Slug flow analysis

Horizontal direction

Slug Flow If the liquid rate is now 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 for well injection.b) Two-Phase flow systemsc) Inadvertent collection in relief lines.

Occurrence:

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 a gravity flow line.

Vertical Direction

Slug Flow A s t h e l i q u i d v o l u m e t r i c r a t e d e c r e a s e s o r t h e g a s r a t e increases, the small bubbles coalesce into large bubbles which o c c u p y a m a j o r p o r t i o n o f t h e p i p e c r o s s - s e c t i o n a l a r e a . Alternating large gas bubbles and liquid slugs move through the p ipe with some smal l bubbles of gas entra ined in the l iquid slugs. Slug flow can occur in either upward or downward vertical flow, but it is usually not initiated during downward flow. However, if slug flow is well established in an upward leg, it will persist in a subsequent downward or hor izontal leg, provided that other conditions remain the same.

Effects of slug flow

S l u g f l o w c a u s e s s e r i o u s p r e s s u r e f l u c t u a t i o n s w h i c h c a n u p s e t t h e p r o c e s s c o n d i t i o n s a n d c a u s e i n c o n s i s t e n t i n s t r u m e n t s e n s i n g . M o r e o v e r , i t c a u s e s 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:

Bend PipeR

AngleSlug impact load fully developed when slug leading edge reaches this cross section.

Impact starts when slug edge reaches this point.

Direction of slug travel

Vacuum Transfer Lines. Condenser Outlet Lines. Reboiler Return Lines. Fired Heater outlets. Boiler Blowdown.

Methods to avoid slug flow

By r e d u c i n g l i n e s i z e s t o a m i n i m u m p e r m i t t e d b y a v a i l a b l e p r e s s u r e differentials.

B y d e s i g n i n g p a r a l l e l p i p e r u n s t h a t w i l l i n c r e a s e f l o w c a p a c i t y w i t h o u t increasing the overall friction loss.

By using a low point effluent drain or bypass or other solutions. B y a r r a n g i n g t h e p i p e c o n f i g u r a t i o n s t o p r o t e c t a g a i n s t s l u g

f l o w . E . g . i n a pocketed line where liquid can collect, slug flow might develop. Hence pocket is to be avoided.

In Caesar II:

a) The length of the fluid cylinder. (The longer the more conservative)b) The inside diameter of the pipe thru which fluid is moving.c) The estimated velocity of the fluid cylinder. (If no other value is available then taken as

between 0.5 and 1.0 times the gas flow velocity.)

The slug flow time history is estimated as shown in the figure below:

R Ѳ/V L-R Ѳ/V R Ѳ/V

Fslug

Time

Force

Slug rise time: RѲ/V

R: Bend Radius

Ѳ: Bend Angle

V: Slug travelling velocity

Slug impact duration: (L-R Ѳ)/V

Slug closing time: Slug rise time

Fslug=mv= (pAV)V=pAV2

P=Fluid density

A=Area of pipe

V=Velocity of vapor

Slug impact rise and fall time=0

Slug impact duration=2L/V

Slug impact Dynamic Load magnitude=rAV2

Where: L – is the estimated length of the slug cylinder

V – is the velocity of the slug

R – is the fluid density, and

A – is the inside area of the pipe.

The basic steps in the slug flow analysis become:

a) Estimate where the maximum problems due to slug flow impact are most likely to occur.

b) Compute the magnitude of the slug flow load at each of the elbows or flow restrictions of concern.

c) 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.

d) 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.

e) Run the spectrum analysis, and review the predicted displacements, forces and stresses.

Not for calculating slug impact loads at bends other than 90 degree;

Change in momentum in coming direction = (1-Ѡβ)pAV2

Change in momentum perpendicular to the incoming direction = (sin β)pAV2

Direction = (sin β)pAV2

Outgoing direction

Direction of slug travel

Incoming direction (Parallel to global axis)

Ѳ

This amplification is often expressed as the dynamic load factor DLF and is defined as the maximum ratio of the dynamic deflection at any time to the deflection which would have resulted from the static application of the load. For structures having essentially one degree-of-freedom and a single load application, the DLF value will range between one and two depending on the time-history of the applied load and the natural frequency of the structure.

There is no way to know for certain, so you make assumptions:

- What could the smallest slug be that I would have to worry about?- What could be the largest slug that they system could handle without destroying itself?- What would be a reasonable slug, midway between the previous two?