Hydrocarbon Processing - May 2021 - 69

Process Controls,
Instrumentation
and Automation
A. GARZA, Endress+Hauser, Greenwood,
Indiana; and S. MILLER, Endress+Hauser,
Rancho Cucamonga, California

Improved measurement of water content
in natural gas
Consumers likely imagine natural gas
as a stream of pure methane. Those who
are closer to the energy industry know
that methane is the major component, but
there can easily be dozens of other substances making up a sizeable portion of
the total volume. Natural gas producers,
pipeline operators and major industrial
users-such as various types of petrochemical plants-have reason to be concerned about what is in the mix (FIG. 1).
Contaminants in natural gas can cause
a variety of problems for pipeline operators and petrochemical plants, especially
when combined with water. This makes
accurate and reliable measurement of water content in natural gas streams critical,
but traditional techniques often fall short.
Fortunately, new analyzers offer muchimproved performance.

fossil fuels, including natural gas-
and they are known for their
toxicity and pollutants produced
during combustion.
* Oxygen degrades amine and some
mercaptans, which are used in
natural gas treatment.
* CO2 dilutes the overall heat value.
The greater problem results when
these contaminants combine with another
contaminant: water. All of them work together with water to produce acids capable
of attacking carbon-steel piping, valves
and other equipment to cause internal corrosion and metal loss over time (FIG. 2).
Natural gas pipelines can corrode from the
outside and inside, but internal metal loss
is more difficult to recognize and measure.

The Pipeline and Hazardous Materials Safety Administration's (PHMSA's)
" Pipeline Corrosion " report summarizes
the situation: " Typically, sales-quality
dry gas will not corrode pipeline interior surfaces. However, natural gas, as it
comes from the well, may contain small
amounts of contaminants such as water,
carbon dioxide and hydrogen sulfide.
If the water condenses, it can react with
the carbon dioxide or hydrogen sulfide to
form an acid that might collect in a low
spot and cause internal corrosion. "
This leaves natural gas producers, pipeline companies and users understandably
concerned about water content-both
liquid and vapor-in the gas flow, since a
pipeline leak or break caused by unchecked

Water exacerbates issues. Natural

gas composition is controlled to some
extent. Wherever natural gas is traded
commercially, there are regulations as to
its chemical content and attributes, such
as calorific value. Local specifications and
ranges vary, but, typically, there are limits
for total sulfur, hydrogen sulfide (H2S),
carbon dioxide (CO2), oxygen and water. This list is not exhaustive, as there are
many other possible components, such as
higher hydrocarbons and other diluent
gases. The common element of these specific components is that they are contaminants and considered undesirable:
* Sulfur and its many compounds
represent the most widely
encountered contaminant in all

FIG. 1. Since natural gas can come from a variety of sources (fossil and renewable), its composition
and attributes vary.
Hydrocarbon Processing | MAY 2021 69

Hydrocarbon Processing - May 2021

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