Chemical Engineering February 2011 - 37

TABLE 3. TYPICAL FRAGMENT RANGES FOR BRITTLE FRACTURE BY
METHOD OF BAKER [6]
Fragment
mass fraction
0.1
0.2
Fragment
mass, kg
186
372
0.5 (2 of equal size) 930
0.6
0.7
0.8
0.9
1,117
1,303
1,489
1,675
Exposed
area, m2
0.2027
0.2027
0.2027
0.2027
0.2027
0.2027
0.2027
and others [8] presented a mean time
between failure (MTBF) for LNG piping
systems of 5.8 × 108 ft-hr.
Probability of one LNG piping system
failure during pneumatic testing =
[(Number of pneumatic test systems)
×(Length of pipe per system)× (Duration
of each pneumatic test)]/ (MTBF)
(10)
The test duration used in one project
was approximately 2 h (1 h at lower
pressures, 15 min at Pdesign, 30 min at
Ptest, and 15 min at Pdesign).
Assuming 200 LNG pneumatic-test
systems and an average of 300 ft of
pipe per test system, the probability
of failure of one LNG piping system
among all those tested during LNG
pneumatic testing is approximately
1 in 5,000. Because of the " bathtub "
shape of failure rate data, the probability
of failure of an LNG piping
system during test may be somewhat
higher. (Graphs of failure rates versus
time typically resemble a bathtub,
with a high initial failure rate, then a
constant and low failure rate during
the expected life of the product, and
finally an increasing failure rate at
end-of-life.)
In the unlikely event of a failure of
an LNG piping system during pneumatic
pressure testing, all personnel
would be protected from blast overpressure
by the conservative exclusion
zones in use. Although one or
more fragments might fall outside the
exclusion zone, most of the fragments
References
1. Mannan, S. M., ed., " Lee's Loss Prevention in
the Process Industries " , 3rd ed., 3 volumes,
Elsevier Butterworth-Heinemann, Oxford,
U. K., 2005.
2. Health and Safety Executive, " Safety in
Pressure Testing " , Guidance Note GS4, UK
Health and Safety Executive, Sudbury, Suffolk,
U. K., 1998.
3. Center for Chemical Process Safety, " Guidelines
for Consequence Analysis of Chemical
Releases " , American Institute of Chemical
Engineers " , New York, N.Y., 1999.
4. Crowl, D. A., Calculating the Energy of Explosion
Using Thermodynamic Availability, J
of Loss Prevention in the Process Industries,
vol. 5, pp. 109-118, 1992.
Velocity,
m/s
109
80.5
64.4
63
62.2
61.8
61.5
Range,
m
1,093
597
429
419
401
399
397
would fall within the exclusion zone,
and the probability of a person being
struck by a fragment is very low.
A confidential and detailed quantitative
risk assessment of a proposed
pneumatic test of a 6-km LNG pipeline
also yielded the conclusion that
the risk of pneumatic testing would be
acceptably low.
When rigorous precautions have
been taken to establish safe testing
procedures and to ensure the integrity
of the piping systems being tested, the
risk of pneumatic pressure testing
of selected LNG piping systems can
be made acceptable. As a minimum,
rigorous attention needs to be given
to the essential considerations listed
in the box, Precautions in pneumatic
pressure testing, p. 33.
Safe exclusion zones
The shock wave from rupture of a
piping system under test could affect
much of the immediate area surrounding
the test, so the exclusion zone
for the test should be at least large
enough to keep the overpressure from
exceeding the criterion selected in
this article of 0.5 psig (0.0345 barg).
The intent is that only those people
conducting the pneumatic pressure
test would be allowed to enter the
exclusion zone and then only under
carefully prescribed conditions.
Conversely, projectiles resulting from
fragmentation of a piping system will
be relatively few in number, and most
would fall relatively close to their origin.
5. Brode, H. L., Blast Wave from a Spherical
Charge, Physics of Fluids, vol. 2, p. 217,
1959.
6. Baker, W. E., and others, " Explosion Hazards
and Evaluation " , Elsevier, New York, N.Y.,
1983.
7. Baker, W. E., Post-Test Assessment of Blast
and Fragment Effects of Explosive Failure of
a Large Steel Containment Shell Model During
Pneumatic Testing, Conference Paper, pp.
289-296, 1985.
8. Atallah, S., and others, Reduction of LNG
Operator Error and Equipment Failure
Rates, Gas Research Institute Report No.
GRI-90/0008, Chicago, Ill., 1990.
The remaining few would be distributed
over a comparatively large area and
would have only a very low probability
of hitting a person. These considerations
form the basis of the logic of using only
overpressure considerations in order to
establish exclusion zones.
■
Edited by Dorothy Lozowski
This article was presented at the 10th Natural
Gas Utilization Symposium and the Spring National
AIChE Meeting, March 21-25, 2010, San
Antonio, Tex.
Authors
Victor H. Edwards is director
of process safety for Aker
Solutions Americas Inc., (3010
Briarpark Drive, Houston, TX
77042; Phone: 713-270-2817;
Fax: 713-270-3195; Émail: vic.
edwards@akersolutions.com).
In his 27 years with Aker,
Edwards' experience includes
process engineering, safety
management and process,
biochemical and environmental
technologies. He has received numerous accolades
in the areas of safety and environmental
engineering, including five DuPont awards, and
has contributed extensively to the engineering literature.
His earlier experience includes assistant
professor of chemical engineering at Cornell University,
an assignment at the National Science
Foundation, pharmaceutical research at Merck,
alternate energy research at United Energy Resources,
visiting professor at Rice University
and process engineering at Fluor Corp. Edwards
earned his B.A.Ch.E from Rice University and his
Ph.D. in chemical engineering from the University
of California at Berkeley. A registered professional
engineer in Texas, he is an AIChE Fellow,
and a member of ACS, AAAS, NFPA, NSPE, and
the N.Y. Academy of Sciences.
Don Sanford is a senior engineering technician
with Aker Solutions Americas Inc. In his 15
years there, Sanford's experience includes piping
materials, piping fabrication, testing, installation,
coatings, thermal-insulation and heat-tracing
specifications. Prior to working with Aker,
Sanford worked with Stearns Rogers, Black &
Veatch, KBR and Raytheon.
Brooke Bonstead is a mechanical
engineer for WorleyParsons
(575 North Dairy
Ashford, Houston, TX 77079).
In her five years in the oiland-gas
EPC industry, Bonstead's
experience includes
serving as a requisitioning
engineer for various mechanical
packages for both onshore
and offshore LNG regassification
and offshore oil production
facilities. She earned her B.S. from Texas
A&M University and is a registered EIT in the
state of Texas.
Larry Skoda is a principal
piping engineer (Email:
Larry.Skoda@SBCGlobal.net;
Phone: 713-372-7174), who
has worked for more than
thirty years as a pipe stress
and piping engineer for companies
designing facilities for
oil and gas, petrochemical and
power companies. His most recent
such employer was Aker
Kvaerner in Houston. Skoda
is a professional engineer who graduated with
a B.S.M.E. from University of Texas at Arlington.
He was a contributor to the piping section of
" Perry's Chemical Engineers' Handbook " .
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