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GAS FIELD ENGINEERING
Gas Gathering and Transportation
CONTENTS
3.1
3.2
3.3
3.4
3.5
3.6
3.7
Introduction
Pipeline Design
Reynolds Number
Relative Roughness
Friction Factors
Pipeline Equations (Weymouth, Panhandle, Modified
Panhandle, Clinedist )
Series, Parallel, and Looped Lines
LESSON LEARNING OUTCOME
At the end of the session, students should be able to:
Apply pipeline flow equations
Design gas transportation, gathering, and distribution
systems.
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3.1 INTRODUCTION
Transmission of natural gas to consumer be divided into three
distinct pipeline units: gathering system, main trunk line
transportation system, and distribution system.
Focuses on design and operation of natural gas pipelines in
onshore and offshore gas fields.
3.2 Pipeline Design
Factors to be considered in the design of long-distance gas
pipe-lines.
the volume and composition of the gas to be transmitted,
the length of the line
the type of terrain to be crossed
maximum elevation of the route
Note: Pipe line must be larger to accommodate the greater
volume of gas.
3.2 Pipeline Design
Several designs are usually made so that the economical one
can be selected.
Maximum capacity of a pipeline is limited by higher
transmission pressures and strong materials.
For economic operation, better to preserve full pipeline
utilization.
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3.2.1 Sizing Pipelines
Capacity of gas transmission is controlled mainly by its size.
Complex equations have been developed for sizing natural
gas pipelines in various flow conditions.
oThe Weymouth equation
oThe Panhandle equation
oThe Modified-Panhandle equation
By using these equations, various combinations of pipe
diameter and wall thickness for a desired rate of gas
throughout can be calculated.
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3.3 Friction Factor
Friction losses:
o Internal losses due to viscosity effects
o losses due to the roughness of the inner wall of the
pipeline
f = f (NRe, eD)
Friction factor is a function of the Reynolds number and of
the relative roughness of pipe.
NRe = Reynolds Number
e
= absolute roughness of pipe
D = diameter of pipe
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3.3 Friction Factor
Equation that relates lost work per unit length of pipe and
the flow variables is
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Reynolds Number
Reynolds number (NRe) is defined as the ratio of fluid
momentum force to viscous shear force.
The Reynolds number can be expressed as a dimensionless
group defined as
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Reynolds Number
Reynolds number is used as a parameter to distinguish
between flow regimes.
Flow Type
Laminar
Critical
Transition
Turbulent
NRe, smooth pipes
< 2000
2000 3000
3000 -4000
> 4000
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Reynolds Number
For all practical purposes, the Reynolds number for
natural gas flow problems may be expressed as
(11.8)
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Relative Roughness
From a microscopic sense, wall roughness is not uniform,
and thus the distance from the peaks to valleys on the wall
surface will vary greatly.
This is measured in terms of absolute roughness, E
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Relative Roughness
eD, is defined as the ratio of the absolute roughness to the
pipe internal diameter:
(11.9)
and D have the same unit.
If roughness not known, take E =0.0006
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Absolute Roughness
Type of Pipe
Aluminiun pipe
Plastic-lined pipe
Commercial steel or wrought iron
Asphalted cast iron
Galvanized iron
Cast iron
Cement-lined
Riveted steel
.
(in.)
0.0002
0.0002- 0.0003
0.0018
0.0048
0.006
0.0102
0.012-0.12
0.036-0.36
Commonly used well tubing and line pipe
New pipe
0.0005-0.0007
12-months old
0.00150
24-months old
0.00175
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3.4 Equation for Friction Factor
Figure is a Moody friction factor chart log-log graph of
(log f) versus (log NRe).
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Laminar Single-Phase Flow
Friction factor for laminar flow can be determined
analytically.
(11.11)
(11.12)
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Turbulent Single-Phase Flow
Out of a number of empirical correlations for friction factors
are available, only the most accurate ones are presented.
For smooth wall pipes in the turbulent flow region.
(11.13)
Valid over a wide range of Reynolds numbers
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Turbulent Single-Phase Flow
For rough pipes fully developed turbulent flow :
Nikuradses Correlation
(11.14)
Note: Velocity profile and pressure gradient are very sensitive to pipe
roughness.
