White paper: Wet Gas Diagnostic with Intelligent Differential Pressure Transmitter
January 2008 – Page 1
Wet Gas Diagnostic with Intelligent Differential
Pressure Transmitter
Dave Wehrs - Program Manager, Pressure Diagnostics
Andrew Klosinski - Application Engineer, Pressure Diagnostics
Emerson Process Management
Introduction
Wet gas is a commonly occurring condition in
the production of natural gas. Wet gas is usually
encountered at the wellhead and after
separators with carryover. Wet gas implies that
a small amount of liquid, usually water, oil and/or
gas condensates, with liquid amounts up to 10%
by volume. Amounts higher than this can be
encountered but are usually regarded as
multiphase flow. With increased interest in well
allocation, flow measurements at the wellhead
or post separator are important and becoming
more prevalent. The wet gas may be metered at
the wellhead prior to commingling of multiple
gas streams prior to the separator, utilizing
common differential producers such as venturis,
orifice plates and V-Cones. Differential
producers will “over-read” the true gas flow rate
due to the presence of the liquid in the fluid
stream. The amount of overreading is directly
correlated to the amount of liquid. The most
common of these liquid content correlations is
the Lockhart-Martinelli (L-M) parameter, which
relates the mass flow of the liquid to the mass
flow of the gas. From the L-M parameter the
amount the meter is overreading can be
determined and subsequently corrected. The
relationship between the L-M parameter and
overreading for a typical venturi, compact orifice
plate and V-Cone is given in Figures One
through Three.
The L-M parameter is usually determined by
running the fluid stream into a test separator.
Once determined, the overreading correction
value is assumed to remain constant until the
well is tested again. This approach is
satisfactory until a significant change in liquid
loading occurs. At this point, the correction
value may no longer be valid and a significant
measurement error incurred. Users would like a
means to measure the actual liquid loading and
would value a means to detect significant
changes in the liquid loading that affect the
overreading correction value.
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Background
Over the past few years, several groups have
investigated the relationship between the noise
signals present in the Differential Pressure (DP)
signal and the amount of liquid in the wet gas
flow. In each case, the investigators’ goal was
to create a wet gas measurement. Nonstandard, high speed differential pressure
transmitters and high speed data acquisition
equipment were used, with data analysis
completed on a PC or similar device.
At about the same time, Emerson Process
Management developed a new generation of
Differential Pressure transmitters with a unique,
scalable architecture. This new pressure
transmitter platform, called the Rosemount
3051S, provides best in class performance and
reliability and also features advancements in
power management that allows the easy
addition of advanced functionality to the base
pressure transmitter. This functionality,
embodied in a feature board, can be ordered as
part of the transmitter or simply added to an
existing transmitter already installed in the field.
Making use of this advanced functionality,
Emerson has developed a unique patented
technology that provides a means for early
detection of abnormal situations in a process
environment. The technology, called Statistical
Process Monitoring (SPM), is based on the
premise that virtually all dynamic processes
have a unique noise or variation signature under
normal operation. Changes in these signatures
may signal that a significant change in the
process, process equipment, or transmitter
installation will occur or has occurred. For
example, the noise source may be equipment in
the process such as pumps, agitators or the
natural variation in the DP value caused by
turbulent flow or any combination thereof.
®
White paper: Wet Gas Diagnostic with Intelligent Differential Pressure Transmitter
January 2008 – Page 2
fieldbus outputs, and were configured to output
pressure data at the maximum rate of 22
updates per second. The data were recorded
using standard PC FOUNDATION fieldbus tools
and analyzed post test using standard Excel
spreadsheets.
The sensing of the unique signature begins with
a high speed sensing device such as the
Rosemount 3051S Pressure Transmitter
equipped with patented software resident in a
HART® Diagnostics or FOUNDATION™ fieldbus
Feature Board. This powerful combination has
the ability to compute statistical parameters that
characterize and quantify the noise or variation
and represent the mean and standard deviation
of the input pressure. Filtering capability is
provided to separate slow changes in the
process due to intentional setpoint changes from
inherent process noise which contains the
variation of interest. The transmitter provides
the statistical parameters to the host system via
HART or FOUNDATION fieldbus communications
as non-primary variables. The transmitter also
has internal software that can be used to
baseline the process noise or signature via a
defined learning process. Once the learning
process is completed, the device itself can
detect changes in process noise and will
communicate an alarm via the 4 – 20 mA output
or alert via HART or FOUNDATION fieldbus.
