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Excessive
corrosion activity can devastate a condenser water system or open process
piping in as little as a few years. Generally co-dependant upon pipe size, a
corrosion rate of 20 mils per year (MPY) can deteriorate ASME A 53 standard 12
in. piping to its minimum safe limit in only 10 years, and its 1-1/2 in.
threaded distributions lines in as little as 2 years or less.
High corrosion rates exceeding 8 MPY are
typically due to under deposit or cell corrosion, microbiologically influenced
corrosion (MIC), and various advanced forms other than general corrosion.
Often, such advanced corrosion activity can develop even though no obvious
physical signs suggest a problem, and where years of corrosion coupon
monitoring show low corrosion rates. Read more about the
limitations of corrosion coupons.
Unfortunately,
all aggressive forms of corrosion are difficult, if not impossible, to control
and correct using standard procedures once the problem has been firmly
established. Most advanced corrosion mechanisms produce wild variations of
pitting or depth penetration into the pipe, extensive iron oxide deposits, and
large tubercles under which the most severe pipe loss is located.
Depending upon the precise corrosion
mechanism, deposits can line the entire pipe interior, exist at only the bottom
area, or develop in random locations. See Technical Bulletin
C-4 about the problems associated with interior
deposits.
It is precisely the
insulating effect of such interior deposits that reduce the corrosion control
benefits from any chemical water treatment program. And since any chemical
inhibitor will likely never even reach the base steel, it becomes virtually
impossible to control a high corrosion condition without first removing those
deposits entirely. See our photo gallery
of various corrosion types.
A high corrosion
rate virtually defines a piping system which has already been heavily damaged.
Therefore, on-line deposit removal measures using an acid, chelant, or other
chemical agent often cannot be safely performed. Most individuals involved in
building operations and plant maintenance can relate stories of catastrophic
piping failures immediately following the introduction of an acid as a pipe
cleaning procedure. We are aware of many such failures in the New York City
area.
In reality, most good quality acid
cleaning products are heavily inhibited to minimize any attack upon the steel
pipe itself, and will dissolve the iron oxide deposits only. Additional metal
inhibitors can be added to further insure that the base steel is protected.
However, any area of weakness held back
by the insulating effects of the deposits themselves will loosen and
potentially produce a leak. The isolation of any weakened areas of pipe is
always advised prior to an acid cleaning, and replacement of such pipe with new
stock is typically recommended prior to any acid cleaning procedure.
Many property
owners and managers therefore find themselves in the difficult position of not
being able to effectively control an advanced corrosion problem without first
removing the existing rust deposits - yet at the same time being unable to
remove those deposits without the likely failure of any weakened areas.
Such a threat is greatest where threaded
joints exist, and where previous failures have already occurred. Whereas a
chemical cleaning may prompt some failures, not acting guarantees even more
failures as the deposits increase in volume and produce even higher corrosion
rates.
There are a
number of reasons for the typically large difference in corrosion rate which
exists between closed and open piping systems. Aside from the greater
vulnerability and threat level to an open cooling tower system due to greater
particulates, aeration, and microbiological influence, open piping systems are
chemically treated using a far lower concentration of corrosion inhibitor.
See Technical Bulletin
# M-12 regarding why closed and open piping systems are treated to different
chemical
concentrations.
Compared to
closed chill or secondary systems, open piping systems are under significantly
greater threats, such as:
- Increased dirt loading due to the air scrubbing function of
the cooling tower. All cooling towers are essentially high capacity, high
efficiency air filters depositing some proportion of the captured dirt and
particulates at the interior pipe wall.
- Increased
biological contamination. Various forms of biological growths, from simple
bacteria to algae and higher multi-cell forms, exist to produce elevated
corrosion levels - either directly or indirectly. MIC is the worst form of
microbiological attack.
- An open piping
system is an oxygen rich and saturated environment conducive to biological
growth. Oxygen is a also key component of most corrosion
mechanisms.
- Cooling tower
systems offer optimum temperature (85-95 º Fahrenheit) for biological
growth to occur.
- Open systems
supply a source of sun energy conductive to algae and most forms of
microbiological growths.
