transition it may be possible to
re-expand the tube deeper in the tubesheet (unless the cracking is occurring very close to the
shell side of the tubesheet). The tube would be expanded using a qualified roll expansion
process, to place the tube into tight contact with the tubesheet. This expansion would
increase the contact pressure between the tube and tubesheet, increasing the resistance to
flow and decreasing or eliminating leakage. This process could be performed either on
existing leaking tubes or preventatively on all tubes in the tubesheet.
Re-expanding tubes that either may be leaking or that could develop leaks in the future could
significantly extend the life of an otherwise good heat exchanger. By re-expanding the tubes,
forced outages can be avoided and damage from the high pressure water spraying on adjacent
tubes and on the shell will be eliminated. The cost to perform tube re-expansions will be
minimal when compared with the cost of replacement heat exchangers and the cost of forced
outages.
Items to Consider for Tube Expansion Repair
The following factors should be considered to determine if tube expansion is possible.
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The portion of the tube to be expanded needs to be determined.
− If leakage is occurring through the end plate, the expander will need to be long enough to
reach the end plate location. The tube should be expanded using a process, such as
hydraulic expansion, that will not lock the tube into the end plate. This expansion will
not only reduce leakage through the plate but also will minimize future tube vibration due
to the tight fit between the tube and plate.
− If leakage is occurring within the tubesheet, due to either weld or tube cracking, a
re-expansion process may be used. This process, typically a roll expansion, will reexpand the tube into the tubesheet to limit or eliminate leakage from the tube to the shell
side of the heat exchanger.
The condition of the remainder of the tube needs to be known. If there are cracks along the
entire tube length then re-expanding the tube alone will not result in an improvement in heat
exchanger performance.
The space available at the tube end to insert the expansion tooling needs to be known.
Usually either a roll or hydraulic expander will be used for this process. Unless a roll
expansion is being performed at the end plate, the usual repair tooling is relatively short,
although there can still be access issues around the tubesheet periphery for hemi-head
channel covers and at pass partition plates.
For tube end plate expansions, the eddy current inspection records need to be reviewed to
determine if there are any tube inside diameter restrictions that would block the expander
from being inserted to the end plate location. The size of the eddy current probe used for the
inspection, plus any other hardware that has been inserted into the tube, can be used to help
determine the tube ID access issues. The potential for any tube end restrictions, that might
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limit tooling insertion into the tube, also need to be known so that tooling can be prepared to
eliminate the restriction.
If it appears that tube expansion is possible, then information will be needed to ensure that the
heat exchanger is properly repaired. The following information is used when planning for tube
expansion.
•
•
Tube expansion will need to be coordinated with eddy current inspection and plug removal.
The tube expander design (diameter and length) needs to be based on the requirements for
the expansion. Before performing tube expansions into heat exchanger tubes, testing needs
to be performed to set the tooling operating parameters. Depending on the type of expansion,
these tests may include setting the rolling torque for tubesheet re-expansions or setting the
hydraulic expansion constants for end plate expansions. In addition, for the tube-intotubesheet re-expansion process, qualification testing should be performed. This would
consist of hydrostatic leak and pressure tests and temperature and pressure cycling. These
tests would verify that the expansion parameters were set correctly for the tube reexpansions.
Conclusions
The costs associated with heat exchanger replacement can be significant. These costs include the
new heat exchanger or tube bundle, the manpower required to remove the old and install the new
heat exchanger components, plant modifications to allow for the removal of the heat exchanger,
and the amount of outage time associated with replacement. In addition, the replacement of a
heat exchanger can adversely affect other work going on in the their vicinity. Because of the
cost and time involved, and if the damage is confined to only the tubing (which is typically the
case), repair of the heat exchanger, through either sleeving or tube expansion, should be
considered. If the tube damage is confined to one general area, there is a good possibility that
the expense of a replacement can be avoided. In addition, the time required to prepare for tube
repair is much less than for replacement (2-6 weeks compared with 18 months), allowing a
decision on repair to be made just before, or even into, an outage.
By removing plugs and installing sleeves, it is possible to return lost heat transfer area to service.
Tubes that would be likely to fail in the near term also can be repaired. This will improve the
performance and reliability of the heat exchanger. The cost to perform the repairs is also much
less than for replacement (usually less than 1/10th the cost). Sleeving has been shown to be a
proven tube repair technique, having been performed since the 1970’s. During this time, tube
repairs have economically extended the useful life of heat exchangers worldwide.
As the number of plugged tubes approaches the upper limits or if damage is consistently
occurring in one area of a heat exchanger, tube repair, through both sleeving and tube
expansions, should be considered to minimize future damage and extend the life of the heat
exchanger.
The following table shows the various heat exchanger repair options and the factors to be
considered when choosing each of the options. Note that the table contains selected criteria for
evaluating component repair versus replacement options. A final decision to implement a
15
particular option should be made on a case by case basis with proper weight given to all factors.
