securely inside the slug flow ‘zone.’ At grade, where the pressure is higher and vapor- isation lower, the operating point would move to the left and stay within slug flow and could possibly enter into the very narrow bubble region before going single phase (no vaporisation). Strategies to get out of the slug flow region are shown in Figure 6 : reduce diameter to increase mass velocity (and pressure drop), reduce vapor- isation and transition to single phase, or increase vaporisa- tion to push into annular/mist flow. • Reduce diameter : For new installations, diameter reduc- tion has been used by the author to prevent slug flow and even in revamp scenarios to eliminate pulsing/vibrating up-comer sections of pipe. In this case, piping changes were not possible since it would incur significant eco- nomic loss to an operating unit • Reduce vaporisation: Calculations indicated that moving into the shallow region of 0 to 0.2 on the 1/ Xtt axis was not possible with the hot feed from the DIB#2 tower. A short field trial of this option was attempted and
1000
Reduce diameter
Design basis
100
Mist
Increase vaporisation
Reduce vaporisation
10
Annular
Bubble
Slug
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0.1
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10
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1/Xtt
Figure 5 Design point shown on the Fair vertical flow map
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Design basis Alt operation 1 Alt operation 2
100
Mist
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Annular
Bubble
Slug
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0.1
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1/Xtt
Figure 6 Fair map with alternate operating points (circle is lower feed temperature, square is higher feed temperature)
The Fair map4 (i.e., John R. Fair flow regime map) for vertical upflow has been used in industry extensively for predicting the flow regime of gas-liquid mixtures in vertical pipes. Several other maps have been developed since its inception, but largely, they are based on the Fair map. Many refining companies have their own flow regime maps that they base their designs on. It is important for the engineer to understand that there are different characteristics for flow in horizontal, vertical up-flow, and vertical down-flow. The flow regime maps are not interchangeable. For the practitioner, it is critical to understand that the lines on the flow regime charts are not well-defined but rather are transitional. The lines on the charts should be considered broad and thick, not narrow, distinct lines. Figure 5 shows the Fair vertical flow map with the design point for the debutaniser feed line illustrated. The point shown is for the design point at the entry into the tower (i.e., pressure = tower pressure + feed distribu- tor pressure drop). It is evident that the design point is
was not successful • Increase vaporisation : Calculations supported the idea to increase the feed preheat. The DIB#1 feed preheater was bypassed, resulting in a higher temperature feed, and the feed temperature was increased by 15°F. The resultant Fair map is shown in Figure 6 . This option was successful. In a new design, all of the strategies above could be used to stay out of slug flow. Reducing diameter and increasing vaporisation would move the operating point diagonally up to the right. To further test the trays and provide some operator security, ways to eliminate the slug flow were reviewed. Increasing the feed temperature is shown as the red box (Alt Operation 1), while reducing the feed tem- perature is shown as the orange circle (Alt Operation 2). Increasing the feed temperature by increments of 5°F was trialled to decrease slugging. After the feed temperature change, the stitching pressure drop profile immediately disappeared. Figure 7 illustrates the resolution of the pressure drop with increasing feed temperature.
6
Revamps 2026
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