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Misleading Indicators

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 corrosion 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 in the form of lost heat transfer and clogged smaller lines.  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.  Originally dissolved, it may precipitate out into solid form, or be removed completely with the blowdown.

• Initial Success

When first installed, most water 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.

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.

• Huge Deposit Volumes Exist

Under even the lowest corrosion rate conditions of perhaps 1-2 mils per year (MPY), substantial rust deposits are inevitable.  Tables 1 and 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.

Table 1

Table 2

Following many years of experience performing ultrasonic evaluations of building and plant related piping systems, CorrView International, LLC has rarely found an older property where internal rust deposits and under deposit corrosion had not developed into 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 recognized 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 1/2 pound of deposits may be removed per filter change or backwash cycle, can only be accomplished over an extended period of time – usually years.

Interior Rust Deposits

• Assume Interior Deposits Exist

It is the opinion of CorrView International, LLC that a certain volume of interior deposits are inevitable unless regularly employing some form of iron oxide removal adjunct 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 and therefore less production.

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.  Aside from the common fact that most water filtration systems are not installed in the most effective location within the system, removing just the suspended component of a corrosion problem has little effect elsewhere.

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.

• An Opposite Effect

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, most water filters will fail to accomplish their intended goal.  This is not to say, however, that the filter purchase and installation will be assumed as anything but a total success.

The incorrect placement of a water filter at the piping system can inadvertently produce a negative result by only capturing the finest particulates.  Installing the filter at a perpendicular take-off point to the main flow of water, very common to most installations, prevents the capture of any larger particulates of greater mass unlikely to stop and turn 90° into the filter inlet – automatically limiting its benefit tremendously.

In such cases, corrosion control chemicals will not reach the base steel to provide protection, new deposits will 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.

• Deposits The True Problem

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 composition 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 style bag filter.  Therefore, a high backwash rate should never be viewed as a reason to slow down or terminate any particulate removal program.

• Close Monitoring Critical

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.  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 water treatment program.  Controlling both new and old corrosion problems can be greatly aided by supplemental filtration if installed and utilized correctly.

© Copyright 2023 – William P. Duncan, CorrView International, LLC