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Tube Analysis Inspection Selection

Tubing Inspection Services

Tubing Inspection Services

No single inspection technique is adequate for all material types; and a single-technology system can only be used for a narrow range of applications. Eddy current testing (ECT) is commonly used to inspect nonferromagnetic materials; remote field testing (RFT) or magnetic flux leakage (MFL) is used for carbon steel and other ferromagnetic materials; and ultrasound (IRIS) Ultrasonic Testing of tubes may be used for accurate thickness measurements of either nonferromagnetic or ferromagnetic materials.

Each technique has its own strengths and weaknesses, and it is not always easy to select the technique that is most appropriate for a specific application; the key is to understand the limitations of each technique and to use each technique for what it is best suited. Very often, a combination of techniques will provide the most effective inspection. The table below contains a guideline for selecting inspection techniques. This guideline does not provide information for every possible situation and does not take into account specific parameters that can affect your application. Every application is unique and your own experience if often the best guideline to follow.

Application

Description

ECT

RFT

MFL

IRIS

Comments

Condensers

Nonferromagnetic materialGenerally, thin tubes made of brass, aluminum brass, cupronickel, or titanium.

X

- ECT is the recommended choice for its speed and its accuracy.

Air Conditioners

Copper with low profile OD finsTypical problem iscircumferential cracking and wear in the land area.

X

- ECT inspection with special probes to detect circumferential cracks.  These probes are normally called AC probes.

Heat Exchangers

Nonferromagnetic material Stainless steel, Cu/Ali

X

X

- ECT is the recommended choice for its speed and accuracy.- IRIS can sometimes be used to confirm the sizing of ID defects.
Mildly ferromagnetic material Duplex steel, SS439, Seacure, Monel

X

X

X

X

- ECT is used only for thin tubes (<1.6 mm) with saturation probes.- For thicker walls, it is better to use RFT or MFL.- IRIS can sometimes be used to confirm the sizing of defects.
Ferromagnetic material Carbon steel, nickel. Typicaldefects are pitting and corrosion.

X

X

X

- RFT, MFL, and IRIS are good, and most of the time complementary.

Air Coolers

Carbon steel tube withaluminum fins Typical defects are ID pitting and OD wear close to the water box.

X

X

- RFT is not effective because the fins disturb the remote field.- MFL is quick and sensitive but does not allow sizing.- IRIS is often used to back up MFL, but has difficulty with OD flaws.

Boilers

Carbon steel tube Typical defects are pitting, wear, and cracking at the tubesheet.

X

X

X

- RFT is good for wear detection and can easily pass the bend.- MFL is good only on straight boiler tubes without swages.- IRIS is good for wear and pits but has no sensitivity in the bend.

Buried Pipes

Carbon steel or cast iron pipes Typical defects are pitting and general corrosion.

X

- RFT is the only effective technique because it does not require cleaning, and it can easily negotiate elbows. However, sensitivity to pitting is limited.

Eddy Current and Remote Field Testing (RFT)

eddy current bobbing coils

Eddy Current Bobbing Coils

There are three basic eddy current test techniques for testing tubes. Selection of the techniques depends on the tube material.

Conventional eddy current testing is applied when testing nonferromagnetic heat exchanger tubes (ASME, 2001). The test is performed with a bobbin coil (like the ones shown to the right) that produces an electromagnetic field in the tube. This allows the inspection to have greater sensitivity on the inside diameter of the tube where defects are most likely to occur and with the use of multiple frequencies we are able to penetrate 100% of tube wall thickness in order to detect flaws at variable depths. When the probe is pulled across a discontinuity, the electromagnetic field is distorted in per portion to the discontinuity size and relationship to the field. This distortion in magnetic field changes the coil impedance that is related to the discontinuity. Because of this coupling effect of the eddy current probe to the material it is crucial to have a properly cleaned tube and a know tube dimension prior to commencing the inspection. The eddy current testing method detects pits, wall loss and cracks. The key component to being able to detect these defects and determine their depths and extent is directly proportional to the calibration standard and probe utilized to set up the equipment. For this reason if the inspector does not have a nearly identical calibration standard to the material being inspected the inspection may not be possible.

saturation probe

Saturation Probe

Full saturation eddy current testing is applied in the testing of partially ferromagnetic and thin ferromagnetic heat exchanger tubes such as monel and some grades of inconel. The bobbin probes, like the one seen here, include a strong magnet that saturates the material magnetically. Once magnetic saturation occurs, testing is done in a manner similar to conventional eddy current testing.

