The black liquor evaporator uses a 7-effect counter flow method to concentrate the mixture. Using the OSI PI monitoring software, they determined the most likely location was in the 4th effect stack.
Figure 3 shows the maps of decibel levels after we conducted the survey. The decibel levels taken on the inside wall area of the stack (Figure 3, at right) dropped as we moved away from the high of 51db, down to 32db.
The survey required the use of a man-lift to access all of the doors. While the contact ultrasound measurements were made, RCM Tech Jim Storey also conducted contact ultrasound thickness testing. Mr. Storey said that the last thickness survey conducted on this stack had been about 5 years ago.
With airborne ultrasound leaks, a large leak usually registers 65-75db at or around 15’ away from the source. Since no other external points had anything higher than 51db we decided to return to that location and investigate further.
Readings rose when we opened the access door (Figure 5). There was a strong correlation between the locations of the leaks and the thinning of the wall of the stack. At the location of the highest ultrasound, 56db, the wall was found to be 0.091” thin, which was the thinnest area found on our initial survey.
With the ultrasound level increasing as we recorded contacts close to the bottom, I changed from the contact probe to the airborne sensor. Ambient ultrasonic db levels were 18db around the stack. When I brought the airborne sensor near the 51db access door, the airborne sensor jumped to 32db – 33db and gave the whooshing sound of rapidly moving air. When I used my hand to separate some of the insulation away from the metal flange( Figures 6 and 7), readings jumped even further, as the airborne sensor levels rose to 36db.
We decided that we would need to have a larger area opened for inspection. Mr. Storey and I went back to confer with other personnel about how we could gain access to the stack below the retaining ring.
As shown in Figure 8, Mr. Storey marked out the approximate dimensions we wanted to have cut away in chalk.
We went back to the Maintenance shop and were introduced to Mr. Jim Rose. He discussed how he would cut the sheet steel away and remove the insulation to give us better access. We also figured out that we would need to have some type of blocking material to immediately cover the holes we expected to find. If we did not, the probability of losing the vacuum was great, thus potentially causing the entire process to shutdown.