- Chemical
corrosion inhibitor concentrations for open systems are generally lower than
closed systems by a factor of 10. Typically, lower recommendations of water
treatment chemicals for an open piping system are not due to a lack of need by
the more stressed and threatened metal components, but rather due to the
economic reality of providing a reasonably effective treatment program
customers are willing to pay.
Given the high
heat transfer efficiency of today's plate and frame heat exchangers, a very
reasonable solution to a high and threatening corrosion rate problem exists by
turning the major portion of the existing piping into a closed system. For the
most severely damaged piping systems, this may often be the last and only
alternative to replacing the entire piping system.
Essentially, the condenser water piping
system is separated in two, preferably at the point nearest the cooling tower
itself, and a plate and frame heat exchanger similar in shape to the below
example installed. A wide range of units exist to accommodate almost any need.
Providing twin units in parallel is advised where a severe corrosion problem
exists, and the need to frequently clean and service the heat exchanger may be
anticipated.
Pumps move the condenser
water from the tower to the heat exchanger on the open or primary side. On the
isolated or secondary side, a second set of pumps move water of a slightly
higher temperature to and from its destination - either package units or main
refrigeration chillers. A reasonable temperature loss of only a few degrees
generally exists.
Such an addition
to the condenser water system requires a significant amount of work and
preparation. Available space for the heat exchanger is required - as is also
the addition of one or more additional condenser water pumps, an added power
requirement, and some piping layout modifications. Typical costs can easily
exceed $250,000.
This is a generally
reasonable expense, however, compared to the alternative of pipe replacement.
The installation of a heat exchanger immediately offers many significant
benefits - the most important being the isolation of the system.
With outside
influences effectively eliminated, it becomes possible to capture and remove
any suspended particulates using an inexpensive side stream bag filter. Adding
dispersing agents or other neutral chemicals to gently remove and suspend
existing deposits offers the option of gradually cleaning the system without
the inherent threat which exists in the use of strong acid cleaners.
Microbiological growths, often the underlying cause of an advanced corrosion
problem, can be significantly suppressed, if not virtually eliminated.
Most importantly, chemical corrosion
inhibitors, which would normally be added in relatively weak concentrations,
and which would be continuously blown down in an open cooling tower loop, will
now remain within the system closed. Significantly higher dosages of chemical
inhibitors not economically feasible to maintain for an open system, will offer
greater penetration through the existing deposits to the metal sub-surface. For
example, instead of a typical maintenance level of 8-10 PPM of molybdate common
for an open cooling system, a far more effective 500 PPM of molybdate can be
maintained.
High concentrations of dual
corrosion inhibitors can also be employed to protect both the anodic and
cathodic areas of the metal - thereby further improving corrosion control. Once
the continuous volume of blowdown water is stopped by closing the system, any
combination of chemicals suddenly becomes not only more effective, but less
costly to use.
Far more advanced corrosion
control products outside the inventory of most water treatment companies are
available to virtually stop most corrosion mechanisms - even those existing
under heavy deposits. While economically prohibitive and technically
questionable to succeed under open system conditions, such options become
available to the building owner or plant operator once a piping system is
closed. Read Technical
Bulletin C-9 about VCI advanced corrosion inhibitors.
Of course, the
existing piping from the cooling tower to the plate and frame heat exchanger,
if not replaced or tested and found suitable for further service, will continue
to present a threat if the fundamental corrosion problem is not corrected - but
to a far lesser degree. The small separation between plates is designed for a
normal amount of particulates, but will clog and require frequent cleaning if
presented with excessive dirt loading. An effective chemical water treatment
program is still required.
Given that the
main supply and return lines between cooling tower and heat exchanger are
typically of larger size and heavier wall thickness, an existing corrosion
problem will have less effect than at the small distribution lines within most
facilities. More aggressive cleaning agents also present far less threat than
to the smaller threaded pipe which may exist in any condenser water or open
piping system.
Many critical
condenser water applications are, in fact, designed from the beginning having a
closed secondary system serving the actual A/C units. Such a system offers an
inherently lower corrosion rate by as much as a factor of ten times, fewer
operating problems overall, and a limit to the water damage which might occur
during a major piping failure.
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