The information listed in this table is for relative comparison purposes only.
Table 1
Repair/Replacement Summary Table
Repair
Option
Application
On-Site Time to
Implement
Lead Time
Required to
Implement
Longevity of
Selected Option
Component
Plus On-Site
Cost
Tube
Plugging
All tube defects, but
limited to ~10% of tubes
before affecting
performance
Minimal time,
typically <1 week
Minimal time,
typically <1 week
Long term repair
Minimal cost
Sleeving
Localized tube defects in
straight tube sections
Moderate time,
typically <3 weeks
Moderate time,
typically <1 month
Moderate to
long term repair
Less than 10%
of bundle or
heat exchanger
replacement
Tube
Expansion
FWH end plate, tube wear
at tube supports, certain
cases of leakage at
tubesheet expansion joints
Minimal time,
typically <2 weeks
Minimal time,
typically <2 weeks
Moderate to
long term repair
Minimal cost
Tube Bundle
Replacement
All tube defects, limited to
those units designed for
replacement bundles
Maximum impact
time, typically
~3 weeks
Moderate to
extended time,
typically >4 months
Long term repair
Less than
component
replacement
All tube and shell defects
Maximum impact
time, typically
~3 weeks
Maximum time
required to
implement, typically
>12 months
Long term repair
Highest cost to
implement
Heat
Exchanger
Replacement
References
1. NMAC Feedwater Heater Maintenance Guide. Charlotte, NC: Electric Power Research
Institute, May 2002. EPRI 1003470.
16
REPAIR vs. REPLACEMENT
Bruce W Schafer
Framatome ANP, Inc.
155 Mill Ridge Road
Lynchburg, VA 24502
(434) 832-3360
[email protected]
HEAT EXCHANGER TUBE SIDE MAINTENANCE –
REPAIR vs. Replacement
Bruce W Schafer
Framatome ANP, Inc.
155 Mill Ridge Road
Lynchburg, VA 24502
(434) 832-3360
[email protected]
Abstract
The traditional method of repairing degraded tubes in shell-and-tube heat exchangers is to
remove the effected tubes from service by plugging. Since heat exchangers are designed with
excess heat transfer capability, approximately 10% of tubes can be plugged before performance
is affected. When the number of plugged tubes becomes excessive, heat exchanger efficiency is
lost, resulting in reduced power output, high system pressure drop, further heat exchanger
damage, or abnormal loads placed on other plant heat exchangers.
As an option to component retubing or replacement, repair methods, including tube sleeving and
tube expansion, have proven to be an effective method to repair defective tubes and keep the
existing heat exchanger in service. For the sleeving process, a new tube section is installed
inside the existing tube to bridge across the degraded area. Tube expansion is used to close off a
gap between the tube and the tubesheet or end plate (to eliminate a leak path) or between the
tube and tube support (to minimize vibration). While not all heat exchangers can be returned to
their original design condition by performing tube repairs, in some instances it may be possible
to get many more years of useful life out of a heat exchanger at a fraction the cost of
replacement.
This paper presents options which the Plant Maintenance Engineer should consider in making
the repair versus replacement decision. This includes the repair options (sleeving and tube
expansion), other conditions within the heat exchanger, and the effect of tube repair on heat
exchanger performance.
Introduction
Traditionally, when maintenance is performed on shell-and-tube heat exchangers, the only
options considered when tube defects are found are to plug tubes and, when the number of plugs
became too great, replace the heat exchanger. The decision to replace the heat exchanger was
based on a number of factors. These included: the number of tubes plugged, the number of
forced outages due to tube damage (and the cost associated with replacing lost power and
repairing the damaged tubes), the impact that the plugged heat exchanger is having on the plant
(due to lost flow or heat transfer surface area), the rate at which tube plugging is occurring, the
availability of funds to replace the heat exchanger, and the expected life of the unit (how much
longer will the unit operate before retirement).
1
From a sampling of industry data, tube failures have been shown to cause between 31% to 87%
(depending on the data source) of the events related to feedwater heaters (1). Since so many of
the failures were related to the tubing, the replacement of an entire heat exchanger due to
damage in one area is an expensive as well as a schedule and manpower intensive option.
The typical means for major heat exchanger repair included complete replacement, rebundling,
and retubing, as described below.
•
•
•
For the replacement option, the entire heat exchanger shell and tube bundle are replaced with
a new unit.
For rebundling, the shell is temporarily removed from the heat exchanger and the old tube
bundle, including, at a minimum, tubes, tube supports, and tubesheet, are removed. A new
tube bundle is inserted and the shell is welded back in place.
For retubing, either the shell (u-tube design) or tube side access cover (straight tubes) is
removed from the heat exchanger and the old tubes are removed from the bundle. New tubes
are then inserted and re-attached to the tubesheet (typically by either mechanical expansion,
welding, or both). In many instances, the existing shell side hardware is used as-is, although
some modifications may be made. Retubing is typically performed on straight tube heat
exchangers, such as condensers and coolers.
