Wall Thickness Measurements in Tubes & Fittings

The determination of the wall thickness of the tubes and fittings in a heater is an essential part of inspection. Thinning deterioration mechanisms can be identified and monitored through wall thickness measurements. The two basic approaches used to determine the wall thickness of piping and tubes are the following:

a.  Nondestructive methods. These include the following:
1.  Measurement by means of ultrasonic, laser or electromagnetic instruments.
2.  Measurement of inside and outside diameter.
3.  Measurement by means of radiation-type instruments or radiography.

b.  Destructive methods. One destructive method is removal of a tube or tube section deep in convection banks and inaccessible for direct measurement of the tube wall. However, with the availability of internal ultrasonic based intelligent pigs to measure wall thickness, the need for destructive examination is lessened.

Corrosion monitoring locations (CMLs) should be selected to provide a means to determine the deterioration extent and to determine the deterioration rate. This usually involves, as a minimum, placing CMLs on all tube passes throughout the firebox. Particular attention should be made to tubes where phase changes occur and where the highest tube metal temperatures are expected. In addition, CMLs should be located on return bends to assess their deterioration. Note that the required thickness for a fitting may be different than that for the tube. For example, the inside radius of a short-radius return bend will have a higher required thickness. Typically, the number of tube thickness points is determined by criteria like a minimum of three points per tube or readings every 5 ft – 6 ft (1.5 m – 1.8 m). Often clean, corrosion-free services require fewer measurements, while high-corrosion services require more measurement points. Although spot thickness readings can identify general thinning, obtaining thickness in a circumferential band will better identify any localized conditions like corrosion grooving. Thickness measurements should be documented and monitored where bulging, sagging and bowing is observed.

Thickness measurements should be recorded and compared to historical readings in the same locations. These wall thicknesses provide a record of the amount of thickness lost, the rate of loss, the remaining corrosion allowance, the adequacy of the remaining thickness for the operating conditions, and the expected rate of loss during the next operating period.

Measuring and recording the thickness of tubes and fittings when they are newly installed is considered important. If this is not done, the first inspection period may not accurately reflect actual corrosion rates. If the installed thickness of the tubes is not available at the time of the first inspection, corrosion loss is usually determined on the assumption that the wall thickness of the new tubes was exactly as specified on the purchase order. This is not always true, and hence an error in the calculation of corrosion rate may result.

The ultrasonic method for obtaining tube-wall thickness is the most commonly used method. For most corrosion inspections, straight-beam ultrasonic techniques are used. The sound is introduced perpendicular to the entrance surface and reflects from the back surface, which is usually more or less parallel to the entrance surface. Proper cleaning of the external oxidation or compensating for the thickness of the oxide layer is essential to properly assess metal loss rates. In many cases, cleaning the oxide will be the only viable way to acquire ultrasonic thickness measurements from the tube’s exterior surface.  Application of internal ultrasonic based intelligent pigs do not require removal of external oxide layers when measuring base metal wall thickness. 
Other methods to determine thinning of tubes that are less accurate than spot ultrasonic or radiographic inspection include local area scanning with electromagnetic acoustic transducer devices (EMAT) and global-tube length inspection using guided ultrasonic wave devices. In the first of these methods, the transducer compares a sample area of known thickness with the same material properties as the tube being examined and then either a hand-held or an automated crawler head is used to scan the tube areas from the OD. If a thin area is detected, follow-up inspection using spot ultrasonic or radiographic inspection is necessary. In the second method known as guided wave, an acoustic wave is introduced into the pipe that travels either axially or longitudinally along the tube. Defective areas as well as welds send back signals to a receiver that are analyzed to determine if flaws exist and at what length along the tube based on time and velocity, then follow-up inspection using spot ultrasonic or radiographic methods is necessary  to confirm whether or not flaws truly exist at the identified locations. Although guided wave does not give thickness measurements of flaws detected, it is valuable in evaluating lengths of tubes where spot examination for localized corrosion would be prohibitive based on the amount of measurements that would be required.

At plugged headers where access to the tube ID is available, many types of calipers are available for measuring the inside diameters of heater tubes, including the simple 36-in. (91.4 cm) mechanical scissors and the 2-point pistol type, the cone or piston type, and the 4-12-point electric type. A caliper equipped to measure several diameters around the circumference of a tube is more likely than others to find the actual maximum inside diameter. Ultrasonic (UT) based intelligent pigging which operates with the ultrasonic transducers off of the surface (immersion method) can also be utilized to acquire inside diameter of heater tubes.  A large amount of data is acquired over the full coil length with this method, enabling the entire coil to be modeled in a 3-dimensional color format, illustrating any damage patterns which may be present.

It is general practice to caliper the inside diameter of a tube at two locations: in the roll and in back of the roll. Since an increase in internal diameter may not be uniform throughout the length of the tube because of erosion, erratic corrosion, bulging, or mechanical damage while cleaning, it is advisable to take several measurements to determine the worst section of each tube. On heaters where the pattern of corrosion is uniform and well established and mechanical damage is known not to exist, measurements for approximately 36 in. (91.4 cm) into the tube may suffice. The roll section of a tube in service should be calipered to locate the maximum inside diameter at any point between the back edge of the tube flare, or the end of the tube if there is no tube flare, and the rear face of the fitting or edge of shoulder left in the tube by the rolling tool.

There are also laser-based profilometry systems, which can provide high accuracy measurement along considerable lengths of the tubes. These devices offer high accuracy but require clean and dry conditions to provide consistent results. These systems are described in 9.6. Thinning at the ends of rolled-in tubes is usually caused by erosion or turbulence that results from change in the flow direction. This type of thinning may also result from frequent rerolling of tubes to stop leakage and will have the same general appearance as a tube with a slight bulge. The loss of wall thickness is not uniform around the circumference. In this type of deterioration, the most thinning usually occurs on the fireside of the tube. This type of corrosion is generally accelerated on the fireside because of the high metal temperature there. Eccentric corrosion may also be caused by external scaling. It is often difficult to determine whether tubes have become eccentric as a result of service, because the condition is not readily detectable by visual inspection of the tube ends. An indication of eccentric corrosion can sometimes be found by measuring several diameters at one location. A reliable means of detection is to measure thickness with ultrasonic-, laser-, or radiographic-type instruments, but these tools can only be used on accessible tubes, usually the radiant tubes. Ultrasonic (UT) based intelligent pigging can be applied to quickly detect and quantify eccentric corrosion damage throughout the convection, cross-over and radiant tubes.  Although this type of corrosion is more common on radiant tubes, it has occurred on convection tubes, usually on those adjacent to the refractory.

Each of the methods to determine wall thickness—measuring the inside and outside diameters of tubes, measuring by means of ultrasonic, laser or electromagnetic instruments, and measuring by means of radiation-type instruments or radiography—can be used to check the thickness of heater tubes.

Electromagnetic techniques cover a broad range of applications including eddy currents, remote field eddy currents and magnetic flux leakage. Each has its own benefits and limitations. Remote field eddy current is commonly used on ferromagnetic tubes. It has benefits in that it can measure wall thickness to within 5% of the nominal wall and will provide indications of other defect mechanisms such as cracking. Most of these techniques are applied using an electromagnetic sensor device that is drawn through the ID of the tube which may require cutting the tube U-bends at the ends to gain access.

No comments: