Slug Flow Analysis

1. INTRODUCTION

2. TYPE OF TWO-PHASE FLOW (FLOW REGIMES IN HORIZONTAL &

VERTICAL)

3. BAKER PAREMETER

4. EXAMPLE OF PROCESS CALC

5. EFFECT OF SLUG FLOW

6. METHODS TO AVOID SLUG FLOW

7. SLUG FORCE CALC.

8. RESTRAINT APPLICATION

9. SLUG FLOW ANALYSIS ON CAESAR - STATIC METHOD

10. SLUG FLOW ANALYSIS ON CAESAR - DYNAMIC METHOD

SLUG FLOW ANALYSIS

Two-phase flow can be defined as the flow in which the fluid occurs in both the liquid and gas phases. It is a very important consideration in process plant piping.

TWO PHASE FLOW

Significance of Two-Phase flow

If Two-phase flow is not considered during design, pressure drop will be underestimated. Higher than the expected pressure drop can reduce plant capacity.

Ignoring Two-phase flow may also lead to poor process control. Two-phase flow to a control valve is difficult to regulate smoothly and reproducibly.

If Two-phase flow is not considered in piping layout & supporting, it will lead to severe vibration causing enormous load on the piping as well as the structure attached to.

Types of Flow

Segregated Flow

Each phase (gas as well as liquid) is continuous.

Types

Stratified Flow

Wave Flow

Annular Flow

Two Phase Flow

Intermittent Flow

Each phase is discontinuous.

Types

• Plug Flow• Slug Flow

Distributed flow

One phase is continuous and the other is discontinuous.

Types

• Bubble Flow (Liquid phase is continuous)• Mist or Dispersed Flow (Gas phase is continuous)

BUBBLE FLOW

In bubble flow, the volumetric liquid rate is high relative to the gas rate. Liquid occupies the bulk of the pipe cross-section and gas flows in the form of small bubbles. The gas and liquid velocities are nearly equal. Due to buoyancy effects, the bubble concentration is higher towards the top of the pipe.

Occurrence

This type of flow can be expected when the vapor content is less than 30% of total weight flow rate.

Flow Regime in Horizontal Pipe

Plug Flow

The transition from bubble flow to plug flow is marked by a change from a distributed to an intermittent flow regime. Holding the volumetric gas rate constant but decreasing the liquid rate causes the bubbles to coalesce, and alternating plugs of gas and liquid flow along the top of the pipe with liquid remaining in the continuous phase along the bottom.


Stratified Flow

Moving from plug to stratified flow represents a transition from intermittent to segregated flow. As the liquid rate continues to decrease but the gas rate is still held constant, the plugs become separate, continuous phases. Gas flows along the top of the pipe and liquid flows along the bottom. The interface between phases is relatively smooth, and the friction occupied by each phase remains constant along the length of the pipe.


Wavy Flow

When the gas rate is now increased in stratified flow, a transition to wavy flow occurs. In this regime, the gas moves appreciably faster than the liquid and the resulting friction at the interface causes waves to be formed. The wave amplitude increases with increasing gas rate.


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.

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.


Annular Flow

At higher velocities, the flow regime transitions from intermittent to segregated. The high gas velocity causes the liquid to flow as an annular film of varying thickness along the pipe wall, while the gas flows as a high-speed core down the middle. There is a significant velocity difference between the two phases. In fact, the gas can shear liquid from the annular film and carry it along in the core as entrained droplets. Due to gravity, the annular film on the wall is thicker at the bottom of the pipe than at the top. This effect diminishes at higher flowing velocities.


Mist/Spray Flow

The mist flow regime evolves from annular flow when the gas velocity increases to a point where a large portion of the annular film is removed from the pipe wall and carried in the gas phase as entrained droplets. Mist flow is also known as spray or dispersed flow. Mist is always accompanied by an annular film, even though it may

be very thin.

Occurrence

This type of flow can be expected when the vapor content is more than 30% of total weight flow rate. E.g. some overhead-condenser and reboiler-return lines.


Flow Regimes in Horizontal Pipe - Transition Phase

Bubble Flow

Bubble flow in vertical pipe is similar to bubble flow in horizontal pipe. The liquid is the continuous phase, with dispersed gas bubbles rising through it. Due to buoyancy effects, the bubble velocity is slightly higher than the liquid velocity during upward flow, but the bubble velocity is slightly lower than the liquid velocity during downward flow.

Flow Regime in Vertical Pipe

Slug Flow

As the liquid volumetric rate decreases or the gas rate increases, the small bubbles coalesce into large bubbles which occupy a major portion of the pipe cross-sectional area. Alternating large gas bubbles and liquid slugs move through the pipe with some small bubbles of gas entrained in the liquid 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 horizontal leg, provided that other conditions remain the same.


Froth/Churn Flow

As the liquid volumetric rate is further reduced or the gas rate is increased, the gas/liquid interface becomes more turbulent. More small gas bubbles mix with the liquid, forming a turbulent, disordered pattern of ever-shortening liquid slugs separated by successively larger gas bubbles, known as froth flow.


Annular Flow

Increasing the gas volumetric flow rate causes the transition to annular flow, where the liquid separation between large gas bubbles disappears and the bubbles coalesce into a continuous, central core of gas. In upward flow, the annular liquid film flow is slowed by gravity, increasing the difference in velocities between the gas and liquid phases. In downward flow, gravity increases the liquid velocity, thereby reducing the phase velocity difference.

With vertical annular flow, the liquid film thickness is more uniform around the circumference of the pipe than with horizontal annular flow.


BAKER PARAMETER

Normally, the two-phase-flow pattern can be established from the chart given below by calculating Baker parameters Bx and By.

