gas flow in pipeline

8/2/2019 Gas Flow in Pipeline

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B Y : M D N O O R B A R I F I N

F A C U L T Y O F C H E M I C A L A N DN A T U R A L R E S O U R C E SE N G I N E E R I N G

U N I V E R S I T I M A L A Y S I A P A H A N G

CHAPTER 4:

GAS FLOW IN PIPELINE


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4.1.1 What is Boundary Layer?

From physical examination, there is a thin layer of fluidadhering to the pipe wall, and that the velocity of this

layer relative to the pipe wall is zero.

This zero-velocity layer affects the successivelayers of flowing fluid.

The idea of a stationary layer offluid, particularly gas, may seemsurprising but it is true even forthe smoothest of pipe materials

4.1 Laminar & Turbulent Flow


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Generally, flow is divided into 2 types:

Laminar flow:where the viscous forces tend toresist fluid movement

predominate, creating a boundary layer whicheffectively extends to the centerof the pipe from the wall

Turbulent Flow: where the viscous forces arerestricted to a thin layer which

extends only a short distancein from the pipe wall.



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Laminar Flow(sometimes called

viscous flow)

The viscous predominateand the entire flow could

be defined as a boundarylayer. This usually occurs

at low velocities.

Its the viscosity of thefluid that determines theresistance to movement

of fluid particles between

parallel layers of thefluid.

The profile is parabolicwhere the velocity of thefluid layers increases fromzero at the pipe wall to a

max. value at the center

Figure 4.3: Velocity Profile for Laminar Flow

AuQ



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L

pdQ

128

4

Hagen-Poiseuille Equation

WhereQ = Volume flow rate

d = Internal diameter of pipe =Fluid viscosityp =pressure loss occurring overlengthL = Pipe length

Two (2) important features of laminar flow:i) The volume flow rate, Q is inversely proportional to fluid viscosity,.ii) The pressure loss, p is indirectly proportional.



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Turbulent Flow A flow where any of the fluid particlesare not travelling in straight parallel

lines.

a) Laminar flow where all the fluid particles are travelling in

straight parallel lines (streamlines)

b) Increasing the average velocity..cause the particlesin the center of the pipe to speed up. The particles in thefluid layers adjacent to the pipe wall are still at zerovelocity..cause the onset of turbulence as shown in b)

c) As the velocity increases..cause more erratic leading to acentral core which is turbulent and an outer layer which isstill laminar as shown in c)

Figure 4.3: The transitionfrom Laminar to turbulentflow



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Summary

1. The boundary layer is that layer near of fluid near to thepipe wall in which the velocity varies from zero at the pipe

wall to the maximum fluid velocity.

2. There are basically 2 types of flow: laminar & turbulent

3. Laminar flow results from the dominance of theviscousforceswithin the fluid

4. The fluid particles travel in straight parallel lines andproduce a parabolic profile

5. Turbulent flow originates in the center of the flow where the

fluid particle velocity is greatest6. In partially turbulent flow, the central core of the fluid is

turbulent and the outer layer, adjacent to the pipe wall, islaminar (the laminar sub-layer)


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..Redv

What are the implications for the flow of natural gas?

Please think in a CRITICAL WAY!!!

How about thekinematic

viscosity, ofthe natural gas?

Whencombined with

a diameter, willtheRe be high?

4.2 Predicting Types of Flow


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4.1 Calculation ofRe for natural gas

For the steady state gas flow, the mass flow rate of gasentering a pipe is equal to the mass flow of gas

leaving the pipe, so the Continuity Equation:


sss uAuAuA 222111

The equation is valid not only for the continuity of mass flow inthe pipe but also for expressing mass flow rate of the same gas


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Exercise4.2.1:

Using the equation below, calculate theRe ofnatural gas flowing in a pipe with an internal

diameter of 100 mm at a rate of 105 m3 (st)/h?

Ans:26295.15


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4.2 Types of Flow in a Gas Supply System

As a general rule the following types of flow can be expected for the givenranges ofRe.

The region between Re = 2000 and Re = 4000 is known as the critical zone.Its so called because the flow cannot easily be defined as either laminar orturbulent, its the point at which the inertia forces are approximately equalto theviscous forces

Re < 2000 Laminar

Re > 4000 PartiallyTurbulent

Re > 107 Fully Turbulent


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Exercise 4.2.2A typical service pipe feeding a single domestic property might consist of a 20 mminternal diameter PE pipe supplying a maximum flowrate of 3 m3 (st)/h?a) Calculate theRefor this customers supply

b) Compare this with:

i. A 180 mm internal diameter low pressure distribution main supplying 250m3 (st)/h of natural gas

ii. A 1000 mm internal diameter transmission pipeline supplying 0.5 x 106

m3

(st)/h

c) What types of flow would you expect in each case?


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4.3 Friction in Turbulent Flow

Figure 4.3: Effect of Pipe Surface onFlow

A- a pipe with a perfectly smooth internal surface

-a laminar sublayer would always completely cover thepipe wall-it would become very thin at high velocities-the main body of turbulence flow would never comeinto contact with the pipe wallB-real pipe with an internal surface, with consists of smallparticles- create the roughness

-laminar sub-layer is thick enough to cover the surfaceroughness-It behaves as a smooth pipeC-the laminar sub-layer has become thinner due toincreasing velocity-Some of the roughness peaks are just protrudingthrough the sub-layer and into the turbulent flow.-So, it is now no longer independent of the internal pipesurface, but it is partly dependent on Re since there isstill a laminar sublayerD-increased velocity has caused the laminar sublayer toshrink even moreAllowing the pipe roughness to protrude further into the

turbulent flow-cause the eddies to form around each particle which


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Turbulent Flow Region Type of Flow Dependent On

Partially developed turbulent Smooth pipe Reynold Number

Partially developed turbulent Transition zone Pipe roughness and

Reynold Number

Fully developed turbulent Rough Pipe Pipe roughness


Smooth pipe Rough pipe

Surface roughness

The degree of roughness is described by its relative roughness:

)(

)(Re

dDiameterPipeInternal

ticlesSurfaceParofHighMeanroughnesslative


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Relative roughness is the mean high of surface particles relative tosome length which is characteristic of the shape of the flow conduit.


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4.3.1 The Moody Diagram

Moody diagram is based on the Darcy friction factor ( ,for laminar flow)

Looking at the Moody Diagram, you should be able to identify:

the laminar flow

the smooth pipe curve

a series of lines representing values for pipe roughness

the critical zone between laminar and turbulent flow

the region of complete turbulence where the rough pipe law applies

the transition zone between the smooth pipe and rough pipe regions

Re

64f


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Exercise 4.3a) Quite often, when renewing an old service pipe to an individual

customer, its possible to insert a small PE pipe through the oldservice pipe to minimize the disruption. A typical installation might

use 16 mm internal diameter PE pipe for a maximum flow rate of 3m3(st)/h. What is the friction factor under these conditions?

b) Now consider a larger PE distribution pipe, say 200 mm I/D with aflow rate of 200 m3 (st)/h. What is the friction factor for this system

NOTE: PE has a typical value of = 0.01 mm.



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4.5 The General Flow Equation

5.0

52

2

2

1

4

1

10

574.7

SLTZ

dpp

fPs

TsQ

L = pipeline length, kmT = Gas temperature, oC

D = Pipeline outside diameterZ = Gas compressibility factor,t = Pipeline wall thickness, mmS = Gas relative density, = Pipeline roughness, mm

gas flow in pipeline

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