Post on 29-Nov-2014
Dew Point Measurements in Natural Gas
Andy BentonTechnical Consultant
53rd Technical SeminarABNT – CB-09
Comite Brasileiro de Gases CombustivesNatal – RN – Brazil15th April 2010
Running order:• What is hydrocarbon and water dew point?
• Why is it important to control and measure dew point in natural gas?
• How can HC dew point be measured?1. Manual visual cooled-mirror dewpointmeter,2. Determination of Liqiud to Gas Ratio (PHLC),3. Equation of state method from GC analysis,4. Direct on-line measurement with Automatic,
Optical Condensation Dewpointmeter.
• Water dew point measurement principle
H2O Dew Point
HC Dew Point
-40 -30 -20 -10 0 +100
1
2
3
4
5
6
7
8
Wholly gaseousHC liquids
present in
gas
Water and
HC liquids
preset in
gas
Temperature, °C
Pressure
MPa
Cricondentherm
Typical Phase Envelope for Northern Europe natural gas
WHAT IS HYDROCARBON DEW POINT?THEORECTICAL DEFINITION: ISO 14532:2005 (Natural gas – Vocabulary) defines hydrocarbon dew point as “The temperature above which no detectable condensation of hydrocarbons occurs at a specified pressure.”
WORKING DEFINITION: API Manual of Petroleum Measurement Standards Chapter 14.1 – Natural Gas Fluids Measurement: “The temperature at which hydrocarbon condensates first begin to form a visible deposit of droplets on a surface, when the gas is cooled at a constant pressure.”
Water & Hydrocarbon Dew Point Reduction Plant
Well Head
Liquid HC (crude) and
bulk water
Mechanical Separator
Glycol Contactor
10-15MPa
J.T. Valve
�P
= 3 to 5
MPa
Low-Temperature
Separator(LTS)
Condensate
(NGL)
Compressor
Transmission
Pipeline
7 to 10 MPa
Water dew point reduction
Hydrocarbon dew point reduction
After extraction from the gas field, natural gas is processed to remove water and heavy hydrocarbon components, before transmission through the pipeline network. This diagram shows a typical plant for the removal of both excess moisture and heavy hydrocarbons
Seabed Pipeline
Why is it Important to Control and Measure Hydrocarbon and Water Dew Point?
Commercial Issues
• Hydrocarbon and water dew point are gas quality parameters specified in the sales contracts agreed between buyer and seller.
• Typical transmission gas specifications (EASEE-gas):
– Hydrocarbon dew point lower than –2 oC at 27 barg
– Water dew point lower than –8 oC at full pipeline
pressure
1/10/20100.7000.555m3/m3d
1/10/2006- 2-oC at 1 – 70 bar (a)HC DP
- 8-oC at 70 bar (a)H2O DP
1/10/20062.5-mol %CO2
1/10/2010[0.01]*-mol %O2
1/10/20066-mg/m3RSH (as S)
1/10/20065-mg/m3H2S + COS (as S)
1/10/200630-mg/m3Total S
1/10/201015.81[13.60]kWh/m3WI
Recommended
implementation dateMaxMinUnitParameter
1/10/20100.7000.555m3/m3d
1/10/2006- 2-oC at 1 – 70 bar (a)HC DP
1/10/2006–Some exceptions- 8-oC at 70 bar (a)H2O DP
1/10/20062.5-mol %CO2
1/10/2010[0.01]*-mol %O2
1/10/20066-mg/m3RSH (as S)
1/10/20065-mg/m3H2S + COS (as S)
1/10/200630-mg/m3Total S
1/10/201015.81[13.60]kWh/m3WI
Recommended
implementation dateMaxMinUnitParameter
EASEE-gas approved gas quality parameters and values
• EASEE-gas: European Association for the Streamlining of Energy Exchange
• Objectives to harmonise business practices across the industry
• 80 cross border points across Europe
Technical Issues
• Measurement of hydrocarbon and water dew point is important in order to:
– Prevent hydrates and condensates – Solid crystalline and liquid formations that limit pipeline capacity and damage compressors and valves.
– Limit pipeline corrosion – Excess water leads to leaks and fractures.
– Achieve Superheat requirements – Fuel gas specifications for gas turbine power plant.
Hydrate formations
Compressor damage – Liquid impact!
HC Dew Point Applications
Hydrocarbon Dew Point Reduction Processing
Natural Gas Dew Point Applications
Gas quality measurementat custody transfer
HC Dew Point Applications:Trans- and Inter-Continental Supply Contracts –Pipeline Interconnections
• Market de-regulation
• Introduction of competitive practices in gas supply and retailing
European Interconnections
U.S. Natural Gas Pipeline Network, 2009
National Distribution of Natural Gas in Brazil
Underground storage facilities to supply transient demand
HC Dew Point Applications:Gas Trading
HC Dew Point Applications:Gas Turbine Electricity Generation
• Avoid liquids in hot gas path
• Uneven flame temperature and length
• Avoid flashbacks
• Turbine manufacturers stipulate 20 to 30°C of superheat for fuel gas to low NOx plant
• Dew-point measurement enables control of fuel gas heaters
• Potential return on investment through reduced parasitic load
How can Hydrocarbon Dew Point be Measured?
