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Severe
corrosion of HVAC piping systems and their related equipment presents the most
potentially damaging losses to any private, industrial or commercial property
next to the threat of fire.
It is
surprising to see, therefore, the relatively low priority often provided for
corrosion control and monitoring of critical HVAC and process piping by many
building owners, operators, and plant engineers. Whereas condenser or open
water corrosion rates of 1-2 mils per year (MPY) were typical only two decades
ago and easy to achieve, it is now common to find system wide corrosion rates
of 5 MPY and greater - with rates occasionally as high as 20 MPY.
Read about changes in
piping trends over the past few
decades.
While certain
unavoidable factors have contributed to generally higher corrosion rates, it
remains clear that the service life of most piping systems could be greatly
extended by donating greater effort to corrosion control. Even though
monitoring corrosion rates through the use of steel corrosion coupons is
somewhat helpful, their accuracy and reliability is often questioned.
Read about the
limitations of corrosion coupons.
To be fair,
the common metal loss terminology in mils per year (MPY) poorly conveys the
true extent of corrosion taking place within the system, and often fails to
warn a plant operator of a serious corrosion problem, and its consequences.
Most building operators and engineers would consider the difference between a 4
MPY and 6 MPY corrosion rate as insignificant, although the net effect of the
higher rate over many years may be a heavy buildup of rust deposits and
premature failure.
The physical size of
the pipe is also a major factor to consider. Smaller diameter pipe begins with
substantially less available wall thickness to corrode, and reaches a failure
point sooner - while larger diameter pipe produces more interior deposits due
to its greater surface area.
This picture
changes, however, as soon as one looks at corrosion in terms of pipe mass or
material weight lost, rather than a MPY value. A low to moderate corrosion rate
of 3 MPY at a 12 in. schedule 40 condenser water pipe for example, while
seemingly minor, actually translates to a physical loss of 38 lbs. of steel per
every 100 linear feet. At 8 MPY, approximately 102 lbs. of metal is lost.
Multiplied by the number of years in service, and the true magnitude of system
corrosion takes on much greater significance than when reported as simply 1, 2,
or 5 mils per year.
A dramatic
example showing the actual weight of steel removed from various pipe sizes at
different corrosion rates is presented below in Table C-1. Although corrosion
rate tests are typically presented in terms of mils per year, the true impact
of that measured rate can only be realized by looking at the actual volume of
metal lost from the piping system. As the piping system wears and the internal
pipe diameter increases, so does the internal surface area and the weight of
metal lost.
For a
typical commercial building property of 33 floors, having a cooling tower at
the roof and a 5,000 ton refrigeration plant in the basement, we can estimate
approximately 1,000 linear feet of 24 in. supply and return piping in service.
From the above table, and based upon a moderate 5 MPY corrosion rate commonly
found today, we can then estimate that approximately 1,210 pounds of steel will
be lost from the steel and distributed into the system for
EACH YEAR of service due to corrosion losses.
For a typical 30 year old property, the
weight of metal lost into solution and blown down, filtered out, settled, or
deposited somewhere within the system is actually enormous. And this value does
not include the hundreds of feet of smaller distribution piping generally
involved.
The damage
caused by a high corrosion problem extends past just the possibility of a leak,
however. In its oxidized form, steel produces approximately 20 to 25 times its
original volume in iron oxide or rust product. This by-product is often found
in horizontal lines and at low flow areas - often accumulating in sufficient
volume to produce under deposit corrosion, heat transfer loss, and eventually
flow rate problems.
Below left is an
example of a 10 year old 12 in. condenser water line suffering from a moderate
corrosion rate of approximately 5 MPY, but a buildup of "tubercular"
deposits along its bottom. Another example at the right shows a 14 year old
condenser system having an accumulation of rust deposits in its 10 in.
horizontal distribution lines from a 6 MPY corrosion rate. Without effective
filtration and other preventative steps, however, such deposits are virtually
inevitable. See Technical Bulletin
C-4 about the problems associated with interior deposits.
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Table C-2,
below, provides an estimate of rust related debris created by the oxidation of
the metal lost, and shows that in general, a tremendous volume of corrosion
product is produced. Using the same 33 floor office building as an example, we
can estimate that as much as 50 cubic feet of rust and iron oxide deposits will
be created from the 1,210 pounds of steel rusted away for EACH YEAR of service. Over a 30 year history, a
significant volume of particulates will be produced to create various secondary
operating problems. Again, effective water filtration is mandatory.
For
condenser or open process water systems, much of this corrosion product will be
lost in the cooling tower blowdown, some will settle in the tower basins and
condenser heads, and some will remain attached to the pipe wall surface.
Corrosion products are often a much
greater threat to closed piping systems, since no outlet exists to remove the
deposits, and due to the much smaller distribution lines which are more
susceptible to deposition. Closed systems generally do not exhibit their
corrosion products like an open system, and therefore receive little attention.
In addition, increased deposits present ideal opportunity and nutrients for
microbiological organisms to grow. See Technical Bulletin
# W-1 about reducing corrosion deposits in closed circulating
systems.
Not only is the
structural integrity of the piping itself threatened by excessive corrosion,
but the resulting corrosion products generally cause secondary problems in the
form of lost heat transfer, biological fouling, microbiologically induced
corrosion (MIC), clogged pipes and abrasive wear to pump seals and
components.
Most
corrosion problems can be avoided by specifying a comprehensive chemical
treatment program using a reliable water treatment contractor. Fully automatic,
water meter activated chemical feed control and dual biocide feeds are an
absolute necessity, and not an option. A supplemental corrosion testing program
and the frequent review of its results are critically important to ensure
satisfactory corrosion control.
Proper
start-up of a condenser water system is often critical, and many examples of
extremely high corrosion loss have been traced back to poor start-up
procedures. The actions of the mechanical contractor, or an inadequate low bid
water treatment package are often found at fault. In short, poor planning and
coordination between the water treatment company, mechanical contractor, and
building owners or operators can result in operating problems for many years
following.
While the evaluation of any
corrosion rate is typically presented in mils per year (MPY), knowing the true
physical loss of pipe in actual weight places a new perspective on any wall
loss evaluation. As the above tables dramatically show, there is very
significant difference in threat level between a 1 MPY, 5 MPY, and 10 MPY
corrosion rate.
©
Copyright
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