gas gathering and transportation

8/10/2019 Gas Gathering and Transportation

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GAS FIELD ENGINEERING

Gas Gathering and Transportation

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CONTENTS

3.1 Introduction

3.2 Pipeline Design

3.3 Reynolds Number

3.4 Relative Roughness

3.5 Friction Factors3.6 Pipeline Equations (Weymouth, Panhandle, Modified

Panhandle, Clinedist )

3.7 Series, Parallel, and Looped Lines

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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 threedistinct pipeline units: gathering system, main trunk linetransportation system, and distribution system.

Focuses on design and operation of natural gas pipelines inonshore and offshore gas fields.

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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.

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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

diameterand 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

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

f =f (NRe, eD)

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Equation that relates lost work per unit length of pipe and

the flow variables is

3.3 Friction Factor

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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 dimensionlessgroup defined as

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Reynolds Number

Reynolds numberis used as a parameterto distinguish

between flow regimes.

Flow Type NRe, smooth pipes

Laminar

Critical

TransitionTurbulent

< 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

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(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,

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Relative Roughness

eD, is defined as the ratio of the absolute roughness to the

pipe internal diameter:

and Dhave the same unit.

If roughness not known, take =0.0006

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(11.9)

Absolute RoughnessTy pe o f P ip e (in.)

Aluminiun pipe 0.0002

Plastic-lined pipe 0.0002- 0.0003

Commercial steel or wrought iron 0.0018

Asphalted cast iron 0.0048

Galvanized iron 0.006

Cast iron 0.0102

Cement-lined 0.012-0.12

Riveted steel 0.036-0.36

Commonly used well tubing and line pipe

New pipe 0.0005-0.0007

12-months old 0.0015024-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

(logf) versus (log NRe).

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Laminar Single-Phase Flow

Friction factor for laminar flow can be determined

analytically.

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(11.11)

(11.12)

Turbulent Single-Phase Flow

Out of a number of empirical correlations for friction factors

are available, only the most accurate ones are presented.

Forsmooth wall pipes in the turbulent flow region.

Valid over a wide range of Reynolds numbers

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(11.13)


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Forrough pipes fully developed turbulent flow :

Nikuradses Correlation

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(11.14)

Note: Velocity profile and pressure gradient are very sensitive to pipe

roughness.


Colebrook equation

Jain equation

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Jain presented an explicit correlation for friction factor.

(11.15)

(11.16)

Applicable to smooth pipes and transition and fully turbulent flow.

Eqn is not explicit in friction factor f. Use Newton-Raphson I terat ion.

Pipeline Equations

Weymouth equation

Panhandle equation

Modified Panhandle equation

Clinedist equation

Weymouth equation is preferred forsmaller-diameterlines

(D < 15 in).

Panhandle equation and the Modified Panhandle equationare betterforlarger-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:

where qh = scf/hr

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(11.22)

(11.24)


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Variables in horizontal pipeline flow equation are;

L = length of pipe (mile)

D = Diameter of pipe(in.)

P1 = upstream pressure(psia)

P2 = downstream pressure(psia)z = compressibility factor

Tb = base temperature(R)

Pb = base pressure (R)


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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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where qh= scf/hr

This form of the Weymouth equation commonly used in the natural gas industry.

D = pipe internal diameter, in

L = Length of pipe, mile

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(11.26)

With this simplification, Eqn(11.22 reduces to

Eqn (11.22) Basic equation, needs trail & error

Weymouth equation


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/hrthrough the pipeline.

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).

Solution

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Example (1 )

The average pressure is:

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Relative roughness:

A. Trial-and-Error Calculation:

First Trial :

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(11.24)

By applying Jain Equation,

(11.16)

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(11.16)

By applying Eqn(11.22)


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Second Trial :

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(11.22)

(11.24)

(11.16)

Third Trial :

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(11.22)

(11.24)

(11.16)

which is close to the previous assumed 1,186,759 cfh

B. Using the Weymouth equation:

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(11.26)


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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 g ravity o f gas = 0.63Upstream pressure = 1 050-psia

Downst reampressure = 430-ps ia

Absolute roughness of pipe= 0.0006-in

Standard t emperature = 60 d eg F

Standard pressure = 14.7 psia

Average z factor = 0.8533

Viscosity of gas = 0.0097

Tolerance limit = 1500

Quiz (3)

Panhandle A Equation-Horizontal Flow

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(11.37)

Panhandle A equation assumes the following Reynolds

number dependent friction factor:

Panhandle A Equation-Horizontal Flow

Then pipeline flow equation is:

where qis the gas flow rate in cfd measured at Tband pb, and

other terms are the same as in the Weymouth equation.

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(11.38)


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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

q = gas flow rate (cfd)Units are same as in Panhandle A eqn:

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(11.39)

(11.40)

Then it takes the form,

Clinedinst Equation-Horizontal Flow

Considers the deviation of natural gas from ideal gas through

integration. It takes the following form:

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(11.41)

Empirical Pipeline Equation

A general non-iterative pipeline flow equation is written as

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 39

(11.42)


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Pipeline Efficiency

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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

Series, Parallel, and Looped Pipelines

Pipelines in Series

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Adding pressure drops for the three segments pipeline in series

Wym Eqn


Pipelines in Series

Consider a three-segment pipeline in a series of total length L

depicted in Figure

(1)

(2)

(3)

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(11.43)

(11.44)

(11.45)


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Adding Eqns: (1), (2) and (3) gives

(4)

OR

(5)

Capacity of a single-diameter (D1) pipeline for the same

pressure drop is expressed as:

(6)

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(11.46)

(11.47)

(11.48)


Dividing Equation (5) (11.47) by Equation (6) (11.48) yields:

Figure (a)

(11.49)

Sketch of series pipeline

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Figure (b) Sketch of parallel pipeline

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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 a6-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 buy1200 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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gas gathering and transportation

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