Note: Descriptions are shown in the official language in which they were submitted.
CA 02838147 2013-12-23
Heat Exchanger Descaling System Using Blowdown
Field of the Invention
The invention relates to systems and methods for descaling or defouling a heat
exchanger and
more specifically to the use of heated caustic water for descaling or
defouling a heat exchanger.
Background
In steam-based thermal recovery operations that are typically aimed at
recovering bitumen or
heavy oil, a longstanding effective approach to raising steam has involved the
use of once-
through steam generators (OTSG). Feedwater to the once-through steam generator
can come
from many sources and, depending upon the properties of the raw water, is
treated to render it
suitable as a feed stream for a OTSG. In general, a OTSG is operated so that
wet steam, typically
around 80% steam quality, is generated, although other levels of steam quality
may be selected.
In some recovery operations, such as those involving steam-assisted gravity
drainage (SAGD),
the wet steam is first separated into its vapor and liquid components by means
of a steam
separator at the outlet of the once-through steam generator. The steam thus
generated is injected
into an oil sands reservoir containing bitumen, or into a reservoir containing
heavy oil. The
steam heats and mobilizes the bitumen or heavy oil. When the mobile
hydrocarbon liquid is
lifted to the surface, it is part of a mixture that also contains water from
condensed steam,
formation water, and various minerals and other constituents which may be
dissolved or
suspended in the mixture, along with vapor and gaseous constituents. After
appropriate gas-
liquid separation followed by treatment of the liquid stream to substantially
segregate produced
water from the produced liquid hydrocarbon constituent, current oilfield
practice often involves
some form of recycling of the produced water.
Heat exchangers, such as a water:boiler feedwater (BFW) heat exchanger or
water:glycol heat
exchanger, are often used to cool produced water for later use. For example,
produced water
may be 125 C and through the heat exchangers is cooled to 90 C. The produced
water that is
being cooled by the heat exchangers generally includes deposits or impurities
such as minerals,
acids, clays etc., which cause the heat exchangers to gradually become fouled
with deposits
referred to as scale. Typically, in hydrocarbon production operations,
produced water is cooled
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CA 02838147 2013-12-23
using the heat exchangers and the scale comprises various organics including
organic acids and
clays that are brown in colour and generally flaky and adhere strongly to the
interior surfaces of
the heat exchanger and associated components and pipes. The fouling or scale
buildup on the
interior of the heat exchanger reduces the heat transfer rate to the produced
water side. The flow
rate of the produced water passing through the heat exchanger is also reduced
as the interior
diameter is reduced by the scale. Over time, the heat transfer rate and the
flow rate are reduced
sufficiently that the heat exchanger must be descaled or defouled in order to
remove the buildup
of deposits to restore the heat transfer ability of the heat exchanger and
restore the flow rate. An
example of the fouling of the interior of a produced water line is shown in
Figure 1 which shows
the interior of a fouled vortex meter in a heat exchanger system. In addition,
Figure 2 shows the
fouling tendencies of three heat exchangers as a function of heat transfer
rate versus run life
which shows how the scale buildup affects the transfer of heat into the
produced water running
through the heat exchangers.
It has been seen in various hydrocarbon operations that there are varying time
periods for heat
exchanger cleaning to achieve suitable results. Typically, the time frame for
cleaning a heat
exchanger is around 30-50% of the run life, where the run life is generally
between 36-48 hours.
This means that for a run life of 36-48 hours the exchanger may have to be
shut down for up to
24 hours in order to restore a suitable operational flow rate of the heat
exchanger. Typical
cleaning of an exchanger often involves the hiring and cost of a third party
contractor to
physically dismantle and remove the build up often through the use of
chemicals. Operation of
the heat exchanger during this time period is halted or at the very least
reduced as water may be
redirected through other heat exchangers, but nevertheless, the overall flow
rate through the heat
exchangers of the operation is reduced during servicing of the heat exchanger.
In addition, at
some point each heat exchanger in the operation must be taken off-line for
cleaning.
Furthermore, the use of chemicals in cleaning creates the need for disposal of
these chemicals
after use.
A need therefore exists to provide a system to obviate or mitigate at least
one disadvantage of the
prior art.
