Table of Content
- What Is Near Field Testing (NFT)?
- The Principle: Working Inside the Transmitter's Near-Field Zone
- Applications: Fin-Fan and Air Cooler Tubes
- Common damage mechanisms identified include:
- When to Use NFT: NFT vs RFT, ECT, and IRIS
- Comparison of Tube Inspection Methods
- Limitations of Near Field Testing
- Conclusion
- Frequently asked questions (FAQs)
Corrosion, erosion and fouling are always present in the heat exchanger, air cooler, fin-fan and boiler systems and gradually weaken the tubing. Many of these assets are subject to high pressure and high temperature, so even if it is only a small area inside that is damaged, it can result in expensive downtime or unplanned failures. Therefore, it is very important to choose the right tube inspection method to ensure proper preventive maintenance.
Near field testing (NFT) is one of the many electromagnetic inspection techniques that has been developed as a specialized solution for the inspection of carbon steel tubing where damage is typically on the inside surface. Compared to the conventional eddy current testing or Remote Field Testing (RFT), NFT is able to focus on the inside wall of ferromagnetic tubes, which is particularly valuable for the inspection of the fin fan and air cooler tubes.
This guide covers how near field testing NDT is used, the reasons for its excellent performance in finned carbon steel tubes, where it can be used, and the limits of which the inspector needs to be aware when choosing it.
What Is Near Field Testing (NFT)?
Near field testing (NFT) is an electromagnetic NDT method to inspect the inside surface of carbon steel tubes for corrosion, erosion, pitting and other defects on the tube's inner wall without extracting fins or causing damage to the tube.
NFT is not to be confused with "near field testing" as it is used in antenna, wireless communication or radar. In NDT, NFT means a certain electromagnetic tube inspection method for ferromagnetic tubes.
NFT intentionally focuses the sensitivity of the inspection on the inner surface of the tube, where the damage typically starts due to the flow. This targeted inspection ability has led to the widespread adoption of NFT as a preferred method of inspection for the following applications: fin-fan coolers in refineries, petrochemical plants, power stations and chemical processing plants.
The Principle: Working Inside the Transmitter's Near-Field Zone
The near field testing concept is often touted simply as “operating in the near field region of the transmitter's field”, but the "why" of this is explained by why this is so different from some other electromagnetic inspection modalities.
NFT employs two nearly adjacent coils mounted within the probe which slides through the tube.
In Remote Field Testing (RFT), the receiver coil is intentionally placed several tube diameters from the transmitter to measure magnetic field that traverses the entire wall of the tube, whereas in NFT, both coils are positioned very close to each other. This setup causes the receiver to only see the electromagnetic field within the near-field zone of the transmitter.
Only the inner part of the wall of the tube is influenced by eddy currents created by the magnetic field of the transmitter. These currents do not penetrate through the full thickness of the wall.
Because the energy is not transmitted through the tube wall, defects on the inside diameter (ID) of the tube greatly affect the signal, and those on the outside diameter (OD) have very little effect.
The primary advantage of NFT in identifying is:
- Internal corrosion
- Flow-assisted erosion
- Localized pitting
- Internal wall thinning
Structural elements like support plates, tube supports and tubesheets cause little interference as it is all inside the inspection field. These can make it difficult to conduct other inspections due to the distortion of magnetic fields or concealment of indications.
This means cleaner inspection data, a lower degree of interpretation for inspectors and enhanced sensitivity to internal damage for inspectors.
The inspection signal that results gives the maintenance team clear and concise information that allows them to know the approximate location of the internal loss and relative severity, before the tubes begin to leak.
Applications: Fin-Fan and Air Cooler Tubes
Near field testing NDT is mainly used to inspect carbon steel finned tubes in air cooled heat exchanger.
These exchangers are commonplace throughout:
- Oil and gas facilities.
- Petrochemical plants
- Refineries
- Fertilizer plants
- Power generation facilities
Coolers with fins pose additional inspection difficulties since the exterior aluminum or steel fins make access to the tube surface difficult. It is impractical and too expensive to remove fins just to examine the fin condition.
The exterior fin structure has virtually no effect on the external inspection process since NFT performs inspection from inside of the tube.
As a result, fin fan tube inspection is one of the best use cases for NFT.
Near Field Array (NFA) extends NFT capabilities by enabling faster inspection of large tube bundles with improved coverage.
→ Read: Sizing Small Volumetric Defects in Fin-Fan Air Cooler Tubes
- Common damage mechanisms identified include:
In addition to air coolers, NFT is also used in:
- Water-cooled heat exchangers
- Boiler tubes
- Carbon steel condenser tubes (CCT).
- Process heat exchangers
- A selection of ferrous piping systems.
Modern inspection systems also include the Near Field Array (NFA) technology, which uses multiple sensing elements to increase inspection coverage, increase productivity and decrease the scanning time for large tube bundles. Near Field Array is especially valuable when conducting large scale shutdown inspections where speed of inspection is a key factor.
When to Use NFT: NFT vs RFT, ECT, and IRIS
The selection of the proper tube inspection method is based on one question:
Will damage be found only on the inside surface or the entire tube wall needs to be evaluated?
If the internal corrosion or erosion is the only concern, NFT offers high sensitivity and has few of the limitations of Remote Field Testing.
If external corrosion or total loss of wall thickness is required for any reason, however, another inspection method may be more suitable.
Tube inspection method selection goes beyond NFT. Here's a complete guide covering all tube inspection roles, techniques and when each method applies in industrial assets.
→ Read: Tube Inspection Role
- Comparison of Tube Inspection Methods
When maintenance engineers are confident that the degradation is taking place on the inside of the tube, NFT is the best solution. Typical applications include cooling water systems, hydrocarbon processing and refinery air coolers where corrosive fluids are flowing through carbon steel tubes.
On the other hand, in the event of external corrosion, fire-side damage or total wall condition needs to be assessed, Remote Field Testing (RFT) or IRIS (Internal Rotary Inspection System) can give a more complete wall evaluation.
Limitations of Near Field Testing
While NFT is a great tool for certain applications it is not a panacea for tube inspection. This is important to know so that inspectors can choose the method that is most suitable for the inspection program.
The biggest drawback is that NFT is able to only look at the inside surface of the tube. Corrosion and external diameter (OD) pitting and external wall loss are rarely detected due to the limited field of view for the inspection.
NFT is also primarily designed for carbon steel and other ferromagnetic tubes. It is not suitable, generally, for stainless steel, copper alloys, titanium, brass or other non-ferrous materials where Eddy Current Testing is more effective.
NFT is not a direct measurement of wall thickness as are ultrasonic inspection techniques. Instead, it is a method of detection of disturbances causing internal metal loss.
Another drawback is that NFT is not a crack detection system. The method is better suited for indication of corrosion, erosion and pitting, but is not optimized for fatigue or stress corrosion cracking.
That's why there are times when NFT is not the only NDT method used, but rather as part of a larger inspection strategy.
Conclusion
Near field testing (NFT) is an important technique used in electromagnetic tube testing that targets only the inside surfaces of carbon steel tubing. Because its operating principle places the transmitter and receiver in the electromagnetic near-field zone, it is highly sensitive to any ID corrosion, erosion or pitting and results in a significant reduction in interference from support plates and tubesheets.
It has the biggest advantage for inspecting fin tubes of fins and air cooler tubes without being able to do it from the outside. NFT is unable to measure damage to external wall surfaces, or determine wall thickness, but rather it can quickly and accurately inspect for internal deterioration in ferromagnetic tubes.
The choice of NFT comes down to the damage mechanism that is anticipated. For situations where the primary concern is internal corrosion, NFT is one of the most effective and convenient methods of inspection.
Frequently asked questions (FAQs)
1. What is the difference between NFT and RFT?
Near Field Testing (NFT) focuses the sensitivity of the inspection on the inside surface of carbon steel tubes by employing transmitter and receiver coils that are very close together. In Remote Field Testing (RFT), the receiver is located at a greater distance from the tube in order to detect both inside and outside defects in the wall, because the electromagnetic field is allowed to penetrate the entire wall.
2. Can Near Field Testing detect external corrosion?
No. NFT is specially developed to measure the inner surface of ferromagnetic tubes. External corrosion, outside diameter pitting, and outside diameter metal loss can usually not be reliably detected due to the nearness of the electromagnetic field near the internal wall.
3. Does NFT work on stainless steel tubes?
No. NFT is mainly used for carbon steel and ferromagnetic tubes. Conventional Eddy Current Testing (ECT) is usually employed to inspect stainless steel, titanium, copper alloys and other non-ferrous materials or other suitable non-destructive testing (NDT) techniques.
4. Why is NFT commonly used for fin-fan coolers?
The difficulties with inspecting outside of the fin-fan coolers are the externally-finned carbon steel tubes. NFT conducts the inspection from inside the tube, and the inspector can determine the internal corrosion and erosion without a fin removal, thus maintenance cost and time can be reduced effectively.
5. What types of defects can Near Field Testing detect?
NFT is useful for the internal corrosion, erosion, pitting, local wall loss and flow assisted corrosion of the inside surface of carbon steel tubes. This is not for the purpose of accurate crack detection or a full thickness measurement.
6. What is Near Field Array (NFA)?
Near Field Array (NFA) is an advanced version of NFT that moves the probes in a way to sense the larger area with multiple sensing elements. It enhances inspection coverage, boosts productivity and reduces the inspection time of large tube bundles like air coolers or heat exchangers in a refinery.
