Hydrocarbon
Dew Point Monitoring of Natural Gas using Field-Mounted
Online Gas Chromatographs
A
practical and reliable approach, highlight Kevin Ernest & Dr. D.
J. Pettigrew.
Introduction
Hydrocarbon Dew Point of Natural Gas
Hydrocarbon
dew point is an important gas quality parameter as it indicates the
temperature and pressure at which hydrocarbon constituents in a natural
gas mixture begin to change phase from gas to liquid. In an ideal pipeline,
all natural gas would remain above the Hydrocarbon dew point and therefore
no hydrocarbon condensates would exist. Hydrocarbon condensates represent
“lost” gas that causes errors in volumetric flow rate, heating
value, and other gas property calculations that are made in custody
transfer measurements. Furthermore, these liquids can cause serious
damage to both the pipeline and the machinery.
that the gas feeds
– such as gas turbine engines. Since the hydrocarbon dew point
temperature can change drastically between different natural gas mixtures
at a given pressure, even historical operating success cannot ensure
against condensate formation in a given pipeline. As natural gas mixtures
become rich with heavier hydrocarbon concentrations, the hydrocarbon
dew point temperature — at which these heavier hydrocarbons will
begin to drop out of the gas as liquids -- can increase dramatically.
Depending on the economics, these rich hydrocarbon components could
be more valuable to the seller as natural gas or as a particular liquid
(such as butanes or propanes). Therefore, in today’s economy, natural
gas distributors and users downstream cannot safely assume that natural
gas concentrations are consistent. This becomes particularly important
during cold weather periods, where natural gas prices are relatively
high, and ambient/pipeline temperatures are low.
Gas Turbine Warranties
Turbine manufacturers, such as General Electric specify that the inlet
gas temperature must be kept 50-60°F above the hydrocarbon dew point
or the turbine warranty is invalidated. This is because maintenance
repairs of $500,000 and above are not uncommon as a result of hot gas
path damage as a result of liquid slugging. Even extremely small amounts
of liquid, if allowed to accumulate in downstream piping can cause damage.
[ 1]
To control against significant condensate formation while optimising
the superheated energy required, gas turbine operators must be able
to monitor hydrocarbon dew point temperature online at the heater inlet.
Relevant Standards
– A.G.A. 4a
Several relevant gas quality specifications are outlined in the new
American Gas Association (AGA) Report No. 4a, “Natural Gas Contract
Measurement and Quality Clauses”. One of the primary purposes of
this report is to “encourage practical limits (ranges) for gas
measurement and gas quality within established commercial tolerances.”
Section 4:
“Quality of gas - since natural gas is generally a very complex
mixture of many gases, vapours, and other contaminants, an understanding
of the nature of these constituents and how they affect the merchantability
of gas is necessary for writing contract quality clauses.”
Other Hydrocarbons
Since natural gas can occur with a great variety of other hydrocarbons,
such as natural gasoline and other crude oil components, it is usually
processed to remove them so that the heat content will be consistent,
and the natural gas will remain in the gaseous state under all of the
conditions that it will be exposed to during transport and distribution.[
2]
Relevant Standards
– API Chapter 14.1
Chapter 14.1 of the API Manual of Petroleum Measurement Standards (MPMS)
addresses, “Collecting and handling of natural gas samples for
custody transfer”. This new standard concentrates on proper sampling
systems and procedures to ensure, among other things, that hydrocarbon
dew point is taken into consideration when samples are collected for
laboratory analysis or when transported to an online GC in the field.
It recommends that steps be taken to ensure that sample gas temperature
remain 20-50°F above the hydrocarbon dew point temperature at all
times during sampling. [ 3]
Online Gas Chromatography
- Pipeline Vs. Process GCs
Online GCs have been commonly used in the natural gas industry on pipelines
since the early 80’s, principally as a means of determining composition
for heating value calculation. ...
