Tubes sustaining creep/stress rupture damage will exhibit diametral growth or sagging. Shining a flashlight along the tube length can be a quick way to identify major bulges, sags and bows. Use of ultrasonic based intelligent pigs can also provide diametral growth measurements throughout the full coil length. Frequently, the diametral growth will be localized to a small section of the tube and appear as a bulge. The bulge may be limited to the hot face of the tube and, therefore, will not be a uniform circumferential growth. Diametral growth occurs from the operating hoop stresses. Localized bulging can result from internal coke deposits causing the wall temperature to become excessive. Another potential cause is flame impingement, which could elevate temperatures. It is not uncommon to find evidence of both since the elevated temperatures can promote coking in some hydrocarbon services. The amount of bulging provides an indication of the extent of damage up to the point of failure.
Tube sagging is another indication of creep damage or short-term overload. Except in this case, the stresses are longitudinal and usually result from inadequate support or excessive temperature. Excessive sagging is often caused by short term overheating because of runaway decoking or loss of flow. In such cases, the tubes should be hardness checked to insure adequate strength remains, and the thickness should be measured. If sagging is caused by long-term creep, a criterion such as 1% – 5% elongation should be used depending on the material.
Tube diameter or circumferential measurements provide an indication of the amount of damage. These measurements can be compared to original measurements to determine the amount of bulging and is often presented as a percentage growth/bulge. Measurements of the circumference can be made with a narrow tape measure often referred to as external “strapping”. This method is limited only to sections of coil which are readily accessible. Other techniques include tube gauges calibrated to specified diameters to determine growth or are set to specific percentage growth and used as “pass-fail” gauges. Tube gauges can be fabricated from thin SS plate (0.10 in. – 0.125 in. [0.25 cm – 0.32 cm]). The cutout dimension is the sum of tube OD, mill out of round tolerance per ASTM-530 (approximately 0.0625 in. [0.16 cm]), and creep/stress rupture growth. Typical gauges are made for 2%, 5%, and 10% creep growth. These measurement techniques may not be precise if the tube is heavily scaled. In these instances, the growth may appear more severe if the scale is not removed. One could increase the tolerance of the tube gauge to account for some scale thickness.
The circumference and diameter can also be accurately determined using crawler devices equipped with special sensors. The sensors transducers sensors can be either laser, ultrasonic or eddy current liftoff types. These specialized external tube crawlers are commonly used when inspecting steam methane/naphtha reformer tubes.
Tube sags are estimated as the distance offset from straight. In reality, the tube is flexible and will have some natural amount of sag and so the measurement will not be precise. Sag measurements can be used similar to diameter measurements to estimate the extent of creep. A significant amount of sag can be tolerated before rupture is a concern and so this condition is not considered serious unless it prevents cleaning or causes headers to jam and wedge against other headers or against the sides of the header compartment. In convection sections, sagging of the tubes in upper rows to a point between those in lower rows can prevent the free passage of flue gas around the tubes. This condition, called nesting, will cause overheating of adjacent tubes and draft loss. If this condition is found, the offending tubes should be replaced.