Tubing Analysis: Heat Exchanger and Boiler Tube Inspection

Why tubes fail

Heat exchanger, condenser, air cooler and boiler tubes have thin walls, work hard and are often hard to see. The damage that ends their lives falls into a few families:

• General wall loss from corrosion or erosion, on the inside, the outside or both.
• Pitting: small, deep, localized corrosion that can go through the wall long before the average thickness looks worrying.
• Fretting and wear where tubes rub against baffles and support plates as the fluid makes them vibrate.
• Inlet-end erosion where turbulent flow enters the tube.
• Cracking, including stress corrosion cracking, and fatigue at supports and tube-to-tubesheet joints.
• Denting and deposits, and in boilers, damage from the fire side and the water side such as hydrogen damage, flow-accelerated corrosion and overheating.

No single technique is best at all of these, and the tube material decides which techniques will work at all.

Matching the technique to the tube

The first question is whether the tube is ferromagnetic. Magnetic materials such as carbon steel and many ferritic stainless steels behave very differently in an electromagnetic field from non-magnetic ones such as copper alloys, austenitic stainless steel and titanium. As a starting point:

• Non-ferromagnetic tubes (copper alloys, austenitic stainless steel, titanium): conventional eddy current testing (ECT) with a bobbin probe, adding rotating or array probes where cracks are expected.
• Ferromagnetic tubes (carbon steel): remote field testing (RFT), with near field testing (NFT) for finned air-cooler tubes and magnetic flux leakage (MFL) where pitting is the main concern.
• Any material, when you need accurate wall thickness: IRIS, the ultrasonic technique, often used to confirm and size what an electromagnetic survey has flagged.

Eddy current: bobbin, rotating and array probes

A bobbin probe is a pair of coils that fits the tube bore and is pulled through at speed, recording signals that an analyst reads for wall loss, pitting, wear at supports and dents. It is fast, which is why ECT is the standard for large non-ferrous bundles such as condensers. Its weakness is cracks that run around the tube, which a bobbin coil can miss. Rotating probes and array probes (ECA) are slower but see cracks in any orientation and give a better picture of what a flaw looks like, so they are used on a sample or to follow up bobbin calls.

Ordinary ECT struggles in carbon steel, because the material's magnetic properties swamp the signal. Techniques that partly saturate the tube magnetically can extend it to some slightly magnetic alloys, but for carbon steel other methods usually do better.

Remote field, near field and MFL for carbon steel

Remote field testing (RFT) uses a low-frequency field that passes through the tube wall twice, from the exciter out and back in to a detector some distance along. It is roughly equally sensitive to wall loss on the inside and outside, and it is the usual screening technique for carbon steel exchanger tubes. It is less sensitive to small pits than to gradual or broad wall loss, and it cannot reliably tell which surface a flaw is on without help.

Near field testing (NFT) is a quick electromagnetic technique for carbon steel tubes, especially finned air-cooler tubes where RFT is hampered by the fins. It is good at finding inside-surface pitting and erosion, but it does not measure outside wall loss.

Magnetic flux leakage (MFL) magnetizes the tube strongly and detects the flux that leaks out at pits and grooves. It responds well to sharp, localized metal loss, and it is another option for carbon steel and finned tubes.

IRIS: the ultrasonic option

IRIS (internal rotary inspection system) puts an ultrasonic probe inside a water-filled tube, firing at a spinning mirror that sweeps the beam all the way around the wall as the probe is withdrawn. It measures remaining wall thickness directly, in any metal, and it tells inside damage from outside damage. That makes it the reference technique for sizing wall loss and for checking what electromagnetic methods have reported.

Its limitations are real. IRIS is slow compared with ECT or RFT, so it is often used on a sample of tubes. It needs very clean tubes, because scale and deposits scatter the sound, so cleaning is part of the cost. It is not a crack-detection technique and it can miss very small pits. Many programs therefore screen with an electromagnetic method and use IRIS to verify and size.

Boiler tubes

Boiler tubes are usually inspected from the outside, during outages. Ultrasonic thickness readings, often taken on a grid across waterwalls and at known problem areas, track wall loss; EMAT scanners, which need no couplant, can cover large areas of waterwall tubing quickly. Specialized ultrasonic techniques look for hydrogen damage and measure internal oxide scale in superheater tubes, where scale growth tracks overheating and remaining creep life. Visual inspection, with attention to swelling, blistering and erosion around sootblowers, is always part of it.

Data analysis matters as much as the probe

Tube inspection is only as good as the person reading the data. Signals from supports, deposits, dents and real damage can look alike, and different analysts can call the same data differently. Good programs use analysts qualified in the specific technique, a written analysis guideline for the equipment, a second independent review of the data, and reports keyed to a tubesheet map, so that findings can be traced, trended from outage to outage and acted on.

Two decisions follow. The first is scope: inspect every tube, or a sample? Sampling saves time and money but can miss localized damage, so it should be designed with the expected damage in mind and expanded when it finds something. The second is the plugging or replacement criterion, a wall-loss threshold that should be set by the owner's engineering before the inspection, not improvised in the field.

Codes and guidance

ASME Section V includes requirements for eddy current examination of non-ferromagnetic heat exchanger tubing, as well as articles on magnetic flux leakage and remote field testing, and carries ASTM practices such as E2096 for remote field examination of ferromagnetic heat exchanger tubes. Which of these apply is usually decided by the owner's specification and, in refineries and chemical plants, the owner's inspection program. Steam generator tubes in nuclear plants are a special case, inspected under ASME Section XI and industry guidelines; see our article on nuclear power.

Frequently asked questions

How NDT LLC can help

NDT LLC helps plants and inspection contractors choose the right tube inspection technique for the material and damage, write and review procedures and analysis guidelines, qualify analysts, and review vendor data independently. Contact us before your next outage. Tube inspection shows up across the process industries; see our articles on refineries, petrochemical plants and pulp and paper mills.