Tray 15
Draw-o
46’
Tray 14
44’
13” - 15”
10” - 18”
13” - 15” 10” - 18”
Tray 13
42’
PA return
Tray 12
40’
8” - 10”
8” - 10”
7” - 9”
7” - 9”
Ring
Tray 11
38’
Tray 10
36’
14” - 18”
Inlets
14” - 18”
7” - 9”
7” - 9”
Draw-o
Tray 9
34’
Sump
Tray 8
32’
9” - 11”
9” - 11”
7” - 9”
7” - 9”
Tray 7
Platform
30’
Tray 6
Figure 3 Centre downcomer gamma scans for the ‘more loaded’ (blue pen) and ‘less loaded’ (red pen) tests
the downcomer needed to be large and lodged at an ori- entation that would block at least 60-80% of the down- comer. Alternatively, the gap that would allow the vapour in needed to be quite large, at least 2 ft². Further, the down- comer gamma scans showed no evidence of vapour break- ing into the centre downcomer from tray 10. This theory was unable to explain the maldistribution of liquid below the diesel draw that was observed in the gamma scans and the tray 8 temperatures. It may be argued that the dislodged panel impeded flow to one side of the tray
more than the other. The problem with this explanation is that there were inlet weirs that would equalise the flow out of the inlet weir sump. In addition, due to the large down- comer clearances, the friction liquid head loss at the down- comer apron was very small, about 0.005in (assuming no blockage), so even a significant blockage would not induce maldistribution to the liquid descending to the lower trays. Maldistribution between panels Quantitative analysis of gamma scans is an invaluable tool for gaining insight into maldistribution between panels. 2 We, therefore, applied it for this troubleshooting endeavour. We calculated froth heights for trays 7 and 9 using Harrison’s method 3 , which is considered accurate for froth height determination. 2 The results are shown in Table 1 . Also included in the table are indices for froth density. The index we used was the minimum transmission count of the peak. The lower the value, the denser the froth. This is a shortcut method. An accurate evaluation involves integra- tion of the peak density along the peak height. 4 This tedious and time-consuming method was not justified here due to time and budget constraints.
Results of quantitative analysis of the trays 7 and 9 gamma scans
Tray
‘Normal
‘Normal Flooded Flooded
operation’ operation’ NE SW
NE
SW
7 Froth height, in
8
11
9
18
7 Density index, counts
400 12.5 500
200
900 13.5 400
170
9 Froth height, in
16
18 50
9 Density index, counts
120
Table 1
18
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