Since the 1970’s, tube sleeving has been used to allow damaged tubes to remain in service. The
sleeves are installed by various means (roll, explosive, or hydraulic expansion, explosively
welded, or press-fit or epoxied in place) over the defective area of the tube. Through the use of
sleeving, which is a low-cost option to retubing, rebundling, or replacement, the useful life of a
heat exchanger can be economically extended. The decision to perform sleeving also can be
made with short notice as opposed to replacement (2-6 weeks compared with 18 months),
possibly allowing repairs to be performed the same outage that the damage is noted.
Tube expansion also can be performed to minimize or eliminate leakage within heat exchangers.
In the tubesheet, tubes can be re-expanded to strengthen the original tube-to-tubesheet joint,
reducing or eliminating leakage and prolonging the life of the heat exchanger. Expansions also
can be made deep within the tube to expand the tube into tube support plates and end plates.
These expansion can reduce tube-to-plate clearance for vibration control or, at end plates, to
minimize steam flow from the high to low pressure side of the plate.
Repair vs. Replace – Factors To Consider
There are numerous factors to consider when deciding whether to repair the tubes in a heat
exchanger or to perform a larger repair scope and rebundle or replace the component. The
following factors should be considered when making the repair vs. replace decision.
•
The budget available for repair or replacement needs to be determined. Typically, the cost of
performing a substantial heat exchanger repair (consisting of plug removal, tube inspection,
tube expansion, and sleeving) is less than 10% of the cost of replacing the unit. Because of
the lower cost, the payback time on the repair option is much shorter than for replacement.
If the heat exchanger is critical to plant operation (either from a safety, efficiency, or power
production standpoint) or is resulting in costly forced outages, it may be possible to justify a
2
repair to the unit in the near-term and a scheduled replacement when a longer outage can be
planned.
•
•
•
•
•
If there are a large number of tube plugs to remove, or if they are difficult to remove
(explosive or welded), then the cost to repair the heat exchanger will increase, and the
scheduled time needed on-site may not fit within the outage window. If it appears that tube
repair may be possible, it may be worthwhile to plug tubes, using removable plugs, until a
certain quantity of tubes are removed from service. At that point the plugs would be
removed and sleeves installed, thereby minimizing the overall maintenance cost.
The location and quantity of the tube defects need to be examined to decide if tube repair is
an option. Tube repair may be appropriate if the damage is limited to a certain area of the
tube, which would allow the use of a short repair sleeve. If the damage is over a significant
portion of the tube, it is possible to install a longer sleeve (up to the full length of the tube) to
ensure that all tube defects are repaired. However, if the u-bend region of the tube is
damaged then tube repair is not possible. Also, it would not be possible to install a sleeve if
a large portion of the tube had damage but there was inadequate clearance for a long sleeve
at the tube end.
One of the more important items to consider when deciding whether a heat exchanger can be
repaired is the condition of the remainder of the heat exchanger. The condition of the shell
side components, such as the impingement plates, tube supports, end plates, and other
structural members, should be in good shape if a long term repair is being planned. An
evaluation also should be made of the shell thickness in areas that are prone to shell
erosion/corrosion. If the tube repair is only a short-term fix, to allow component operation
until a replacement heat exchanger can be installed, the condition of the shell side is not as
critical.
The life expectancy of the power plant needs to be factored into the decision to repair or
replace a heat exchanger. If the only problem with the heat exchanger is in one section of the
tube, and the expected run time on the unit is relatively short, it would be advantageous to
repair rather than replace the heat exchanger since it will be very difficult to pay back the
cost for replacement over the remaining plant life.
The outage time required to repair a heat exchanger, even when tube and shell side
inspections are performed, is typically much less than for replacement. In addition, very few,
if any, plant modifications need to be made to make the repairs. This allows other work to be
performed in the vicinity of the heat exchanger.
Along with the shorter outage duration, the site support required for repair is much less.
Usually, there are no shell or head modifications required since all work can usually be
performed through the manways and pass partition plates. Less repair equipment is required,
resulting in less space being needed in the area of the heat exchanger for setup and storage.
In addition, the time required to prepare for tube repair is much less than for replacement (26 weeks compared with 18 months), allowing a decision on repair to be made just before, or
even during, an outage.
At nuclear plants, the added cost for the disposal of radioactively contaminated heat
exchangers must be taken into account. Before disposal, there is the cost of surveying the
3
•
heat exchangers for release and, if contamination is found, they must either be
decontaminated or disposed of as radioactive waste. Tube repairs can eliminate these costs.
If the heat exchanger is being replaced to eliminate detrimental materials in the cooling
system (i.e. copper in the condensate/feedwater system) then tube sleeving will not be
beneficial. The only practical solution would be to retube/rebundle/replace to change out the
tube material.
