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Installing a
water filter for any larger piping system is often a capitol decision costing
$25,000 or more. In most cases, it is in direct response to a recognized
corrosion problem - where its function may play an even greater role in
protecting millions of dollars of equipment, product, revenue, or
infrastructure.
What often occurs,
however, is that the filter removes a certain volume of dirt and particulates
within its first few weeks of being installed, but then slows dramatically in
its backwashing cycle or cleaning frequency. This change is sometimes
interpreted incorrectly as either a failure of the unit's operation or design,
but more likely - that the corrosion problem has been resolved. The filter
installation is viewed as a total success, and the problem is
forgotten.
In many cases, a filtration
unit, and especially a high efficiency sand filter or basket/bag housing type
filter, will clean up the fine suspended particulates and turbidity of the
water to produce a noticeable improvement in clarity, color, and appearance.
This improved water quality, in turn, is perceived as an improvement in
removing deposits from the piping also - which is very rarely the case.
See Technical Bulletin
M-10 regarding the difference between clean water and clean pipe.
First suspicion is always that the
filtering unit is not functioning correctly. Closer inspection and trouble
shooting, however, will typically verify the unit's proper operation - to leave
further questions remaining. In fact, the problem actually rests in two
areas:
First is the
chemical treatment program. Since years of operation may have deposited
hundreds and perhaps thousands of pounds of iron oxide corrosion product at the
interior piping surface prior to the filter unit being installed, it is
impossible to remove those deposits using filtration alone, and in only a few
weeks or months, possibly even years. Deposits are usually hardened in place
and will resist most mild detergent cleaners.
See Technical Bulletin
C-1 regarding the volume of corrosion product created at different corrosion
rates.
Every filtration system
therefore requires supplemental chemical agents to loosen and re-suspend the
attached deposits back into the water flow and maintain it moving in
circulation. Ideally, efforts to re-suspend the particulates into the water
flow should be effective, but not exceed the reasonable capture rate of the
filter.
A review of all options with the
water treatment contractor should help in choosing a safe but effective
chemical capable of loosening the deposits and maintaining them suspended in
solution. Removing a sample of deposit laden pipe for lab analysis is almost
mandatory. A laboratory trial cleaning of the proposed chemical against those
same pipe deposits will even better demonstrate the likely degree of success,
and potential damage caused to the pipe itself.
Above all, CVI recommends avoiding any
strong acid or alkali cleaners except where thorough ultrasonic and
nondestructive testing has been performed to identify any weakened areas of
pipe, joints, A/C units, etc. Corrosion coupons of both steel and copper are
especially important during any cleaning procedure in order to monitor for an
increase in wall loss.
A second and
possibly more important reason for a low efficiency problem is the filter
installation itself, since very often, the piping layout to the filter or its
location will dramatically reduce its
effectiveness.
This is not a concern for
full flow filtration units since they must, by definition, be in line. Still,
full flow filters are most effective if located on the supply riser downfeed
from the cooling tower, and should ideally be located at the very bottom of the
system.
Critically
important to all filter installations are the basic laws of physics and
inertia, and the fact that particles have the interest to continue their travel
in a straight line relative to their mass.
Very light particles are
more likely to change direction and move with the water flow, while heavier
particles and larger chips of iron oxide and scale, for example, are more
likely to continue in a straight
line.
For side stream
filtration, the actual location of the filter is far more important - with
placement of the take-off inlet line greatly influencing its capture
efficiency. Most often, side stream 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. Since almost all pumps have coarse
strainers at their inlets, larger particulates are captured there, or shattered
into smaller particulates for re-suspension into the system.
In such cases,
the take-off connections to and from the filter are typically oriented
perpendicular to the flow of water, and at either 3 or 9 o'clock positions for
horizontal lines - and sometimes even at 12 o'clock. This side take-off
arrangement assumes that any suspended particulates will literally stop dead
and change direction 90 degrees at the point of highest velocity within the
entire piping system - an impossible event.
Virtually no large particulates are
likely to reach the filter under such conditions, and will instead travel
straight past the filter inlet to settle in some other lower flow area of the
system prior to ever being captured. Furthermore, many circulating pumps are
not located at the absolute bottom of the piping system, where the
concentration of particulates is usually the
greatest.
Water flow in the area of a pump
header will be quite turbulent, and therefore provide slight benefit by
randomly forcing larger particles into the filter inlet. However, if the filter
take-off is connected after an extended horizontal run, the water flow may
become more laminar, and thereby concentrate particulates at the center and
bottom of the pipe - again away from any side oriented filter inlet.
