The entire heating coil should be given a thorough visual inspection. Visual inspection is a fundamental technique to help identify the effects of deterioration, actual defects, and an indication of potential defects or weaknesses in the tubes, crossovers, fittings, and connections, including blowdown, steam, pressure gauge, vents, and thermowell connections. Conditions found by visual examination are typically followed by a more detailed inspection to assess the degree of deterioration. It provides means to focus inspection efforts.
Because of the arrangement of the tubes and refractory walls, visual inspection of the external surfaces of the tube is usually restricted to the fireside of the radiant tubes. Special attention should be given to the following locations:
b. In vertical heaters, the area from the firebox floor to approximately 20 ft (6 m) above the firebox floor.
c. Entry and exit points through the tube sheets of inlet and outlet tubes.
d. Tube supports, hangers and guides (inspect for deformation and cracking).
All tubes rolled into fittings should be examined for leakage in the rolled joint. Leaks in tube rolls and around plugs can often be found by observing the location of coke or oily deposits around headers when the heater is removed from service. An examination should also be made when the coil is under test pressure. The inspection should be visual and should in some cases be supplemented by feeling the tube at the rear face of the fitting for indications of leakage.
Visual inspection can sometimes be facilitated by holding a small mirror between the tube sheet and the fitting to obtain a view of the juncture between the tube and the fitting. Roll leaks will often not become detectable until a coil has been under pressure for 10 – 15 min. Leakage in the tube rolls can be either a nuisance or a serious problem, depending on the process and the operating conditions of the heater. Where there is no formation of coke, the leak may be stopped by rerolling the tube. Roll leakage is serious, however, in the case of a heater that is subject to coking and that operates at high pressure-temperature conditions or in poisonous or highly explosive vapor service, including phenol or hydrogen service. Oil leaking between the fitting and the outside surface of the tube can result in the formation of coke. This coke formation continues with service, and the force of the coke buildup can be sufficient to cause partial collapse of the tube end and to allow the tube to slip in the fitting. Under these conditions, leakage cannot be corrected by rerolling because the serration in the fitting’s tube seat is full of coke, and the mechanical strength of the rolled joint is not improved by the rerolling operation.
In the case of rolled-on fittings, the internal surface should be inspected visually for signs of deterioration and to ascertain the fittings’ general physical condition. With sectional, streamlined fittings, the housing section (the part the tube is rolled in) should be examined for undercutting, the width and condition of the U-bend seats, and excessive erosion and thinning of the housing in the annular space (the section of the housing between the end of the tube and the inside edge of the U-bend seat). The inside surfaces of the U-bend should be examined for thinning and to ascertain their general condition. With solid fittings, the body section should be examined for undercutting, the width and condition of the plug seat, and erosion and thinning of the barrel section of the body (the cylindrical section with the plug seat at one end and the tube seat at the other end) and the cross port (the connecting section between the two barrel sections).
The seating face on U-bends and plugs should be examined for corrosion, and the width of the seat should be checked against the width of the seat in the housing or body sections. If there is not a tight fit between the U-bends and the housing for the entire width of the seating surface or if the width of the seating surface is longer on one member, member erosion will be severe. This same condition should be checked on solid fittings at the closure area between the fitting body and the plug. Fittings should be examined to determine the fit and depth of seating between the U-bend or plug and the main body of the fitting. If the fitting seat has become enlarged through service, the U-bend or plug can protrude so deeply into the fitting that it is not possible to head up and get a tight joint when the fitting is under pressure. In the case of a sectional fitting, the end of the U-bend will contact the end of the tube or the tube stop, depending on the type of tube seat used. In the case of a solid fitting, the ears on the plug will contact the outside face of the fitting.
In some cases, a rolled-in tube may also be welded to the fitting. There are two basic reasons for welding a tube to the fitting: (a) to stop leakage by means of a seal weld and (b) to improve the efficiency of the rolled joint by means of a strength weld. The use of a strength weld warrants careful consideration and justification. The types of defects that are commonly found are cracking, slag, and porosity in the weld. Any welding between the tube and the fitting, regardless of its basic purpose, should be examined carefully. Review by a materials or welding engineer is recommended before any welding to the tubes.
The exterior surfaces of the fitting body and the holding members should be inspected visually. The types of deterioration commonly found on the external surface of fittings are cracking, distortion, and mechanical wear. Cracking is usually confined to the fitting body or, in the case of welded fittings, to the welded joint. Locations in the fitting body that should be examined for cracking include the area around the plug or U-bend seat, the juncture of an ear or horseshoe holding section and the main body, and the ear or horseshoe section itself. If conditions warrant, a visual inspection of cracks can be supplemented by a range of applicable surface, near surface or volumetric NDT techniques.
Visual inspection of the ears, the holding members, and the dogs and caps of the holding members is performed primarily to detect distortion and wear, to determine whether there is a proper fit or contact, and to ascertain whether the strength of the fitting has been affected. The threaded portion of the holding screw and the dog or cap should be examined for excessive wear. Distortion that is not apparent to the eye may prevent proper assembly. The plug or U-bend seat in the fitting should be examined for enlargement, deviations from roundness, change in the width of the seat, and damage to the seating surfaces. The tightness of this joint depends on these four conditions.
For welded fittings, visual inspection is limited to the external surfaces and to the weld attaching the fitting to the tube. The accessible external surfaces of the fitting should be examined closely for any indications of defects, particularly cracks in welds. The inspection of welds should cover a band of 1 in. – 2 in. (2.5 cm – 5 cm) on each side of the weld. Cracks may develop and remain entirely within the weld, or they may start in the weld and run out into the tube or fitting. The inspection of the heat-affected zone and adjacent parent metal is important. It is of paramount importance in the case of alloy welding. The visual inspection of a weld may be supplemented by a range of applicable surface, near surface or volumetric NDT techniques.
Cross-over sections of tubing used to connect sections of coil may be located outside of the firebox or enclosure but should not be overlooked during inspection of the heater. Movement of the several parts of the coil and changes in temperature can cause stress and fatigue. The surfaces of the tubing, especially bend section surfaces, should be examined for cracks.