Heat Exchanger Repair Options
There have always been options available to either repair or replace heat exchanger tubes in the
event that tube leakage or degradation is present. The initial option, after the problem tubes have
been located (either through non-destructive examinations, such as eddy current testing, visual
inspections, or leak tests) is to plug the tube. Depending on the type of service and operating
pressures of the heat exchanger, various types of plugs are employed. These include tapered
fiber and metal pin plugs, rubber compression plugs, two piece ring and pin plugs, two piece
serrated ring and pin plugs (installed with a hydraulic cylinder), welded plugs, and explosively
welded plugs. In addition to the tube end plug, there also may be a stabilizer rod or cable that is
inserted into the tube to minimize future tube vibration damage.
At the beginning of the life of a heat exchanger, inserting a few plugs into damaged tubes has
little effect on the performance of the heat exchanger. However, if heat exchanger problems
continue, and the number of plugs increases significantly, it is possible that the heat exchanger
will eventually reach a point that it will not handle the full load that is placed on it. This is due
to a combination of loss of heat transfer area and the increased pressure drop. In addition, as the
number of plugged tubes increases, abnormal temperature conditions (either hot or cold spots)
may be set up in the heat exchanger. These conditions can result in an acceleration of tube
damage, creating a faster demise of the heat exchanger.
Once the number of plugs reaches a unacceptable level, the heat exchanger will need to be
repaired, replaced, or bypassed. However, bypassing the unit is usually not recommended, at
least for a long time period, since it will result in a loss of efficiency and heat transfer area.
Also, the heat load from the bypassed heat exchanger will be transferred to another heat
exchanger in the string, resulting in greater than normal operating flow rates and higher
degradation in that heater.
The following sections show the options that can be used to replace or repair the entire heat
exchanger or just the tubes.
Retubing
If the unit has straight tubes, good access, and the remaining components (shell, tube supports,
internal structural pieces) of the heat exchanger are in good shape, the tubes can be replaced.
The old tubes are removed from the unit and new ones, typically manufactured from an
improved material, are inserted, and then expanded, into place. Insertion of the new tubes is
shown in Figure 1. In addition to performing retubing to replace damaged tubes, retubing has
been performed to eliminate detrimental materials (such as copper from condenser tubes) to
4
minimize damage to other equipment within the plant (nuclear steam generators or fossil
boilers).
Figure 1
Condenser Retubing
Rebundling
Some heat exchangers are designed to be rebundled rather than replaced. For these units the
entire tube bundle, including tubes, tubesheet, and tube supports are replaced, as shown in
Figure 2. The original shell and any other internal structural pieces would be reused (although
any necessary internal repairs could be made when the shell was removed). The new tube
bundle can be manufactured to ensure that original design problems with the existing unit are
corrected. However, the same basic design must be maintained since the new bundle must fit
within the existing heat exchanger shell. Rebundling costs about 15-25% more than retubing (1).
Figure 2
Heat Exchanger Rebundling
5
Replacement
A third and typically widely used option is to replace the entire heat exchanger, as shown in
Figure 3. Full replacement allows alternate tube materials, changes in heat transfer area, and
structural changes to be employed, including added clearances in areas where erosion or other
problems may be occurring, to ensure that the current heat exchanger problems do not re-occur
in the future. However, the cost associated with a full replacement is the greatest of the three
options, about 5% more than for rebundling (1). In addition, there are no guarantees that the new
heat exchanger design will not have new, unanticipated problems.
Figure 3
Heat Exchanger Replacement
Sleeving
An alternate approach to retubing, rebundling, or replacement of a heat exchanger is to install
sleeves over the defective portions of the tubes The sleeve consists of a smaller diameter piece
of tubing that is inserted into the parent tube and positioned over the tube defects. After
insertion, each end of the sleeve is expanded into the parent tube material. These expansions
serve the dual function of structurally anchoring the sleeve into the tube and providing a leak
limiting path, allowing the sleeve to become the new pressure boundary for the tube. This means
that a sleeved tube can have a 100% through-wall indication and still remain in-service, since the
sleeve is now the new structural and pressure boundary. The installation of the sleeve into the
tube will allow the majority of the tube’s heat transfer area and flow to be maintained.
If heat exchanger repair by sleeving is a possibility then a strategy needs to be used to prepare
for future repair. It may be cost effective to plug a quantity of tubes, per the non-destructive
examination results, each outage using a removable plug. When the quantity of plugged tubes
reaches a certain level the plugs can be removed and sleeves installed. Using this approach will
minimize the cost and time during each inspection outage while allowing the maximum tube
repair later in the heat exchanger’s life.
6
There are three types of sleeves that are installed into heat exchanger tubes. These are full
length, partial length structural, and partial length barrier sleeves. The three types are discussed
below. Figure 4 shows the sleeve layout.
