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Home::Environment
How Specialty Gases Differ from Industrial Gases
Author : Bob Davis
When it comes to compressed gases, there is often confusion over
the difference between industrial gases (sometimes referred to
as commodity or bulk gases) and specialty gases (sometimes
referred to as cylinder gases, although industrial gases can
also be supplied in cylinders). The Compressed Gas Association
(CGA), who sets standards to which suppliers of all types of
compressed gases conform, defines its mission as being
“dedicated to the development and promotion of safety standards
and safe practices in the industrial gas industry.” In a broad
sense, in that most compressed gases are used for some sort of
industrial application, all could be considered to be industrial
gases. So to define the true difference between industrial gases
and specialty gases, one must look beyond the application to
other factors such as complexity, level of purity and certainty
of composition.
According to the CGA compressed gases are often grouped into
five loosely defined families: atmospheric; fuel; refrigerant;
poisonous; and those having no obvious ties to any of the other
families. Assignment to these families is somewhat arbitrary and
typically based on the origin, use or chemical structure of a
gas. Specialty gases can belong to any of these five families.
Essentially, they are industrial gases taken to a higher level.
The dictionary describes one of the definitions of the word
specialty as: an unusual, distinctive, or superior mark or
quality. Specialty gases then, can be defined as high-quality
gases for specific applications that are prepared using
laboratory analysis and other preparation methods in order to
quantify, minimize or eliminate unknown or undesirable
characteristics within the gas. Regarding specialty gas
mixtures, precise blending is also necessary to achieve very
specific concentration values for the components contained
within the mixture.
Specialty pure gases Pure gases are considered to be specialty
gases when they are used as support gases for laboratory
instruments such as chromatographs, mass spectrometers and other
various types of analyzers and detectors. Manufacturers of these
types of highly sensitive instruments normally specify the
purity level of pure gases to be used with their instruments.
For example, high-purity, moisture-free helium is often used as
a carrier gas in these instruments. When unwanted impurities are
present, performance of a laboratory instrument may be
compromised, or the instrument itself may be damaged. A good
rule of thumb is, when purity (sometimes as high as 99.9999%)
and/or quantification of trace impurities is an issue, a pure
gas is considered to be a specialty pure. Specialty pure gases
are used in the manufacturing of semiconductors and other
closely controlled applications as well. They may also be used
to assess and monitor the integrity of a bulk pure gas. Carbon
dioxide is a good example. Beverage-quality CO2, as used in the
manufacture of soft drinks, can be classified as being more of a
bulk-type gas because it is used in large quantities. However,
because purity is a health concern, a specialty pure CO2, in
which all trace impurities have been carefully quantified, is
needed to calibrate instruments used to monitor the purity of
the bulk CO2.
Specialty gas mixtures Many specialty gases are actually gas
mixtures that contain individual components. They are frequently
used with various types of analyzers for process control and
regulatory compliance. Some specialty mixtures are somewhat
“standard” and may contain only three or four components, such
as nitric oxide and sulfur dioxide mixtures that are used by
utility companies to calibrate Continuous Emissions Monitors
(CEMs). Others may be quite complex, containing as many as 30 or
more components. Usually, a specialty gas mixture is prepared
using a Standard Reference Material (SRM) in order to validate
accurate measurement of the mixture’s components. This provides
what is known as traceability to a known measurement standard
from a recognized metrology institution such as the National
Institute of Standards and Technology (NIST). Specialty mixtures
typically have components measured in percentages,
parts-per-million and parts-per-billion.
Laboratory analysis to quantify all components and impurities in
a specialty mixture is nearly always critical. A formal document
known as a Certificate of Accuracy or Certificate of Analysis is
provided for each cylinder containing a specialty mixture, and
also for some specialty pure gases. This certificate specifies
the concentration values for all contents, as well as other
important information such the method of blending, type of
laboratory analysis and reference standard used to prepare the
mixture and expiration date. Expiration date refers to the
length of time the components of a mixture remain at their
certified concentrations within the specified tolerances.
Depending on the stability of the components, shelf life can
vary from as little as six months to two years or more. Special
cylinder preparation processes, such as Scott’s Aculife cylinder
inerting treatments, can be used to condition cylinder interior
walls in order to extend a mixture’s shelf life.
Specialty gases are typically not used in nearly as large a
quantity as industrial gases and are supplied in steel or
aluminum high-pressure cylinders containing up to 3000 pounds of
pressure per square inch/gauge (psig). Hence, they are sometimes
referred to as cylinder gases or bottled gases. The cylinder
itself is typically not included in the price of the specialty
gas it contains and must be returned to the gas supplier when
the gas has been depleted. A nominal monthly cylinder rental is
usually charged until the cylinder is returned. Many specialty
gases are also available in small, portable and non-returnable
cylinders such as Scott’s SCOTTY Transportables. Other
specialized containers include lecture bottles that are often
used in laboratories and floating piston-type cylinders that are
used to contain volatile liquid phase mixtures.
The cost of specialization Due to blending technology, cylinder
preparation, laboratory analysis and statistical quality control
necessary to produce specialty gases, cost is much higher than
for lower grade industrial gases. An A-size cylinder containing
218 cubic feet of a low grade of helium suitable for filling
party balloons might cost little more than $50. The same
cylinder containing 99.9999% pure research grade helium, with a
total impurity of less than one part-per-million (1 ppm), would
cost about $500. That’s still a bargain considering 144 cubic
feet of a three-component EPA Protocol mixture having an
analytical accuracy of 1% may cost as much as $1,500. As with
any other specialized product, the end cost of a particular
specialty pure or gas mixture is largely determined by the
degree of difficulty and complexity involved in its preparation.
Considerations when purchasing specialty gases Purchasing
specialty gases can be a daunting task. Because of today’s
bottom line-oriented business climate, one might consider
selecting a specialty gas product based strictly on price. Be
careful! While in some cases organizations such as the EPA may
dictate minimum accuracy and manufacturing processes for certain
gas mixtures, there are few industry-wide standards for
specialty gas quality. Blending, analytical and cylinder
preparation procedures vary between suppliers of specialty
gases. Moreover, suppliers do not always use common nomenclature
when describing their products. Even when product names are the
same, the characteristics of the gases can be quite different.
The best advice is to carefully evaluate your application needs
before purchasing. Then talk with a specialty gas expert to be
sure you fully understand how the characteristics of a
particular pure gas or gas mixture will either meet or possibly
compromise your application. Remember also that most specialty
gases require the use of specialized delivery equipment that is
constructed of materials that will protect gas purity and
integrity.
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