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Water Filtration

The importance of water filtration is becoming known in terms of reducing corrosion activity as well as to improving heat transfer efficiency.  Even under the most effective chemical treatment control conditions producing lowest possible corrosion rates, steel pipe is still oxidized into producing potentially hundreds and even thousands of pounds of deposits or more over decades of service.  A 5 MPY corrosion rate acting against 12 in. schedule 40 pipe actually removes an astounding 64 pounds of steel per every 100 ft. per year.  Oxidized, that 64 lbs. of steel translates into 1.6 cu. ft. of iron oxide rust.

This website offers overwhelming evidence to the reality of what corrosion activity produces in terms of internal deposits to any circulating system, and to a long term threat rarely seen or recognized until a failure occurs.

Our other Technical Bulletins related to water filtration well illustrate common problems associated with water filtration systems – from inappropriate equipment choices to its wrong installation on the system.  In the overwhelming number of examples that we see from our ultrasonic investigations, poor and inappropriate installation is the primary mistake resulting in little to no benefit following a typically large capital expense.

• Background

We provide full background and explanation on this problem in our other technical bulletins related to water filtration.  Of those, Technical Bulletin WF-03 is especially relevant to this issue and important to review.

As we have stated, most water filtration units are installed where most convenient and at least cost, rather than where they will produce the greatest benefit.  While the physical placement of the unit body is rarely important, its take-off point from the piping system is absolutely critical.  In many examples, filtration units are supplied with little to no direction toward their best placement – thereby leaving installation to building operators, piping contractors, and facility managers not at all familiar with water flow mechanics.  In others, installation directions from the sales representative or manufacturer are 100% dead wrong – resulting in the same end result.

As stated previously, the benefit of any water filter in controlling a particulate problem depends upon five basic elements:

Factors Influencing Water Filtration Effectiveness

• Filter Capacity

Full flow or side stream are the main choices.  Full flow filtration is always preferred but rarely possible – especially at larger piping systems.  Effective filtration at full flow rates means large units at extremely high cost, as well as potential flow restriction.  The installation of a full flow centrifugal separator allows very coarse particulate removal at low pressure drop, but is not what would be considered true filtration.

Full flow filtration is more appropriate to open water condenser systems, whereas side stream filtration is more appropriate to closed systems.
Side stream filtration is typically at high removal efficiencies but at very low flows.  Potentially high dirt loading of any high efficiency side stream filter installed to an open condenser water system typically demands automatic back washing over manual cartridge or filter element replacement in terms of both maintenance demand and material costs.

• Filtration Type

While full flow filtration can be accomplished by any filter type if large enough, they are typically strainer element types or centrifugal.  Automatic backwashing or automatic blowdown is a must.  The capture of smaller diameter low micron particulates is generally not possible.

For side stream units, the choice is commonly sand type pool filter units or more effective centrifugal / sand media hybrids having higher particulate capture verses basket and bag filtration types.  Both types are capable of sub-micron filtration at lower flows.

• Particulate Capture Efficiency

Rust particulates dramatically range in size between the invisible sub-micron and fist sized.  What works well for a coarse particulate problem, centrifugal filtration for example, will not provide any benefit in removing a 1 micron threat.  At the same time, a high efficiency sand filter may become totally overwhelmed tying to handle very coarse particulates.

A particle size analysis is somewhat useful in any filter selection process, although the source of the sample for analysis is often taken from a moving stream where the larger solid particulates are not represented.

Most filtration representatives and manufacturers will address the fine micron range and easier to remove particulates taken from a moving sample but ignore the overwhelming majority of the problem adhered to the side walls, bottom, and other low flow dead end areas – thereby also ignoring the overwhelming majority of the problem.

• Flow Rate

Filtering the greatest flow of water is always desired, with economics always the limiting factor.  In virtually every example there is a trade off to consider.

For any open water system, we would prefer a full flow 100 micron retention capable centrifugal filter at 1,000 GPM over a 0.5 micron rated side stream sand filter handling only 50 GPM.