Figure 4
Heat Exchanger Sleeve Designs
Full Length Sleeve
These sleeves are installed from one end of the tube to the other in straight tubed heat
exchangers. After insertion, the full length of the sleeve is expanded into the parent tube. This
step serves the dual purpose of maintaining heat transfer as high as possible (typically 75%-90%)
while minimizing flow pressure drop through the tube. After the full length expansion step,
shown in Figure 5, the sleeve ends are trimmed flush with the existing tube ends and the sleeve
is roll expanded into the tubesheet.
The full length sleeve is typically used in a condenser or cooling water heat exchanger when the
tubes have multiple defects along their length. Full length sleeving is an attractive option when a
relatively small percentage of the tubes require repair. Through sleeving, the majority of the
tube heat transfer area can be left in service, resulting in a heat exchanger that is close to its asdesigned condition.
Full length sleeving is comparable in many ways to retubing in the methods employed to install
the sleeves. However, since removal of the existing tube is not required, and the typical number
of tubes that will be full length sleeved are below the number that would be retubed, the cost for
7
material and manhours are much less than for retubing, making sleeving a cost-effective option
to return and keep tubes in service.
Figure 5
Full Length Sleeve Expansion
Partial Length Structural Sleeve
This type of sleeve is used to repair shorter defects in the tube. The sleeve can be installed
anywhere along the straight length of the tube. Various methods are used to expand the sleeve in
place. These include roll expansion (both in the tubesheet and in the freespan portion of the
tube), hydraulic expansion in the freespan portion of the tube, and full length expansion. These
expansion types are discussed below. The installation of a hydraulically expanded sleeve is
shown in Figure 6.
•
•
If one end of the sleeve is in the tubesheet, a torque-controlled roll expansion will be made.
This expansion is similar to the original tube-to-tubesheet roll. Freespan roll expansions are
made to either a torque controlled setting or to a diameter controlled hardstop setting.
Usually, freespan roll expansions are only used when the sleeve length is relatively short,
since it can be difficult to insert a roll expander deep into the tube. Both the tubesheet and
freespan roll expansion parameters are set so that they can provide both the structural and
leakage requirements for the sleeve.
For sleeves installed deep within the tube, a hydraulic expansion device is used to connect
the sleeve to the tube. The expander consists of multiple plastic bladders that are filled with
high pressure water. As the water pressure increases, the bladders expanded against the
inside of the sleeve, pushing the sleeve into the tube. The expansion process, which is
computer controlled, continues until either a preset volume of water or a preset pressure is
reached. At this point the sleeve is properly expanded and the bladders are depressurized.
Hydraulic expansions can be made anywhere along the tube length since the expander is
connected to flexible high pressure tubing and is not restricted by tube end access. The
8
•
expansion parameters are qualified to meet the proper structural and leakage requirements for
the sleeve.
Full length expansions are not usually used for structural or leak limiting purposes but
instead are used to improve heat transfer and flow through the sleeve and to close the annulus
between the sleeve and tube. The full length expansion is made by placing a tool, with seals
on each end, into the sleeve. The inside of the sleeve is filled and then pressurized with
water to a preset pressure setting, expanding the sleeve into tight contact with the tube. After
the full length expansion is made, the ends of the sleeve are typically either roll or
hydraulically expanded to form the structural and leak limiting sleeve-to-tube joint.
Many times, the partial length structural sleeves are used to repair indications at one particular
area of the tube, such as wear damage at tube support locations, cracking in roll transitions, or
pitting indications at one discreet location along the tube length. Longer versions of these
sleeves also have been used to repair an entire damaged section of a heat exchanger, such as a
desuperheater or drain cooler section of a feedwater heater. Because of the wide variety of uses,
the sleeve length can range from as short as 1 foot to over 12 feet in length.
Qualification testing is performed on the structural sleeves to ensure that they can withstand the
design temperature and pressure conditions imposed on them. The test results must show that
the sleeve will be the new pressure boundary even with a 100% through-wall indication in the
parent tube. Sleeves of this type, using mechanical expansions (roll and hydraulic), have reliably
been in-service for more than 15 years.
Figure 6
Partial Length – Hydraulically Expanded Structural Sleeve Installation
Partial Length Barrier Sleeve
These sleeves, also known as shields, are used at the ends of the tubes to act as a barrier to tube
end erosion. These sleeves are usually very thing, are not designed to act as a pressure boundary
or structural repair, and are installed in areas of high turbulence. The materials for these sleeves
are compatible with the existing tube material and may include plastic inserts. The sleeves are
9
either roll or hydraulic expanded or pressed or epoxied in place. If tube end erosion is occurring,
or is expected to occur, the use of these tube end sleeves will protect and prolong the life of the
parent tube, although over time tube erosion may begin to occur at the end of the sleeve. Many
heat exchanger tube ends have been protected with shields, significantly prolonging the life of
the tubes.
