Technical Bulletin # C-05
Proof That Draining Down Condenser
Water Piping
Will More Than Double Its Corrosion Rate
THE PROBLEM:
It is a generally recognized fact that
fully drained or partially drained piping systems are far more susceptible to
corrosion than systems containing treated water, or even untreated water. Given
a moist environment in combination with the presence of abundant air and
oxygen, exposed piping has been documented to corrode at a rate two to ten
times that of other water filled pipe of the same type, and located within the
same circulating system. Condenser water systems suffer the greatest.
In cases where condenser water piping is
drained down within the interior of a building to protect it from freezing, it
is common to measure significantly higher corrosion rates at the rooftop or
outside level. In many cases, roof level piping may require replacement many
decades before the remainder of the system. The buckets of scale typically
removed from strainers and condenser heads every spring start-up are partially
the result of such higher off-season corrosion activity.
Ironically, corrosion coupon racks, often
the only form of corrosion monitoring used, are rarely installed in outdoor or
roof level locations. And if they are, the coupons are generally removed during
the off-season. See Technical Bulletin
# C-01 about the limitations of corrosion coupons.
Of course, corrosion at the outer surface
of the exposed rooftop pipe will also occur if not properly coated and
protected - a maintenance problem often identified as a contributing factor to
an overall higher measured corrosion rate. However, it is generally the piping
interior which, having been totally or partially drained over many years,
places the piping at greatest risk of advanced failure.
The below comparison of wall thickness
measurements and estimated corrosion rates from our ultrasonic pipe analysis of
a New York City commercial property clearly illustrates the differences which,
to some degree, always exists between drained and filled piping at the same
exact condenser water system.
In this example, the left side of the
page represents test results taken from an ultrasonic evaluation of a section
of 18 in. extra heavy condenser water pipe located in the sub-basement machine
room area of a New York City office building, and never drained of treated
water.
The right side of the page represents the
same exact pipe at the 30th floor shaftway, where it has been drained every
single year during its five month winter season. The pipe is the same in all
respects, has an initially specified wall thickness of 0.500 in., a history of
excellent water treatment maintenance, and has been in service for over 45
years. Descriptions of the various charts and bar graphs precede each set of
report data.
Comparison # 1 - Wall Thickness Measurements
In a direct comparison of current wall
thickness measurements, test results show significantly lower remaining pipe
metal at the drained test site to the right. Also, the greater deviation
between thickness measurements at the drained piping illustrates the much
higher level of pitting activity at that location. Please note the differing
scales for wall thickness at the left side of each graph.
Comparison # 2 - Average Corrosion Rates
The below calculations are based upon the
average of all recorded ultrasonic wall thickness measurements taken above.
Differences in average measured pipe thickness, corrosion rate, and remaining
pipe life life are dramatic. Annual draining of this system has actually
increased the average corrosion rate of the roof level piping
FOUR TIMES over that of the rest of the system. As
a result, the lifetime of the piping has been reduced by nearly
TEN FOLD!
Comparison # 3 - Maximum Corrosion Rates
The below tables show the same basic set
of calculations as in Comparison # 2, except that they are based upon the
lowest measured wall thickness value of each set. Such data represents a weak
link or worst case scenario, and offers an estimate of when the most aggressive
corrosion will deteriorate the piping past its safe recommended limit.
Shown here, differences in corrosion
rate, retirement date, etc. are even more pronounced. In fact, the minimum wall
thickness of the filled pipe significantly exceeds the standard specifications
of any new pipe installed today, while the upper pipe exists at nearly
HALF that value.
Comparison # 4 - Original vs. Remaining
Values
While the original pipe wall thickness
and minimum allowable thickness values remain constant for both test locations,
a major difference is obvious in the amount of pipe remaining. For the subject
property, testing indicates that the minimum measured pipe wall thickness of
the drained piping is nearing its minimum allowable safe operating limit.
THE SOLUTION:
There are currently only three feasible
ways to reduce the corrosion rate within a system which is partially or fully
drained during part of the year. The easiest, though least effective, is to
increase the level of the standard chemical inhibitor 2, 3 or more times just
prior to draining. This leaves a heavier coating of standard inhibitor on the
piping to provide partial protection against oxidation.
A second method is to introduce a
supplemental chemical inhibitor specifically formulated for the purpose of
preventing corrosion during lay-up periods. Numerous formulations exist,
including the newest development of powders called Vapor Corrosion Inhibitors
(VCI). These place a protective and penetrating layer of inhibitor on the the
surface of the metal to provide virtually complete corrosion control.
See Technical Bulletin
# P-05 for further information on VCI corrosion inhibitors.
A third rarely used, but highly effective
method, is to fill the empty system with a blanket of nitrogen gas, displacing
the air and oxygen, and stopping the corrosion process almost entirely.
An extremely critical step prior to any
lay-up procedure is to chemically clean and sterilize the entire system. By
removing unwanted rust, dirt, and biological matter, all of the above inhibitor
methods will work much more effectively due to the increased amount of contact
between inhibitor and base metal.
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