Wastage

By analysing the scans, different types of tube characteristics, wastage patterns or profiles can be documented and effectively used to make immediate decisions during shut downs and in planning maintenance programs. 

The following discussion of wastage profiles is a result of observations made during many years of inspection. The classifications have been kept relatively broad as many permutations of these basic classes have been observed. It is also nor uncommon for two types of wastage profile to be evident on the same tube. 

Figure 1 is a cross section of a typical generator tube showing the tube sheet, the location of both the soft and hard roll marks and the typical location of wastage. 

Most recovery boilers use 63,5 mm OD carbon steel tubes in the generating bank. With a few exceptions these tubes are swaged at the ends to 50,8 mm. When the 63,5 mm raw tube is manufactured it is subject to a lot of specifications i.e. ASME. There are no specifications for the swaged end of the tube. This is unfortunate as the swaged part of the tube is subjected to further mechanical deformation dtuing the rolling procedure and is located in a wastage zone of tire recovery boiler. 

Near drum wastage is the most prevalent type of wastage documented. It cart be found anywhere in the generating bank but is most common on the wall tubes, in tlic row immediately next to soot blower lanes and in the hot and cold row s. Three types of near drum w astagc have been documented. 

The wastage is 2,6 mm below the nominal wall thickness of 4.2 mm. On the rare occasion this type of wastage may also penetrate into the tube sheet for up to 6 mm. 

Wastage all around the tube is seldom seen near the tube sheet in recovery boilers. Figure 9 shows a sean of a tube that has gradual thinning around the full circumference of the tube with the wastage stopping at the tube sheet. 

Fig. 8 Scan showing single quadrant wastage. Fig. 9 Circumference and localised wastage. Highly located...

Highly localised wa.stage also does not occur very often. It is more representative of a crevice than mass wastage. 

Fig. 10 Locali.scd wastage adjacent to the tube Fig. II Scan showing above the drum sheet. wastage.

Fig. 12 Tube with severe two sided wastage. Fig. 13 Expanded plot of fig. 15, showing two...

Wastage does not have to be dramatic to be easily observed on a full coverage scan. Figure 14 and 15 show a tube with 0,2 ram of eccentricity that has been in service for two years and has 0,15 mm of wastage near the drum. New tubes are good indicators to determine wastage rates as the scans are very clean without noise of uneven rough corrosion. 

Fig 14 Scan of a tube in service for two years Fig 15 Expanded scan of a new tube with 0,15 with wastage near the drum. mm of wastage. 

Depending upon the random orientation of en eccentric tube relative to the wastage zone, the tube may have to be replaced years sooner than it normally would have, had the tube conformed to a concentric specification. 

Fig. 16 Wastage on the thick side of an Fig. 17 Two sided wastage on the thick side eccentric tube. of an eccentric tube.

When wastage is located on the thin side of an eccentric tube, the life of the lube may be dramatically shortened because the material loss is subtracted from the thinnest part of the tube. 

Tile detection of this type of wastage is very straight forward as there is an exaggerated increase in the amplitude of lube w all eccentricity. The amount of wastage can be estimated by visualising the original wave shape and subtracting the measured minimum value. 

Wastage will also occur on that part of an eccentric tube that represents the nominal thickness of the tube. 

Pitch Control. Resinous constituents of wood cause problems in paper machines by sticking to the rollers and causing spots or holes in the paper the worst cases cause paper webs to mpture. Costly stoppages, wastages, and quaUty problems because of these resinous substances can be avoided by using Upases (84). 

Let us consider a cooler in the equipment flow sheet. The required rate of heat removal is known from the balances, and the rate of cooling water can be calculated once the inlet and outlet temperatures of the water have been specified. The calculation of the consumption of other utilities is also straightforward. Allowances should be made for wastage. 

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