70 BNOINEERINO SECTION Mr. G. P. Bhargava presented the following paper. Paper AN INVESTIGATION INTO THE CAUSES OF EVAPORATOR TUBE FAILURE G. P. BHARGAVA AND R. C. SHARMA National Sugar Institute, Kanpur, India During the course of technical advisory work the authors came across a typical case of evaporator tube failure and this paper deals with the investigations carried out and gives suggestions which may be helpful to Sugar Factory Engineers. During hydraulic pressure testingof a quadruple effect evaporator 490 tubes were found to be leaking. The evaporator had a heating surface of 6000 sq. ft. with equal bodies 5'-10" dia. having 603 tubes of 1-15/16"0. d. x 5'-1. 15/16" long, 16 S. W. G., the pitch being 2-7/16". There was a central dowdtake 1'-39" dia. The four incondensable gas outlets were 718" bore-3 connections from the top and one from the bottom in each calandria. The steam inlet to the 1st body was 8" bore, and the vapour pipes between vessels 14, 16", 18" & 24" bore, respectively. The quadruple evaporator had been working in parallel with a triple effect evaporator of 5500 sq. ft. heating surface for the past twenty years. The working result for the last season are given in Table I. TABLE I S. No. December 1. Cane crushed mds. 404,680 2. Hours worked 636.9 3. Cane crushedlhr. mds. 760 4. Mixed Juice % cane 83.4 5. Evaporation % 69.32 6. ph Clear Juice 6.8-6.85 7. ~rix January February March 5,89,784 454,500 2,99,497 686.9 563.6 378.7 845.5 807.0 790.9 I 86.4 86.3 87.4 67.64 65.94 66.31 i 6.8 6.75-6.8 6.7-6.8 Clear Juice 15.32 15.33 ' 15.74 16.22 8. Brix Syrup 49.47 47.37 46.21 48.15 From the above it will be seen that the ph of the clear juice was maintained closely around 6.8. At the time of the hydraulic test of the bodies 385 tubes in the 2nd body and 55 tubes in each of the 3rd and 4th vessels were fouhd defective. Leakage of water through the tubes varied from leaf perspiration to fine spray under pressures varying from atmospheric to 40 ibs./sq. in. Both hand and electric scrapers had been used for cleaning purposes, r
a. P. BHARQAVA, R. C. SHARMA 71 and an examination of these showed that they were of satisfactory design. The inside surfaces of the tubes didnot show any mechanical damage due to scraping. The defective tubes were taken out from the bodies and weighed. New brass tubes of 70130 composition taken from the stores were also weighed, the weights differing as indicated in the Table 11, and giving an average weight for new 1-15/16" o. d. tubes of 1.351 lbs. per ft. length. Table I11 gives the weights of old tubes removed from the evaporator, and the percentage decrease in weights compared with new tubes. TABLE I1 WT. OF NEW TUBES 1-15/16" 0. D. X 5'-2.1/8" LONG Tube Weight grams lbs. Wt./ft. of length No. 1. 3145 6.95 1.342 lbs. 2. 3133 6.92 1.337,, 3. 3237 7.14 1.378,, 4. 3157 6.97 1.347,, Average weight : 1.351 lbs./ft. length. Weight for length of 5'-1.15/16"=6.95 lbs. TABLE I11 WT. OF OLD TUBES 1.15/16" 0. D. x 5/-1.15j16"~0~~ % Tube Weight Wt./ft. Difference in wt. %Decrease No. Gms. lbs. length taking 1.351 lbs. in as standard weight 1. 2481 5.47 1.08 0.271 20.2 2. 2596 5.72 1.13 0.221 16.4 3. 2524 5.57 1.10 0.251 18.6 4. 2660 5.87 1.16 0.191 14.1 5. 2409 5.32 1.05 0.301 22.2 6. 2498 5.50 1.08 0.264 19.5 7. 2252 4.98 0.985 0.366 26.3 8. 2542 5.62 1.11 0.241 17.8 9. 2778 6.12 1.21 0.141 10.4 10. 2632 5.80 1.146 0.205 15.2 11. 2524 5.57 1.10 0.251 18.6 12. 2687 5.925 1.17 0.181 13.4 13. 2553 5.63 1.11 0.241 17.8 14. 2599 5.72 1.13 0.221 16.3 It was found that in all cases tubes were attacked towards the top ends, particularly.below the portion where they were expanded into the top tube plate. The attack extended only a few inches down the tubes, the remainder of the
72 ENGINEERING SECTION outside surface being free from attack. Test pieces of equal length, cut from corresponding positions in the length of various old tubes, were found to be of equal weight, which showed that the relative wear at different portions of the old tubes was the same. It appears that the local wastage of the brass tubes was due to the condensate carryinginto the calandria appreciable quantities of incondensable gases, principally ammonia and with possibly sulphur dioxide, carbon dioxide, and oxygenin solution. The most dangerous of these gases is ammonia which being light, rises to the top of the steam space, and dissolves in the condensate adhering to the top tube surface, thereby producing a corrosive solution which attacked the tubes. The first body of the evaporator received exhaust steam in the calandria and as this did not contain corrosive gases no tubes in this body were attacked. Theintensity of attack was greatest in the second body and less in the third and fourth. Figure 1. In Fig. 1 at (a) & (b) are shown cut tube pieces, the finc crack in (a) and the Gin holes in (b) in he portion below the top tube plates being clearly visible. After cutting the defective end of some of the tubes it was found under hydraulic pressure test that the remainder of the tube lengths could safely withstand a pressure of 250 Ibs./sq. in. I~I view of this sound condition of the major portions of the tubes it was advised that these could be used again by building them upto their original lengths by the addition of new pieces of tube welded on by means of " Easy Flo " welding rods. The method employed is to turn down the cut ends of the tubes for a distance of 118" to 3/16" removing half of the metal
1 G. P. BHARGAVA, R. C. SHARMA 73 thickness so as to produce a sliding fit overlapping joint as shown in Fig. 1 (c). " Easy Flo " rods and flux are then used for gas welding the joint to give a smooth finish inside and outside as shownin Fig. 1 (d). This enables the tubes to be refitted through the holes in the tube plates and avoids any difficulty being experienced later when cleaning with wire brushes or scrapers. The cost of a new tube of 1-15/16" o. d. x 5'-1.15/16", 16 S. W. G. was about Rs. 12181-, while the cost of repairing old tubes by the method suggested, was about -181-, including material, labour, power, gas, etc. A welder can easily braze 100 tubes in a shift of eight hours and a turner could prepare this number of tubes in the same period. It was found that about two inches of 0.06" dia. " Easy Flo " rod was consumed per joint making the cost of material about two annas. In order to test that pieces of tube, jointed-by the method described, would withstand high pressure, the test arrangement shown in Figure 2 was used. The jointed pieces were subjected to an oil pressure which was steadily raised to 500 lbs./sq. in. without any leakage occurring. After they have been welded, tubes which are to be used again should be tested to 200 lbs./sq. in. before refitting in the evaporator. Fig. 3 shows an apparatus which was designed for this purpose. Fig. 2. As a precaution against further deterioration of the tubes by the action of the incondensable gases it was recommended that two of the gas vent pipes in and from the calandrias be increased from 718" bore to lg" bore and arranged as shown in Fig. 4. It was advised that the third pipe be connected directly to the top tube plate as shown in Fig. 5, so that gases accumulating there could be drawn off directly. Attention was also drawn to the need for cleaning.out the
ENGINEERING SECTION DISCUSSION i