Items to Consider for Tube Repair
Prior to choosing to perform tube sleeving, the following factors should be considered.
•
•
•
The length, location, and quantity of tube defects that would require sleeving need to be
determined. If the defects are in one or a few short areas then either a single or a couple of
partial length sleeves could be used. However, if the defects are spaced throughout the
length of the tube, then the only option would be a full length sleeve.
The parent tube in the area where the sleeve will be expanded is to be defect free. This will
insure the highest sleeve-to-tube joint integrity. Also, the tube support designations must be
clearly identified to insure that the sleeve is installed at the correct location along the tube
length. This is especially true in areas where there may be skipped baffles and the tube only
touches every other support plate.
The condition of the remainder of the tube away from the sleevable defects needs to be
known. If there are u-bend defects that may require plugging then the tube should not be
sleeved. Sleeving is an option if the remainder of the tube is in good shape.
The space available at the tube end to insert a sleeve and its installation tooling needs to be
known, as shown in Figure 7. If a short, partial length sleeve is being used, the amount of
space required is not as critical, although there can still be access issues around the tubesheet
periphery for hemi-head channel covers and at pass partition plates. However, if a full length
sleeve is required, there will need to be a significant amount of clearance from the tubesheet
face.
Figure 7
Required Clearance for Sleeve Installation
•
Inspection records need to be reviewed to determine if there are any tube inside diameter
(ID) restrictions that would block the sleeve from being inserted to the target location. The
size of the eddy current probe used for the inspection, plus any other hardware that has been
inserted into the tube, can be used to help determine the tube ID access issues.
10
•
•
The post-sleeving tube inspection requirements need to be considered. Typically, the ability
to inspect the tube beyond a sleeve is not a significant issue. While the presence of the
sleeve reduces the inside diameter of the tube, which will result in the need for a smaller
inspection probe, the probe will remain large enough to detect pluggable tube indications
(usually greater than 40%), however small indications may go undetected.
As part of the post-sleeve inspection, the sleeve and its attachment to the tube should be
examined. There is no need to inspect the section of the parent tube between the sleeve
expansions since this is no longer part of the pressure boundary.
If tube cleaning is to be performed in the heat exchanger, then the type of sleeve to be
installed needs to be evaluated. If on-line cleaning is performed, the sleeve size cannot
restrict the passage of the balls or brushes. For off-line cleaning, the projectiles need to pass
through the sleeve without becoming stuck. Many sleeves that are installed in tubes that
require cleaning are full length expanded to ensure the best results for the cleaning
equipment.
If it appears that tube sleeving is possible, then information will be needed to ensure that the heat
exchanger is properly repaired. The following information is used when planning for sleeving.
•
•
•
•
•
•
Tube sleeving will need to be coordinated with eddy current inspection and plug removal.
If it is expected that sleeving may be performed, then it is important that the proper sleeve
material be purchased in advance of the job.
The sleeve material needs to be compatible with the heat exchanger parent tubing and with
the water chemistry within the heat exchanger. The galvanic corrosion potential between the
sleeve and tube needs to be determined. Also, effects of crevice corrosion between the
sleeve and tube, in the heat exchanger water chemistry, need to be considered to determine if
sleeving is a viable repair option.
The sleeve dimensions need to fit the heat exchanger operating and design conditions plus
any restrictions within the tube ID. The sleeve outside diameter (OD) is to be designed to fit
into the tube but must be long enough to limit the amount of sleeve expansion. The sleeve
wall thickness needs to be sized for the heat exchanger operating parameters, including any
ASME Code minimum wall thickness calculations, if needed. The sleeve length must be
long enough to span the expected tube defects but needs to be sized to fit any tube end
clearance restrictions.
Before installing sleeves into heat exchanger tubes, testing needs to be performed to set the
installation parameters. Depending on the type of sleeve being used, these tests may include
setting the rolling torque, hydraulic expansion constants, and full length expansion pressure.
In addition, depending on the application for the sleeve, there may be a need to do
qualification testing, which would consist of hydrostatic leak and pressure tests and
temperature and pressure cycling. These tests would verify that the expansion parameters
were set correctly for the sleeve application.
If a large quantity of sleeves are being installed, it may be necessary to calculate the heat
transfer and flow loss due to sleeving. These calculations will give a sleeve-to-plug ratio that
can be used to determine the expected improvement in heat exchanger performance after
sleeving is complete (and tubes have been returned to service, if applicable).
11
•
The sleeve may need to be full-length expanded based on the heat exchanger operating
environment. However, the production rates for sleeve installation are lower when full
length expansions are performed. While full length expansion is typically not needed in
many applications, such as most feedwater heaters, it should be considered for the following.
