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Water Dew Point Measurement in
Natural Gas and its Traceability
Andrea Peruzzi - VSL
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Pag. 2
Water dew-point measurement
in natural gas and its traceability
Andrea Peruzzi and Rien Bosma
EUROPEAN FLOW MEASUREMENT WORKSHOP
Noordwijk, 18 March 2015
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Pag. 3
Outline
The measurement of the water dew-point of natural gas:
• The water dew-point temperature of a moist gas
• The natural gas chain
• The measurement of the water dew-point of natural gas along the chain
• Sensor technologies
Traceability of water dew-point measurements:
• Primary standard: high-pressure dew-point generator
o Principle of operation
o Design
o Construction
o Use
Investigation performances of Al2O3 sensors
o Response time
o Stability on time
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Pag. 4
Water dew-point of a moist gas
Dew-point (frost-point) temperature of a moist gas: the temperature at which a sample
of a moist gas must be cooled, at constant pressure, for water vapour to condense into
water (ice)
Why does the water vapour start condensing?
Because, when cooling a moist gas, the water vapour contained in it eventually reaches
saturation with respect to liquid water (ice)
Dew Frost
(In this case the moist gas is atmospheric air)
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Pag. 5
Given a sample of moist natural gas:
TDP = f(xW, P) (xW = mole fraction of water in the sample, P = pressure)
• remove water → lower xW → lower TDP
• reduce P → lower TDP (but same xW)
TDP after pressure reduction (after expansion) requires the knowledge of the
enhancement factor f(P, TDP):
Transforming TDP → xW requires the knowledge of the pressure P and the enhancement
factor f(P, TDP):
Water dew-point of natural gas
P
TeTPfx DPWDP
W
)(),(
EUROPEAN FLOW MEASUREMENT WORKSHOP
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2
222
1
111 )(),()(),(
P
TeTPf
P
TeTPfx DPWDPDPWDP
W
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Pag. 6
Extraction Processing Transport Distribution Users
Offshore or on-shore
gas field
Gas processing plant:
dehydration and
removal/separation of
various components
High-pressure
pipeline or LNG
Distribution to local
networks through
medium pressure
pipeline
- Power plants (35%)
- Large customers (10%)
- Retailers (25%)
- Storage
EUROPEAN FLOW MEASUREMENT WORKSHOP
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Natural gas chain
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Pag. 7
Extraction/
Production Processing Transport Distribution Users
Offshore or on-shore gas
field
Gas plant: dehydration
and removal/separation of
various components
High-pressure pipeline or
LNG
Distribution to local
networks through medium
pressure pipeline
- Power plants (35%)
- Retailers (25%)
- Large customers (10%)
- Storage
EUROPEAN FLOW MEASUREMENT WORKSHOP
Noordwijk, 18 March 2015
Natural gas chain
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Pag. 8
Natural gas processing plant
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Extracted gas is not suitable for high-pressure transmission and consumption
Dehydration: H2O must be removed at a level below a specified contractual value
• Safety: remove H2O to avoid corrosion, condensation and hydrates formation in
the pipeline → lower TDP
• Removing H2O costs money (each °C of TDP means M€ in plant operation costs)
Find the optimal balance between satisfying contractual value and cost effective
operation of the plant
Accurate measurement of TDP is crucial
Acid gases removal (CO2 and H2S)
Dehydration Mercury removal
Nitrogen rejection
NGLs recovery
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Pag. 9
Measurement at gas processing
plant: glycol dehydration
Measurement point: Glycol contactor
Typical conditions: TDP < -30 °C at 5 to 8 MPa
Requirements:
• fast wet-up response
• protected against glycol or other liquid
contaminants
• immune to chemical attack from H2S,
mercaptans and other sulphides
Accuracy: 1°C TDP (declared by manufacturer)
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Pag.
Extraction/
Production Processing Transport Distribution Users
Offshore or on-shore gas
field
Gas plant: dehydration
and removal/separation of
various components
High-pressure pipeline or
LNG
Distribution to local
networks through medium
pressure pipeline
- Power plants (35%)
- Retailers (25%)
- Large customers (10%)
- Storage
EUROPEAN FLOW MEASUREMENT WORKSHOP
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Natural gas chain
10
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Pag. 11 EUROPEAN FLOW MEASUREMENT WORKSHOP
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Long distance transportation of
natural gas?
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Pag.
Natural gas transport
12 EUROPEAN FLOW MEASUREMENT WORKSHOP
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Transport:
• High-pressure pipeline (80%)
• LNG (20%)
In both cases:
• Pipeline custody transfer contractual requirements
o Pipeline: TDP < -10 °C at 6.5 MPa
EU (EASEE – Madrid Forum: TDP < -8 °C at 7 MPa)
• LNG liquefaction plant: TDP < -80 °C at 7 MPa
Measurement requirement:
• Fast wet-up response
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Pag. 13
Measurement of trace moisture
before LNG liquefaction
Measurement point: molecular sieve columns
Typical requirement: TDP < -80 °C at 7 MPa (0.1 ppmV)
Sample point: middle bed of each tower and outlet of each tower
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Pag. 14
LNG chain
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Pag.
