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Over 20 years of
experience in chemical water treatment and ultrasonic pipe testing has provided
CorrView International an excellent understanding of many common corrosion
related problems found at commercial office properties and plant facilities.
Indeed, we see the same corrosion problems again and again - which indicates to
us some clear and unmistakable trends for different piping systems and
operating conditions.
We offer below some
general guidelines of where corrosion problems might exist and why. Many
corrosion scenarios are linked to others obviously in similar cause or effect.
Although the corrosion issue is complex, and may present itself in many
different forms, it often begins due to some simple initiating factor - a lack
of chemical treatment, or a faulty start-up of the system, for example. The
presence of a corrosion problem in any particular area should always raise
concern and the need for further investigation.
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Higher corrosion rates are often found at the return lines for
open process or cooling tower systems. This difference does not commonly exist
in closed systems.
Sorting a large set of testing data based upon flow
direction for a condenser water loop will often show this difference quite
clearly - with 1-3 MPY higher return side corrosion rates not
uncommon.
One suggested cause is that the higher temperatures returning
to the cooling tower naturally accelerates the corrosion activity, and/or that
it provides a more suitable thermal environment for microbiological growth.
Another is that the iron oxide created at the supply side piping then
has opportunity to migrate downstream to the return side - where it is more
likely to deposit and create higher corrosion conditions.
See Technical Bulletin
C-4 about the problems associated with interior
deposits.
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This is an obvious problem well recognized by most experienced
plant engineers and property owners - and which becomes most pronounced as
either the length of the horizontal or vertical pipe run
increases.
Particulates, and especially those heavier components, settle
out in the horizontal lines based upon the length of pipe, flow velocity, and
pipe diameter. We consider interior deposits to be one of the most serious
problems affecting any piping system due to the secondary damage usually
created.
This problem exists for open and closed systems both, but more
so obviously at open cooling systems due to the greater source of airborne
particulates, microbiological content, and due to the naturally higher
corrosion rates of an open system.
See Table C-1 for the
actual pounds of metal lost at various corrosion rates.
For
tall commercial building properties, the vertical risers often show
substantially less corrosion and pitting than the horizontal runs. Longer
vertical runs increase the difficulty for particulates to migrate upward and
therefore horizontal settlement increases. Larger volumes of pipe also produce
a greater volume of iron oxide deposit which inevitably settles
elsewhere.
For large process and manufacturing plants spanning acres of
floor space, particulates entering any length of pipe may not remain suspended,
but instead settle throughout the distribution lines. This problem is amplified
as the piping branches off into smallest lines, and as the distance from the
circulating pumps increases - thereby lowering flow velocity.
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Three separate corrosion scenarios can
exist to produce a noticeable difference between pipe located at the top and
bottom of the same system.
The first involves the settlement of
particulates into the lowest points of the system to produce an under deposit
condition with substantially higher corrosion rates.
Second involves the
layout of the piping system, and whether significant enough differences in flow
rate exist to influence the effectiveness of the chemical treatment program
and/or cause the deposition of particulates.
Depending upon whether the
system is partially drained over the cold weather months, a third possibility
may be the substantially higher corrosion which occurs at any pipe which is
drained and left open to the atmosphere. This is a common problem at roof level
piping. See Technical Bulletin
# C-3 about increased corrosion activity during winter or temporary drain
down.
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Drained pipe is usually linked to the
most serious corrosion losses. In many cases, a high corrosion condition will
be traced back to years earlier - when some event required the temporary or
extended draining of some or all of the pipe. In comparison, drained piping
produces significantly greater metal loss than pipe filled with fresh and
untreated water.
The degree to which draining causes damage is directly
related to the infiltration of fresh air into the pipe. Therefore, it is common
to find a high wall loss at the cooling tower side, and a sudden increase in
wall thickness after an isolating valve, or as the pipe travels further toward
a dead end. See Technical Bulletin
# C-3 about increased corrosion activity during drain
down.
Any effort to maintain a piping system filled with
chemically treated water is highly recommended. This includes insulating and
heat tracing outdoor lines in colder climates, and installing critical
isolating valves. Another option is to use extremely effective VCI chemical
inhibitors where possible. See Technical Bulletin
# C-9 for further information on VCI corrosion
inhibitors.
