ID PITTING: INSIDE DIAMETER PITTING
This can be caused from poor water treatment, scale getting lodged into the tube and vibrating back and forth, ID Deposits, or from tube manufacturing.
When ID Pitting has occurred under deposits, this defect can be very deep and small in diameter and can be very difficult to accurately depth analyze, and almost impossible if the deposit is magnetic.
OD PITTING: OUTSIDE DIAMETER PITTING
This does not normally exist without corrosion being associated with it. OD pits can be directly from manufacturing if there is no corrosion present. OD pits may also be mistaken for Zipper Cracks.
FRETTING: ALSO KNOWN AS SADDLE WEAR
This can be caused by excess machine vibration, surging, the drilled hole in the support plate was drilled to large, or sometimes water flow problems. This type of defect is one of the most unpredictable defects due to the work hardening that takes place at the support plate area because of the continual tube vibration. The result of these two factors may cause the tube to rupture at any time. This defect is tube wear at the tube support plate. Once this phenomena starts, the rate of damage increases exponentially.
This type of defect is known to be directly related to tube manufacturing, It is not known if this type of defect propagates deeper into the tube wall, but if it does, it would be due to machine operation such as thermal stress, machine vibration, or machine start ups and shutdowns. These are longitudinal defects that can be anywhere from 1/16" to the full length of the tube, and can be anywhere around the circumference of the tube. It is believed that Zipper Cracks are caused by laps or seams in the material before tube processing. During the tube processing operation, stresses are set up in the tube which eventually cause these laps or seams to open up, or propagate deeper during machine operation. This would explain the different depths of Zipper Cracks found.
If this particular defect was caused by the way the tube was manufactured, then all Zipper Cracks would be very close to the same depth, and also in the same area along the tube length. Eddy Currents are also very sensitive to most Zipper Cracks due to the fact that the coils are very sensitive to any defects that disrupts the coils current in a perpendicular direction.
**NOTE: A ZIPPER CRACK THAT EXTENDS THE FULL LENGTH OF A TUBE CAN NOT BE DETECTED BY EDDY CURRENT, ASSUMING THAT IT'S DEPTH IS CONSTANT. THIS APPLIES TO FINNED TUBES ONLY.
This is a machine manufactured defect. Each tube is rolled in the evaporator, at the support plate to stop tube vibration that creats FRETTING at the tube support plate. Over Rolling happens when too much pressure has been applied to the rolling machine, causing the copper tube to be EXPANDED into the support plate. There are different degrees of Over Rolling. Over Rolling at the support plate may cause a stress riser, therefore causing a crack at a support plate. This is impossible to detect because of the carbon steel support plate being very close to the crack defect. This can mask the resistive value given by the defect in Eddy Current analysis
This is a machine manufacturer defect. The rolling process is the same as Over Rolling, but has a physical dimension difference. When the actual expansion does not line up with the support plate longitudinally, and the roll just catches the edge of the support plate, or sometimes does not hit the support plate at all. This type of manufacturing defect may also cause a stress riser.
This type of defect is self explanatory, and only occurs in the Evaporator under running conditions. Determining the extent of FREEZE BULGING is very difficult because of the inconsistency and the nature of the defect. The best way to determine the extent of bulging is to have actual samples to compare signals with. A high percentage of FREEZE BULGES are normally considered PLUGGABLE.
OD DEPOSITS: OUTSIDE DIAMETER DEPOSITS
Outside Diameter Deposits are caused from moisture that has been in the system for some time, rust from machine manufacturing, or from tube manufacturing. An OD Deposit can look like a defect at normal test frequency, but can be recognized at a proper frequency. OD Deposits can also mask defects on the outside, or inside of the tube.
ID DEPOSITS: INSIDE DIAMETER DEPOSITS
Inside Diameter Deposits are caused from silty water, slow water flow, poor water treatment, lack of proper tube cleaning, or the unit has been stored for the season with silty water. ID Deposits can cause serious ID Pitting, and can make EDDY CURRENT testing impossible, if the deposit is magnetic.
This type of indication normally happens in the same area along the length of a tube and is the same for most all tubes in that bundle. This is from tube processing and can be mistaken for many ID defect, OD defects, and Dents in the tube bundle if frequency comparison is not properly utilized. These are referenced as tube manufacturing tool impressions.
OD Corrosion is sometimes difficult to determine the exact extent of damage, but is easily found. The by-product of corrosion is copper oxide, which has a resistive value to Eddy Current Testing that gives an OD Defect signal. OD Corrosion is caused by excessive moisture, over a period of time in the system, and can effect both Condenser and Evaporator. OD Corrosion is most common in the larger units, but does happen in smaller units also. This type of defect in a worse case can set up stress corrosion cracking.
There are many mechanical variances that can cause this type of defect. Tube manufacturing, tube installation at manufacturing, chiller operation, or a combination of any of the three. Due to the nature of circumferential cracks, and the rarity of this occurrence, detection of this type of defect is virtually impossible with Eddy Current testing.
In some units there are indications that can not be accurately analyzed. In some cases, it may be better to plug these tubes just to be safe, unless otherwise stated in the final report. Even the best of Eddy Current equipment, some circumstances still hinder the analyzing capabilities that would normally take place for most Chillers. It is also a general understanding that Eddy Current is not a 100% type of inspection. This is due to the nature of Eddy Current Testing, and to all the different variables in Chiller Tubes. Basically this section is to cover the remaining types of indications not yet recognized.