The V-Cone was tested at four flow rates (750,
663, 500 and 250 m³/hr) at 15 bar and Liquid
Volume Fractions (LVF) from 0 to 4.5%. The
Venturi was tested only at 60 bar, three flow
rates (427, 285 and 142 m³/hr) and identical
LVF’s. The 405C was also tested at 60 bar, four
flow rates (200, 175, 150 and 100 m³/hr), and
LVF’s from 0 to 8.1%. All tests were performed
over a Lockhart-Martinelli range of 0 to
approximately 0.3, the range generally used to
define wet gas.
Each test run lasted at least 90 seconds and at
least two data runs were taken at each test
condition. After each test run, the DP mean and
standard deviation were calculated for each data
set. In addition, the average gas and liquid flow
were determined along with line pressure and
temperature.
With the design of this transmitter and feature
board complete, Emerson initiated a test
program to determine if this technology could be
applied to the detection of significant changes to
the liquid loading of wet gas.
Test Results and Analysis
The test data for each differential producer were
overlaid on a flow map to illustrate the location
of their predicted flow regimes. For this
discussion the Shell Flow Pattern Map, a
commonly used representation of two-phase
horizontal flow, was used. Shell created this
map using experimental data by observing the
flow conditions at varying gas and liquid Froude
values.
Test Program Description
Two traditional wet gas differential producers
and a new, increasingly popular differential
producer, the Rosemount 405C Compact
Orifice, were tested in August/September 2005
at TÜV NEL’s wet gas test facility in East
Kilbride, Scotland, UK. The traditional
differential producers consisted of a 0.75 beta
100mm (4”) McCrometer V-Cone and a 0.75
beta 100mm (4”) Seiko venturi. The 405C was a
0.4 beta 100mm (4”) device. The DP signal was
measured by a 250” Rosemount 3051S
Differential Pressure Transmitter (0.75 beta
units) or a 1000” Rosemount 3051S (0.4 beta
unit). Both transmitters had FOUNDATION
Frg =
U sg
SuperficialGasVelocity
=
LiquidGravityForce
gD
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®
The gas and liquid Froude numbers are
dimensionless numbers calculated by the
square root ratio of fluid velocities (whether gas
or liquid) as if the single phase completely filled
the pipe to the gravity force on the other fluid.
The ratio of the liquid Froude number to the gas
Froude number equates to the LockhartMartinelli number. The equations are as follows:
ρg
ρl − ρ g
=
mg
A gD
1
ρ g (ρl − ρ g )
(1)
White paper: Wet Gas Diagnostic with Intelligent Differential Pressure Transmitter
January 2008 – Page 3
Frl =
U
SuperficialLiquidVelocity
= sl
LiquidGravityForce
gD
ρl − ρ g
Frl
= Lockhart − Martinelli
Frg
Where g is the gravitational constant, A is the
internal pipe cross-sectional area, D is the pipe
inner diameter, U represents the superficial
velocity for gas or liquid and ρ is the fluid density
for either gas or liquid.
As the Flow Pattern Map was created based on
experimental observations, the boundaries
illustrated serve only as predictions of the
indicated flow regime and carry with them a
degree of uncertainty. Conditions such as
pressure, temperature, flow rates, fluid
properties, pipe diameter and primary element
1
geometry all contribute to this uncertainty .
In Figure 4, as an example, the 0.75 beta Seiko
venturi test data at 60 bar are shown. The data
are represented in lines of constant LVF by
varying superficial gas and liquid velocities. As
seen, the bulk of the data lies in the AnnularMist flow regime with a small portion of the low
flow rate test points falling into the Stratified
regime. The relative proximity of the process
data to flow boundaries is significant in
understanding the performance of the SPM
diagnostic and will be discussed further.
