MOSITURE
MEASUREMENT IN NATURAL GAS
Bayu Nurwinanto
ABSTRACT
The
measurement of moisture content in natural gas is extremely important, from a
technical perspective and in order to ensure conformance to contractual
specifications. Typically, this measurement is
one of the most difficult to perform successfully - natural gas sources are
generally dirty, corrosive, heavily moisture laden and at high pressure. Moisture removal is a key stage of natural
gas processing prior to sale and its efficiency is important in order to
satisfy fiscal contractual obligations, to ensure that the gas is safe to
transport and has the right properties for subsequent use.
This
paper discusses the technology of natural gas processing and the demands it
puts on the humidity measurement industry, both technically and
commercially. This paper also describes
the methods that may be employed to effect a measurement in natural gas and to
ensure integrity and longevity.
Actual case histories
are referred to within the paper as examples of good and bad practice.
INTRODUCTION
Natural gas extracted from
underground sources is saturated with liquid water and heavier molecular weight
hydrocarbon components. In order to meet the requirements for a clean, dry,
wholly gaseous fuel suitable for transmission through pipelines and
distribution for burning by end users, the gas must go through several stages
of processing, including the removal of entrained liquids from the gas,
followed by drying to reduce water vapour content. The dehydration of natural gas is critical to
the successful operation of the production facility and the whole distribution
train through to the end user. The presence
of water vapour in concentrations above a few 10s of parts per million has
potentially disastrous consequences. The
lifetime of a pipeline is governed by the rate at which corrosion occurs which
is directly linked to the available moisture in the gas which promotes
oxidation. In addition, the formation of
hydrates can reduce pipeline flow capacities, even leading to blockages, and
potential damage to process filters, valves and compressors. Such hydrates are the combination of
excessive water vapour with liquid hydrocarbons, which may condense out of the
gas in the course of transmission, to form emulsions that, under process
pressure conditions, are solid masses.
Furthermore, in the processing of gas prior to transmission, a cold
temperature separator is most often used to extract the heavier molecular
components to avoid the formation of such hydrocarbon liquid condensates at
prevailing pipeline operating temperatures that change with climate. The drying of natural gas to a dew point
lower than the operating temperature of the chiller plant is of obvious
importance to prevent freeze up problems, causing flow restriction, with
resulting consequences in terms of plant operating efficiency.
For these reasons it is standard practice at
natural gas production facilities, both on- and offshore, to measure the
moisture content in natural gas on a continuous, on-line basis at critical
points to ensure successful processing and efficient, reliable plant
operation. The successful design,
installation and operation of industrial hygrometers for such applications
requires special consideration to be given to the particular nature and
composition of the gas being measured and the processing techniques being
utilised.
Dehydration Process
The
most common processing technique for drying natural gas is that of simple
mechanical separator, to divide the gas from the liquids of the two phase flow
coming from the gas field, followed by glycol dehydration. Here a riser tower has an array of spray
nozzles around it’s circumference through which glycol is injected, as a liquid
desiccant, into the gas stream flowing up through the tower. The adsorption process results in
moisture-laden glycol that coalesces into globules that are naturally forced,
through flow dynamics, outward towards the wall of the tower. The liquid glycol is collected in trays,
piped out of the tower and is regenerated by heating to evaporate the absorbed
moisture prior to re-injection in a continuous operating, re-circulating
loop. Such glycol contactors, as they
are termed, are specified to achieve a moisture content of less than 3
Lb./MMSCF (pounds of moisture per million standard cubic feet of gas) under
normal operating conditions.
The
high flow velocity of gas through the contactor leads to possible carry through
of glycol mist. Consideration must be
given to this characteristic if the application of a hygrometer is to be
successful in monitoring the performance of the dehydration process. Contamination of the moisture sensor or
sample handling system results in a serious deadening in response for the
analyser due to the moisture adsorption/desorption capacity of the glycol. A conventional coalescing filter with fibre
element positioned at the front end of the sampling system can effectively
protect the moisture sensor from contamination but will not solve response
problems unless any collected liquids are flushed out of the filter housing by
a continuous flow from the drain port (Fig. 1). A membrane type filter offers the best protection in such glycol
applications but is restricted to a maximum operating pressure of 10 MPa. These
filters also work on a bypass flow arrangement but use a micro-porous membrane
of fluorocarbon material to offer superior protection.
 |
Figure
1. Schematic
Diagram of
a Typical Moisture Analyser
for
Natural Gas Dehydration Plant |
In addition to protection against
liquid contamination, it is suggested by some suppliers of instrument sample
filters for natural gas applications that an absorbent material, in the form of
an in-line column of activated charcoal through which the sample flows, is used
to remove glycol vapour. In our
experience the presence of glycol vapour, which has a low vapour pressure (130Pa
at 53°C)
and corresponding low maximum trace concentration, has little detrimental
effect on the performance of sensing technologies like the Ceramic Moisture
Sensor used for such measurements.
