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The effectiveness
of any water filtering system is often promoted by their manufacturer, and
viewed by most clients, in terms of water clarity produced. With sand filters
generally providing the highest particle removal efficiency, it is not unusual
to see claims of sub-micron particle retention used as the major selling point.
Although not an absolute rule, most rust
fouled piping systems will show a higher than normal iron oxide content under a
laboratory analysis - both in dissolved and suspended forms. In its suspended
form, the rust typically settles out in the lower floor piping and at other
horizontal and lower flow areas - thereby causing notice. It may also impart a
brown or yellow tint to the water depending on particulate concentration, and
dissolved iron content. Condenser, process, or open water systems suffer the
greatest deposit problems for the obvious reasons. Dissolved, it may
precipitate out into solid form, or be removed completely out with the
blowdown.
When first
installed, most filters will see a heavy amount of backwash or filter element
changes. This continues as the water in circulation is gradually cleaned.
Backwashing every 10 minutes may eventually decline to once every day, and then
once every few days. Such progress is totally dependant upon particle retention
and filter size in relation to the overall piping system - with larger or full
flow filters obviously providing a quicker response. It is also greatly
dependent upon the placement of the filter and its piping arrangement.
See Technical Bulletin
W-3 regarding filter installation recommendations.
At some point however, the water will
become cleaner, backwashing slows, and such progress is viewed as the solution
to a previous corrosion problem. This is very rarely true, however.
Under even the
lowest corrosion rate conditions of perhaps 1-2 mils per year (MPY),
substantial rust deposits are inevitable. Tables C-1 and C-2, shown below,
document the metal loss at various corrosion rates, and the resulting deposits
of iron oxide which always present a threat to any piping system.
As a result, oxidized steel deposits
accumulate within the pipe to create more serious under deposit pitting
conditions, and are usually well established before other symptoms such as
blocked strainers, dirty cooling tower pans, and turbid or discolored water
appear. Given a moderate corrosion rate of 5 MPY, it is entirely possible for a
large 12 in. condenser water system to accumulate thousands of pounds of rust
deposit buildup after just 10 years of operation.
Following many
years of experience performing ultrasonic evaluations of building and plant
related piping systems, CVI has rarely found an older property where deposits
and under deposit corrosion were not developing to be an operating problem to
some degree. Such problems often remain undiscovered, and it is only after
entrance is made into the system that the problem is usually discovered and
some form of remediation considered.
While
not necessarily to the degree shown in the below photographic archives of past
investigations, we can suggest that some volume of interior deposits will exist
for any property in service for ten years or longer. In most cases, clean water
will be produced after a few weeks of adding supplemental water filtration,
though in reality - the true problem still remains.
The sheer volume of rust material present
makes it obvious that any cleanup operation involving filtration, where perhaps
one pound of deposits may be removed per filter change or backwash cycle, can
only be accomplished over an extended period of time - usually years.
It is the opinion
of CorrView International that a certain volume of interior deposits are
inevitable unless regularly employing some form of iron oxide removal chemical.
Therefore, deposits should be assumed present in an open circulating system of
any age unless proof exists otherwise.
Deposits also are a serious threat to
closed recirculating systems, especially since they are hidden from view. They
remain of less concern simply due to the lower corrosion rates involved.
See Technical Bulletin
C-4 regarding the general threat of interior pipe deposits.
Piping systems which are known
to be fouled with deposits provide the best evidence that clean water is not a
reliable indication of system condition. In many examples, heavy layers of iron
oxide at 1/2 in. or more will exist throughout an entire piping system, yet
show little or no evidence of problem in terms of poor water clarity or iron
content.
It is exactly for this reason
that corrosion coupons, which exist in an isolated stream of clean water, so
often fail to provide an accurate indication of the corrosion activity taking
place at the pipe itself. Read more about the
limitations of corrosion coupons.
Ironically, the
addition of a water filtration system can actually be detrimental to a building
property or process plant by eliminating the visual signals that would normally
prompt further investigative action. Unless some form of chemical dispersing
agent is employed, and is effective at resuspending the iron oxide
accumulations back into the water flow for removal, any water filter is simply
not performing its intended role.
The
incorrect placement of a water filter at the piping system can produce the most
negative effect by only allowing the capture of finest particulates. Installing
the filter at a perpendicular take-off point to the main flow of water, for
example, would clearly prevent the capture of any larger particulates unlikely
to stop and turn into the filter inlet - automatically limiting its benefit
tremendously. See Technical Bulletin
W-3 about why filtering systems fail to
perform.
In such cases,
corrosion control chemicals continue to be prevented from reaching the base
steel to provide protection, new deposits accumulate at a greater rate than are
removed, and therefore the problem for which the filter was originally
installed not only continues, but accelerates. Unfortunately, we find this to
be a common occurrence, and perhaps an overlooked problem or misunderstanding
of events by water treatment companies, filtration experts, and their clients
alike.
In fact, the
effectiveness of any water filter is wholly dependent upon the ability of the
water treatment contractor to accurately identify the interior deposit problem
and provide an effective and safe gradual chemically based removal program.
With this in place, the water filter should be expected to perform at a
significantly higher backwash rate until such time as either the deposit
removal program declines in effectiveness, or the problem is resolved.
Where automatic backwashing filters are used,
the backwash rinse water can be post filtered and reclaimed if necessary
through the use of a small basket or bag filter. Therefore, a high backwash
rate should never be viewed as a reason to slow down or terminate any
particulate removal program.
Monitoring the
effectiveness of any water filter and deposit removal effort can only be
accomplished via the regular visual inspection of the piping interior. This is
most easily performed using removable spool pieces throughout the piping
system, and constructed of original and deposit laden pipe.
See Technical Bulletin
C-11 about the information available from spool piece
monitoring.
Laboratory analysis
of water quality again only addresses the water itself and will show suspended
and dissolved iron - thereby providing some indication of chemical
effectiveness. Corrosion coupons provide no possible information since they, by
definition are new, and contain no surface deposits.
Maintaining a photographic record of the
pipe interior at regular intervals, and at various locations throughout the
piping system, will assist greatly in evaluating any filtration / deposit
removal program.
A chemical water
treatment program should not only control new corrosion, but address any
existing corrosion deposits. Clearly, most corrosion problems can be directly
linked back to a deficient and sometimes non-existent water treatment program.
Controlling both new and old corrosion problems can be greatly aided by
supplemental filtration if installed and utilized correctly.
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Copyright
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