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IRIS detects hidden defects in vertical heat exchanger

TesTex NDT Ltd carried out an Eddy Current Test (ECT) on a vertical heat exchanger at a Fertilizer plant. The unit tube bundle was manufactured from Stainless Steel 304 & therefore the preferred technique for this type of exchanger was ECT.  No in-service failures or issues had been reported to TesTex prior to the inspection commencing.  A sample of tubes (219 = 21% of total) were inspected using the TesTex developed TX4400 ECT system.  On analysis of the data Row 1 Tube 21 (indicated by red arrow) displayed reportable wall loss (equal to or in excess of 20% Nominal Wall Thickness).  The wall loss detected was to the external tube wall & measured by the ECT system at 34% loss of the NWT.

Below are the Tubesheet Map results for the initial inspection.

After consulting with the client a further inspection using an Internal Rotary Inspection System (IRIS) was carried out to establish the accuracy of the ECT results.  As the IRIS system operates on an Ultrasonic principle a more accurate sizing of flaws would be expected.  In comparison to the ECT result (34%) a wall loss of 39% was measured by the IRIS system.  During the initial IRIS inspection sample it became evident that some large internal wall losses were present in the unit which had not been detected by the ECT system.  Due to this a sample of tubes (239 = 23% of total) were inspected with IRIS with the results showing that 146 tubes displayed reportable wall loss indications. IRIS is more sensitive to flaw detection so the reportable threshold is set at 10% of NWT, however the largest wall loss was detected at 77% of NWT.

Below are the Tubesheet Map results for the IRIS inspection.

Fig 1:- IRIS image of Row 1 Tube 21 indicating erosion of the external tube wall (highlighted by red border) up to a depth of 39% NWT.

Fig 2:- IRIS image of Row 5 Tube 36 indicating internal localised wall loss up to a depth of 46% NWT.

In conclusion, the correct technique for this heat exchanger was requested by the client, however ECT, in isolation, was not detecting all the flaws that were present.  Many techniques have shortcomings & in this case, as the internal wall losses were located close to the lower tubesheet, they were not able to be detected using ECT.  The reason for this is that the Eddy Current signal was being saturated by the large increase in material mass, in this case the lower tubesheet & therefore a blind spot was created.  If a client suspects that potential flaws may be in close proximity to tubesheets it might be advisable to consider a combination of inspection techniques rather than just the obvious one.