Frequently Asked Questions according tubes and pipes testing.

1. What is the general recommendation of NDT method for tubing inspection?





Tube inspection is typically broken into two categories: ferrous and non-ferrous. Ferromagnetic materials refer to materials with magnetic properties such as carbon steel and 400 series stainless steel. Nonferromagnetic materials refer to materials with nonmagnetic properties such as cooper, brass, Inconel and most stainless steels. Many of air cooled exchangers contain finned tubes. Some of these are monometalic but some are bimetalic. Depends from configuration it is very important during choosing most adequate method and technique. The following table shows the techniques that are used for the different tubes materials





	Technique Table	ET	RFT	XRFT	MFL	IRIS	LOTIS
1	Ferromagnetic: carbon & low alloy steel, Chrome-Moly, SeaCure, Duplex		X		X	X	X
2	Ferromagnetic tube with non-ferromagnetic fins: aluminum, copper, stainless steel			X	X	X	X
3	Non-ferromagnetic: Stainless Steel, Copper, Aluminum & Titanium alloys, Inconel*	X				X	X
4	Non-ferromagnetic with fins, monometallic tubes	X				X	X
5	Non-ferromagnetic with fins, bimetallic tubes	X				X	X
* - some non-ferromagnetic alloys can be also partially ferromagnetic - take a special attention in choosing proper method due to possible big interpretaion mistake




Table 1. Useful different NDT method for tube inspection depends from type of metal.





2. What is detectability of NDT methods for different type of defects?




Detectability of different NDT methods depends also from type of tested materials. Look into two below tables for basic information. Please note that this information regards to performing testing from ID surface without access from OD surface. If we conduct testing from OD situation will be inverse.





	Damage Mechanism	Conventional Eddy Current*	RFT	XRFT	MFL	IRIS (UT)	LOTIS (Laser)
1	I.D. General Wall Loss	Yes	Yes (Limited)	N/A	N/A	Yes	Yes
2	O.D. General Wall Loss	Yes	N/A	N/A	N/A	Yes	N/A
3	I.D. Pitting	Yes	N/A	N/A	N/A	Yes (Limited pit size)	Yes
4	O.D. Pitting	Yes	N/A	N/A	N/A	Yes	No
5	I.D. Cracking, Stress Corrosion Cracking	Yes	N/A	N/A	N/A	No	Yes (Limited)
6	O.D. Cracking, Stress Corrosion Cracking	Yes	N/A	N/A	N/A	No	No
7	Volumetric Flaws	Yes	N/A	N/A	N/A	Yes (Limited)	N/A
* - Eddy Current contains also some very specific techniques like Full or Partially Saturated ET, Motor Rotating ET, Array ET, Pulsed ET, Saturated Low Frequency ET (SLOFEC)




Table 2. Technique Flaw Detection Table for Non-ferromagnetic Tubing.






	Damage Mechanism	Conventional Eddy Current*	RFT	XRFT	MFL	IRIS (UT)	LOTIS (Laser)
1	I.D. General Wall Loss	Yes (Limited)	Yes	Yes	Yes (Limited)	Yes	Yes (Limited)
2	O.D. General Wall Loss	N/A	Yes	Yes	Yes (Limited)	Yes	N/A
3	I.D. Pitting	Yes (Limited)	Yes	Yes	Yes	Yes (Limited pit size)	Yes
4	O.D. Pitting	N/A	Yes	Yes	Yes	Yes	No
5	I.D. Cracking, Stress Corrosion Cracking	Yes (Limited)	Yes	Yes	No	No	Yes (Limited)
6	O.D. Cracking, Stress Corrosion Cracking	N/A	Yes	Yes	No	No	No
7	Volumetric Flaws	No	Yes	Yes	No	Yes (Limited)	N/A
* - Eddy Current contains also some very specific techniques like Full or Partially Saturated ET, Motor Rotating ET, Array ET, Pulsed ET, Saturated Low Frequency ET (SLOFEC)




Table 3. Technique Flaw Detection Table for Ferromagnetic Tubing.





3. We have been performed pressure leak testing and any leakages found. Should I do Eddy Current tubes testing in heat exchangers?




Leak testing is important but you know that any leaks just now. Eddy Current Testing giving you much more information about quality of tubes and wall losses, cracks, pittings, wear scar etc. These defects can propagate during service and leaks can occur later. Proper management of this information giving chanse to avoid very expensive not planned outages.




4. What kind of information I shall send you to prepare quotation for tube testing?




For different material and dimensions of tubes we using different probes. Before sending offer and to be ready for mobilisation we need basic technical information like:



• Material
• Outside Diameter
• Inside Diameter
• Wall Thickness
• Number of tubes
• Length of tubes
• Straight or U-bend
• Finned or not
• Cleanness level




5. Do you testing also U-bend part of tubes?




U-bend part of tubes is possible testing also but generally using only ET due to short and flexible bobin probes. Cross section of bended parts must be as a ring not as a elipse. Minimum radius shall be not less than 300mm. Other NDT methods for testing U-bends are not applicable because too long and rigit probes:





6. What a cleanness level required for different method and who can perform this cleaning?




For eddy current testing and other electromagnetic methods it is require general cleaning like during typical outage for increasing quality of heat transfer factor. This cleaning can be made but approved local company with certificate for utilisation waste oil products. Many times our representative starting mobilistaion few days earlier and cooperate with team conducting cleaning.

Unfortunately for IRIS method and LOTIS ID surface must be clean on the top level. Surface shall be without any scale or rust. LOTIS require additionally removing any oil or water drops.





7. We have about 50 heat exchangers in instalation with hundreds of tubes and planning for outage. This will be the first time after 5 years of service. How we can arrange tube testing during short period of 3-4 weeks?




50 heat exchanger for 3-4 week it is really huge amount and checking 100% percent it is very difficult in this period but also not needed. 5 years in sercice it is important time for the first survey. The best way is to choose for testing most critical heat exchangers if you know which one are critical or to take all heat exchangers but for inspection choose only 20% of tubes. Results of this testing will give you important information about general quality level. If some tubes will be in not acceptable conditions than testing shall be expand up to 50 or 100%.




8. How looks typical report for tube testing?





Typical report for tube testing contains 3 general parts. The first part of report contains all basic technical information about dimension, material, construction, identification numbers and client.



The second part is a Tube Sheet Diagram (TSD) with the view of all tubes on the front sheet of heatexchangers. Using few different colours and marks or letters client can see image of general condition. Different colours and letters inform us about existing defects and size in the rate to nominal wall thicknes of tubes. Range of wall thickness loses can be adjusted to customers requirement.



The last part of report is the information about all findings, size and percentage to wall thickness relation. Position in the length of tube can be measure from the TSD or from the nearest Tube Support Pate. Other information like Angle and Voltage of signal is rather for Supervisor for additional checking data analysis





Additionally we can add some very specific comments or conclusion like:

• statistical charts
• recommendation for the next outage
• additional cross sections showing position of defects
• Historical Management and Planing System (HMAPS) with historical data and quality changes during service