Utilizing coarse full flow filtration and finer side stream filtration is an excellent filtering combination.

• Biological Contamination

This is more of a concern to open water systems where new organisms are constantly introduced through the cooling tower.  Microbiological growths can coagulate sand and other fine media sufficiently to require replacement.  Fine slotted screens and bag elements are also vulnerable to this constant maintenance problem.

The issue of microbiological contamination of sand filtration media is now being viewed by many as a potential health issue.

• Inlet Supply Water

Of the above, we consider the take-off site for any water filter as the most critical element related to its effectiveness or failure to perform.  Differences in capture efficiency are rarely as important as placing the filter at the most optimal location to receive the highest dirt loading.

Most often, side stream filtering units are installed across the suction and discharge headers of the pumps in order to utilize their 40-80 PSI differential to move the water through the filter without an additional pump.  Typically, the take-off point to the filter is perpendicular to flow.  Since almost all pumps have coarse strainers at their inlets, largest particulates may be captured there, or shattered into smaller particulates for re-suspension into the system.

We illustrate this very common installation problem in the photographs below.  In the top left photo, the inlet take-off to the side stream filter is actually at the inside radius of a 90° elbow, with the water flow direction approaching from the rear left.  Any rust particulates would be forced to the opposite side of the pipe at right and glide along its outward radius wall – never even reaching in proximity of the filter inlet.  No possible benefit of this water filter is therefore realized, as it exists in name only. An identical condition is shown at top right at another refrigeration plant.

Further below left and right, we show another common filter take-off at the 3 o’clock position, better of course than the previous two examples, but unlikely to capture rust particles flowing through this 12 in. chill water line.  At longer horizontal lengths, water flow becomes laminar (fairly smooth and steady with parallel layers having different velocities relative to each other) rather than remaining turbulent.  This form of laminar flow prevents random movement from bringing a rust particle to the take-off filter suction.  With any larger rust particle or deposit falling by gravity to move along the bottom surface of this pipe, such side capture ports are almost totally ineffective.  And yet, it is the most common installation specification in use.

Improper And Ineffective Installation

In the below example we show a side stream filter installation which has accomplished absolutely nothing since its purchase.  Installed as an afterthought to various secondary systems at an older building property, this filter was parallel piped into a side stream chemical feed tank.  At left, the supply take-off for the chemical tank is shown located at the top of the pump header – no concern where water is only required to move chemicals, but worthless in terms of capturing particulates from the system likely moving along the bottom.

With the chemical feed tank and filter installed across the room at approximately 12 ft. away, there were ten 90° elbows separating the pump header and filter unit itself.  A final tangential take-off to this greatly undersized basket filter ensured that any rust particulate entering the line would travel straight down into the chemical feed tank rather than being captured by the filter. Its 3/4 in. piping was totally inadequate to any filtration requirement.

• No Chance Of Success

As an older building having documented low corrosion activity, substantial interior rust deposits would still be expected.  In fact, building engineering defined plugged coil conditions which have developed in the over 50 years of building service.  The filters installed approximately 10 years ago in response to clogged coils unfortunately accomplished nothing.

Our recommendation toward effective filtration was in two parts.  First, modify the piping layout to produce a take-off point to the filter having the greatest volume of rust available for capture.  This was easily defined at the end of the circulating pump discharge header where the water flow takes a 90° upward turn into the building.

The 6 in. line would require removal at the end and then welding in a new tee in its place.  With the water moving normally upward, any particulates already having velocity from the pumps and forward inertia will move directly into the straight end of the tee.  The 6 in. line is extended to produce a dead leg and then elbowed downward and reduced to the inlet size of the water filter.

Installing a larger basket filter having a 1-1/2 inlet and outlet was also necessary.  In the revised format, the new 6 in. tee would be turned downward and reduced to 2 in. size and valved for maintenance of the filter.

Considering together the many factors affecting water filtration, it becomes clear that any purchase requires substantial thought and planning.  Still, few water filtration units are properly installed to gain the most from their use.

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