Chemical Engineering August 2021 - 38

H2 Partial Pressure, kPa
1,200 0 1,000 2,000
1,000
800
0.5 Mo
600
Carbon steel
400
3,000 4,000 5,000
2.25Cr -Mo
1.5Cr - Mo
1-Cr-Mo
6,000
600
500
400
300
200
0 100 200 300 400 500 600 700 800 900 1,000
H2 Partial Pressure, psi
FIGURE 3. This modified Nelson diagram (adapted from API 941) can be
used for selecting the right material for hydrogen service at different
temperatures
H2 in air at 1 atm = 1.9×10-8 Btu
(0.02 mJ).
Explosive limits are the maximum
and minimum concentrations of
vapor, mist, or dust in air or oxygen
at which explosions occur. The size
and geometry of the environment as
well as the concentration of the fuel
control the limits. " Detonation limit "
is sometimes used as a synonym for
" explosive limits. "
The explosive limits for a mixture of
H2 in air are 18.3 vol.% (lower limit)
and 59 vol.% (upper limit).
Piping materials
In the design of piping systems (Figure
1), one of the first steps in the
process will be to determine the material
of construction needed for each
fluid service. And each fluid service
will be categorized in accordance
with ASME B31.3 para. 300(b)(1),
which states that, " The owner is also
responsible for designating piping in
Category D, Category M, High Pressure,
and High Purity Fluid Services,
and for determining if a specific Quality
System is to be employed. "
Categorizing a fluid service identifies
the degree of examination and
the types of examination that are required
along with a number of other
requirements based on a fluid's category.
The owner's responsibility for
this will typically fall to the owner's
engineering department or an outsourced
engineer.
And while the B31.3 Process Piping
Code does not tell an owner
what material to use for specific fluid
services, it does give guidance for
a material's strength, thickness and
joining requirements. There are also
36
two
statements
within the Introduction
of the Code
that are explicit
stating that:
*
" It is the
owner's responsibility
to select the
Code Section that
most nearly applies
to a proposed piping
installation. "
*
" The Code
is not a design
handbook. Many
decisions that must
be made to produce a sound piping
installation are not specifi ed in detail
within this Code. The Code does
not serve as a substitute for sound
engineering judgments by the owner
and the designer. "
And to expand on the first bullet
point above, B31.3 Para. 300(b)
(1) also states that, " The owner of a
piping installation shall have overall
responsibility for compliance with
this Code, and for establishing the
requirements for design, construction,
examination, inspection and
testing that will govern the entire
fluid handling or process installation
of which the piping is a part. " Having
therefore set a few ground rules for
responsibilities and the requirement
for designating fluid service categories,
let's see where gaseous hydrogen
fits into all of this.
Because of, and aside from, its
tendency as a leak-prone, volatile
fluid, gaseous hydrogen can be, in
accordance with B31.3 fluid service
categories, considered either a Normal
Fluid service or a Category M
Pipe
Fittings
Flanges
Bolts
Gaskets
Valves
Gate
Globe
in
fluid service. As mentioned above,
categorizing a fluid service is the
owner's call, so long as it fits within
a selected Category guideline as
described
in B31.3, para. 300.2
Definitions, under " fluid service. "
Following are definitions of both
Normal Fluid service and Category
M Fluid service:
" Normal Fluid Service: a fluid service
pertaining to most piping covered
by this Code, i.e., not subject
to the rules for Category D, Category
M, Elevated Temperature, High Pressure,
or High Purity Fluid Service.
Category M Fluid Service: a fluid
service in which both of the following
apply:
(1) the fluid is so highly toxic that
a single exposure to a very small
quantity of the fluid, caused by leakage,
can produce serious irreversible
harm to persons on breathing or
bodily contact, even when prompt
restorative measures are taken
(2) after consideration of piping design,
experience, service conditions,
and location, the owner determines
that
the requirements
for Normal
Fluid Service do not sufficiently provide
the leak tightness required to
protect personnel from exposure. "
In the above definition for Category
M, gaseous hydrogen does not meet
the criteria for item (1) because it is
not considered a toxic fluid. But the
Code leaves the door open to enable
the assignment of a fluid service as
Category M by applying paragraph
(2) in stating that after due consideration
of " ...piping design, experience,
service conditions, and location... "
the owner can indeed make
the determination that the requirements
for Normal Fluid Service do
TABLE 2. EXAMPLE SPECIFICATION FOR H2 SERVICE
Sch. 80, C.S., ASTM A106 Gr. B
Sch. 80, B.W., carbon steel, ASTM A234 Gr. WPB, ASME B16.9
R.F. Welding neck, carbon steel, ASTM A105, ASME B16.5, 300 Class
Stud bolts, ASTM A193 Gr B7, with nuts, ASTM A194 Gr 2H
(Manufacturer) or equal gasket, 0.125-in. thick. 316 stainless-steel serrated
solid-metal core, with spiral-wound flexible graphite both sides, and integral
centering ring
B.W. or flanged, bellows seal gate valve, welded bonnet, stainless-steel bellows,
test port - (Manufacturer) #B-(size code)-2-06-4T-02-TS or equal.
B.W. or flanged, bellows seal globe valve, welded bonnet, stainless-steel
bellows, test port - (Manufacturer) # B-(size code)-2-07-4T-02-TS or equal.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
AUGUST 2021
Temperature, °F
Temperature, ºC
http://WWW.CHEMENGONLINE.COM
Chemical Engineering August 2021

Table of Contents for the Digital Edition of Chemical Engineering August 2021