Summary of the Invention
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CA 02838147 2013-12-23
In one aspect, the present disclosure provides a descaling system for
descaling or defouling a
heat exchanger utilized in a produced water system of a steam-based thermal
hydrocarbon
recovery operation. Wet steam exiting a OTSG, or other suitable steam
generator, is separated
into its substantially vapor and substantially liquid components by means of a
steam separator at
the outlet or downstream of the OTSG. The vapor component exiting the steam
separator,
comprising substantially 100% steam quality, also known as dry saturated
steam, is injected into
the reservoir. The liquid component exiting the steam separator is referred to
as blowdown, and
contains, in concentrated form, substantially all of the impurities that were
originally in the
feedwater. A portion of blowdown is maintained to ensure minimal to no scaling
in the OTSG.
The blowdown, with its high impurity levels, is generally disposed of and is
considered a waste
product.
In another aspect, it has been determined that blowdown may be used to remove
the scale
buildup or fouling, referred to as descaling or defouling, in the heat
exchanger, returning the
exchanger to operational parameters. As a result, a system has been developed
to utilize the
blowdown that is typically wasted and instead reuse the blowdown in descaling
or defouling the
heat exchangers of a hydrocarbon production operation in less time than is
traditionally required
to service the heat exchanger. In addition to reducing the downtime or offline
time of the heat
exchanger, the reuse of the blowdown, in certain embodiments, may also reduce
the net amount
of water or energy consumed by the steam-based thermal hydrocarbon recovery
operation.
In one illustrative non-limiting embodiment, the blowdown may be taken from a
point further
downstream of the steam separator in the produced water system where some heat
captured in
the blowdown has been removed from the blowdown for reuse, thereby reducing
the net cost of
heat exchanger cleaning as this further downstream blowdown may be purely a
waste stream and
the energy contained therein is lost.
In one embodiment of the present invention there is provided a system for
descaling a heat
exchanger in a produced water system of a hydrocarbon production operation
using caustic
heated water to descale the heat exchanger, the system comprising:
a) a heat exchanger for acting on a produced water;
b) a caustic heated water supply; steam separator in fluid communication with
a steam
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generator for separating steam of a steam quality suitable for injection into
a hydrocarbon
reservoir and blowdown of a lower steam quality, the blowdown being caustic;
c) a caustic heated water loop in fluid communication with the caustic heated
water
supply, the caustic heated water loop loop also in fluid communication with
the heat exchanger
for directing the caustic heating water through the heat exchanger for
descaling the heat
exchanger;
d) a valve for controlling the rate of flow of the caustic heated water
through the heat
exchanger; and
e) a valve for preventing flow of a produced water through the heat exchanger
during
In a further embodiment of a system or systems as outlined above, the caustic
heated water is
blowdown and the system further comprises:
g) a steam separator in fluid communication with a steam generator for
separating steam
and wherein the caustic heated water loop is a blowdown loop in fluid
communication
with the steam separator for collecting the caustic blowdown, the blowdown
loop also in fluid
communication with the heat exchanger for directing the caustic blowdown
through the heat
In a further embodiment of a system or systems as outlined above, the caustic
heated water has a
pH of at least about 10.5.
steam quality of 40% or lower.
In a further embodiment of a system or systems as outlined above, the caustic
heated water has a
saturated liquid temperature of about 310 C or lower.
In a further embodiment of a system or systems as outlined above, the system
comprises a
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CA 02838147 2013-12-23
plurality of heat exchangers, each heat exchanger in an isolatable cleaning
loop in
communication with the caustic heated water loop, thereby allowing individual
isolation of one
or more heat exchangers with the caustic heated water loop for descaling the
one or more
isolated heat exchangers.
In a further embodiment of a system or systems as outlined above, the system
further comprises
a pressure letdown valve for reducing the pressure of the blowdown before
inlet to the heat
exchanger to ensure the blowdown is at a pressure below an operating maximum
of the heat
exchanger.
In a further embodiment of a system or systems as outlined above, the system
further comprises
a blowdown cleaning tank for removing precipitate from the blowdown following
descaling of
the heat exchanger.
In a further embodiment of a system or systems as outlined above, the caustic
heated water loop
comprises valving allowing for the blowdown to be passed through the heat
exchanger in either a
direction of the process flow or in a reverse direction to the process flow.