From our
extensive involvement in ultrasonic pipe testing, we have well documented the
volume of iron oxide and other particulate deposits which commonly accumulates
in condenser water and other HVAC piping systems. Given even a low corrosion
rate of 1-2 mils per year, hundreds and possibly thousands of pounds of rust
deposits can accumulate over one or two decades, and will remain attached to
the pipe without some mechanism to remove them. A not uncommon 20% loss of wall
thickness at 12 in. Schedule 40 condenser water pipe actually means that 10.7
lbs. of steel has been lost per every linear foot.
See Technical Bulletin
# C-1 for the actual pounds of metal lost and deposits created at various
corrosion rates.
For an 800 ft.
riser system, that same 20% thickness loss means that approximately 8,560 lbs.
of steel has been oxidized and transformed into a less dense mass of iron oxide
having approximately 18 times its original volume. And now, some percentage of
that volume will still exist within the piping system to cause further
deterioration.See Technical Bulletin
C-4 about the problems associated with interior
deposits.
Any expectation of
removing such internal corrosion, or even a small proportion of it through the
use of a side take-off intake to any by-pass filter, is virtually impossible.
At most, such an installation will only provide the most minimal removal of the
smallest suspended particulates, leaving the overwhelming balance behind. An
improperly installed filtration unit will not even keep up with the rate of new
rust deposits naturally cerated.
Were the
removal of such a high volume of debris, scale, rust, and other particulates
successful, new problems would be created at the drain lines where the backwash
was directed. Our experience has shown it rare for any automatic filter
installation to include a settling tank to collect the backwashed deposits,
with almost all backwash lines piped directly to drain. Yet, clogged floor
drains and sumps, which would occur if hundreds of pounds of rust deposits were
directly discharged into them, rarely, if ever occur.
In one way, piping the backwash directly
to drain eliminates any evidence of filter efficiency, but by definition of not
creating drain line problems, defines that the filter is not as effective as
likely planned. The below photo, taken of an automatic side stream filter
installation having a collection and settling tank for backwash deposits, is a
prime example.
Seen from a downward
inside view, this 3 ft. diameter x 4 ft. high tank had collected so few
deposits that not even the entire bottom of the settling tank was covered. The
filter, one of the most efffective automatic units on the market, removed
perhaps only 1 lb. of rust in over 18 months of 24 hour/day operation of an
open condenser water system known to be heavy with particulates.
Given that the supply and return
distribution piping removed from the system were found clogged almost 50% with
interior deposits, and that thousands of pounts of particulates were known to
exist throughout the 36 floor office property, this investment in filtration
was shown to be virtually worthless. The purchase of one of the best automatic
filters on the market, installed at a 2 in. perpendicular take-off to a 18 in.
vertical supply riser, ultimately accomplished nothing.
A much better
alternative is to install the filter at the very bottom of the supply line from
the cooling tower, or at the bottom of the downfeed line in the case of a
closed system. This uses gravity to advantage in not only directing the
particulates downward, but with increased force. With a higher system pressure
normally found at the return piping upward, this configuration requires a
supplemental circulating pump at additional installation, energy, and
maintenance costs.
The piping layout is
critical in itself. Whenever possible, the filter inlet should be in a straight
and downward direction from either the cooling tower or closed system downfeed
piping. This takes advantage of the inertial forces which tend to move all
particulates in a straight line whenever the water changes direction. If
possible, we recommend replacing a basement or lowest level downfeed elbow with
a full size tee instead, and then reducing the tee diameter to the size of the
filter pipe inlet.
Extending the bottom
of the tee piping in its original dimension approximately 3-4 ft. provides a
collection chamber or dead leg, and even further improvement to filtering
efficiency. Expanding this same collection piping to a greater diameter, 6 in.
to 24 in. for example, immediately drops the water velocity at this end point -
thereby producing greater settling and even more filtration benefit at
relatively low cost.
The layout of the
piping is also a major factor. Where low flows and/or long horizontal runs
exist, particulates, even if they are loosened by the action of a chemical
agent, may not migrate back into the main flow stream to be picked up by the
filtering device. The most effective and sophisticated filtering device becomes
virtually useless if insufficient flow exists to move the particulates to its
collection inlet. Therefore checking flow velocity is always important to
secure whether booster pumps may be needed.
In our
experience, we rarely find filtering systems installed to provide maximum
effectiveness, nor a chemical treatment program even addressing the question of
existing interior deposits. Normally, after spending $25,000 or more on the
filtration unit, corners are usually cut to reduce installation and operating
costs, and only a small degree of benefit is realized as a
result.
While the selection of the
filtering device itself is a priority concern, CVI recommends providing an
equal amount of attention to its placement, installation, and chemical
treatment program. A realistic assessment of pipe quality, volume of deposits,
and threat level to the system are also important areas to consider - meaning
that ultrasonic and metallurgical testing are a must.
Combined, the proper combination of
planning and engineering can turn an otherwise mediocre performing water filter
into a unit that makes substantial improvement to building or plant
operations.
©
Copyright
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