In the series on Non-Destructive Testing (NDT) for the HVAC sector, I have so far highlighted oil analysis, followed by vibration analysis and infrared thermography. This time, let’s look at eddy current testing.
The what and how of it
Introduced in the 1950s, eddy current tube analysis has become the dominant NDT used globally to inspect nonferrous shell and tube heat exchangers used throughout the military, nuclear, heavy equipment, comfort cooling, cogeneration/power, pulp/paper mill, process and the HVAC chiller industries.
The science of eddy current tube testing is known as Electromagnetic Inductance. The test instrument controls the frequency (F) fed into the eddy current probe. As the probe passes through the tube, eddy current flow is induced on both ID/OD surfaces/subsurfaces. Any change in tube wall thickness and/or metallurgical microstructure disrupt the steady electromagnetic eddy current flow, causing an impedance change that results in instantaneous impulses (flying dot/lissajous signals) on the instrument monitor.
Eddy current analysis has been endorsed and actively utilised by chiller manufacturers, controls groups, independent service providers, mechanical contractors, engineering firms, property management groups and insurance providers worldwide
Phase manipulation and amplitude (gain) are used to process the impulses. Along with F, these test parameters are controlled by the field technician/analyst. This demands that the on-site individual possess the necessary expertise to properly calibrate and actively interpret the signals as they are manifested on the screen.
Benefits and applications
Eddy current inspection offers:
• Quantitative/qualitative surface/subsurface analysis of defects on both ID and OD (Outer diametre and inner diametre) orientations
• Identification of alloy composition, hardness, specification, wall thickness and geometrical changes, and can help indicate tube cleanliness
• A way to easily overcome in-service conditions typical to chillers
• Accurate, reliable, reproducible results due to calibrated and certified test procedures
• Portable, efficient, and cost-effective analysis
These attributes have driven eddy current testing’s value and popularity, and need to be consistently used as a part of predictive/preventative maintenance (P/PM), warranties, service agreements, annuals, conversions/retrofits, insurance protocols, quality control, due diligence, litigation, commissioning, and even, water treatment programmes.
Eddy current analysis has been endorsed and actively utilised by chiller manufacturers, controls groups, independent service, mechanical contractors, engineering firms, property management groups and insurance providers worldwide.
Training and expertise for testing
In the Gulf region, I recommend EC testing and analysis annually on chiller condenser tubes, due to harsh atmospheric conditions in the region
Personnel who are engaged in testing are certified to Level I (trainee), Level II (technician), or Level III (analyst), depending on their individual company’s definition standards, and based on the individual’s knowledge, education, background, training and experience.
The American Society for Nondestructive Testing (ASNT) utilises the governing document SNT-TC-1A Recommended Practice, allowing firms providing NDT services to define, train, educate and certify their personnel to Level I, II or III standards. As such, certification records of test personnel are always available to the client to verify credibility, reputation, longevity and expertise.
In the Gulf region, I recommend EC testing and analysis annually on chiller condenser tubes, due to harsh atmospheric conditions in the region. I also recommend that when purchasing new water-cooled chillers, an EC analysis is performed at the factory on the evaporator and condenser tubes and, subsequently, during chiller start-up and commissioning. This could help:
• Save the chiller owner substantial amount of money
• Assign accountability, should the tubes be damaged during flushing or improper flushing of the chilled water and condenser water loops, as proof that chiller tubes were in good condition at the factory, and that improper start-up was performed
• Prove that defective tubes were installed at the factory
I encourage readers of this column to submit any questions on chiller maintenance or troubleshooting problems. I would be happy to address them in future columns.
CPI Industry accepts no liability for the views or opinions expressed in this column, or for the consequences of any actions taken on the basis of the information provided here.
Dan Mizesko is the Managing Partner of Al Shirawi US Chiller Services. He can be contacted at email@example.com.