For analysis purposes a very useful parameter is
the ratio of the DP standard deviation (σ) to the
DP mean (X). In many DP flow applications, this
ratio has been found to remain relatively
constant as the flow rate increases. This
behavior is true for both gas and liquid flows.
This ratio (σ/X) versus the mean DP was plotted
on an X/Y chart for each primary element for a
given line pressure.
As an example to discuss the test results,
reference Figure 5. The data points for a given
gas flow rate at various LVF’s are indicated by
the same style symbol and color. A trendline for
each gas flow rate is also provided. The black
arcs represent trendlines for constant LVF
values at different flow rates. In this figure, the
relationships between the σ/X ratio and the
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ρl
=
ml
A gD
1
ρl (ρl − ρ g )
®
(2)
(3)
mean DP value for the 0.75 beta Seiko venturi at
60 bar are shown. As expected, per the DP flow
square root relationship, the DP increases with
higher gas flow rates at fixed LVF’s as the σ/X
ratio remains relatively constant. However, for a
given increase in LVF percent, the σ/X ratio
responds at a significantly higher rate. For this
primary element at 60 bar, the slope (sensitivity)
of σ/X ratio to increasing LVF is the highest at
the lowest gas flow rate (140 m³/hr). With
increasing flow rates the σ/X ratio maintains its
correlation to LVF but experiences a slight
decrease in sensitivity to LVF changes.
Referencing Figure 4, the Flow Pattern Map for
the venturi, data contained in the Stratified
region demonstrate far less sensitivity to
changes in LVF relative to data in the AnnularMist regime.
The results for the 0.4 beta 405C compact
orifice plate at 60 bar are shown in Figure 7.
While less sensitive than the venturi in terms of
percent, the σ/X ratio clearly displays the same
relationship to increases in gas flow rate and
LVF. This is particularly evident with LVF’s
above 2.9%. The geometry of this primary
element differs greatly from the venturi and may
allow it a degree of sensitivity to wet gas
detection even when operating around the
Stratified regime boundary.
Figure 9 shows the results for the 0.75 beta VCone at 15 bar. Excellent sensitivity of the σ/X
ratio is indicated at the three gas flow rates
tested. At the low flow rate, LVF conditions
above 1.47% result in significant increases to
the σ/X ratio with this behavior once again
attributed to the flow conditions straddling the
Stratified boundary.
White paper: Wet Gas Diagnostic with Intelligent Differential Pressure Transmitter
January 2008 – Page 4
Conclusion
The test results for the three different differential
producers indicate that the value of the ratio of
DP signal standard deviation to the mean DP
can provide a means to detect changing Liquid
Volume Fractions in a wet gas flow stream. This
ratio remains relatively constant with gas flow
rate changes and fixed LVF, but increases
significantly with increasing LVF and constant
gas flow rates. Factors such as primary element
geometry and flow regime have been shown to
influence the ability of the transmitter to
distinguish LVF’s primarily at low flow rates.
The sensitivity of this relationship is strongest at
lower gas flow rates and decreases at higher
gas flow rates.
®
Today, the Rosemount 3051S
Pressure Transmitter can output the
required SPM parameters via HART or
FOUNDATION fieldbus digital communication
protocols needed to determine these factors
described in this document. Having these
parameters can readily provide users with
additional insight into their process conditions.
With this capability, gas field operators can have
an indication of significant changes in LVF that
affect the overreading correction values for
wellhead flow measurements or separator
operation. In the former case, gas wells may
only need to be tested when a change is
detected by the transmitter rather than on a fixed
periodic basis avoiding the costly efforts
associated with retesting the well.