Thus BX depends on

1. Weight-flow ratio

2. Physical properties of liquid and vapor phases

Note: It is independent of pipe size

BAKER PARAMETER

l

l

l

vl

v

lx W

WB

31

32

531

Thus BY depends on

1. Vapor-phase flow rate 2. Vapor and liquid densities3. Pipe sizePractical significance of pipe size is by changing pipe diameter the type of flow might be changed which changes the friction losses.

vl

vy

A

WB

16.2

l

l

v

l

v

l

W

W

A

Flow rate of vapor (lb/h)

Internal cross section of the pipe (ft²)

Flow rate of liquid (lb/h)

Surface tension of liquid (dyn/cm)

Liquid density (lb/ft3)

Vapor density (lb/ft3)

Viscosity of liquid (cP)

Determine a reasonable pipe size, downstream of the control valve for the piping configuration shown. The available pressure differential, ΔP, is10 psi ,including that for the control valve. The two-phase flow data in the line after the control valve are:

Liquid Vapor

Flow (W) lb/h 59,033 9,336

Density (ρ) lb/ft³ 31.2 1.85

Viscosity (μ) cP 0.11 0.0105

Surface tension (σ) dyn/cm 5.07 -

Process Calculation - Example - Line Sizing / Configuration

From Baker Parameter equations, we can calculate bakers parameters for several sizes of Sch-40 pipe, including and larger than 4”. Using the Baker coordinates, we establish the flow region from the fig.Pipe size Baker coordinates Flow Region(in) BX BY 8 245 7,650 Bubble/slug6 245 13,250 Bubble4 245 30,000 Bubble

As 8” line creates slug flow, it should be avoided. Using appropriate formulae for calculating the pressure drop for the type of flow, it is found that the pressure drop is higher at 4” line. Hence 6” is the optimum line size

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.

Boiler Blowdown.

Methods to avoid slug flow

• By reducing line sizes to a minimum permitted by available pressure

differentials.

• 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. E.g. in a

pocketed line where liquid can collect, slug flow might develop. Hence pocket is

to be avoided.

Should a slug for condition be considered, the forces on an elbow may be determined as follows:

The static force on a bend due to steady fluid flow is

For a 90° Elbow (φ = 90),

where ρ = density of the liquid (lb/ft3)

V = Velocity of vapor (ft/sec)

A = Area of cross-section (ft²)

g = gravitational constant (32.2 ft/sec²)

The Stresses and forces caused by the impact of slug will be 1.5 - 2.0 times the static force.

Considering DLF = 1.5 for impact loading (as per Fluor Design Guide),

Slug Flow Calculation

2sin

2 2 g

AVF

g

AVF

2414.1

g

AVF

212.2

• Restraints designed for slug flow will allow movement due to normal operation conditions; however the design will be primarily for excessive movement due to impact the slug. All changes of pipe run direction will require restraint e.g. the loop shall be restrained as shown. In case of any negative effect of the restraint in thermal case, sufficient gap may be introduced.

• Standard guides and anchors are normally inadequate for restraining slug flow. Special design is required.

• Free standing structural “T” supports are also inadequate for restraining slug flow. These supports are usually quite tall and tend to offer little resistance to horizontal loads. Consequently, even though they are not overstresses from the effects of the load, they do not sufficiently dampen the impact and the resultant displacement of the pipe at the restraint point can be a serious problem.

Restraint Application

Slug Flow Analysis - Static Method (using CAESAR II)

Slug Flow Analysis - Static Method (using CAESAR II)

Load Cases to be built up

Input Procedure

In the absence of process parameters, it is preferable to use the density of vapor for stress calculation as well as to find out the minimum frequency of the system. However, the hydrostatic load may be used for the design of supports. This is applicable wherein hanger selection is not required. Systems which have hanger design requirement, it is advisable to refer all relevant process data and find out the equivalent density of the fluid which is more realistic approach.

Two Phase Flow

Normally, Slug force is treated as an impulsive force. This method is based on the assumption that the slug traverses the elbow and then suddenly drops to zero again. It is also assumed that the slug is formed across the pipe full cross section. The duration of slug is calculated based on the length of slug and stream velocity. This results in short duration impulsive loads on the pipe.

The slug size is calculated based on the length of the pipe before the elbow & liquid volume fraction i.e. Length of liquid slug = Length of pipe * Liquid volume fraction

Duration of slug is calculated as,Length of liquid slug

Time duration = Velocity of flow

The slug periodicity is calculated as,

Length of liquid slug + Length of gas slugPeriodicity of slug =

Velocity of flow

Slug Flow Analysis - Response Spectra Method (Dynamic analysis using CAESAR II- Based on previous experience -Not FD procedure)

SLUGFLOW_CALC.xls

Force on the elbow due to fluid flow is calculated as,

FAXIAL = V2 A (1- cos )FORTHOGONAL = V2 A sin

WhereFAXIAL Force on elbow in axial direction ( N )FORTHOGONAL Force on elbow in orthogonal direction ( N ) Density of liquid ( Kg/ m3 )A Internal cross-sectional area of pipe in ( m2)V Stream velocity ( m/Sec ) Angle of bend ( Degree )

It is assumed that the elbow is subjected to force due to liquid slug and drops to a smaller value based on the density of gas, after some duration as the liquid traverses the elbow. This load is treated as a rectangular pulse load with the duration and periodicity calculated as above. The no. of cycles are given as per maximum limit of CAESAR-II package.

Natural frequencies of the piping system is calculated for the few modes up to frequency cut-off of 33 Hz.

Slug Flow Analysis - Response Spectra Method (Dynamic analysis using CAESAR II)

Slug Flow Analysis - Dynamic Method (using CAESAR II)Response Spectra - Input

Slug Flow Analysis

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