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Microbiologically
Influenced Corrosion, commonly referred to as MIC, is unquestionably the most
serious corrosion threat to a piping system any building or property owner can
face. While it is found in many different piping systems, it is most commonly
identified in open condenser water or process water systems due to the
favorable environmental conditions normally existing. An open condenser water
system offers abundant nutrients, oxygen, sunlight, lower treatment chemical
levels, rough interior surface, and low flow areas to promote the growth of
those microorganism that can often lead to MIC
contamination.
MIC has the potential to
produce extraordinary corrosion rates of 25 mils per year (MPY) and more -
which is sufficient to destroy a piping system in just a few years. Once an MIC
infection is well established, controlling the problem can cost hundreds of
thousands of dollars, and years of specialized cleanings, treatments, and close
monitoring.
Many authorities agree that
in most cases, MIC can never be completely eliminated from a piping system, but
only suppressed to some tolerable point. For any piping system where MIC has
said to have been eliminated through chemical use, reinfection is likely to
occur. The longer an MIC condition exists, the more damage is caused, the more
the system is weakened, and the more difficult and costly it is to correct - if
repairs are even possible. See Technical Bulletin
# C-5 for more about the threat of MIC.
The only
treatment used in the control and elimination of MIC is chemical sterilization.
Chlorine is probably the most common and effective sterilizing agent employed,
with high doses required for a certain amount of contact time in order to
achieve success. Chlorine is extremely corrosive to the various pipe metals and
elastomers found in most piping systems, however, and usually requires
accepting the trade-off of solving one corrosion problem against creating
another.
Bromine, chlorine dioxide,
hydrogen peroxide and other commonly available and more cost effective
sterilizing agents have all been used to varying success, but often their
effectiveness is dependant upon not their killing properties alone, but in
combination with the conditions established within the piping system itself.
Most chemical water treatment suppliers offer their own proprietary sterilizing
agents for treating MIC as well, since most common alternating biocides are
capable of only suppressing bacterial growth, not eliminating
it.
After possibly years of a
microbiological corrosion problem, however, a large mass of organic and
inorganic material is likely to exist attached to the pipe surfaces, and
especially in areas of low flow. Packed into dead ends, against closed valve
seats, and in the multitude of remote areas available for debris to settle
within a typical open condenser water system, this material provides its own
natural barrier to any chemical cleaning method. Installing a good water
filtration system is mandatory, therefore, once an MIC condition is
confirmed.
Effective chemical cleaning
then requires a repeated combination of chemical sterilization followed by
chemical cleaning of the resulting dead debris so that the sterilizing agent
can finally penetrate fully into the microbiological mass. The failure to
thoroughly expose every possible extreme within a piping system to the
sterilizing agent will unquestionably lead to reinfection of the piping
system.
For larger piping systems having
straight run vertical risers, the application of high pressure water jet
cleaning combined with a good sterilizing agent in the water stream can very
dramatically remove thousands of pounds of combined rust and microbiological
matter in relatively short time. The majority of the MIC infection can thereby
be eliminated, leaving a greatly reduced chemical cleaning effort at the
smaller piping only. See Technical Bulletin
# M-3 for more information about high pressure water jet
cleaning.
Another excellent
alternative is the application of the most effective oxidizing agent known for
killing MIC - ozone. Requiring a high initial equipment and installation cost,
ozone cannot be adapted to by a microbiological entity, as can occur with some
other chemical sterilizing compounds. Ozone is generated on site as a gas, and
due to its rapid decomposition and reaction with elements within the water, can
literally take weeks for any ozone residue to propagate throughout the piping
system.
In the proper concentration,
ozone will eliminate an MIC problem entirely. However, undersizing of the
equipment to reduce costs is a common mistake, resulting in a perceived failure
of the process itself. In many areas of the U.S., carefully controlled ozone is
applied as a single source chemical corrosion and biocide treatment with
claimed low corrosion rates. However, in higher concentrations, it can produce
excessive corrosion and pitting against metal surfaces.
For that reason, a thorough advance
evaluation of any sterilizing agent for its corrosion potential is always
recommended prior to use. Careful short term corrosion monitoring of any
chemical cleaning agent is also required.
Heat, in the form
of fire, has been used throughout millennium to safeguard against infection and
disease. Two of the most significant achievements in modern medicine were the
discovery of the microbial world, followed by recognizing the need to sterilize
medical instruments used in medical and surgical
procedures.
Sterilization may be
accomplished by high temperatures alone, or through the use of"wet or
moist" heat in the form of steam. Dry heat sterilization generally requires
higher temperatures to be effective, and yet may be ineffective at destroying
spores and other resistant biological forms capable of surviving in a dry and
desiccated state. A benefit of steam sterilization is its ability to penetrate
into spores and desiccated microorganisms to produce complete sterilization at
lower temperatures. It is also more effective at sterilizing porous surfaces.