1. Manual Visual Dew Point Method –“Dew Scope”
2. Determination of Liquid to Gas Ratio (PHLC)
3. Equation of State Method Using GC Composition Analysis
4. Automatic Optical Condensation Method
Hc Dew Point Measurement Methods
G
a
sP
P
1. Manual Visual Cooled-Mirror Dew-Point Checker
• Bureau of Mines, ASTM/API method
Images courtesy Ametek®
PROS CONS
+ The traditional and widely
used measurement
technique
+ The de facto standard for
hydrocarbon dew point
measurement
+ Low capital investment -
Instrument costs $5,000 to
$10,000
– Periodic spot checking
only
– ‘Subjective’, operator
dependant measurement
of variable sensitive and
repeatability
– Labor intensive, therefore
high running costs
Manual Visual Cooled-Mirror Dew-Point Checker
Gas
Gas Outlet
Liquid Collection
Separator
Cooling Coil
Pressure and flow controls
P
PGas Inlet
Drain
Temperature
Controlled Bath
2. Determination of Liquid to Gas Ratio
PROS CONS
+ A direct measurement method
where contractual specifications
stipulate a liquid to gas ratio
(Potential Hydrocarbon Liquid
Content to ISO 6570)
+ An absolute expression of the
condensate formation potential
of the gas at a certain
temperature and pressure
(grams/cubic metre of gas)
– Impractical for field
application and installation
– High cost ($100,000 plus)
– Not measuring Hc Dew Point,
the parameter most
commonly used to control
this aspect of gas quality
– Averaging analysis technique
leads to slow response to
process changes
Determination of Liquid to Gas Ratio
• GCs provide Calorific Value (BTU)
• Custody transfer is based on BTU and thus there is a large installed base
• Field units are C6+ with a few C9+
• Equations of State (EOS) can give a calculated HC dew point BUT sensitivity of analysis governs accuracy of results
3. Equation of State Method Using GC Analysis
Here is an illustration of how sensitivity of analysis directly effect calculated HcDP estimations.
Why?
ISO 23874 (2006) ‘Natural Gas – GasChromatographic requirements for hydrocarbon dew point calculation’
• “be capable of measuring alkenes up to and including dodecane”,
•“be capable of measuring individual alkenes at a concentration of 0.000 000 1 (0.1 ppm)”
• “be capable of measuring individual alkenes at a concentration of 0.000 000 1 (0.1 ppm)”
•“measure all hydrocarbons in the range C5 to C12”
PROS
+ Potential to combine a number of gas quality/fiscal metering parameters into one analyzer
+ The components contributing to a high dew point level may be identified and so help to determine the reason/source.
+ Possibility to provide a theoretical phase envelope curve
CONS
- Accuracy is dependant on correct use of specialized reference gases
- Indirect method of determining HcDP
- The limit of analysis sensitivity can produce large HcDP errors
- Trained staff required to maintain proper performance
- High initial and operating costs plus the analyzer house, and personnel costs.
EOS Method Using GC Analysis
4. Automatic, Optical Condensation Dewpointmeter
• It is possible to automate the optical condensation measurement principle to give an on-line measurement of hydrocarbon dew point that is reproducible, free from operator bias and drift.
• In Michell’s case, this is achieved with Condumax II Hydrocarbon Dew-Point Analyser, that utilises a patented detection technique known as the Dark Spot Principle.
Cooled Mirror Principle of Operation
Water Dew or IceWater Dew or IceWater Dew or IceWater Dew or Ice
Sample gasSample gasSample gasSample gas
AUTOMATIC, OPTICAL CONDENSATION DEWPOINTMETER
Main difficulties in making such a measurement:
• Natural gas condensate forms an ‘invisible’ low surface tension film
• Fractional condensation
Where is the hydrocarbon dew point?
Liquid to Gas Ratio Curve
• Surface Tension –Water is very High
• Hydrocarbons are very low
HcDP is a different image than a water dew point to a Chilled/Cooled Mirror
• Water dew point sensors are mirrors
• HcDP sensors should be rough
• When water condenses on the mirror by cooling it, the reflected light is diffused.
• HcDP actually forms a reflective surface
So optical detection surfaces are used?