References
1
“Wet Gas Flow Metering With Gas Meter Technologies” by Steven, R., CEESI
Figures
Venturi / .75β
Wet Gas Characteristic
Over-reading (sqrt(DPtp/DPgas)
1.50
1.40
1.30
1.20
60bar / 143m^3/hr / 4.8m/s
1.10
60bar / 285m^3/hr / 9.6m/s
60bar / 425m^3/hr / 14.4m/s
1.00
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Lockhart-Martinelli Parameter
Figure 1: Relationship between Lockhart-Martinelli parameter and overreading for a typical Venturi
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White paper: Wet Gas Diagnostic with Intelligent Differential Pressure Transmitter
January 2008 – Page 5
®
Rosemount Model 405C / .40β Conditioning Orifice
Wet Gas Characteristic
Over-reading (sqrt(DPtp/DPgas)
1.50
1.40
1.30
1.20
60 bar / 100m^3hr / 3.4m/s
1.10
60 bar / 125m^3hr / 5.1m/s
60 bar / 150m^3hr / 5.9m/s
60 bar / 200m^3hr / 6.8m/s
1.00
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Lockhart-Martinelli Parameter
Figure 2: Relationship between Lockhart-Martinelli parameter and overreading for a
Compact Orifice Plate
V-Cone / .65β
Wet Gas Characteristic
Over-reading (sqrt(DPtp/DPgas)
1.50
1.40
1.30
1.20
15bar / 300m^3/hr / 10.1m/s
15bar / 100m^3/hr / 3.4m/s
15bar / 500m^3/hr / 16.9m/s
1.10
60bar / 100m^3/hr / 3.4m/s
60bar / 250m^3/hr / 8.4m/s
1.00
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Lockhart-Martinelli Parameter
Figure 3: Relationship between Lockhart-Martinelli parameter and overreading for a typical V-Cone
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White paper: Wet Gas Diagnostic with Intelligent Differential Pressure Transmitter
January 2008 – Page 6
®
NEL Wet Gas - Shell Flow Pattern Map
0.75B Venturi @ 60 Bar
10
Liquid Froude Number
SLUG
1
LVF 8.20%
LVF 6.90%
LVF 5.60%
LVF 4.30%
LVF 2.90%
STRATIFIED
0.1
LVF 1.47%
LVF 0.77%
XLM=0.1
XLM=1
0.01
0.01
ANNULARMIST
XLM=0.01
0.1
1
10
Gas Froude Number
60 bar, 140 m3/hr
60 bar, 285 m3/hr
60 bar, 425 m3/hr
Figure 4: Shell Flow Pattern Map for 0.75 beta Seiko Venturi at 60 bar
0.75 Beta Seiko Venturi, 60 bar
Differential Signal Variability
2.50%
LVF = 6.90%
2.00%
142.5 m3/h
LVF = 5.60%
285 m3/h
LVF = 2.90%
Constant Flowrate,
Increasing LVF
Standard Deviation of DP / Mean DP (%)
LVF = 8.20%
1.50%
LVF = 4.30%
426.5 m3/h
142.5m3/hr
285m3/hr
426.5m3/hr
LVF = 0
LVF = 1.47%
LVF = 0.77%
1.00%
LVF = 1.47%
LVF = 2.90%
LVF = 4.3%
LVF = 0.77%
LVF = 5.6%
LVF = 6.9%
0.50%
LVF = 8.2%
LVF = 0%
Increasing Flowrate,
Constant LVF
0.00%
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
Differential Pressure (in WC)
Figure 5: DP signal variability for 0.75 beta Seiko Venturi at 60 bar
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White paper: Wet Gas Diagnostic with Intelligent Differential Pressure Transmitter
January 2008 – Page 7
®
NEL Wet Gas - Shell Flow Pattern Map
0.4B 405C @ 60 Bar
10
Liquid Froude Number
SLUG
SLUG
1
LVF 3.73%
LVF 8.10%
LVF 6.94%
LVF 5.63%
LVF 4.20%
STRATIFIED
LVF 2.90%
0.1
LVF 1.47%
LVF 0.74%
XLM=1
XLM=0.1
0.01
0.01
