Chemical Engineering November 2013 - 39

FIGURE 4. Bell-end socket-welded elbows
are not compatible with Category
M fluids
have been reported in the chemical
literature should be used):
(i) Oral toxicity. A liquid with an
LD50 for acute oral toxicity of not
more than 500 mg/kg or a solid
with an LD50 for acute oral toxicity
of not more than 200 mg/kg
(ii) Dermal toxicity. A material
with an LD50 for acute dermal toxicity
of not more than 1,000 mg/kg
(iii) Inhalation toxicity. Either
a dust or mist with an LC50 for
acute toxicity on inhalation of not
more than 10 mg/L; or a material
with a saturated vapor concentration
in air at 68°F greater than or
equal to one-fifth of the LC50 for
acute toxicity on inhalation of vapors
and with an LC50 for acute
toxicity on inhalation of vapors of
not more than 5,000 mL/mm3
Or (B), the substance is an irritating
material, with properties similar
to tear gas, which causes extreme irritation,
especially in confined spaces.
NFPA. The NFPA's Chemical Hazardous
Rating System places chemicals
into three groups: health, flammability
and reactivity. Within those groups are
twelve classes. The only group identifying
potential Category M Fluids is the
health group. Within the health group,
there are four classifications. The classification
identifying possible Category
M Fluids is number 4 - Danger: May
be fatal on short exposure. Specialized
protective equipment is required.
DOT and NFPA. Using both the Hazardous
Material Table in 49 CFR and
NFPA's Table of Chemical Ratings a
determination of possible Category
M Fluids can readily be made. Any
chemical flowing through pipe or tubing
listed as either a " 4 " in the Health
column of NFPA's Table of Chemical
Ratings, or as a " Forbidden " , " 2.3, " or
" 6.1 " in the " Hazard Class or Division "
column of the Hazardous Materials
Table in 49 CFR could be considered a
Category M Fluid.
While these two resources provide
essentially the same information, they
do so with two different objectives,
and therefore do not always agree. For
example, NFPA considers bromine a
Class 3 (Warning: corrosive or toxic,
avoid skin contact or inhalation).
While the DOT, under 49 CFR, considers
bromine a Class 6.1 (Poisonous materials).
NFPA considers chloroform a
Class 2 (Warning: May be harmful if
inhaled or absorbed), while the DOT
considers chloroform a Class 6.1.
Regardless of whether or not they
agree on the hazard level of a chemical,
if a chemical is indicated as a 4 by
NFPA, or " Forbidden " 2.3, or 6.1 by the
DOT, it should be considered a potential
Category M Fluid.
Declaring a fluid service to be classified
as Category M places added requirements
on the design, fabrication,
construction, and examination of the
piping. These additional requirements
are identified in ASME B31.3 Chapter
VIII - Piping for Category M Fluid
Service. The added requirements
themselves are not significant; however
B31.3 Chapter VIII, like the rest
of the code, does not go into the details
for piping layout requirements and
other detailed design considerations.
There are various aspects of piping
design that go beyond the scope and
dictates of a code. And therein lies the
point at which good design and engineering
practice supplement the sound
basis of code requirements and blends
with the owner's particular needs and
a facility's proprietary specifications.
The B31 series of codes address the
requirements necessary to assure system
safety and integrity by specifying
material limits, stipulating fabrication
requirements, providing minimal
examination requirements, and so on.
They do include some essential design
guidelines when needed to achieve
that assurance. They cannot specify
routing requirements or when to use
double-containment piping systems,
determine accessibility, or whether
this type of piping can or cannot run
underground, in addition to other
proprietary design elements. This degree
of detail is owner- or site-specific.
Even within a large, multi-facility
corporation, these requirements may
vary from plant to plant.
Whenever a chemical, designated as
a Category M Fluid, is made part of a
project, there should be a well-defined
basis of design established by the
owner or engineer in order to convey
to the designer a predetermined set of
design parameters.
Code requirements
The introduction to the ASME-B31.3
piping code states that " The designer is
cautioned that the code is not a design
handbook... " In stating this, ASME is
explaining to the reader in very few
words that this code will not provide
pointers on the best way to route a piping
system, how to support piping, or
the amount of slope a pipeline might
require. It will instead provide criteria,
methods and formulas for such things
as how to determine maximum loads
on piping and ways to test installed
piping for leak integrity. Just as the
piping code provides the criteria for
welding, but does not explain how to
weld, it also provides the minimum
requirements necessary for a safe piping
system, but does not tell engineers
how to design it.
The 2012 ASME B31.3 Piping Code
is composed mainly of ten chapters
and nineteen appendices. The first six
chapters are considered the base code.
Supplementary to the base code are
four additional chapters that pertain
to the specific needs of piping systems
with conditional requirements that
vary from the content of the base code.
The supplemental chapters, VII
through X, use the same paragraph
numbers as the base code, along with
a prefix that identifies the particular
chapter with which it is associated.
In the case of Chapter VIII, for
Category M Fluid Service, the paragraph
prefix is " M. " For example, the
base code paragraph 302.2.4 would be
M302.2.4 in Chapter VIII. If there are
no additional requirements in a paragraph
beyond that of the base code,
then there is a statement that clarifies
that. For instance, there are no
additional requirements in paragraph
M302.4 beyond what is required in
the base code. The statement under
M302.4 in Chapter VIII is therefore
" Paragraph 302.4 applies in its entirety, "
meaning that the requirements
stated in paragraph 302.4 (of the base
code) are sufficient for the supplemental
requirements in Chapter VIII.
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Chemical Engineering November 2013

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