Tuesday, April 6, 2010

Re: [MW:4716] Other NDT Test

Dear Sakib,

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.

2. The proximity of the coils to 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.

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 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.

Regards,


Yadav Shiva Chelliya,
Inspection Engineer,
Doha Qatar,
+974-6166465



On Tue, Apr 6, 2010 at 8:21 AM, Sakib <sakib321@gmail.com> wrote:
but is ET is not for tube inspection>???

personnel is asking for alternate of hydrotest for shell side


On Tue, Apr 6, 2010 at 10:05 AM, Yadav Shiva <luvshiv2002@gmail.com> wrote:
Dear Pokya,

There is a leak test other than Hydrostatic test on Heat Exchangers. But these possibility depends upon the severity , pressure , flow we can go with Eddy current testing. As the static equipment get eroded heavily(as on Visual Inspection) we can go with Eddy current testing. Depends upon the Client as it is costly to carry out such Inspections.
Since NDT such as DP can be carried out on Tube to Sheet if it is a seal joint.

Regards,
Yadav Shiva Chelliya,
Inspection Engineer,
Doha Qatar,
+974-6166465


On Tue, Apr 6, 2010 at 7:09 AM, pokya tx <pokya84@gmail.com> wrote:
Dear all,

Are there any leak test other than Hydrostatic test to check on Heat Exchanger shell side leaking.

Thanks.

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with Regards


SAQIB JALAL
Inspection Engineer
PakArab Fertilizer Limited
Khanewal Road , Multan
office number +92-61-9061 EXT 3110
mobile number +92-321-4043556

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