The
measurement principle of such sensors is adsorption/desorption of water
molecules into a hygroscopic layer between two conductive electrical plates - A
substrate layer beneath and a porous top plate exposed to the flowing sample
and through which moisture molecules freely permeate to maintain a natural
equilibrium of moisture content. The variation of moisture adsorbed into the
hygroscopic layer results in a corresponding change in the dielectric between
the conductive plates and thus the ability to use this principle for continuous
on-line measurement. However, the
Glycol molecule has similarities to that of water in that both have polar
covalent bonds with an unequal sharing of electrons between a bond of oxygen
and hydrogen atoms which, as a result, become negatively and positively charged
respectively.
As such both water
and glycol molecules possess the potential to cause a response in the sensor if
adsorbed into the hygroscopic layer as the oxygen atoms are attracted to the
positively charged regions of the hygroscopic layer. However, the maximum possible concentration
of glycol vapour is extremely low relative to that of moisture which means
that, given effective filtration to remove glycol mist as prescribed above, any
effects to measurement accuracy are negligible.
However, the use of in-line adsorption cartridges, in addition to a
filter, to remove such glycol vapour can have serious detrimental side effects.
Activated charcoal is an effective desiccant with a capacity to adsorb moisture
in addition to glycol vapour and as such will cause significant damping of the
changes in moisture content of the analysis sample in response to process
variations. If the process gas becomes drier, the activated charcoal will tend
to act as a water source at the moisture analyser inlet and if the process gas
gets wetter, the charcoal will desiccate it, giving a falsely dry reading.
Hydrocarbon
dew point
For
natural gas there are two dew-point temperatures of relevance, the water dew
point, as we discuss here, and the hydrocarbon dewpoint. The latter is quite simply the temperature at
which liquid hydrocarbons condense out of the gas upon cooling. Such liquid hydrocarbons comprise the heavier
molecular weight components of the gas composition, typically butane and
higher. This parameter, as with water
dew point, requires dedicated processing plant (in the form of condensing
chillers) and purpose designed measurement instrumentation. However, the significance to the measurement
of water dew point arises if a moisture analyser using a condensing dew-point
measurement technique is utilised such as a Bureau of Mines Apparatus1. This form of manual visual cooled mirror
dewpointmeter, and any other type of automated, condensing dew-point analyser,
may give confusing results when used for water dew-point measurement in natural
gas. This is because of the difficulty in observing the water dew point
separately from that of hydrocarbons and glycol that are highly likely to
condense on the mirror surface at a higher temperature than the water dew point
(Fig. 2). The use of a sensor based on a
non-condensing measurement principle, such as the Ceramic Moisture Sensor,
avoids this difficulty as it does not employ a condensation measurement
technique and therefore will not suffer from such cross-measurement effects.
 |
Figure 2. Hydrocarbon and Water Dew Point Variation
with
Pressure for
a Typical Natural Gas Composition |
Conversion from measured dew point to moisture content
The
pressure of natural gas is typically 4 to 8 MPa in processing plant and
on-shore transmission whilst gas entering offshore pipelines is often
compressed to 16 MPa or higher. In any
dew-point analysis the influence of gas pressure must be considered. The Michell Ceramic Moisture Sensor as with
its predecessors, the older aluminium oxide technologies, adsorb moisture in
equilibrium with the gas sample flow to which it is exposed and thus exhibits a
response to variations in water vapour pressure. Water vapour pressure is directly related to
dew point, which enables such sensors to be calibrated accurately and easily in
the parameter of dew point. The
relationship between partial pressure of water vapour and dew point remains
consistent irrespective of total gas pressure and the composition of the dry
gas components. Thus such a sensor
calibrated by the instrument manufacturers on known dew point calibration
gases, usually performed at atmospheric pressure, can be applied to accurately
determine the dew point of any process gas at any chosen analysis pressure. The parameter of water dew point is
the most widely used parameter to stipulate this element of gas quality in
contractual supply specifications between gas producers and pipeline operators
through to end customers. However, in
some specifications for process plant such as glycol dehydration contactors as
well as pipeline operations it is more common for a maximum permissible
moisture content to be stipulated. The
conversion from measured dew point to moisture content needs specific
consideration to be given to the non-ideal behaviour of high-pressure natural
gas that requires the use of enhancement factors when performing the conversion
from measured dew point at known analysis pressure to moisture content. A
number of sources of such conversion data, originating mainly from work carried
out at IGT2, Chicago, in the 1950’s are in common usage today and
are reproduced in a current ASTM standard3. Data is only provided down to -40°C
dew point, limiting its applicability in colder climate regions, where
specifications for moisture content are stricter in order to avoid the
potential for condensation to occur with the associated problems discussed
earlier. Furthermore, where desiccant
columns are used as second stage dehydration plant, which is a necessity, fore
instance, on a natural gas liquefaction plant to prevent freeze up inside the cryogenic
process, then moisture levels need to be less than 1 ppmV, typically lower than
0.1 ppmV, which equates to less than -70°C
dew point at process conditions.