Measurement at re-gasification
terminal
Typical requirement: TDP < -10 °C at 7 MPa (leakage indicator in heat-exchangers)
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Pag. 16 EUROPEAN FLOW MEASUREMENT WORKSHOP
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Sensor technologies
Different measurement principles:
• Impedance sensors: polymeric and metal oxyde sensors (Al2O3
(Easydew, Michell), P2O5 (Accupoint LP 2, Meeco), polymer
(E+E))
• Spectroscopic analyzers (Aurora, GE)
Calibrated at atmospheric pressure and in nitrogen or air gas
(conditions completely different from field conditions)
Investigation of the performances of sensors and analyzers:
• Response time
• Stability on time (drift)
• Pressure dependence
See: “An investigation of the comparative performance of diverse humidity
sensing techniques in natural gas”, J.G. Gallegos et al., Journal of Natural Gas
Science and Engineering, 23 (2015) 407-416
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Pag. 17 EUROPEAN FLOW MEASUREMENT WORKSHOP
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Primary dew-point generator
Source of traceability for water dew-point temperature measurement:
primary dew-point generator
Thermodynamically-based: • A gas stream (air, N2, CH4, …) …
• … is saturated with water vapour…
• … by flowing the gas over a plane surface of water (or ice)…
• … at known temperature TS and pressure PS
• If the gas is fully saturated in the saturator, the dew-point temperature of
the moist gas drawn from the saturator is:
Dry gas Moist gas
Saturator at
{TS, PS}
Water
SDP TT
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Pag. 18
Design criteria: • Maximize turbulent flow
• Reduced tilt of water surface
• Acceptable pressure drop
Design features:
• Channel
• Segments and bends (180° each)
• Dams and barriers
Choice of design parameters: • Depth of the channel: 30 mm
• Number of segments and bends: 20
• Number of dams and barriers: 20
• Water depth: 7 mm
Saturator design
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Pag. 19
HPDP generator construction
Saturator
Ethanol
reservoir
Ethanol
bath
Precooler
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Pag. 20 20
Pressure
controller
Syringe
pump
Presat
Saturator
Precooler
Ethanol
bath
SPRT
Pressure
CMH
MBW
373 LX
CMH
MBW
373 HPX
CMH
MBW
973
N2
CH4
CMH Pressure Range Temperature range
MBW 973 0.1 MPa -60 °C to +20 °C
MBW 373 HPX 0.1 MPa to 10 MPa -80 °C to +20 °C
MBW 373 LX 0.1 MPa to 0.25 MPa -95 °C to +20 °C
Validation
Monitor differential response of CMHs when:
• changing saturator flow rate while keeping fixed flow rate at CMHs
• changing dew-point of inlet gas
Validation of HPDP generator
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Pag. 21
Pressure
controller
Syringe
pump
Presat
Saturator
Precooler
Ethanol
bath
SPRT
Pressure
CMH
MBW
373 LX
CMH
MBW
373 HPX
CMH
MBW
973
N2
CH4
PPC = 6.000 MPa P973 = 1078 hPa
PAtm = 1024 hPa
PS = 6.000 MPa
TS = 14.97 ⁰C
PHPX = 6.000 MPa
TREF1 = 14.97 ⁰C
PAtm = 1024 hPa
PAtm = 1024 hPa
PAtm = 1024 hPa
PLX = 1037 hPa
TREF2 = -30.72 ⁰C
ΦS = ΦHPX + ΦLX+ ΦX
ΦHPX = 2 l/min
ΦLX = 0.5 l/min
ΦX
Validation of HPDP generator (CH4, TDP = +15⁰C, P = 6 MPa)
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Validation of HPDP generator (CH4, TDP = +15⁰C, P = 6 MPa)
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Pag. 23
Pressure
controller
Syringe
pump
Presat
Saturator
Precooler
Ethanol
bath
SPRT
Pressure
CMH
MBW
373 LX
CMH
MBW
373 HPX
CMH
MBW
973
N2
CH4
PPC = 5.997 MPa P973 = 1066 hPa
PAtm = 1013 hPa
PS = 5.997 MPa
TS = -14.95 ⁰C
PHPX = 5.997 MPa
TREF1 = -14.95 ⁰C
PAtm = 1013 hPa
PAtm = 1013 hPa
PAtm = 1013 hPa
PLX = 1028 hPa
TREF2 = -50.71 ⁰C
ΦS = ΦHPX + ΦLX+ ΦX
ΦHPX = 2 l/min
ΦLX = 0.5 l/min
ΦX
Validation of HPDP generator (CH4, TDP = -15⁰C, P = 6 MPa)
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Pag. 24
Validation of HPDP generator (CH4, TDP = -15⁰C, P = 6 MPa)
Measurement cycle:
N2 → CH4 → N2
CMHs start drifting
after switching to CH4
Hydrate Formation!
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Hydrate formation line
Stable response CMH’s : No hydrate formation
Drift in response CMH’s : Hydrate formation
Setup can be used at low temperatures, but only for low pressures (< 2 MPa)
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Impedance sensor technology
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Hygroscopic non-conductive layer (< 1 µm)
Two conductive layers (porous top layer 0.1 µm)
Base ceramic substrate
Absorption of water vapour
Fast wet-up response
Pressure rating: 30 MPa
Accuracy: 1°C TDP (declared by manufacturer)
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Investigation of Al2O3 dew-point
sensors
Dry-to-wet response time of Al2O3 sensors versus the water mole fraction in nitrogen. The
graph is obtained from dew-point temperatures in the range -60 ºC to +3 ºC and pressure up
to 6 MPa
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Pag. 28 EUROPEAN FLOW MEASUREMENT WORKSHOP
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Investigation of Al2O3 dew-point
sensors (time drift)
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Pag. 29 EUROPEAN FLOW MEASUREMENT WORKSHOP
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Conclusions
The measurement of the water dew-point of natural gas is required at
different points along the natural gas chain with an accuracy of 1 °C
The water dew-point Td of natural gas is measured on a continuous
on-line basis at high pressure with humidity sensors and analyzers
Sensors and analyzers are calibrated only at atmospheric pressure
and in air or nitrogen
The extrapolation of the calibration to field conditions (line pressure of
8 MPa and natural gas) translates into uncertainties of up to 10 °C
Calibration of sensors at field conditions is strongly recommended