− if tube ID cleaning needs to routinely be performed
− if a long sleeve is being inserted that would severely restrict the tube’s heat transfer or
flow
− if the tube-to-sleeve crevice needs to be eliminated in a hostile water chemistry
environment
− if there are large eddy current probe fill factor restrictions
Heat Exchanger Tube Expansion Repair
In addition to sleeving, it is possible to expand the tube to improve the heat exchanger
performance. These tube repairs can minimize further tube damage and maximize the useful life
of the heat exchanger. Two methods of tube expansion can be performed. One is to expand
deep within the tube to close off a leak path between the tube and the end plate. The other is to
re-expand the tube into the tubesheet to minimize tube-to-shell side leakage.
Tube-to-End Plate Expansion
In some heat exchangers, typically feedwater heaters, there are internal plates which separate one
zone of the heat exchanger from another (usually condensing [steam] from drain cooler [liquid]).
Due to the pressure differential across the plate, and the different temperatures and phases
between the two sections, it is important that leakage not occur through the plate. However, in
some feedwater heaters, the plate design is too thin, resulting in leakage of steam from the
condensing to the drain cooler zones, as shown in Figure 8. When this occurs there is erosion of
the end plate and tube vibration due to the high steam velocities and the steam condensing to
liquid in the drain cooler region. The vibration causes wear at the tube supports which can lead
to tube failure. The leakage of steam also increases the drain cooler temperature, resulting in a
less efficient heat exchanger.
12
Figure 8
End Plate Leakage in a Feedwater Heater
Expanding the tube can reduce the gap between the tube and the end plate. The expansion can
be performed using either a roll or hydraulic expander. Once the expander is in position the tube
is expanded until it contacts the end plate. An accurate expansion, which does not over-expand
the tube into the plate (the tube needs to be able to slide in the plate after expansion so that it
does not buckle during heatup/cooldown), needs to be performed. This can be achieved by using
a computer controlled hydraulic expansion that automatically shuts off the pressurization system
when it detects that the tube has contacted the plate.
After the tubes are expanded into the end plate, the steam flow is minimized or eliminated,
reducing the drain cooler temperatures and increases plant efficiency. Further tube damage, in
the form of tube wear and adjacent tubes impacting on one another, will be reduced to nearly
zero and the vibration operating stresses will be reduced significantly. The life of the heat
exchanger will be increased at a minimal cost as compared with replacement.
Tube-to-Tubesheet Expansion
In some heat exchanger designs, with a certain combination of materials, leaks develop between
the tube and tubesheet. In many low pressure units, the tube is only expanded into the tubesheet,
with no subsequent weld. Many of the leaks that occur in these units are the result of a
fabrication error and can be corrected by re-expanding the joint to the correct expansion size.
However, leakage occasionally occurs in high pressure heat exchangers, typically feedwater
heaters, even when the tubes have been welded to the tubesheet. The two prime causes of this
leakage are in areas where the original tube-to-tubesheet weld has either cracked or eroded due
to flow (in the case of soft materials, such as carbon steel) or where there is a crack in a tube-totubesheet expansion transition.
13
•
•
For the first case it may be possible to re-expand the tube using a qualified roll expansion
process. The expansion would increase the contact pressure between the tube and tubesheet,
increasing the resistance to flow and decreasing or eliminating leakage. This process could
be performed on existing leaking tubes or preventatively on all tubes in the tubesheet.
If cracking is occurring at the original tube expansion transition it may be possible to
re-expand the tube deeper in the tubesheet (unless the cracking is occurring very close to the
shell side of the tubesheet). The tube would be expanded using a qualified roll expansion
process, to place the tube into tight contact with the tubesheet. This expansion would
increase the contact pressure between the tube and tubesheet, increasing the resistance to
flow and decreasing or eliminating leakage. This process could be performed either on
existing leaking tubes or preventatively on all tubes in the tubesheet.
Re-expanding tubes that either may be leaking or that could develop leaks in the future could
significantly extend the life of an otherwise good heat exchanger. By re-expanding the tubes,
forced outages can be avoided and damage from the high pressure water spraying on adjacent
tubes and on the shell will be eliminated. The cost to perform tube re-expansions will be
minimal when compared with the cost of replacement heat exchangers and the cost of forced
outages.
Items to Consider for Tube Expansion Repair
The following factors should be considered to determine if tube expansion is possible.
•
•
•
•
The portion of the tube to be expanded needs to be determined.
− If leakage is occurring through the end plate, the expander will need to be long enough to
reach the end plate location. The tube should be expanded using a process, such as
hydraulic expansion, that will not lock the tube into the end plate. This expansion will
not only reduce leakage through the plate but also will minimize future tube vibration due
to the tight fit between the tube and plate.
− If leakage is occurring within the tubesheet, due to either weld or tube cracking, a
re-expansion process may be used. This process, typically a roll expansion, will reexpand the tube into the tubesheet to limit or eliminate leakage from the tube to the shell
side of the heat exchanger.
