Chemical Engineering August 2021 - 40

fect in contributing to hydrogen attack.
In selecting carbon steel for
use in operating temperatures not
exceeding 500°F, the partial pressure
of hydrogen can exceed in access
of 1,000 psig.
The Figure 3 diagram indicates the
choice of steel warranted to avoid
hydrogen attack as a function of operating
temperature and partial pressure
of hydrogen. Austenitic stainless
steels are not susceptible to HTHA
and are a satisfactory material at all
temperatures and pressures.
With a proven track record, the
most practical material used in hydrogen
service is 316/316L austenitic
stainless steel. While it is advised to
post weld heat treat (PWHT) carbon
steel to bake out any residual hydrogen
from the welding process and decrease
the post weld hardness in the
heat affected zone (HAZ) this is not
required for austenitic stainless steel.
Heat treatment and thermal heat
effect caused by welding have little
effect on the mechanical properties
of austenitic stainless steel. However,
the mechanical properties, such as
strength and hardness of austenitic
stainless steels, can be increased
by cold working. When bending and
forming
austenitic stainless-steel
pipe, its mechanical properties will
be altered, including a reduction in
the material's ductility.
If cold forming is required of
austenitic stainless steel, a full solution
anneal
(heating
to
around
1,045°C followed by quenching or
rapid cooling) will restore the material's
mechanical properties to their
somewhat original values. This will
also remove alloy segregation, sensitization,
and sigma phase realized
after cold working. In performing
the solution annealing process, you
are cautioned that the rapid cooling
might well
introduce residual
stresses back into the material if not
done properly.
For a selection of material that is
acceptable for H2 service, Tables
GR-2.1.1-1 " Material Specification
Index for Piping and Pipe Components "
and GR-2.1.1-2 " Material
Specification Index for Pipelines "
found in ASME B31.12 Hydrogen
Piping and Pipelines are a good
place to start.
38
TABLE 3. LEAK RATE ACCUMULATION
Leak rate
Unit of measure
10-4
10-3
10-2
10-1
cm3/s
cm3/s
cm3/s
cm3/s
Mitigating leaks
With a standard atomic weight of
1.008 atomic mass units (amu), hydrogen
is the lightest and smallest
element in the periodic tables, giving
it a high propensity to leak; with,
I might add, potentially devastating
results. It is therefore imperative
that a gaseous piping system be
designed to limit mechanical type
joints and enhance those joints that
are truly necessary.
In limiting potential leak points, the
system should be entirely welded,
with the exception of flanged joints
at equipment, inline components
and valving. Threaded joints should
be avoided as much as possible, if
not entirely. If threaded joints cannot
be avoided for some reason, it
is recommended that they be fully
engaged without thread sealant then
seal welded. Piping joints should
be buttwelded, and post-weld heat
treatment (PWHT) when carbonsteel
pipe is used. After welding, pipe
within the heat-affected zone (HAZ)
becomes susceptible to hydrogen
attack, even at ambient temperatures.
And while hydrogen attack
mainly occurs at elevated temperatures,
the PWHT step will reduce, if
not eliminate, that possibility entirely,
even at ambient conditions.
The weak point in an all-welded
system will be the flange joint. In
order to provide a high degree
of integrity at flanged joints, the
Kammprofile gasket (Figure 4), or
some form of that gasket design,
should be considered. Manufactured
in much the same way by multiple
manufacturers, it is a very forgiving
gasket. It consists of a serrated solid
metal ring sandwiched between a
soft, deformable sealing material.
The serrations concentrate bolt load
on a smaller area to provide a tight
seal at lower stress. Its design allows
it to compensate for irregularities in
the flange surfaces as well as fluctuations
in service conditions.
Hours to
accumulate 1ft3
78,669
7,866.9
786.7
78.7
ft3/yr
0.11
1.11
11.13
111.31
Another consideration in system
integrity is the valving. Leaks around
valve stem packing and body flanges
is a real concern. In an effort to prevent
this, the selection of bellows
seal valves are recommended.
Using a 1-in. sch. 80 carbon steel
pipe, in our following example, per
ASTM A106 Gr B with allowance for
manufacturing tolerance, corrosion,
and mechanical allowances the
maximum allowable working pressure
(MAWP) can be calculated for
temperatures up to 300°F in a twostep
process. (Note: The reason it
states, " ...for temperatures up to
300ºF... " is due to the fact that the
allowable stress (S) for ASTM A106
Gr B material begins to deteriorate
when the temperature exceeds
300ºF. (S), in Equation (1) would
therefore need to be adjusted for
temperatures above 300ºF.)
Referring to Equation (1), the first
step requires calculating for the theoretical
burst pressure of the pipe.
P = (2 × T × S)/D
(1)
Where:
D = Outside diameter of pipe, in.
P = Burst pressure, psig
Pa = Maximum Allowable Working
Pressure (MAWP), psig
S = Maximum allowable stress for the
material at design temperature, psi
T = Pipe wall thickness minus mechanical
allowance, corrosion allowance,
and manufacturing tolerance, in.
The second part of the process
is to calculate for the maximum allowable
working pressure, Pa, for
the pipe by applying a safety factor,
Sf, against the results for P in accordance
with Equation (2):
Pa = P/Sf
(2)
Therefore, in using the 1-in. sch.
80 material mentioned above, the
calculation for the burst pressure
would look like the following:
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