Chemical Engineering August 2021 - 42

step procedure wherein the first step
is a pneumatic leak test for pressure
integrity followed by a sensitive leak
test for leak integrity. Both tests are
in accordance with ASME B31.3, ensuring
the integrity of the joints with a
close approximation of the H2 service
gas and its leak potential.
Aside from hydrogen's potential
volatility, its molecular size, with an
atomic mass of 1.008 amu, creates
a containment problem. (The reason
for the follow-up sensitive leak
test.) Helium, with an atomic mass of
4.002602 amu, provides a very close
approximation of that containment
problem. Nitrogen, with an atomic
mass of 14.00674 amu, would not
provide the same assurances found
when using helium as the test gas.
I would recommend against using a
heavier gas for the leak test, such as
nitrogen, by then extrapolating the
results of a nitrogen leak rate to determine
what the leak rate might be
if it were H2.
When testing potentially volatile
or lethal piping systems for leaks, a
maximum allowable leak rate should
be predetermined and specified. The
quantitative aspect of the accumulation
of H2 from a leak can be assessed
under two separate criteria:
1. Is the piping inside a building?
a. Is the building well ventilated?
b. Are there any potential ignition
sources within close proximity to
the piping?
c. Is the piping in an area that
could pocket and accumulate hydrogen
emission?
2. Is the piping in open air?
a. Are there potential ignition sources
within close proximity to the piping?
Two different acceptable leak rates
could be established for inside piping
systems and outside piping systems.
The high diffusion rate of helium
makes it difficult to test for minute
quantities of helium on an inside installation,
and even more difficult on
an outside installation. For that reason,
and because H2 would probably
not have a place to accumulate outside,
a higher leak rate could be tolerated
for an outside installation.
Detecting leaks can be accomplished
with a soapy water solution
like Snoop. However, this is not the
best method for locating the rela40
tively
small leaks that could occur
with helium. Nor does it provide a
means to quantify the leak rate.
Using a helium probe (spectrometer),
leak rates can be determined to a
level of 10-6 cm3/s. This allows leaks
to not only be located but to be quantified
as well. Determining and specifying
the maximum allowable leak rate
for gaseous hydrogen is a plant- or
owner-specific issue that is based on
a plant by plant circumstance.
If it is determined that a single maximum
allowable leak rate would apply
to both inside and outside installations,
then the basis for a worse case
inside installation would determine
the maximum allowable leak rate. In
making that determination, a scenario
would have to be created whereas a
leak would occur at a joint, at an assumed
leak rate, inside a building, in
still air, with a vaulted or penthousetype
ceiling; a space above the leak
where H2 could accumulate.
Assuming good design practices
have been followed, the main concern
regarding this discussion is with
a gaseous H2 discharge from a leak
accumulating in an enclosed building.
Table 3 lends some perspective
when assessing the magnitude of a
given leak rate.
When setting a value for an allowable
leak rate, it should be assumed
that good design practices may not
be adhered to. Even though a good
design, particularly where H2 is concerned,
would not allow a building to
be designed without good ventilation,
and would not allow a penthouse type
ceiling without ventilation, it should be
assumed otherwise; a worst case
scenario, if you will.
As a reference, ASME B31.3 -
Process Piping provides for a Sensitive
Leak Test in Para. 345.8, subparagraph
345.8.2 Method, in which
it states that, " The test shall be the
Bubble Test - Direct Pressure Technique
in accordance with ASME
BPVC, Section V, Article 10, Mandatory
Appendix I or another leak test
method that has a demonstrated
sensitivity not less than 10−3 std.
mL/s under test conditions. "
Final remarks
Whether you're designing a lowpressure
gaseous H2 distribution
system at ambient temperatures or a
resid-hydrotreater system operating
in the neighborhood of 3,000 psig at
600°F, engineering and construction
need to go into it with a good understanding
of the many risk nuances
that H2 brings to the table.
As mentioned a number of times,
H2 is very volatile and unforgiving.
Do the design and construction
as if you were the one who had to
work around such a system every
day. That kind of mindset brings a
whole new perspective to what you
are doing.
The list of codes, standards and a
regulation in Table 1 provide a wealth
of information regarding the design
and construction of hydrogen piping
systems. I would recommend
gaining access to the information
contained in these volumes prior to
getting involved with designing and
constructing a H2 system. Understanding
the nuances in H2 piping
design helps you avoid the pitfalls
you might otherwise overlook. ■
Edited by Gerald Ondrey
Author
W. M. (Bill) Huitt has been involved
in industrial piping design,
engineering and construction
since 1965. Positions have included
design engineer,
piping
design instructor, project engineer,
project supervisor, piping department
supervisor,
engineering
manager and president of W. M.
Huitt Co. (P.O. Box 31154, St.
Louis, MO 63131-0154; Phone: 1-314-966-8919;
Email: wmhuitt@aol.com; Website: www.wmhuittco.
com), a piping consulting firm founded in 1987. His experience
covers both the engineering and construction
fields and crosses industry lines to include petroleum
refining, chemical, petrochemical, pharmaceutical, pulp
& paper, nuclear power, biofuel and coal gasification. He
has written numerous specifications, procedures on design
and construction, guidelines, papers, and magazine
articles on the topic of piping design and engineering.
Huitt has also written " Bioprocessing Piping and Equipment
Design - A companion guide for the ASME BPE
Standard. " He is a past member of ISPE (International
Society of Pharmaceutical Engineers), CSI (Construction
Specifications Institute) and a current and active member
of ASME (American Society of Mechanical Engineers).
He is a member of the B31.3 section committee,
Chair of B31.3 Subgroup H on High Purity Piping, Vice
Chair of ASME BPE subcommittee on Certification, a
member of three other ASME-BPE subcommittees and
is active on several Task Groups. Huitt is also a member
of the ASME Board on Conformity Assessment for BPE
Certification, a member of the A13 Standards Committee
for Standard A13.1 Scheme for the Identification of
Piping Systems, a member of the API (American Petroleum
Institute) Task Group for RP-2611, and he serves
on two corporate specification review boards. He has
also authored the training program and provides training
to ASME consultants for auditing fitting manufacturers
applying for ASME BPE Certification.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM AUGUST 2021
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Chemical Engineering August 2021

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