6. On single-furnace boilers, a lane more than 1 1/2 inches wide may allow overheating of the superheater and of the superheater supports. If a large lane (1 1/2 tubes wide or wider) exists near the superheater outlet header end of the boiler, the married tubes that caused such a large lane should be renewed.
To identify the cause of the tube failure by visual inspection, you will need to know something about the various ways in which tubes rupture, warp, blister, and otherwise show damage. Tube failures must be reported, and they must be reported in standard terminology. The following sections of this chapter deal with the inspection techniques required for determining the causes of tube failure and with the various ways in which boiler tube damage is classified and identified.
The inspection techniques required for determining the cause of tube failure must naturally vary according to the nature of the problem. For example, a rupture in a fire row tube can usually be described adequately on the basis of simple visual observation, but the cause of damage to a tube that is deep in the tube bank cannot usually be determined without removing the intervening tubes. When a blistered tube suggests a waterside deposit, the nature and extent of the deposit can be determined only by removing and splitting the tube so that the waterside can be examined.
Relatively simple equipment is required for the field inspection of damaged or fouled pressure parts. Equipment for this purpose should include the following: (1) devices for measuring tube diameters, depth of pits, and thickness of deposits; (2) instruments for separating deposits and corrosion products - a sharp knife, a chisel, a steel scribe, or a vise to crack deposits loose from the tube samples; (3) an approved type of portable light; (4) a supply of clean bottles for collecting samples of deposits; and (5) a mirror for viewing relatively inaccessible places.
Many of these items of equipment can be improvised if necessary. For example, a simple gauge for measuring the depth of waterside pits may be made by pushing a straight pin or a paper clip through a 3- by 5-inch card so that the point of the pin or clip projects beyond the card, at right angles to the card. Such an improvised depth gauge is shown in figure 12-8. A section of string can be wrapped around a deformed tube and then laid along a ruler to obtain a measure of tube enlargement or tube thinning. Of course, special tools such as calipers, depth gauges, and scale thickness indicators give more accurate results and should be used if they are available; but the improvised tools, if used with care, can also give good results.
The classification of boiler tube damage is considered here under four major classifications: (1) fireside cavities and scars, (2) waterside cavities and scars, (3) tube deformities and fractures, and (4) tube deposits.
FIRESIDE CAVITIES AND SCARS on the tube firesides often indicate the reasons for tube failure. The term circumferential groove is used to describe the metal loss that occurs in bands or stripes around the circumference of a tube. Fireside grooving of this type often occurs at the header ends of horizontal tubes such as superheater tubes. The most common cause of this damage is leakage from tube seats higher in the tube bank. The grooving occurs as the water runs
Figure 12-8. - Improvised depth gauge.Continue Reading