In a further embodiment of a system or systems as outlined above, the system
further comprises:
a blowdown exchanger in fluid communication with the blowdown loop for
removing
some heat from the blowdown and producing a secondary caustic blowdown having
a lower
steam quality than the blowdown generated by the steam separator; and
a secondary blowdown loop in fluid communication with the blowdown exchanger
for
collecting the secondary caustic blowdown, the secondary blowdown loop also in
fluid
communication with the one or more heat exchangers for directing the secondary
caustic
blowdown through the one or more heat exchangers for descaling the one or more
heat
exchangers.
In a further embodiment of a system or systems as outlined above, the
secondary caustic
blowdown has a steam quality of 20% or lower.
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CA 02838147 2013-12-23
In a further embodiment of a system or systems as outlined above, the
secondary caustic
blowdown has a saturated liquid temperature of about 170 C or lower.
In another embodiment of the present invention there is provided a method of
descaling a heat
exchanger for use with produced water generated from a hydrocarbon production
operation, the
method comprising:
i) generating a caustic heated water from a hydrocarbon production operation;
ii) bringing the heat exchanger offline; and
iii) passing the caustic heated water through the heat exchanger.
In a further embodiment of a method or methods as outlined above, the method
further
comprises:
iv) reducing the pressure of the caustic heated water to a level below the
operating
threshold of the heat exchanger before step iii).
In a further embodiment of a method or methods as outlined above, the caustic
heated water is
blowdown generated from a steam generator/separator system.
In a further embodiment of a method or methods as outlined above, the caustic
heated water has
a pH of about 10.5 or higher.
In a further embodiment of a method or methods as outlined above, step iii) is
carried out for
between about 1 and 12 hours.
In a further embodiment of a method or methods as outlined above, step iii) is
carried out for
between about 2 and 4 hours.
In a further embodiment of a method or methods as outlined above, step iii) is
carried out for
about 1 hour.
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CA 02838147 2013-12-23
Brief Description of the Drawings
Figure 1 is a photo showing the fouling or scale-buildup of a vortex meter in
a produced water
line;
Figure 2 is a graph showing the fouling tendencies as a function of heat
transfer in three different
water/BFW heat exchangers;
Figure 3 is a graph showing the cleaning trends as a function of heat transfer
illustrating the
effect of cleaning or descaling using one embodiment of a system of the
present invention on two
heat exchangers;
Figure 4 is a schematic of one embodiment of a system for cleaning a heat
exchanger using
blowdown;
Figure 5 is a schematic of another embodiment of a system for cleaning a
series of heat
exchangers using blowdown;
Figure 6 is a chart showing the change in average heat transfer versus
cleaning time of both
primary blowdown (V-2020) and secondary blowdown (E-2030);
Figure 7 is a chart showing clean heat transfer (HT) coefficient versus
cleaning time,
demonstrating that cleaning times as low as 1 hour have been shown to be
effective in at least
partially cleaning the heat exchangers wherein the clean HT coefficient is a
function of other
variables;
Figure 8 is a chart showing Differential Fouling Factor Diff R versus Dirty
Fouling Factor R; and
Figure 9 is a chart showing Diff R versus R in reaction to cleaning time.
Detailed Description
For the purposes of this disclosure, the terms "descaling" and "defouling" may
be used
interchangeably and refer to the removal or at least partial removal of
deposit or scale buildup or
fouling that occurs as a result of the operation of a heat exchanger with
produced water passing
therethrough. It will be appreciated that the scale or fouling may be
comprised of a number of
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CA 02838147 2013-12-23
different compounds including for example organics and/or organic acids and
clays. Just as the
terms "descaling" and "defouling" may be used interchangeably, the terms
"scale", "fouling"
and "deposits" may also be used interchangeably for the purposes of this
disclosure.
It will also be appreciated that the term "steam" encompasses heated water, as
at various
operating temperatures and pressures heated water may be in the vapour phase
and unless
otherwise indicated, the terms "steam" and "heated water" are not intended to
be mutually
exclusive. It is typical to observe both phases of vapour and fluid in the
blowdown as would be
appreciated.