Today, contaminated instruments can be
returned to being 100% free of any microbiological or viral contamination
through the standardized practice of "autoclaving." Autoclaving involves
placing any surgical instruments or materials to be sterilized in a steam
sterilizer, otherwise known as an autoclave, for a defined period of time and
at a specific temperature and pressure. Once such requirements are met, all
viruses, bacteria, and spores are guaranteed to have been totally destroyed or
rendered inert, and the autoclaved items suitable again for medical service
without any threat of infection.
Standard
conditions for autoclaving requires bringing the temperature of the autoclave
vessel to 250° F (121° C), by raising the pressure to 15 PSI. This
temperature and pressure combination must then be held at or above this setting
for 20 minutes or more in order to guarantee complete sterilization.
Recognizing that
every piping system is essentially a pressurized vessel capable of achieving
high temperatures, we can apply the same theory of sterilization used in
medical autoclaves to a piping system contaminated by MIC or any
microbiological entity. Most building properties already have such a sterilized
condition in their steam services piping. Sterilizing a condenser water or
other open water piping system is therefore a relatively simple, short term
procedure.
For any metal piping system,
handling the pressure requirement of 20 PSI is easily achieved, and does not
matter if the pressure is created by water, air, gas, or steam. Temperatures of
250° F are similarly no limitation to a steel or copper piping system.
Draining of the piping system is mandatory, of course, but with only a short
contact time of a few hours to bring the pipe up to the required temperature
for its 20 minute duration, shutdown time is greatly
minimized.
An adequate source of medium
pressure steam is typically available to any building or plant property, and is
available at almost negligible cost in comparison to chemical sterilizing
alternatives. Temporary installation of one or more adequately sized steam
supply lines to the piping system will be required, and a professional
engineering review of proposed plans and procedures is strongly recommended in
all cases.
Sterilization of a open
circulating loop does require a large amount of advance planning in order to
identify the temperature limitations of any equipment, and the isolation of
vulnerable equipment, components, or sensors, etc. Since condensate will be
created as the steam cools, it is necessary to plan its quick and adequate
removal through the temporary installation of steam traps or drains. Therefore,
an accurate piping layout and thorough knowledge of the piping distribution
plan is the very first step in checking feasibility.
We strongly
recommend against applying heat sterilization to any Victaulic or gasket
clamped piping systems. Refrigeration machines, or any equipment which would
produce an expansion due to the application of heat should be adequately
isolated or blanked off from the main piping system. Depending upon the size of
the piping system and its valving arrangement, it may be possible to sterilize
it section by section, understanding however that to open any area of
non-sterilized pipe to sterilized areas will re-infect the cleaned
pipe.
A full walk through survey of the
piping system is required in order to identify any obvious concerns. Open
piping at the cooling tower will require valving off, or the installation of
blank plates at the pans and sump in order to sterilize the most extreme limits
of the system. Advance planning and the installation of temporary vents at
various extremes of the system are necessary in order to enable the steam
supply to reach the full extent of the piping and raise the temperature of the
pipe sufficiently.
Clearly, smaller piping
systems are significantly easier to sterilize than larger systems. For large
piping systems, measuring the surface temperature of the pipe is recommended in
order to ensure that adequate time has passed to allow the mass of the pipe to
reach the required temperature of 250° F. Pressure gauges are typically
available throughout a piping system to monitor
pressure.
All valves, future taps, and
piping distribution lines not used for the isolation of critical equipment
should be opened in order to maximize the reach of the steam throughout the
system. Various test methods are available that will provide engineers with a
clear indication of whether sterilization at the various points and extremes of
the piping system has been accomplished. Infrared testing is one such method
capable of quickly determining the surface temperature of the pipe.
Once completed,
the entire piping system is allowed to thoroughly cool, and chemically treated
fresh water re-introduced. Given the large volume of now dead microbiological
debris, combined with other rust deposits, a standard chemical cleaning should
be immediately performed. A large volume of debris should be expected dependant
upon the degree of microbiological infection and condition of the system, and
therefore, multiple chemical cleanings may be necessary.
Any MIC contamination is likely to have
removed substantial pipe metal, which will now exist as a less dense iron oxide
combined with the dead organic material. Spare strainers and a full engineering
staff is recommended to handle the effects that the debris will have on the
operating equipment.
Overall, the
application of steam to correct an MIC contamination problem is primarily a
question of thorough preparation and good advance planning. There is no high
cost expenditure for sterilizing chemicals, manpower, and re-treatment
inhibitors, and no chemicals necessary other than those to clean the system of
debris once sterilization is complete. Steam sterilizing costs are mostly in
labor and temporary piping. Whereas chemical sterilization can take months to
perform and still not produce the desired result, steam sterilization is
guaranteed to work within a few hours if planned and performed properly.
©
Copyright
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