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For a wide variety of reasons,
threaded pipe almost always shows the first sign of a corrosion problem. This
is primarily because as much as 65% of the available pipe wall is cut away
during the threading process itself on day one.
See our thread loss
table. This small diameter pipe also offers inherently less wall
thickness - thereby an already thin material is reduced even
further.
Threaded pipe usually exists at lower flow areas, and at the
furthermost extremes of the system at the process equipment or A/C units. Here,
flow rates are the lowest, or may be periodically shut down altogether - two
additional factors commonly associated with higher corrosion
rates.
Threaded pipe almost always involves brass valves or a transition
to copper pipe. This often creates a galvanic corrosion condition at the
threads since, in the majority of examples, dielectric insulators are
absent.
Gaps at the pipe fittings also offer opportunity for
particulates and microbiological growths to collect and produce a localized
corrosion problem - once again focused at the very weakest point of the system.
While adequate pipe may exist along 99% of its length, a failure at the threads
usually means the end of service for the entire pipe length.
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Any piping investigation, if thorough,
is likely to identify some higher degree of corrosion at the horizontal and/or
lower floor piping. This is obviously due to the settlement of particulates in
this area, and the secondary corrosion effects such particulates
create.
In many of those cases, a substantial increase in corrosion
activity and wall loss can be measured at the bottom and lower sides of the
horizontal runs. Primarily dependent upon flow velocity, particulates settle,
and tubercles of iron oxide develop to produce extremely high under deposit
corrosion rates of 25 MPY or greater.
View a good example of
this problem.
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No flow areas of piping can
demonstrate dramatically different corrosion scenarios. Where isolated and
truly stagnant, such as exists for a fire standpipe system, a small amount of
corrosion takes place and then further corrosion activity virtually ceases.
It's not unusual, therefore, to measure remaining wall thickness values near
new pipe specifications in such cases.
Where new water is introduced in
to a dead end, by-pass, or isolated pipe section, results can be the opposite,
and extremely high corrosion activity often becomes established. By definition,
such areas of pipe are never tested for corrosion - as no flow exists to route
water to and from a corrosion coupon.
Any by-pass loops, especially
those having the downstream side closed by a valve, are high priority areas for
severe pitting corrosion to develop. This occurs when particulates enter and
settle out in the pipe in high quantities to produce severe under deposit
pitting, and is quite common.
Pipe serving lead and lag equipment, or
where the water flow is shut down when the equipment is de-energized, is
especially vulnerable to much higher corrosion activity.
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Higher corrosion rates are often found
at the new pipe following a pipe repair, replacement, or extension to an
existing system. This is especially common at condenser or open process water
systems.
One easy explanation is the better quality and higher corrosion
resistance of pipe produced decades ago. Its not unusual to measure a 1-2 MPY
corrosion rate at 40 year old pipe, and a 4-5 MPY corrosion rate at newly
installed pipe within the same recirculating system.Review a summary of
piping quality, operating, and design changes which have
occurred.
In many examples, iron oxide and other particulates
quickly migrate into the new pipe to produce an accelerated under deposit
condition. Where new pipe is installed downstream of older constricted pipe,
the greater volume of the new pipe slightly drops the velocity and therefore
allows more settling of particulates.
Galvanic activity between new and
clean pipe and old and deposit laden pipe is also recognized as occurring -
although the mechanism is not completely understood. |
In many cases,
identifying a corrosion problem is simply a question of looking in the right
direction. Quite often, no problem is known about, nor even suspected, until
some special interest is raised. Under the worst case scenario, a problem may
exist for years and exhibit no indication to the property owner, operators, or
chemical treatment contractor.
Ultrasonic
testing excels as the most valuable corrosion monitoring tool for the purpose
of finding hidden faults since it provides a quick and low cost method of
determining wall thickness. Coupled with a thorough data analysis, ultrasound
can provide an almost complete understanding of piping system integrity. Areas
of concern raised can them be confirmed or further investigated through
metallurgical testing or other methods.
As
long as sufficient testing is performed by skilled and experienced personnel,
and as long as key problem areas as outlined above are addressed, ultrasound
will produce a thorough and reliable system evaluation.
See Technical Bulletin
P-7 for more about the level of information provided by ultrasonic
testing.
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