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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.
4.1 Laminar & Turbulent Flow
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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
4.1 Laminar & Turbulent Flow
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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.
4.1 Laminar & Turbulent Flow
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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
4.1 Laminar & Turbulent Flow
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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:
4.2 Predicting Types of Flow
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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4.2 Predicting Types of Flow
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 Predicting Types of Flow
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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4.2 Predicting Types of Flow
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
4.3 Friction in Turbulent Flow
Smooth pipe Rough pipe
Surface roughness
The degree of roughness is described by its relative roughness:
)(
)(Re
dDiameterPipeInternal
ticlesSurfaceParofHighMeanroughnesslative
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4.3 Friction in Turbulent Flow
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 Friction in Turbulent Flow
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.
4.3 Friction in Turbulent Flow
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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