EDDY CURRENT TUBE ANALYSIS
HEAT EXCHANGER FAILURE...MYTH OR FACT?
Heat exchange systems are designed with practically no risk of failure
when they are new. In fact, some components have such nominal exposure that they are
not apt to fail for years, if ever. After all, they have no moving parts. Is
it a myth then, that these vessels fail, catastrophically resulting in extended downtime
and loss of revenues for their owners? No unfortunately it is not.
Although these vessels are not as susceptible to defects in their early years, they begin to suffer increasing failure rates at about 8 years old. Ironically, it is the action of their "nonmoving" parts that cause the highest percentage of trouble. The tubes within these vessels rest on steel supports (sheets) of about 1/4 or 1/2 inches thick and spaced the length of the vessels. As the tubes expand and contract from temperature changes, some erosion results. However, the most damage is caused by vibration of the tubes against their steel supports as fluid flows through them at high velocity (up to 1400 feet per minute). As this wear continues, erosion begins to accelerate and the risk of failure becomes dangerously high. Wear is discriminant, however, in that all tubes are not subject to the same degree. In fact, some show no signs of erosion. Nonetheless, the tubes that are worn destroy the vessel's integrity and reliability.
Although this mechanical wear is the most common cause of tube failure, 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 acid which attack the copper tubes. They also etch the steel supports, widening the gap between them and the tubes, allowing more vibration.
In air conditioning chiller sections, freon boiling off around the tubes causes O.D. pitting.
In condensers and other heat exchangers the physical accumulation of hardness, lime, ferrous deposits and other elements causes corrosion cells which result in I.D. pitting action. Often, the attack continues until there is a hole in the tube.
Other forms of corrosion and erosion that are equally fatal but not as common are: galvanic, corrosion, impingement, deposits attack, crevice corrosion, waterline attack, dezincification, exfoliation, fatigue, stress corrosion cracking, etc.
Unquestionably, heat exchanger vessels do deteriorate and fail.
There is a myth however - the common belief that you have to wait for tubes to fail before you know which ones to replace. In other words, 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; so, there are still defective tubes within the vessel that could fail 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 and chemical reaction 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.
Through the 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
The service described herein will interest anyone who is affected by the failure of:
*Heat Exchangers* *Air Conditioners*
*Process Coolers* *Condensers*
Heat exchanger tubes fatigue, corrode, erode, bulge, freeze and just plain wear out. Through conventional inspection techniques (visual, pressure test, etc.) it is impossible to detect a deteriorating tube until a catastrophic failure occurred and thousands of dollars lost.
By employing eddy current evaluation, the risk of such a failure is manageable.
Hartford Steam Boiler Inspection and Insurance Company has had literally millions of dollars of experience with tube failure. They believe that eddy current evaluation "plays a vital role in good preventative maintenance". Indeed, their 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". Their company magazine, The Locomotive, continues by noting that:
"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 vibrational 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."
"Also, selected replacement of a few tubes will typically restore the unit to full reliability."
MANAGING THE RISK
The principal of eddy current testing has been employed in various fields on non-destructive evaluation (NDE) for years. However, 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. Impulse are fed back to a console, telling the tube's condition.
After 100% of them are probed, an intelligent decision is then made as to which, if
any, tubes are to 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 fiberglass shell or "probe". An alternating current of 2 Hz to 300 Hz is applied to the primary coil and generates a magnetic field. This field, in turn, causes eddy currents to flow in the tube. The magnitude and depth of these eddy currents are dependent upon:
1. The strength of the magnetic field.
2. The proximity of the coils to the tube.
3. 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 is solely dependent on the vessel's tube
characteristics, particularly permeability.
The induced eddy current themselves set up a secondary magnetic field which is counter to that established by the probe. Since 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; i.e., 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 its interpreted by the operator. Those 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.
Our experienced Analysts can analyze on the average of 750 to 1200 tubes per day depending on length and cleanliness.
24 HOURS A DAY
People with field experience and product knowledge to assist you.
For Information and Assistance
CALL (800) 356-1932 * (909) 613-9000 * FAX (909) 590-3446
People with field experience and product knowledge to assist you.
CONDENSER & CHILLER SERVICES, INC. * 13382 BENSON AVE. * CHINO, CALIFORNIA 91710
California Lic. No. 493008
Nevada Lic No. 41229
Washington Lic. No. CONDECSO94Q5