It will also be appreciated that although reference is made to the descaling
of a heat exchanger,
the descaling or partial descaling of piping and/or components associated with
the heat
exchanger may also be carried out to some level.
It has been determined that blowdown may be used to remove the scale buildup
or fouling in a
heat exchanger that operates with production water passing therethrough.
Blowdown from the steam separator generally comprises a heated water and/or
steam under
pressure. The heated water and/or steam is of a temperature either
insufficient for use in the
hydrocarbon production operation or is below a predetermined quality threshold
of the steam
separator and is partitioned by the steam separator and deemed as blowdown.
The blowdown is
typically caustic in nature and generally has a pH is some systems of from
about 10.5-12. The
pH may however be outside of this range. The blowdown being concentrated with
high impurity
levels is generally caustic as a result of the presence of those impurities.
Caustic heated water including blowdown has been shown to be useful in
removing fouling
buildup, primarily organic acids and clays, and returning heat exchangers to
clean operating
conditions. Figure 3 shows a graph of three heat exchangers and their heat
transfer rates over
time as they gradually become fouled and are then cleaned using a system
according to the
present invention. As can be seen, following blowdown cleaning, heat transfer
rates are restored
to initial or near-initial conditions.
Test results have shown that blowdown may be used to chemically remove fouling
in heat
exchangers. As a result, a system has been developed to utilize caustic heated
water, such as
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CA 02838147 2013-12-23
blowdown, that is typically wasted and to instead reuse the blowdown in
descaling the heat
exchangers. In this way, the energy contained within the blowdown is not
wasted but rather
reused.
One non-limiting embodiment of a descaling system that uses blowdown for
descaling a heat
exchanger in a hydrocarbon production operation is shown generally in the
schematic of a
blowdown cleaning system 12 in Figure 4. Wet steam from a steam generator 8 is
input to a
steam separator 10. It will be appreciated that steam may be provided by a
plurality of steam
generators such as OTSG's which may be arranged in series. Generally, the
input to the steam
separator 10 has a steam quality lower than 100% and generally, but without
wishing to be
limiting, the steam quality may be around about 80%. The steam separator 10
separates steam
into two components, dry saturated steam for use in downhole injection into a
well, for example
in a steam-assisted gravity drainage (SAGD) operation, and blowdown. In one
embodiment, the
blowdown comprises a steam having a steam quality of 40% or lower. In another
embodiment,
the blowdown may be characterized as 310 C saturated liquid. Of course, it
will be appreciated
that a range of temperatures can be achieved by controlling the system
pressure and that the
pressure drop across a control valve of the steam separator 10 can result in
two-phase flow. The
blowdown exiting the steam separator 10 is caustic and in one embodiment, the
blowdown may
have a pH of about 10.5-12. Introduction of the blowdown into the cleaning
loop occurs via the
operation of valve 100.
The blowdown water may then be directed through a heat exchanger 30 for
descaling the heat
exchanger 30.
As is shown in Figure 4, the piping and valving of the blowdown cleaning
system 12, may be
configured to facilitate cleaning of the heat exchanger 30 by the flow of
blowdown in the
direction of the process flow or in the reverse direction (otherwise known as
backwash flow). In
one embodiment, normal cleaning may be achieved via backwash flow. Flow of
blowdown may
be controlled through various valves in the process flow piping and the
blowdown piping. It will
be appreciated that any suitable valving arrangement may be used for carrying
out the recycling
of the blowdown through the blowdown cleaning system 12 for descaling heat
exchanger 30.
The non-limiting schematic of the system 12 shown in Figure 4 is arranged to
allow for caustic
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CA 02838147 2013-12-23
heated water, referred to as the cleaning flow, to be optionally with or
against produced water
flow through the heat exchanger 30. For example, to operate blowdown cleaning
fluid against
the flow of produced water or to provide a backwash flow, valves 102, 105, 107
and 108 are
closed while valves 103, 104, 109 and 106 are opened. Alternatively, to
operate blowdown with
the flow of produced water, valves 102, 103, 106 and 105 are opened and valves
109, 104, 107
and 108 are closed.