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Turbulent Single-Phase Flow
Colebrook equation
(11.15)
Applicable to smooth pipes and transition and fully turbulent flow.
Eqn is not explicit in friction factor f. Use Newton-Raphson Iteration.
Jain equation
(11.16)
Jain presented an explicit correlation for friction factor.
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Pipeline Equations
Weymouth equation
Panhandle equation
Modified Panhandle equation
Clinedist equation
Weymouth equation is preferred for smaller-diameter lines
(D < 15 in).
Panhandle equation and the Modified Panhandle equation
are better for larger-sized lines.
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Weymouth Equation for Horizontal Flow
Basic pipeline flow equation for steady state horizontal flow
where unit of gas flow rate is in scfh(standard cubic feet/hour)
is:
(11.22)
(11.24)
where qh = scf/hr
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Weymouth Equation for Horizontal Flow
Variables in horizontal pipeline flow equation are;
L
D
P1
P2
z
Tb
Pb
=
=
=
=
=
=
=
length of pipe (mile)
Diameter of pipe(in.)
upstream pressure(psia)
downstream pressure(psia)
compressibility factor
base temperature(R)
base pressure (R)
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Weymouth Equation for Horizontal Flow
When applying the above Eqn (11.22), trial and error
calculation procedure is needed.
To eliminate trial and error calculation, Weymouth proposed
that f varies as a function of diameter in inches as follows:
(11.25)
With this simplification, Eqn (11.22) reduces to
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Weymouth Equation for Horizontal Flow
Eqn (11.22) Basic equation, needs trail & error
With this simplification, Eqn(11.22 reduces to
(11.26)
Weymouth equation
where qh = scf/hr
D = pipe internal diameter, in
L = Length of pipe, mile
This form of the Weymouth equation commonly used in the natural gas industry.
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Weymouth Equation for Horizontal Flow
Assumptions for use of the Weymouth equation including
no mechanical work,
steady flow,
isothermal flow,
Constant compressibility factor,
horizontal flow,
and no kinetic energy
change.
These assumptions can affect accuracy of calculation results.
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Example (1 )
For the following data given for a horizontal pipeline, predict gas
flow rate in cubic ft/hr through the pipeline.
Solution
The problem can be solved using (a)Equation (11.22) with the
trial-and-error method for friction factor, and (b) Weymouth
equation without the Reynolds number-dependent friction
factor(Eqn 11.26).
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Example (1 )
The average pressure is:
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Relative roughness:
A. Trial-and-Error Calculation:
First Trial :
(11.24)
By applying Jain Equation,
(11.16)
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(11.16)
By applying Eqn(11.22)
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(11.22)
Second Trial :
(11.24)
(11.16)
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(11.22)
Third Trial :
(11.24)
(11.16)
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which is close to the previous assumed 1,186,759 cfh
B. Using the Weymouth equation:
(11.26)
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Quiz (3)
For the following data given for a horizontal pipeline, predict
gas flow rate in ft3/hr through the pipeline by applying
example(1) with trial and error method for friction factor
calculation , and (2) Weymouth Equation(11.26).
Diameter of pipeline
= 16 in
Length
= 190 miles
Average temperature
= 80 deg F
Specific gravity of gas
=
0.63
Upstream pressure
= 1050-psia
Downstream pressure
= 430-psia
Absolute roughness of pipe= 0.0006-in
Standard temperature
= 60 deg F
Standard pressure
= 14.7 psia
Average z factor
=
0.8533
Viscosity of gas
=
0.0097
Tolerance limit
=
1500
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Panhandle A Equation-Horizontal Flow
Panhandle A equation assumes the following Reynolds
number dependent friction factor:
(11.37)
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Panhandle A Equation-Horizontal Flow
Then pipeline flow equation is:
(11.38)
where q is the gas flow rate in cfd measured at Tb and pb, and
other terms are the same as in the Weymouth equation.
