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Dirt,
microbiological material, iron oxide deposits and scale will negatively impact
almost any piping system if left to accumulate. It is a naturally occurring
problem every building operator and plant engineer will likely face at some
point during their career. In some cases, the problem may be minor enough to
not even be realized. In its worst form, however, corrosion can produce
accelerated pipe loss, leaks and the premature failure of a piping
system.
With corrosion always present to
some degree, the most common operating problems are usually those associated
with deposit build-up, lost heat transfer efficiency, and higher operating
costs. In cases where corrosion activity has been high over an extended period
of time, interior deposits can actually accumulate to the point of preventing
adequate flow, yet with the more serious threat actually existing due to the
loss of wall thickness. See Technical Bulletin
C-4 regarding the threat of interior deposits.
In condenser and
chill water piping, an interior deposit build-up as thin as 1/8 in. can provide
the ideal environment for under deposit corrosion, cell corrosion and/or
microbiologically influenced corrosion (MIC) to develop. Most often associated
with condenser water or open process water systems under certain conditions,
MIC is a well recognized threat to virtually all piping systems from domestic
water to fire sprinkler service.
The below
example, taken from an actual case history, graphically illustrates the volume
of deposits which can accumulate over time. Here, lightly scraping an
approximate 36 sq. in. section of 10 in. pipe interior produced the volume of
iron oxide debris shown, and easily confirmed the source of material clogging
the strainers and condenser water tubes. A rough estimate of deposit volume,
based upon the weight of material removed from one localized pipe section,
suggested that over 1,500 lbs. of iron oxide material likely
existed.
Due to an extended drain down for
new chiller replacement, iron oxide which had accumulated over 35 years of
operation began to loosen and travel throughout the condenser water system. The
known volume of interior deposits made it clear that millions of dollars of new
HVAC equipment would be at risk if such deposits were not removed before
returning the original 1965 pipe to new service.
Ultrasonic
testing showed that the above piping system had deteriorated from schedule 80
thickness specifications to just below schedule 40 at a moderate 5 MPY
corrosion rate. Corrosion activity was generalized at the surface - leaving the
pipe in good overall physical condition. Such a finding is unusual, however,
since in most other cases, deep localized pitting is far more common. For many
building owners and plant managers, the loss caused by a high corrosion
condition is only discovered after it has progressed past the point where the
pipe is suitable for further reliable service.
In examples where
ultrasonic examination can be verified with a view of the pipe interior, we
have consistently found the highest wall loss hidden beneath the largest
tubercles or deposits. The below photographs clearly document a heavy wall loss
due to an aggressive under deposit corrosion condition - in this case producing
a severe localized wall loss of 0.150 in. or more. This loss can be seen at the
left side wall of the pipe for both photographs.
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Aside from total
pipe replacement, removing any long established deposits usually requires a
continuing, multistage chemical cleaning program using aggressive and
potentially dangerous chemical agents - chemicals which can attack the base
metal as well as the problem itself. Some of the worst and most expensive
piping failures we are aware of have involved the use of strong acids during
on-line cleaning operations.
The fact is
that a deposit fouled piping system also defines a physically weakened piping
system. Even though a corrosion failure in progress will take place eventually,
a multitude of piping failures occurring throughout a building property, and
within minutes of adding an acid or aggressive cleaner to a piping system, is
something to avoid by all means. A "Controlled Disaster," termed by some
chemical cleanout companies to define a limited pipe failure threat, is still a
disaster by any definition once the water begins pouring
down.
Various acids and chelating
chemicals do offer great benefit, and often present the only cleaning option
available. This is especially true for more complicated piping layouts and
smaller diameter piping. However, their use requires a careful preliminary
assessment of the piping to identify any weakened areas. Threaded joints are
always a concern, as are Victaulic or clamped fittings.
CorrView
International, as well as most other authorities, have well documented the
relationship between interior deposits and long term leaks and secondary
operating problems. We believe that once iron oxide deposits have been
established at the interior pipe surface, there is little alternative except to
remove them. The thinking at the operations and management if typically
different - with concern for preempting a leak condition often setting aside
any consideration for taking corrective
action.
But while a chemical cleaning may
indeed cause leaks, allowing the deposits to remain will produce even greater
long term damage. Some chemical formulations offer safer removal options, and
are worth investigating. Long term methods of deposit removal also exist using
filtration, although they will not provide the immediate results often
necessary. See Technical Bulletin
W-4 regarding various types of filters
available.
Compared to the many
chemical options that produce varying results with always a clear threat, one
non-chemical method offers perhaps the best and safest cleaning
alternative.
High pressure
water jet cleaning is a technology well proven in over thirty years of removing
iron oxide and various other forms of hardened surface deposits. It can safely
loosen and eliminate virtually all types of deposits simply through the action
of multiple high pressure water nozzles - washing away all deposits and
cleaning the interior piping virtually down to the bare metal.