Extrapolations of the IGT data have been used by Michell Instruments and
other moisture analyser manufacturers but this introduces increased uncertainty
of measurement and can lead to disputes at custody transfer between gas
producers, pipeline operators and end customers if different conversion data is
used. Major European natural gas companies
are now tending to use a new harmonised standard4 covering the full
range of measurement. The conclusion of
this work, leading to a consistent approach amongst gas companies and
instrumentation suppliers, has an added importance given the increased prevalence
of inter-country pipelines. For applications
requiring units of moisture content to be indicated then the conversion is
usually performed in the measurement electronics of the hygrometer where the
measured dew point, at a known pressure, is converted to the measurement unit
desired by the natural gas company.
Units in regular use include Lb./MMSCFD (gas industry in USA and also
world wide users of American designed processing plant) and mg/std.m³ (European
specifications). Knowing the pressure of
analysis for the primary dew point measurement is critical to the accuracy of
the unit conversion as a percentage error in defining the analysis pressure
will directly transpose into the same percentage error in calculating the
moisture content. The analysir pressure
should therefore either be fixed by a ‘peak shaving’ pressure regulator set to
the minimum line pressure level or by on-line measurement of the analysis
condition using a pressure transducer to provide a real time input of the
variations with line pressure into the calculation (Fig. 1).
Sour
natural gas
Further
difficulty may also be experienced in interpreting water dewpoint measurements
made in natural gas if a conversion to units of moisture content is required
and if the gas composition contains a significant amount of carbon dioxide
and/or hydrogen sulphide. Such ‘sour’
natural gas, as it is termed, is found in many gas fields in current
production. The amount of moisture
required to reach saturation water vapour pressure in H2S and CO2
is considerably higher than for moisture in methane or a ‘sweet’ natural gas
composition at the same temperature. As
such the water dew point measured in a sour gas, irrespective of the
measurement principle applied, will be significantly lower than for a sweet gas
containing the same moisture content.
This needs to be compensated for, using published data5, and
thus requires knowledge of the concentration of sour gas components. A
typical example for a sour natural gas project can be taken from a major
producer in Northern Germany. Here a
specification for maximum permissible moisture content is set at 50 mgH2O/std.m³ (equivalent to around -12°C dew point at line pressure of 7 MPa
for sweet gas but lower than –20oC dew point for gas which is very
sour). This limit is set by safety
standards to control the severe corrosion problems that are associated with
sour gas and is enforced by The Bergamt, a German Federal authority. For natural gas from such sour fields the
concentration of H2S can be as high as 33% (mol) in extreme cases
and 9 to 15%(mol) is average. Sour
natural gas production facilities include de-sulphurisation plant at an early
stage in the processing with sulphur production being a major part of the
operation. Handling such gases prior to
de-sulphurisation means careful consideration to the design of the processing
plant and moisture analyser alike. The
obvious corrosion difficulties are compounded by the characteristic of H2S
to promote sulphide stress cracking6 in metallic materials and the
potentially fatal consequences of any leakage of sour gas as H2S,
which cannot be detected by the human sense of smell above 200 ppm
concentration, is highly toxic attacking the nervous system. The application of a moisture
analyser for sour gas measurement requires a sensing technology that can
perform satisfactorily in these most aggressive of gas media. The Ceramic Moisture Sensor has been
successfully used in these applications for some years. The materials selected for the active device
of this sensor are exclusively ceramics and base metals that are inert by
nature and offer good resilience to chemical attack. This combined with the robust design and
careful selection of materials for the construction of the other gas-wetted
parts of the sensor affords reliable service in this application. Experience has been gained over the last five
years with two types of installation method. The first
installation type is the conventional method used for a natural gas
installation, that of remote sampling with a sample conditioning system (Fig.
1) but in such sour gas applications all sample wetted components must be
selected strictly in accordance with NACE6 requirements, with
significant cost implications. In this
case the sample flow exhausting from the system is taken to a flare where the
toxic gas is rendered safe by burning.
