The tubes within these vessels rest on steel supports (sheets) of about 1/4- or 1/2-inch thick and spaced the length of the vessels. As the tubes expand and contract from temperature changes, some erosion results. The most damage is caused by vibration of the tubes against their steel supports as fluid flows through them at a high velocity (up to 1,400 feet per minute). As this wear continues, erosion begins to accelerate and the risk of failure becomes dangerously high.

However, all tubes are not subject to the same degree of wear. In fact, some show no signs of erosion. Nonetheless, tubes that are worn destroy the vessel's integrity and reliability.

CHEMICAL DETERIORATION

Chemical deterioration such as corrosion is equally damaging. In an air conditioning system, for example, refrigerant will break down under certain conditions of temperature and moisture, forming hydrofluoric and hydrochloric acids that attack copper tubes. They also etch the steel supports, widening the gap between supports and the tubes, allowing more vibration.

In air conditioning chiller sections, refrigerant boiling off around the tubes causes outside diameter (od) pitting. In condensers and other heat exchangers, the physical accumulation of hardness, lime, ferrous deposits, and other elements cause corrosion cells that result in inside diameter (id) pitting action. The attack may continue until there is a hole in the tube.

Other forms of corrosion and erosion that are not as common are galvanic, corrosion, impingement, deposit attack, crevice corrosion, water line attack, de-zincification, exfoliation, fatigue, stress corrosion cracking, etc.

Unquestionably, heat exchanger vessels do deteriorate and fail. But it is a myth that you have to wait for tubes to fail before you know which ones to replace. This myth says catastrophe is unavoidable unless you replace all tubes, good and bad, before they fail.

There have been attempts to disprove this myth by pulling a random sample of tubes, identifying areas where tubes have worn excessively, and pulling the suspicious ones. Since a number of good tubes are needlessly replaced, this approach is expensive. Furthermore, even after paying this high price, the vessel's integrity is unchanged. Tube wear does not follow a uniform pattern; there are still defective tubes within the vessel that could fail at any moment - perhaps the tube next to one that was replaced.

The objective is to identify each defective tube individually. Then, prior to vibration or metallurgical/chemical reactions taking their toll, the high-risk tubes can be replaced, in effect renewing the vessel nearly to its original condition and restoring its integrity without wasting good tubes.

TECHNOLOGY ADVANCES

Through advancements in electronic technology, this objective can be realized. A proven technique of vessel testing exists that provides a profile of all tubes in the vessel without damaging the tubes: eddy current analysis.

Through conventional inspection techniques (visual, pressure test, etc.) it may still be difficult to detect a deteriorating tube until a catastrophic failure occurs and thousands of dollars are spent. By employing eddy current evaluation, the risk of such a failure is manageable.

Hartford Steam Boiler Inspection and Insurance Co. has had literally millions of dollars of experience with tube failure. Company officials have said that eddy current evaluation "plays a vital role in good preventive maintenance." Indeed, the company's loss prevention requires "that all insured absorption machines of 100 tons in capacity or more, and over five years old, be subjected to an eddy current analysis of the tubes."

Hartford Steam's publication, The Locomotive, further notes, "Eddy current evaluation has successfully been used in air conditioning chillers and condensers, petrochemical process vessels, heat exchanger tubing, and utility steam turbine surface condensers," as well as "on submarines for almost 30 years by the nuclear Navy.

"Eddy current inspection can detect tube corrosion, pits and vibration wear before leaks and a resulting unscheduled shutdown can occur. The savings realized in leak avoidance, retubing costs, and avoidance of recurring leaks should far outweigh the cost of the inspection."

MANAGING THE RISK

The principle of eddy current testing has been employed in various fields on nondestructive evaluation (NDE) for years. In the heat exchanger field, unique problems associated with dissimilar metals and limited access had to be overcome. Now, with eddy current analysis, all manner of tube defects are detectable in heat exchangers with similar accuracy as the mills experience when testing new tubes for minute flaws.

The tubes in a heat exchanger are tested by insertion of a probe the full tube length. Impulses are fed back to a console, which shows the tube's condition. After all of the tubes have been probed, a decision is made as to which should be replaced.

The equipment used in tube inspection is an impedance bridge. The inductive legs of the bridge are a primary and secondary coil encased within a fiber glass shell or probe. An alternating current of 2-300 Hz, applied to the primary coil, generates a magnetic field. This field, in turn, causes eddy currents to flow in the tube. The magnitude and depth of these eddy currents depend upon:

The strength of the magnetic field.

The proximity of the coils to the tube.

The magnetic permeability of the tube.

The first and second factors are fixed when the analyzer is calibrated. Then, the depth of eddy current penetration depends upon the vessel's tube characteristics, particularly permeability.

The induced eddy currents set up a secondary magnetic field that is counter to that established by the probe. Because the tube's permeability varies with wall thickness, the continuity of this counter force is broken by flaws or wear in the tube.

As the probe is inserted and wall thickness differences are encountered, the change in counter force creates a voltage impulse. This is fed back through the secondary coil of the probe to a console. If the tube is worn or has a hole, feedback will reflect this. The impulse results in unbalanced voltage across the impedance bridge.

By applying phase discrimination to the amplified unbalanced voltage, it is possible, with the proper selection of frequency, to qualitatively project defects on an oscilloscope screen. Each defect, with its own identifying wave form, is interpreted by the operator. Defects that are consistently discerned are holes, "leakers," split fins, chip marks, eroded/corroded areas (on both the inside and outside of the tube), lap seams, presence of "tramp" metal, dents, and areas incorrectly expanded. Once the defective tubes are identified, they are selectively removed and replaced with new ones.

Mike Golondzinier is with Condenser and Chiller Services, California Finned Tube Co. He can be reached at 800-356-1932 or www.ccs-tubes.com.

Publication date: 06/05/2006