The condition of the remainder of the tube needs to be known. If there are cracks along the
entire tube length then re-expanding the tube alone will not result in an improvement in heat
exchanger performance.
The space available at the tube end to insert the expansion tooling needs to be known.
Usually either a roll or hydraulic expander will be used for this process. Unless a roll
expansion is being performed at the end plate, the usual repair tooling is relatively short,
although there can still be access issues around the tubesheet periphery for hemi-head
channel covers and at pass partition plates.
For tube end plate expansions, the eddy current inspection records need to be reviewed to
determine if there are any tube inside diameter restrictions that would block the expander
from being inserted to the end plate location. The size of the eddy current probe used for the
inspection, plus any other hardware that has been inserted into the tube, can be used to help
determine the tube ID access issues. The potential for any tube end restrictions, that might
14
limit tooling insertion into the tube, also need to be known so that tooling can be prepared to
eliminate the restriction.
If it appears that tube expansion is possible, then information will be needed to ensure that the
heat exchanger is properly repaired. The following information is used when planning for tube
expansion.
•
•
Tube expansion will need to be coordinated with eddy current inspection and plug removal.
The tube expander design (diameter and length) needs to be based on the requirements for
the expansion. Before performing tube expansions into heat exchanger tubes, testing needs
to be performed to set the tooling operating parameters. Depending on the type of expansion,
these tests may include setting the rolling torque for tubesheet re-expansions or setting the
hydraulic expansion constants for end plate expansions. In addition, for the tube-intotubesheet re-expansion process, qualification testing should be performed. This would
consist of hydrostatic leak and pressure tests and temperature and pressure cycling. These
tests would verify that the expansion parameters were set correctly for the tube reexpansions.
Conclusions
The costs associated with heat exchanger replacement can be significant. These costs include the
new heat exchanger or tube bundle, the manpower required to remove the old and install the new
heat exchanger components, plant modifications to allow for the removal of the heat exchanger,
and the amount of outage time associated with replacement. In addition, the replacement of a
heat exchanger can adversely affect other work going on in the their vicinity. Because of the
cost and time involved, and if the damage is confined to only the tubing (which is typically the
case), repair of the heat exchanger, through either sleeving or tube expansion, should be
considered. If the tube damage is confined to one general area, there is a good possibility that
the expense of a replacement can be avoided. In addition, the time required to prepare for tube
repair is much less than for replacement (2-6 weeks compared with 18 months), allowing a
decision on repair to be made just before, or even into, an outage.
By removing plugs and installing sleeves, it is possible to return lost heat transfer area to service.
Tubes that would be likely to fail in the near term also can be repaired. This will improve the
performance and reliability of the heat exchanger. The cost to perform the repairs is also much
less than for replacement (usually less than 1/10th the cost). Sleeving has been shown to be a
proven tube repair technique, having been performed since the 1970’s. During this time, tube
repairs have economically extended the useful life of heat exchangers worldwide.
As the number of plugged tubes approaches the upper limits or if damage is consistently
occurring in one area of a heat exchanger, tube repair, through both sleeving and tube
expansions, should be considered to minimize future damage and extend the life of the heat
exchanger.
The following table shows the various heat exchanger repair options and the factors to be
considered when choosing each of the options. Note that the table contains selected criteria for
evaluating component repair versus replacement options. A final decision to implement a
15
particular option should be made on a case by case basis with proper weight given to all factors.
The information listed in this table is for relative comparison purposes only.
Table 1
Repair/Replacement Summary Table
Repair
Option
Application
On-Site Time to
Implement
Lead Time
Required to
Implement
Longevity of
Selected Option
Component
Plus On-Site
Cost
Tube
Plugging
All tube defects, but
limited to ~10% of tubes
before affecting
performance
Minimal time,
typically <1 week
Minimal time,
typically <1 week
Long term repair
Minimal cost
Sleeving
Localized tube defects in
straight tube sections
Moderate time,
typically <3 weeks
Moderate time,
typically <1 month
Moderate to
long term repair
Less than 10%
of bundle or
heat exchanger
replacement
Tube
Expansion
FWH end plate, tube wear
at tube supports, certain
cases of leakage at
tubesheet expansion joints
Minimal time,
typically <2 weeks
Minimal time,
typically <2 weeks
Moderate to
long term repair
Minimal cost
Tube Bundle
Replacement
All tube defects, limited to
those units designed for
replacement bundles
Maximum impact
time, typically
~3 weeks
Moderate to
extended time,
typically >4 months
Long term repair
Less than
component
replacement
All tube and shell defects
Maximum impact
time, typically
~3 weeks
Maximum time
required to
implement, typically
>12 months
Long term repair
Highest cost to
implement
Heat
Exchanger
Replacement
References
1. NMAC Feedwater Heater Maintenance Guide. Charlotte, NC: Electric Power Research
Institute, May 2002. EPRI 1003470.
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