If necessary, a pump (shown in Figure 5 at 210) may be added to increase the
flow rate of the
blowdown. It will be appreciated that a throttle valve and flow meter may be
used to control the
flow rate of the blowdown through the heat exchanger 30. In addition, a
pressure letdown valve
may be used to reduce the pressure of the blowdown to ensure that the pressure
is low enough so
that the cleaning fluids do not exceed the mechanical design pressures and
temperature of the
exchanger. In one embodiment, the exchanger has a maximum design operating
temperature of
170 C and the pressure may be reduced until flashing of the blowdown generates
a vapour
fraction and the temperature is reduced to approximately 165 C.
An optional injection quill may be included in the system to allow for
chemicals to be added to
the cleaning stream as required or desired. For example, an additional caustic
or caustics may be
added to the blowdown so as to increase the pH of the blowdown. Typically
caustics may
include those caustics, for example NaOH, commonly used in SAGD water
treatment systems
for produced water or brackish water, for example, in a weak acid cation (WAC)
unit. In another
aspect, a surfactant may be added to the blowdown.
In one embodiment discussed in more detail below with reference to Figure 5,
when a series of
heat exchangers are used in the produced water system, as is typical of a SAGD
operation, the
system may be adapted so that each heat exchanger may be individually taken
offline for
cleaning while produced water is either directed to the remaining heat
exchangers or to a spare
heat exchanger to ensure operation capacity is not reduced during descaling.
In systems
including multiple heat exchangers, sufficient piping and valving may be
provided to isolate each
heat exchanger in a cleaning loop so that each heat exchanger may be cleaned
individually while
the remaining heat exchangers may be used or individually taken offline for
maintenance.
Likewise, in systems including multiple heat exchangers, sufficient piping and
valving may be
CA 02838147 2013-12-23
provided to isolate more than one heat exchanger for cleaning at a time.
Also shown in Figure 4 is a blowdown exchanger 20 for further recovering some
heat from the
blowdown. In one embodiment, blowdown that is redirected upstream of the
blowdown
exchanger 20 via valve 100, although having a higher temperature, is less
efficient for the overall
system. In contrast, if some heat energy can be recovered by the blowdown
exchanger 20 and
provided to the boiler feedwater (BFW) system, less heat is lost to the
cleaning loop. It has been
determined that secondary blowdown produced from the blowdown exchanger 20 may
also be
used in descaling the heat exchanger 30. Although the secondary blowdown is of
a lower
temperature and steam quality, for example a steam quality of about 20% or
lower, it is
sufficiently hot and sufficiently caustic to function in descaling the heat
exchanger. Otherwise,
no heat is recovered from the secondary blowdown produced by the blowdown
exchanger 20 and
the secondary blowdown must therefore be disposed of as a waste steam. By
redirecting the
secondary blowdown through valve 101 and utilizing the secondary blowdown to
descale heat
exchanger 30, efficiency is gained and there is generally little or no
opportunity cost for using
this secondary blowdown. The secondary blowdown, in one embodiment, may be
characterized
as about 170 C sub-cooled liquid wherein the temperature is process dependent.
The sub-cooled
liquid is generally a single phase liquid in most cases but can achieve two
phases at lower system
pressures. One advantage of using the secondary blowdown is that there can be
an increased
flow potential due to reduced pressure loss in the liquid flow. In addition,
the high grade heat is
recoverable prior to use of the secondary blowdown as a feed source for
descaling. Furthermore,
the secondary blowdown does not need to be disposed of, and there is a
reduction on the load of
the utility system thereby allowing for an increase in facility performance.
The neutralized blowdown, including scale, may be recycled or it may disposed
of and optionally
sent to the facility for channeling to suitable disposal facilities or
equipment. An optional pump
may be added to further assist in the transfer of the neutralized blowdown for
disposal as shown
in Figure 4. Neutralized blowdown may optionally be directed to a skim tank
for capture as well
or alternatively.
Further, use of secondary blowdown has been observed to be effective in
reducing or mitigating
water hammer within the system. Because the water hammer event generally
occurs at low
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CA 02838147 2013-12-23
pressure (-800 kPa), this event may be reduced by sub-cooling the secondary
blowdown.
