If the liquid rate is increased, a transition from segregated to intermittent flow occurs. The higher liquid rate causes the wave crests to touch the top of the pipe and form frothy slugs. The velocity of these liquid slugs and the alternating large gas bubbles is greater than the average liquid velocity. The large gas bubbles occupy nearly the whole pipe cross-sectional area I.E.
a) Low quality steam flow well injection.
b) Two-Phase flow systems.
c) Inadvertent collection in relief lines.
This type of flow may occur in a pocketed line between an overhead condenser at grade and an elevated reflux drum. Discharge lines from pressure safety valves, rupture discs may have slug flow. Slug flow will not occur in gravity flow line.
Effects of slug flow
Slug flow causes serious pressure fluctuations which can upset the process conditions and cause inconsistent instrument sensing. Moreover , it causes vibration especially at vessel inlets, pipe bends , valves and other flow restrictions. This can lead to equipment deterioration and operating problems.
Special cases of slug flow
Vacuum Transfer Lines
Condenser Outlet Lines
Reboiler Return Lines
Fired heater outlets
Methods to avoid slug flow
By reducing line sizes to a minimum permitted by available pressure
By designing parallel pipe runs that will increase flow capacity without increasing the overall friction loss.
By using a low point effluent drain or bypass or other solutions.
By arranging the pipe configurations to protect against slug flow. Eg. in a pocketed line where liquid can collect, slug flow might develop. Hence pocket is to be avoided.
The basic steps in the slug flow analysis become
Estimate where the maximum problems due to slug flow impact are most likely to occur.
Compare the magnitude of the slug flow load at each of the elbows or flow restrictions of concern.
Estimate the time waveform of the slug flow load, and use the pulse table generator to get a response spectrum for each different slug flow time waveform.
Define the forces spectrum shock load cases. For elbows that are very close, the user may wish to apply the loads from both in the same load case.
Run the spectrum analysis, and review the predicted displacements, forces and stresses.