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Home::Environment
How Specialty Gases Differ from Industrial Gases
Author : Bob Jefferys
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.
This article is copyrighted by Scott Gases
(http://www.scottgas.com). It may not be reproduced in whole or
in part and may not be posted on other websites, without the
express written permission of the author who may be contacted
via email at scottgas@digitalbrandexpressions.com
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