Remote field testing (RFT) is applied in the testing of ferromagnetic heat exchanger tubes such as those made of carbon steel. The test is performed with a bobbin coil (shown to the right) that transmits an electromagnetic field in the tube. These coils look similar to those for saturation only the housing is longer and there are two connections to the testing unit as opposed to one. Remote field testing is limited to detection of large discontinuities and severe wall loss.

Tube Sheet Map

Tube Sheet Map

The testing of heat exchangers (represented as a tubesheet to the right) is the number one application of eddy current testing in industrial plants. Heat exchangers include condensers, general plant exchangers, feedwater heaters, air coolers and lube oil coolers. Both conventional and remote field testing are used. Conventional eddy current testing is used for testing nonferromagnetic tubing, such as stainless steel, copper nickel alloys and titanium. Remote field testing is applicable for testing ferromagnetic tubing, such as carbon steel and nickel. Conventional eddy current testing is a fast, reliable and accurate method for detecting discontinuities in tubing. Tests can be done at pulling speeds of up to 1.8 m/s (6 ft/s). Remote field testing is limited to the detection of larger discontinuities and test speed is limited to approximately 0.3 m/s (1 ft/s). A specialized version of conventional eddy current testing is full saturation. This technique is applicable for thin ferromagnetic tubes such as SA-268 steel in condenser tubes and partially ferromagnetic tubing materials such as nickel copper alloy, SA-789 steel and SA-790.

Corestar RPC Gun

Corestar RPC Gun

A unique problem in heat exchangers can be leaks in the tubesheet roll. This can occur when overrolling of the tubes under the tubesheet causes circumferential cracking. In addition, there can be circumferential cracks just behind the tubesheet. Such cracks can easily be missed by bobbin probes and require a special technique. Cracks under the tubesheet are detected using surface probes. These probes can be simple handheld surface probes or motorized probes similar to the ones used for bolt hole testing in the Eddy Current Aircraft industry such as the specialized unit shown to the left.

Another consideration when choosing probes for inspections whether it be conventional eddy current, full saturation eddy current or RFT is the presence of bends in the tubes or not. If bends are present, flexible head probes are then used. Flexible eddy current probes allow for U-bend inspection but proper planning must be taken prior to the inspector showing up to your job site.

Corestar Omni 200R

Corestar Omni 200R

Bobbing Coil Test Lead

Bobbing Coil Test Lead

Saturation Probe

Saturation Probe

Here at Aqualified we use only the best equipment (CoreStar OMNI-200R shown to the above) for every facet of eddy current and RFT tubing analysis. All heat exchangers are map in the data base management software and stored with the acquired data for future trending.

Signal Tube Analysis Standard

Signal Tube Analysis Standard

Multi Frequency Tube Analysis

Multi Frequency Tube Analysis

Eddy Current Auto Analysis

Eddy Current Auto Analysis

Shown are some screen shots of data acquired from a test standard (top left and center) and from an actual tube (top right).

So, how do you get Aqualified to perform your Tube analysis inspection? Give us a call and one of our tubing specialists can walk you through the process.

When requesting a tubing job it is beneficial to have very specific information about your job when contacting us to save you time.

Some of the basic things we will need to know are:

  1. Material type
  2. Number of tubes
  3. Dimension of tubes
  4. Are there bends in your tubes
  5. Are they finned
  6. Are they welded or rolled
  7. Has the unit been previously tested
  8. What specification, code, procedure or standard is your unit being inspected to.
  9. Is there an original U-1, engineering drawing or tube diagram available.

If you are unsure of these questions feel free to call us to speak to a friendly tubing analysis expert.


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