Shell Research
Detection Principle Development Sequence
Dry
Light
Sou
rce
COOLED MIRROR
Shell Research
COOLED MIRROR
H2O Wetted
Light
Sou
rce Detector
� Good sensitivity for water dewpoint
Detection Principle Development Sequence
Shell Research
HC Wetted
Light
Sou
rce Detector
X Totally insensitive for HC dewpoint
COOLED MIRROR
Detection Principle Development Sequence
Shell Research
ETCHED SURFACE
Dry
Light
Sou
rce
Detection Principle Development Sequence
Shell Research
ETCHED SURFACE
HC Wetted
Light
Sou
rce Detector
X Poor sensitivity for HC dewpoint
Detection Principle Development Sequence
Shell Research
ETCHED SURFACE WITH CONICAL DEPRESSION
Dry
Light S
ource Detector
Detection Principle Development Sequence
Shell Research
HC Wetted
Light S
ource Detector
ETCHED SURFACE WITH CONICAL DEPRESSION
Detection Principle Development Sequence
� Excellent sensitivity for HC dew point
Michell DARK SPOT™ detection technique
Dark Spot™ Detector Images
Dry Sensor
Dark Spot™ Detector Images 2
Some Condensate beginning to form
Dark Spot™ Detector Images 3
More Condensate forming –This is the dew point image
Dark Spot™ Detector Images 4
Excess Condensate – This image is below the dew point
Dark Spot™ Detector Images 5
Advanced Condensation – This image is well below the dew point
Typica
l 10 m
inute m
easurement cy
cles
-50
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0.0
50
.0
100
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150
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200
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250
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300
.0
350
.0
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Tim
e
Op
tical S
urfa
ce ('M
irror') T
em
pera
ture
, deg
rees F
Dete
ctio
n S
ign
al C
han
ge, m
V
CONDUMAX II HYDROCARBON DEW-POINT ANALYSER
• High Sensitivity - To better than 5 mg/m3 using DARK SPOT Technique
• Facility to produce an ‘LGR plot’ -‘Sensitivity Calibration’ routine
Confronting the issues:
• Decoupling the sample flow rate/cooling rate inter-relationship - fixed volume sample analysis
CONDUMAX HYDROCARBON DEWPOINTMETER (circa 1985-2004)
CONDUMAX II Combined Hydrocarbon and Water Dew-Point Analyser
• Automatic System
• ‘Dark Spot’ HCDP measurement
• Ceramic Moisture Sensor (optional WDP)
• EEx d ATEX approval
• Verification routines, low maintenance
CONDUMAX II Hydrocarbon Dew-Point Analyser
Main Unit
Remote Interface
Sampling System
Sensor Cell Assembly
Sensor Cell exploded ……
Condumax II
• Self-contained Measuring system
• 0.5 deg C HCDP accuracy
• Touch screen operation
• Modbus RTU communications
• 2 x 4-20 mA output
• Active X controls for plant integration
CONDUMAX II –Measurement range capability
Depression Graph Condumax II Sensor Cells
Pressure with natural gas at 27 barg.
-6.6
-21.7
-34.3
-49
-63-70
-60
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
1 2 3 4 5
Tem
pera
ture
, d
eg
ree C
amb Maximum cooling depression
CONDUMAX II – Simplicity….
Condumax II - Installation made simple:
� Wholly hazardous area installation – No safe area requirement
� Direct installation in field locations – No analyser house requirement
� 90-264Vac, 50/60Hz, 200 Watt, is the only utility requirement
� No cooling air or gas required
� Hermetically sealed Peltier cooler - No dry gas purge required
• Close to the sample tap
• Sample tap on top quadrant of pipe
• Heat-traced and insulated sample tubing
• Sun shade over instrument housing, or in-doors
Typical Field Installation
Pros Cons
+ Direct fundamental ‘objective’
measurement with high sensitivity
and repeatability
+ Stand alone operation – Built-in
verification routines
+ No special maintenance staff
requirements or certified reference
cylinder gases
+ Its like having your own skilled
technician taking a API/ASTM
Bureau of Mines reading every 10
minutes, 24/7.
– Significant initial investment but very low operating costs
AUTOMATIC HcDP ANALYZER
Condumax II – Water Dew Point
• Water dew-point measurement
• Michell Ceramic Moisture Sensor
• Line pressure transducer
• Accuracy +/- 1°C WDP
• 206 barg rated
• Sour gas compatible
Michell Ceramic Moisture Sensor
Ceramic Substrate
Conductive Layer
Porous ConductiveLayer
HygroscopicActive Layer
Michell Ceramic Moisture Sensor
0
5
10
15
20
25
30
35
40
45
50
-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20
Dewpoint, deg. C
Sensor Im
pedance, K o
hm
s
Condumax IIIndustry proven sensor technology
• Michell Ceramic Moisture Sensor
• Measurement range: 0.001 to 30,000 ppmVCalibrated: -100 to +20oC dew point.Response to -130 and up to +30oCdp
• Accuracy: +/-10% of moisture content reading
• 300 barg rated, immune to pressure shock
• Chemically inert, resilient to extreme sour gas (30% H2S)
Condumax IICalibration Integrity
• UKAS accredited calibration laboratory
• Certified calibration traceable to NPL (UK) and NIST (USA)
• Unique cost effective MichellCalibration Exchange Serviceavailable to all users globally
Thanks you for attending
Any questions?
Pls contact ENGEZER
www.engezer.com.br