XLM=0.01
0.1
ANNULARMIST
1
10
Gas Froude Number
60 bar, 100 m3/hr
60 bar, 150 m3/hr
60 bar, 175 m3/hr
60 bar, 200 m3/hr
Figure 6: Shell Flow Pattern Map for 0.4 beta Rosemount 405C compact orifice plate at 60 bar
0.4 Beta Rosemount 405C, 60 Bar
Differential Pressure Variability
1.75%
Standard Deviation of DP/ Mean DP (%)
LVF = 8.10%
100m3/h
150m3/h
175m3/h
200m3/h
100m3/hr
150m3/hr
175m3/hr
200m3/hr
LVF = 0%
LVF = 0.74%
LVF = 1.47%
LVF = 2.90%
LVF = 4.20%
LVF = 5.63%
LVF = 6.94%
LVF = 8.10%
LVF = 6.94%
1.50%
LVF = 5.63%
1.25%
LVF = 4.20%
LVF = 0.74%
LVF = 2.90%
LVF = 0%
LVF = 1.47%
1.00%
0.75%
0
200
400
600
800
1000
1200
Differential Pressure (inWC)
Figure 7: DP signal variability for 0.4 beta Rosemount 405C compact orifice plate at 60 bar
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White paper: Wet Gas Diagnostic with Intelligent Differential Pressure Transmitter
January 2008 – Page 8
®
NEL Wet Gas - Shell Flow Pattern Map
0.75B V-Cone @ 15 Bar
Liquid Froude Number (Frl)
10
SLUG
LVF 3.75%
1
LVF 4.46%
LVF 3.02%
LVF 2.28%
LVF 1.53%
STRATIFIED
0.1
LVF 0.77%
LVF 0.39%
XLM=0.1
XLM=1
0.01
0.01
XLM=0.01
0.1
ANNULARMIST
1
10
Gas Froude Number (Frg)
15 bar, 250 m3/hr
15 bar, 500 m3/hr
15 bar, 660 m3/hr
15 bar, 750 m3/hr
Figure 8: Shell Flow Pattern Map for 0.75 beta McCrometer V-Cone at 15 bar
0.75 Beta V-Cone, 15 bar
Differential Pressure Signal Variability
4.00%
LVF = 4.46%
LVF = 3.75%
Standard Deviation of DP / Mean DP
3.50%
250 m3/hr
500m3/hr
663m3/hr
750m3/hr
250m3/hr
500m3/hr
663m3/hr
750m3/hr
LVF = 0%
LVF = 0.39%
LVF = 0.77%
LVF = 1.53%
LVF = 2.28%
LVF = 3.02%
LVF = 3.75%
LVF = 4.46%
LVF = 3.02%
3.00%
LVF = 2.28%
2.50%
LVF = 1.53%
2.00%
LVF = 0.77%
1.50%
LVF = 0%
LVF = 0.39%
1.00%
0.50%
0.0
50.0
100.0
150.0
200.0
250.0
Differential Pressure (inWC)
Figure 9: DP signal variability for 0.75 beta McCrometer V-Cone at 15 bar
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White paper: Wet Gas Diagnostic with Intelligent Differential Pressure Transmitter
January 2008 – Page 9
®
NEL Wet Gas - Shell Flow Pattern Map
0.75B V-Cone @ 60 Bar
Liquid Froude Number (Frl)
10
SLUG
1
LVF 8.20%
LVF 6.94%
LVF 5.63%
LVF 4.28%
LVF 2.90%
STRATIFIED
0.1
LVF 1.47%
LVF 0.77%
XLM=0.1
XLM=1
0.01
0.01
XLM=0.01
0.1
ANNULARMIST
1
10
Gas Froude Number (Frg)
60 bar, 125 m3/hr
60 bar, 250 m3/hr
60 bar, 375 m3/hr
Figure 10: Shell Flow Pattern Map for 0.75 beta McCrometer V-Cone at 60 bar
0.75 Beta McCrometer V-Cone, 60 Bar
Differential Pressure Signal Variability
3.00%
126m3/hr
Standard Deviation of DP / Mean DP (%)
LVF = 8.2%
250m3/hr
375m3/hr
LVF = 6.94%
250m3/hr
2.50%
LVF = 5.63%
375m3/hr
126m3/hr
LVF = 4.28%
LVF = 0%
2.00%
LVF = 0.77%
LVF = 2.90%
LVF = 1.47%
LVF = 2.90%
LVF = 4.28%
LVF = 0 - 1.47%
1.50%
LVF = 5.63%
LVF = 6.94%
LVF = 8.20%
1.00%
0.0
50.0
100.0
150.0
200.0
250.0
Differential Pressure (inWC)
Figure 11: DP signal variability for 0.75 beta McCrometer V-Cone at 60 bar
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