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8/9/2019 Presentation ORC

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NATURAL GAS MEASURMENT (CONT’D)

The method of measuring liquid or gas by

volume is referred to as volumetricmeasurement.

Producers account for gas in units of mcf(1000 cu.ft)

The total mass of a substance in a cu ft of gasdepends partly on its absolute pressure (psia)and partly on temperature (oR).

The American Petroleum Institute (API) and

American Gas Association (AGA), since 1967,have used 14.73 psia and 60of (520oR) as theirstandard (base) conditions.


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ORIFICE METER

This is a type of differential pressure method of meteringgas

The orifice meters compound of two elements

• Primary Element

• Secondary Element


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PRIMARY ELEMENT

The primary element is the differential pressure-

producing device. It composed of the followingparts:

The meter tube – a length of special pipe through which the gas flows.

The orifice plate holding and positioning device-an orifice flange or an orifice filting installed as anintegral part of the meter tube to hold the orificeplate in a position perpendicular and concentric to

the flow of gas. The orifice plate- a flat circular plate with acentrally bored, sharp-edged orifice machined toan exact.


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Predetermined dimension that forms acalibrated restriction to the flow of gas throughthe meter tube and is the source of thedifferential.

Pressure taps – Precisely located holesthrough the pipe walls or orifice plate may bemeasured.

Straightening vanes – a device that may beinserted in the upstream section of the metertube swirling in the gas stream.


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SECONDARY ELEMENT

The secondary element is called the differential

gauge and is connected to the upstream anddownstream pressure taps of the primaryelement.

It measures the differential pressure and static

pressure which are recorded on a circular chart.


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BASIC ORIFICE METER EQUATION

From the general energy conservation equation

for any points in the meter tube

∫ ∫ ∫ −−=++2

1

2

1

2

1lwwdz

gc

g

gc

udvVdp

Where,

v = sepcific volume = cu ft/ibmp = pressure, ibf/sq ftu = average linear – flow velocity ft/secgc = conversion factor: 32.17 (ibm/ibf) (ft/sec

2)

g = acceleration due to gravity ft/sec2z = vertical distance above datum, ft

w = work done by flowing fluid, ft ibf/ibmLw = work energy lost due to frictional effect, ft-

ibf/ibm.


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BASIC ORIFICE METER EQUATION (CONT’D) From the assumption, dx = o, w = o

O gcduU Vdp =+∫ ∫

2

1

2

1

From the above equation, it can be shown that,

−

∆

=

z T

hp DC

P

T q

g b

b

γ β 41

59.2182

2

Therefore,

f w g phC q 1=


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GENERALIZED ORIFICE FLOW EQUITATION

The generalized orifice flow equation is given as:

f wh P hC q

1=

f wT h

Where,qg is the gas flow rate, scf/hr

h w is the differential pressure (in inches of water @ 60o

f)pf is the static pressure (in psia)

CI is the orifice flow constant

Is the pressure extension.

This constant is a function of several other orificeconstants given by:

CI = (F b) (Fr) (Y) (Fpb) (Ftb) (Fg) (Fpv) (Fm) (Ft) (Fa)


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GENERALIZED ORIFICE FLOW EQUATION (CONT’D)

Where

F b = Basic orifice factor

Fr = Reynolds number factor (viscosity)

Y = Expansion factor

Fpb = Pressure base factor (14.73) contracted pressure base

Ftb = Temperature bases factor (T b/520)


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Ftf= Flowing temperature factor

Fg= Specitic gravity facator,

Fpv = Super compressibility factor

Fm = Manometer factor for me-cury meter.

Ft= Guage location factor

Fa = Orifice thermal expansion factor

= 1 + [0.000,018,5 (of – 68)] 304 and 316 stainless steel

=1 + [0.000, 015, 9 (of – 68)] monel

etemperatur flowactual +400/520

g γ 1

z

Zb


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QUESTION ON METERING Given an orifice meter equipped with flange taps withstatic pressure from the down stream taps and the

following:D1= line size = 8.071 inches of actual 1D

D2 = orifice size = 1.000 inch

Flowing temperature = 65of

Ambient temperature = 70

o

fP b = contract pressure base = 14.65 psia

Temperature base = 50of = 510.oRγg = Specific gravity = 0.570

H w

= total heating value = 999.1Btu cu.ft

Xn = mole fraction nitrogen content = 0.011

Xc = mole fraction of carbon (iv) oxide content = 0.000

H w average differential head = 50 inches water

Pf= average downstream gauge pressure = 370 psia Calculate the orifice flow constant and the uantit of


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0008.1

1239.01301.0/int

),(25.