The alternative installation type is direct insertion ‘in-line’ with the
Ceramic Moisture Sensor being mounted at the tip of a specially adapted,
stainless steel probe assembly that inserts directly into the process
pipeline. The advantages of this
installation arrangement is that the sour gas remains in the pipeline, so
removing the risks and costs associated with deploying a conventional sampling
in such a sour gas application. The
speed of response for such an installation is extremely fast but a major
disadvantage is the difficulty involved in removing the sensor probe assembly
from the pipeline that is required for periodic maintenance of the sensor
calibration. This, as for all sour gas
applications of the Ceramic Moisture Sensor, is recommended on a six monthly
schedule, which, as these sensors are fully interchangeable, is achieved by
exchange of the sensor in use for a freshly calibrated sensor carrying a detailed
certification of calibration. A further
disadvantage is the lack of protection to glycol contamination that is afforded
by such direct insertion.
Some
Examples of Successful Applications
Glycol Dehydration – Underground Gas Storage Facility
Michell’s
Ceramic Moisture Sensor has been implemented successfully to measure water dew
point in high-pressure natural gas stored in large, natural underground storage
caverns in mainland Europe. Storage pressures of up to 30 MPa mean that the gas
must be very dry in order to prevent condensation occurring. Both water and
hydrocarbon dew-point temperatures are measured during filling (pressurisation)
and usage (de-pressurisation) cycles. The storage facility is used to meet
demand in peak periods and may be non-active in the summer months. Therefore
reliability is a vital factor, as the most likely time for a moisture analyser
to fail is when there is no gas flow and
a corrosive, moist or contaminated, stale sample is presented to the sensor for
an extended period of time.
Fiscal Metering at Gas Transfer Points – European Pipelines
Michell’s
Cermet and Cermet II IS Hygrometer systems are used for the continuous
measurement of the moisture content in natural gas that is transferred from one
transmission company to another, normally across country borders. In one
particular case, natural gas from a Russian source is measured after glycol
dehydration at its transfer point into Germany, where the dew point must be
lower than 0 oC at any line pressure, up to 10MPa. Performance of the on-line Cermet II IS
measurement system is validated by regular reference against a
regularly-calibrated CERMAX IS portable hygrometer. At any such transfer point
it is vital that the gas dew point is measured, particularly if there is to be
mixing of gases from different sources. Whilst it is possible to calculate the
resultant dew point from a volumetric mixture of two or more sources of gas,
practical verification is required for contractual conformity.
Sour Gas Moisture Measurement
At
a site in Germany, a special application has been successfully fulfilled, for
measurement in sour natural gas. At this site the NH3 and H2S
concentrations are as high as 33%, but the Michell Ceramic Moisture Sensor has
been proven in practice to operate successfully over a number of years. Special
materials of construction have been used to ensure that electrical contacts to
the Moisture Sensor are not corroded by the high NH3 and H2S
content, though the sensor active surface is of the standard Michell design. In
the past, other analysers had a maximum operating life of only a few weeks and
in some cases just hours or days. With the Michell Ceramic Moisture Sensor the
calibration interval has been extended to six months.
LNG Production
Gas
exporting countries have a need to verify the quality of natural gas prior to
liquefaction, for bulk transportation by sea. Similarly, the importing gas
company will need to verify the quality of the LNG as-delivered. Michell has
successfully implemented its Ceramic Moisture Sensor technology on many LNG
plants, particularly in the Middle East region, for measurement of sub-ppm
moisture levels at relatively high line pressures. This application is
reasonably simple, as the LNG has been processed to remove heavy hydrocarbons, most
of the moisture and is usually very low in H2S and other corrosive
components.
Low Pressure Measurements at Consumer Points
Often
forgotten, the end-point for much natural gas is in the consumer chain at low
pressure. Here, there is normally a specification that requires the dew point
to be lower than –26 oC. Two factors are important. The first is to
ensure that the calorific value of the end-user gas meets specification. Excess
moisture will effectively reduce the CV and therefore must be kept to an
acceptably low level. Second, the dew point must be low enough to ensure that
no condensation (and therefore subsequent pipeline freezing and possible
fracture) can occur in winter conditions. In some countries where winter
temperatures dictate, the specification may be even more stringent. Michell’s
CERMAX IS portable hygrometer is used to rapidly determine the moisture content
in low-pressure consumer gas lines to determine contractual conformance and to
help trace leaks. Michell has also supplied many gas authorities with humidity
calibration equipment to enable local calibration of field instruments,
maintaining a traceability chain back to UK, US and other national humidity
standards.
CONCLUSION
The
application of moisture analysers for the measurement of natural gas is not
simple and straightforward. There are
many aspects to be considered that are unique to natural gas and that can
greatly affect the reliability of both the instrument in service and of the measurement
data that it provides. Detailed
consultation between instrument manufacturer and their customers in the natural
gas industry is required to device the best solution to each individual
application.
