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Corrosion’s Added Impact Against Drained Pipe

It is a generally recognized fact that fully drained or partially drained piping systems are far more susceptible to corrosion than those containing treated water, or even untreated water.  Given a moist environment in combination with the presence of abundant air and oxygen, partially drained pipe 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 or open water systems clearly suffer the greatest in Northern climates where draining is often unavoidable.

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 those cases, roof level piping will require replacement many decades before the remainder of the system.  The buckets of iron oxide and scale typically removed from strainers, condenser heads, and heat exchanger tubes at every spring start-up are partially the result of such higher off-season corrosion activity.

Although most such systems are intended to be only drained sufficiently inside the building to avoid freezing, a surprisingly large number of systems are full drained either due to error or some other maintenance reason.  Draining 2-3 floors within the building is often judged by a 15 psi reduction in static pressure in the basement, yet for many investigations we pursue, the drain was open and simply never closed.  Just the slow leak of water from a loose pump seal can drain down an entire condenser water riser over a winter season. 

• Coupon Testing Impossible

Corrosion coupon racks, typically the only form of corrosion monitoring used, cannot be employed at the most vulnerable area of drained down pipe due to a lack of water flow.  They are rarely installed in outdoor or at roof level locations since no one raises that interest.  Of course, corrosion at the outer surface of the exposed rooftop pipe will also occur if not properly coated, insulated, and protected – a common maintenance problem often identified as a contributing factor to an overall higher measured corrosion rate.  However, it is most often the pipe’s interior, having been totally or partially drained over many years, which places the system at greatest risk of advanced failure.

We provide a useful handout on the errors associated with corrosion coupons Here.

• Case History

The below comparison of wall thickness measurements and estimated corrosion rates, taken from an actual 1996 ultrasonic piping investigation of a New York City commercial property, clearly illustrates the differences which, to some degree, always exists between drained and filled piping within the same condenser water system.  The building is well maintained with a very conscientious and knowledgeable engineering staff.

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 mechanical equipment room area of a 42 story New York City office building, and never drained of chemically treated water.

The right side of the page represents the same exact pipe at the 30th floor outdoor shaftway, where it has been drained every year during its five month winter season.  The pipe is the same in all respects – having been ASTM A 53 seamless extra heavy stock with an initially specified wall thickness of 0.500 in., a solid history of excellent water treatment maintenance, and having been in service for 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, shown below 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 – required due to the large difference in wall thickness.

Filled Pipe	Drained Pipe

• • • • • Comparison # 2 – Average Corrosion Rates

The below calculations are based upon the average of all recorded ultrasonic wall thickness measurements illustrated above.  Differences in average measured pipe thickness, corrosion rate, and remaining pipe life are dramatic.  As shown, the annual draining of this system has actually increased the average corrosion rate of the roof level piping four times that of the rest of the system.  As a result, the lifetime expectancy of that piping has been reduced by nearly ten fold!

Filled Pipe	Drained Pipe

• • • • • 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 activity will deteriorate the piping past its safe recommended limit.

Shown below, differences in corrosion rate, percentage of loss, and retirement date are even more pronounced.  In fact, the minimum wall thickness of the constantly filled pipe significantly exceeds the standard specifications of any new pipe installed today, while the upper pipe exists at nearly half that value.

Filled Pipe	Drained Pipe

• • • • • 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 wall remaining.  For the subject property, ultrasonic testing indicated that the minimum measured pipe wall thickness of the drained piping is nearing its minimum allowable safe operating limit.  Yet the water filled pipe offers almost unlimited remaining service.

Filled Pipe	Drained Pipe

• • • • • Comparison # 5 – Corrosion Rate vs. Location

The below graph is based upon data taken from a separate ultrasonic investigation of a 37 floor condenser water piping system at a downtown New York City office building.  The blue line represents the average corrosion rate based upon all wall thickness measurements; the red line represents the highest corrosion rate based upon the lowest individual wall thickness.  Since the condenser piping was installed exposed to the elements within an outside stairway, it had been drained completely to the bottom for all 39 prior winter seasons.

Ultrasonic thickness testing was performed at the roof and at each floor down to the basement.  As expected, testing showed a high corrosion and pitting condition throughout most areas.  When the test results were sorted based upon the physical floor location, highest to lowest, we found a clear trend – with noticeably higher corrosion and pitting activity at the upper areas of the pipe, and the only low and uniform examples of corrosion found at the lowest 4-5 floors.  Similar to many past investigations of drained vs. filled pipe, an almost 10 to 1 relationship was shown to exist.

