Hydrocarbon Processing - May 2021 - 71

Process Controls, Instrumentation and Automation
indicate and they overtreat it, adding cost.
The first situation could contribute to a
safety incident, whereas the second hurts
profitability. The clear need to solve both
challenges is a water content measurement that is correct and consistent.
Traditional tunable diode laser analyzers. From a theoretical standpoint, a

tunable diode laser absorption spectroscopy (TDLAS) analyzer is arguably the
most effective mechanism to measure water content in natural gas. Using an infrared
wavelength laser, it is possible to isolate the
very distinct peaks in the wavelength absorption spectrum that are indicating water and other components in the stream.
This means that the analyzer can provide a
water content measurement unaffected by
glycol, methanol, amine or H2S. While this
underlying capability is a matter of physics, putting it to work in a way that is practical and usable in a typical operating environment is challenging for manufacturers.
TDLAS analyzers are, by nature, very
stable and rarely need calibration. The
sensor itself is not subject to drift or
problems from chemical contamination.
However, the supporting mechanisms can
challenge effective operation. For example, a TDLAS analyzer is optical in nature,
so designs employ mirrors and lenses to
direct and focus the beam from the source
to the detector (FIG. 3).
The sample gas flows through the
measurement cell-the space where the
beam passes. When the sample contains
free liquids, there is an opportunity for
flooding the cell. If the mirrors become
coated, then the beam can be attenuated
or blocked. When such problems eventually occur, the cell requires service; however, it may take operators some time to
realize this type of problem.
These difficulties stem largely from
the sample conditioning system. It must
capture contaminants and deliver a clean
sample to the measurement cell, but it
must not affect the water content. The
system must also control temperature and
pressure of the sample gas to ensure accuracy and repeatability. When servicing
is necessary, some analyzer designs can be
very complex or impossible to disassemble, with extensive tweaking required as
part of reassembly. Some analyzers call for
matched sets of components, requiring expensive replacement-part kits, or the end
user is forced to send the analyzer back to

the factory for repair. This can detrimentally lower measurement uptime availability in critical custody-transfer points.
Fortunately, TDLAS analyzer designs
have been evolving to provide operational
simplicity, while delivering more sophisticated analysis.
Analyzer improvements. TDLAS analyzer improvements have concentrated on
three main areas. These include:
* Transmitter electronics, including
human-machine interface
(HMI), data presentation, system
connectivity and diagnostics
* Modular construction to simplify
serviceability, including a sampling
system, an optical enclosure,
a sample cell and electronics
* Enclosure protection for mounting
versatility.
A field-mounted analyzer depends
on its transmitter to perform a very wide
range of functions. It must support the
basic metrology calculations to provide
excellent accuracy, linearity and repeatability. It must also provide internal HMI
support for its local display, along with
connectivity to send its data to a larger
supervisory control and data acquisition
(SCADA) system or another automation host system. The ability to program
functions easily and intuitively makes all
the difference for ease of use and enables
high measurement uptime, so this capability must be provided.
Desired connectivity options have expanded to include web server capabilities,
extending the range of remote data access
via the internet to any device capable of
hosting a web browser, such as a laptop,
smartphone or tablet. This capability is

particularly critical when an analyzer is
deployed in a location not easily accessible by technicians and operators. When
remote access is combined with internal
data storage, it is a simple matter for authorized users to upload gas analysis data
from extended periods of time.
In many respects, the most important
new capability that the transmitter supports is internal diagnostics to indicate
how the unit is functioning (FIG. 4). This
determines when problems are developing that can cause poor readings or a complete outage. When severe enough, these
situations must trigger alarms to call for
immediate maintenance attention. Continuous evaluation and reporting should

FIG. 3. A gas sample must be contained in
the space where the beam passes through,
so a dirty sample can coat optical surfaces.

TABLE 1. Compounds frequently found in natural gas streams that can damage
electrochemical measurement methods or make them ineffective (but that
do not affect a TDLAS analyzer)
Gas phase
contaminant
Methanol

Aluminum
oxide

Phosphorus
pentoxide

Quartz
crystal

Chilled
mirror

TDL sensor

Glycol

Amine

Mercury

H2S

Hydrogen chloride

Chlorine

Ammonia

X = Analyzer unaffected * = Can cause permanent damage to the sensor O = Can cause slow or inaccurate readings

Hydrocarbon Processing | MAY 2021

Hydrocarbon Processing - May 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - May 2021