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Tube Maintenance end of the tube is the measurement to the tube leak . As the plug continues in , the flow will reverse as soon as the line of the reverse chamfer passes the failure .
• Longitudinal crack : If the leak is a longitudinal crack , the air flow will start to come out the tube end which plug was fed through initially as soon as the rear chamfer passes the crack . This is the start of the crack and should be marked on the wire or pulling snake attached to the probe . Then as the probe continues in , the air will flow out both tube ends until the line of the front chamfer passes the other end of the crack cutting off the air flow to the tube end beyond the plug . This is the other end of the crack , and the wire should be marked a second time . The distance between these two marks less the length of the straight portion of the plug body is the length of the crack . The length from the line of the front chamfer to the first mark is the depth into the tube where the leak occurred .
• Ruptured tube : If the plug stops short of the leak and gets stuck , the tube has been damaged likely due to a rupture or otherwise deformed in some way . This type of failure generally indicates an area of adjacent tube damage or leaks . If the tube is shattered the probe will go in , air will then come out from both sides of the tube ends , and the plug will then hit a solid obstruction ( a broken tube end , a baffle plate , etc .). Slowly and easily withdraw the probe in that case . This is a rough measurement of rupture location . Tube stabilizing rod for the ruptured tube and insurance plugs for the adjacent tubes are highly recommended for this sort of tube failure scenario .
• If a leak occurs in the outer tubes , this plug can negotiate the ‘ U ’ bends and locate the leaks . However , if the leak occurs in a tube near the centerline of the bundle , the probe cannot negotiate the tight inner bends .
Non-destructive examination ( NDE ) An NDE such as Eddy Current Test ( ET ) can identify the location of most defects , determine if the defect is ID or OD originated and estimate the size of the defect . ET has the advantage of having relatively fast inspection rate and can be used to evaluate heater condition deterioration by comparing current result with past inspections to trend defect growth rates . ET vendors will typically provide a color coded tubesheet map that shows the location of tubes with defects , the amount of wall loss determined by the test and , the location of the defect down the length of the tube ( i . e ., is the defect within the Condensing Zone or Desuperheating Zone or Subcooling Zone or at a baffle location ). A successful ET result depends on operator experience and proficiency , fill factor ( percentage of tube ID cross section filled by the probe ), calibration of the probe relative to tube material etc . In certain instances , ET can miss through-wall failure ; therefore , it is advised to take caution when using ET results as the only basis for tube plugging criteria .
48-1 48-28
0-19 ( 1054 Tubes , 52 %) 20-39 ( 2 Tubes , 0 %) 20-39 OD ( 0 Tubes , 0 %) 40-59 ( 29 Tubes , 1 %)
40-1 40-48
40-59 OD ( 0 Tubes , 0 %) 60-79 ( 134 Tubes , 7 %) 60-79 OD ( 11 Tubes , 1 %) 80-100 ( 720 Tubes , 35 %)
Tubesheet map . Source : Testex NDT
Viewed from South
Legend (% Wall Loss )
Tube to tubesheet leak : Shell side pressure test It is important to differentiate between tube failures versus tube to tubesheet joint failures . Repair strategies are varying on this determination . Leaks in tube to tubesheet joint can be detected by shell side pressure test . Shell side pressure test is done by draining the tubes completely , blowing them dry , flooding the shell side with water , and then pressurizing shell side with service air ( 25-30 psi ). Then check the tubesheet for leaking tubes . If any leak exists , pressurized water will find a way through the tube to tubesheet joint , and leakage will be apparent through the ligament surrounding the tube . This method can also detect leaks in the tube length , span beyond the tubesheet joint . In that case leakage will be visible through tube inlet , instead of outer circumferential joint . When the shell is pressurized with air , a soap bubble solution can be applied to the tubesheet face to quickly identify leaks . The advantage of the shell side leak test :
• It is much easier to identify the leaking tubes and
• Existing tube plugs in failed tubes can also be tested to ensure they are holding .
30-1 30-48
EVERY FIFTH ROW IS HIGHLIGHTED IN RED
20-1 20-48
10-1 10-38
80-100 OD ( 12 Tubes , 1 %) Inaccessible ( 0 Tubes , 0 %) Not Tested ( 0 Tubes , 0 %) Plug ( 68 Tubes , 3 %) Block ( 0 Tubes , 0 %)
1-1 1-10
About the author
Mohammad Rahman has over a decade of experience in the chemical , power , and nuclear industries as a maintenance , reliability , and physical asset management professional . He has extensive experience in leading reliability improvement projects in a wide range of static and rotating equipment . www . heat-exchanger-world . com Heat Exchanger World September 2023
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