Although the entire piping system was drained, the higher loss of pipe wall thickness at the upper floors was attributed to the abundance of fresh air and oxygen migrating through the cooling tower openings and down the risers.  The lowest sections of pipe, as expected, showed the lowest corrosion activity for the exact opposite reason.  Similar results have been found in many other CVI investigations, as well as by other investigators.

Useful Recommendations

There are currently only four feasible methods toward reducing the corrosion rate within a piping system which is partially or fully drained over any period.  One or more may apply depending upon conditions.

• Increase Inhibitor Level

The easiest, though least effective measure, is to greatly increase the level of the standard chemical inhibitor just prior to draining.  This theoretically leaves a heavier coating of rust protection on the piping to provide partial protection against oxidation while drained.  In reality, we have found little benefit through this action, and most properties which do follow such procedures generally still experience an excessive corrosion loss at drained pipe locations.

Most water treatment companies offer special lay-up inhibitors, although it is difficult to judge their effectiveness in the field.  Application is often sporadic, or related to the chemical supplier that season.  We have never failed to identify far greater corrosion activity at a roof location regardless of the form of standard lay-up chemical treatment provided.

• Vapor Corrosion Inhibitors

A second and extremely effective method is to introduce a supplemental chemical rust inhibitor specifically formulated for the purpose of preventing corrosion to steel pipe during lay-up periods.  Numerous formulations exist, including the newest development of powders called Vapor Corrosion Inhibitors (VCI).

Shown at left, a small concentration of powder totally protects this highly vulnerable fine steel wool at right above and below the water line, as well as at its liquid to vapor phase.  At left, the same steel wool without protection.  After 8 years, the treated steel wool sample at right shows no corrosion activity whatsoever.

These gas producing products place a protective and penetrating layer of corrosion inhibitor on the surface of the metal to provide virtually total corrosion control for up to two years and more, as shown.  They are sublimating chemicals, meaning that the material passes from solid directly to a gas phase with moving through a liquid phase – similar to carbon dioxide, otherwise known as “dry ice.”

They require a general, though not airtight, sealing of the piping system in order to allow the protective gas produced to migrate throughout the piping system.  This is easily accomplished at the refrigeration side due to the many valves commonly installed.  Protecting all roof level pipe right up to the sump and return drip pans is best accomplished by installing temporary inflatable sealing test plugs at all openings.  The powder itself can be introduced into the system through various methods.

We highly recommend Cortec products.  Further information is available directly at www.cortecvci.com.

• Nitrogen Gas

A third rarely used, but 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.  Its effectiveness relies upon being able to drive all air from the system to replace it with only nitrogen, which is the primary difficulty.  Purging air through a 1/8 in. hose is easily accomplished since the air is pushed out in a laminar flow with the nitrogen behind.  As pipe size increases, the nitrogen is diluted into the air and the air released to allow more nitrogen input itself contains nitrogen.

For larger diameter piping systems of 8 in. and greater, an extended period of nitrogen purging will be required before reaching a concentration capable of stopping the corrosion process.  This procedure requires an airtight piping system, which may be difficult to achieve at the cooling tower end – the area most in need.  It is almost never used in HVAC applications.

• System Drying

Since corrosion cannot continue without a presence of moisture, reducing that moisture content below a certain dew point will inevitably stop or substantially lower all corrosion losses.  Drying, however, is often difficult due to the physical configuration of most piping systems, and especially for open systems at the cooling tower itself, which would require temporary closure.  Areas of the system containing larger reservoirs of water can take months to dry.  For clamped piping systems, substantial water will remain at all horizontal lines due to the groove which raises an internal bump trapping water along the bottom.  Water will also remain at the end to end gap common to clamped systems.  Vertical risers are therefore the easiest to address most effectively.

Drying can be accomplished by one or a combination of heating, desiccant use, or the introduction of heated ultra dry air to 100 °F into the system.  Custom configured equipment is available.

• Clean System Is Essential

An extremely important step prior to any lay-up procedure is to chemically clean and sterilize the entire system.  Annual sterilization of all open condenser water systems is a generally good recommendation for all such systems.  By removing unwanted rust, dirt, and microbiological matter, the above inhibitor methods will work much more effectively due to the increased amount of contact between the chemical inhibitor and base metal.

High corrosion and pitting rates present a significant threat to every building or plant property which drains its piping for even short intervals.  Most often, that wall loss is not recognized until a failure occurs, or interest to learn the source of large start-up rust deposits is raised.  It is a problem often not addressed due to a combination of non-awareness, physical difficulty, questionable effectiveness of prior corrective measures, and an expenditure benefiting only a potentially limited amount of the overall piping system.

© Copyright 2024 – William P. Duncan, CorrView International, LLC