Figure 5 is a schematic of another embodiment of a descaling system shown
generally as 300
according to the present invention. This embodiment includes a plurality of
heat exchangers
215, 220 and 225, for example, production water:BFW and/or production
water:glycol heat
exchangers, which may be isolated in a cleaning loop for individual offline
cleaning while the
remaining heat exchangers may remain online and operational. The system of
Figure 5 further
includes a blowdown pump 210 as well as a blowdown cleaning tank 200 for
cleaning blowdown
water following descaling. The non-limiting schematic of the system 300 shown
in Figure 5 is
arranged to allow for cleaning flow to be optionally with or against produced
water flow through
the heat exchangers 215, 220 and 225. For example, to take heat exchanger 220
offline and
operate blowdown cleaning fluid against produced water flow (to provide a
backwash flow
through heat exchanger 220) while keeping heat exchangers 225 and 215 online,
valves 237,
240, 242 and 245 are kept open. This allows blowdown to pass through heat
exchanger 220 in a
blackwash direction. Valves 230, 232, 233 and 235 are kept open to allow heat
exchangers 215
and 225 to remain operational. Valves 231 and 234 are closed to take heat
exchanger 220 offline
for cleaning. Valves 236, 238, 239 and 241 are closed to ensure blowdown does
not pass through
heat exchangers 215 and 225. It will be appreciated that through the opening
or closing of these
valves, any of the three heat exchangers may be isolated, taken offline and
cleaned while the
remaining heat exchangers are kept online. Likewise, more than one heat
exchanger may be
taken offline while the remaining heat exchanger(s) remain online. A further
heat exchanger (not
shown) may be added to take over for any of the other heat exchangers taken
offline to maintain
facility capacity.
The system shown in Figure 5 may optionally further include piping and valving
for using
secondary blowdown produced by the blowdown exchanger 20 (shown in Figure 4).
Figure 6 shows the relationship between the cleaning time and the heat
transfer rate. As can be
seen, heat transfer rates have been shown to be partially restored in as
little as 2 hours of
cleaning time. Cleaning, however, has been shown to be effective after as
little as 1 hour as
shown in Figure 7. Furthermore, heat transfer rates are restored in less than
the typical 30-50%
cleaning time relative to run time as has been historically observed by other
traditional cleaning
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CA 02838147 2013-12-23
methods. There are varying time periods for cleaning to achieve optimal
results, as outlined
above. Typically, the time frame for cleaning an exchanger is around 30-50% of
the run life
using traditional methods, where the run life is generally between 36-48 hours
before fouling
reduces the heat transfer rate to a point where cleaning is necessary. As
shown for example in
Figures 7-9, using a system as disclosed herein, effective cleaning times have
been observed
from about 2-9 hours using blowdown, or about 12-15 hours using secondary
blowdown in order
to return the heat exchanger to suitable operating heat transfer rate
conditions. In various
circumstances, cleaning times of as little as 1 hour may result in sufficient
descaling to allow for
operation of the heat exchangers.
In operation including trials and pilot projects, improved cleaning time,
relative to conventional
cleaning techniques outlined above, has been achieved after a thousand cleans
using the primary
blowdown source, highlighting the reproducibility of the system as described
herein.
Without wishing to be limiting, a typical cleaning flow rate is about 4 tonnes
per hour and a
range of 1-6 tonnes per hour has been implemented. Further, the temperature of
the primary
blowdown may be in the range of about 130-160 C.
It will be appreciated that the caustic blowdown may have a pH lower than 10.5
and provide
descaling functionality. A lower pH may have a lower degree of corrosiveness
and therefore
may be slower in descaling a heat exchanger. All levels of causticity are
within the use of the
term "caustic" for the purposes of this disclosure, provided the level of
causticity is sufficient to
at least partially descale the heat exchanger and return it to operational
parameters. It should also
be appreciated that the longer a heat exchanger must be offline, the less
efficient the descaling
becomes as heat exchange cannot take place.
It will be appreciated that in addition to a continuous cleaning strategy, a
"flush-then-soak"
arrangement is also contemplated and may be carried out using the systems
disclosed herein. In
such an arrangement, the heat exchangers may be put through an alternative
soak and flush state.
It will be appreciated that the embodiments detailed above are not intended to
be limiting in any
way and are for illustrative purposes intended for one of skill in the art.
Modifications may be
made to the piping, valving and other steam generating and separating
components or system
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without departing from the invention.
14