Since there are no acids, alkalis or any
chemicals used whatsoever, there is no threat of attacking and removing the
base metal. The pressure against the pipe itself presents little if any threat.
Unlike chemical cleaning, which generally requires an extended period of time
to dissolve and flush the deposits and then neutralize the system, high
pressure water jet cleaning provides instant results - allowing the piping
system to be placed back in service
immediately.
High pressure cleaning
removes the heaviest and most strongly attached deposits via multiple passes of
various size water jets - solving problems that would be virtually impossible
to address using other physical methods.
The below left
side photograph shows the buildup of rust deposits within an actual section of
45 year old, 6 in. diameter galvanized steel domestic water riser. Note the
larger and lighter colored iron nodules or tubercles mixed throughout the
darker colored deposits.
The right side
photograph of the exact same section of pipe shows that all traces of iron
oxide deposits have been completely removed after two quick cleaning passes
using a 3,500 PSI water jet. No base metal has been removed from the pipe
itself.
Note the widespread pitting of the
interior pipe surface which has been revealed following the removal of the
interior deposits. Yet the remaining wall thickness, in this specific example,
can be seen as quite acceptable. Such localized pitting activity is common
where deposits have been allowed to accumulate, and is one of the most common
causes of pipe failure. Removing such deposits in order that the anti-corrosion
chemicals can reach the base metal, should be considered an absolute priority
action for any fouled piping system.
Interior surface
deposits can either produce generalized and shallow deterioration of the pipe
surface, or it can produce very deep localized pitting as shown in the below
photograph. Removing a large tubercle revealed two penetrations of
approximately 0.125 in. and 0.075 in., with virtually no other corrosive attack
anywhere in the immediate area. Such a corrosion scenario is far more
threatening since all potential corrosion activity focuses at one or a few
surface locations.
For a New York
City client, approximately 15 floors of 8 in. supply and return condenser water
piping was pressure cleaned using 10,000 PSI water jet. Prior ultrasonic
testing showed that 35 years of use had reduced the original schedule 80 piping
to just below schedule 40 specifications, and that long service life existed
once the existing heavy deposits were removed. Long remaining service life was
not expected, however, if the deposits remained in place, and the corrosion and
pitting rate increased.
During the
cleaning process, small samples of the rinse water were regularly taken and
collected into one container. A sample of that collection is shown below left.
By testing its solids content, CVI estimated that over 2,500 lbs. of material
was washed away in addition to approximately 400 pounds of solids collected in
a settling container. Overall, we estimated that approximately 1/2 oz. of iron
oxide was removed for every gallon of water used in the process. The below
right side photo, taken after 48 hours of settling time, demonstrates the
volume of rust removed per volume of rinse water.
While the
benefits are many, high pressure water jet cleaning is, unfortunately, not
applicable for all piping applications. It is best suited to straight runs of
vertical piping and to heights not exceeding 300 ft. Given that many site
preparations may limit its use, consideration must be made for the following:
- The system
must be off-line and completely drained.
- Access must be
available either through a sufficiently sized valve, spool piece, header,
flanged connection or temporary opening cut into the pipe in order to introduce
the cleaning apparatus and hose.
- Multiple
access points may be necessary depending upon the piping layout.
- The lowest
section of the piping must be open in order to drain the cleaning jet water and
the debris which will be removed. Water volume can be high, exceeding 40
gallons per minute, and must be adequately removed using pumps. Ideally, the
rinse water should be drained into a settling tank in order that heavier
deposits can be physically removed as solid waste prior to being discharged
into the drain.
- The actual
water pressure generating unit is substantial in size, and is typically powered
by a gas fired motor. Water carrying lines can reach 500 ft. - which is often
enough to reach from the pressure head to work location.
- Adequate water
supply in the form of a 2 in. fire hose connection is generally necessary.
- Horizontal and
vertical lines can be cleaned with equal effectiveness, though with greater
difficulty for any horizontal piping. The water jet is self-propelling over
hundreds of feet and will navigate most turns and elbows. Attempting to
navigate past two elbows is often difficult and often requires a new access
point.
The use of high
pressure water jet offers significant advantages depending upon the layout of
the piping system. It can provide an immediate solution to a deposit problem in
some instances, or address the majority of a deposit problem for others. Given
the high volume of deposits that generally will exist at the main lines - a
combination of using water jet to clean the larger diameter piping and then a
mild chemical cleaner for the distribution lines may be the best alternative to
such a problem.
Removing perhaps 80% of
the problem from the main lines and risers using water jet will dramatically
reduce the requirement for both water filtration and chemical cleaners. Overall
savings are also significant in comparison to the high costs associated with
most chemical cleaning agents.
©
Copyright
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