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Panhandle B Equation-Horizontal Flow
(Modified Panhandle)
Panhandle B equation is most widely used one for long
transmission and delivery lines, it assumes that f varies as
(11.39)
Then it takes the form,
(11.40)
q = gas flow rate (cfd)
Units are same as in Panhandle A eqn:
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Clinedinst Equation-Horizontal Flow
Considers the deviation of natural gas from ideal gas through
integration. It takes the following form:
(11.41)
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Empirical Pipeline Equation
A general non-iterative pipeline flow equation is written as
(11.42)
q in cfd
The values of the constants are given in Table for the different
pipeline flow equations.
Table Constants for Empirical Pipeline Flow Equations
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Pipeline Efficiency
E in the equation denotes Pipeline Efficiency Factor
Pipeline flow equations are developed for 100% efficient
condition
In real case, water, condensate, scale etc in the line
E represents the actual flow rate as a fraction of theoretical
flow rate
E ~ 0.85 0.95 represent a clean line
Some Typical Values for E is shown in the Table
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Series, Parallel, and Looped Pipelines
Wym Eqn
Pipelines in Series
Adding pressure drops for the three segments pipeline in series
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Series, Parallel, and Looped Pipelines
Pipelines in Series
Consider a three-segment pipeline in a series of total length L
depicted in Figure
(1)
(11.43)
(2)
(11.44)
(3)
(11.45)
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Series, Parallel, and Looped Pipelines
Adding Eqns: (1), (2) and (3) gives
(4) (11.46)
OR
(5)(11.47)
Capacity of a single-diameter (D1) pipeline for the same
pressure drop is expressed as:
(6) (11.48)
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Series, Parallel, and Looped Pipelines
Dividing Equation (5) (11.47) by Equation (6) (11.48) yields:
(11.49)
Figure (a)
Sketch of series pipeline
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Series, Parallel, and Looped Pipelines
Figure (b) Sketch of parallel pipeline
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Series, Parallel, and Looped Pipelines
Pipelines in Parallel
Weymouth Equation for any 1 segment
( 11.50)
Figure (b), Applying the Weymouth equation to each of the
three segments gives:
( 11.53)
Dividing Eqn. (11.53) by (11.50)
( 11.54)
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Series, Parallel, and Looped Pipelines
Looped Pipelines
Consider a three-segment looped pipeline depicted in Figure (c ).
(11.59)
Figure ( c) Sketch of looped pipeline
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Series, Parallel, and Looped Pipelines
Capacity of a single diameter(D3) pipeline is expressed as
(11.60)
Dividing Eqn. (11.59) by (11.60)
(11.61)
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Example (11.2 )
Consider a 4-in pipeline that is 10 miles long. Assuming that the
compression and delivery pressures will maintain unchanged,
calculate gas capacity increases by using the following measures
of improvement: (a) Replace three miles of the 4-in pipeline by a
6-in pipeline segment; (b) Place a 6-in parallel pipeline to share
gas transmission; and (c) Loop three miles of the 4-in pipeline
with a 6-in pipeline segment.
Solution
(a) This problem can be solved with Equation (11.49)
L = 10 mi
L1 = 7 mi
L2 = 3 mi
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D1 = 4 in
D2 = 6 in
= 1.1668, or 16.68% increase in flow capacity
(b) This problem can be solved with Equation (11.54)
D1 = 4 in
D2 = 6 in
= 3.9483, or 294.83% increase in flow capacity
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(c) This problem can be solved with Equation (11.61)
L = 10 mi
L1 = 3 mi
L3 = 7 mi
D1 = 4 in
D2 = 6 in
= 1.1791, or 17.91%
increase in flow capacity
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QUIZZ(1)
QUIZ(4)
1. Your customer from Thailand Company PTTEP is
currently buying 700 MMSCFD of gas from you. The
company is mentioning that they want more gas to buy
1200 MMSCFD by next year. The length of pipe line is
500 miles from your gas field. It is impossible to install a
new larger pipe line within one year. What is your
opinion for solving this issue?. The important point is to
meet their requirement gas volume.
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QUIZZ(1)
ASSIGNMENT 2
1. Explain, in your words, the natural gas prices(up to 2013)
and its scope in the oil and gas industry of Malaysia, and
compare the results with other companies around the
world, with references.
2. What is the difference between Natural Gas and LNG?
Explain the scope of LNG in Malaysia
To be submitted individually not later than 28 Feb 2013
5:00pm.
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Thank You
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Q&A
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