0005.149000.01

64.138

0680.011

0680.0

1071.81,23.

38.200

1071.81,22.

64.13865.1437050

1239.0071.8

000.1

2

1

2

=

==

=+=

+=+=

=

−−

=

−−

=+=

===

γ

β

γ

β

and ph for erpolating by

pressure staticdownstream for Table !rom

ph

b !

b

line Dinanin plateina for Table !rom

!

line Dmanin plateina for table !rom

" ph

D

D

f w

f w

r

b

f w


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)()()()()()()()()()()(

tan

9993.0

)36.(int

,570.070,370

054.1),(35.

1,35.

3245.1

570.0,34.

9952.0

65,33.

9808.0

50,32.

0055.1

65.14,31.

1

at m pv g tf tb pbr b

m

g

o

f

pr

pv

g

tb

o

tb

o

pb

b

! ! ! ! ! ! ! ! # ! ! C

ist cons floworificetheThen

!

Tableerpolationby

and f etemperatur ambient psig P !or

! a Table !rom

! Table !rom

!

gravity specific for Table !rom

!

f etemperatur flowing for Table !rom

!

f baseetemperatur for Table !rom

f

P for Table !rom

=

=

===

=

==

==

==

=

==

γ


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hr $scf

hr ft

"

phC qh

hour one for flowof ratetheThrn "

C

f w

/05.37

/052,37

4.3845025.267

,25.267)9993.0()0254.1()3245.1()9952.0(

)9808.0)0055.1()0008.1()0005.1()38.200(

3

===

=

=

=


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CHART

Two principal types of chart are:

Uniform scale direct – reading chart for differential

pressure – line are spaced on equal distance

Square – root chart – reads the square root of the

percentage of the full scale range of the gauge.

DIRECT – READING CHART

Circular charts for recordity the differential andstatic pressure guages are usually 12 in. in

diameter

Scale ranges for fuel gas metering are given below.


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Common differential pressure ranges, in.

of water

Common static pressure ranges,

psig

0 to 10

0 to 20

0 to 50

0 to 100

0 to 200

0 to 100

0 to 250

0 to 500

0 to 1000

0 to 2500


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SQUARE – ROOT CHART For this type of chart, a chart factor (CF) may bedefined as:

100

$%C! =

Actual pressure

rangemeter "C%

C! "C%

2

2

10

)(

=

=

Where, MR = Meter Range CR = Chart Reading


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EXAMPLE ON CHART Given that for a 50in by 100 Ib gauge, the differential pressuregauge Rh = 50 in and static pressure; Rp = 100 psi. assume square

root chart readings; differential = 7.2 in and static = 9.4 psi.Calculate the actual differential and static pressure.

SOLUTION

psia "

% "reading chart

pf

pressure&tatic

water in "

% "reading chart

h

pressureal Differenti

p

hw

36.8810010

4.9

10

.92.255019

2.7

10

,

2

2

2

2

=

=

=

=

=

=


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VENTURI METER

The venturi tube operates on exactly the same

principle as the orifice meter. Discharge coefficients of venturi are larger than

those for orifices and vary from about 0.94 to

0.99. It gives a definite improvement in power losses

over an orifice and is often indicated for

measuring very large flow rates where power

losses can become economically significant.

The initial higher cost of a venturi over an orifice

may thus be offset by reduced operating costs.


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INSTRUMENT CALIBRATION Calibration entails the adjustment of a measurementdevice so that the value from the measurement device

agrees with the value from a standard. The international science organization (ISO) hasdeveloped a number of standards specially directed tocalibration of measurement devices. Within most companies, the responsibility for

calibration measurement device is delegated to a sspecific department. The frequency of calibration is normallypredetermined but earlier action may be dictated if

the values from the measurement device becomesuspected. Calibration of some measurement devices involvescomparing the measured value with the value fromthe working standard.


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