H2Tech - Q2 2021 - 46

CHEMICAL AND FERTILIZER PRODUCTION

-148
200

-58

Temperature, °F

122

212

302
148

180

133

160

118
Curve A

140

Curve B

DBTT
100
80

103
89

Lower toughness at
comparable temperature

120
Energy, J

ature (DBTT) for steels. Depressurization
will lower temperatures, and repressurization controls may be needed to ensure that
the stresses are acceptable. In older plants,
refrigeration chillers may not meet existing guidelines, or they may be operated at
lower-than-DBTT temperatures.
NH3 in refrigeration service or in
cryogenic storage systems typically operate in a cold condition. NH3 's saturation
temperature at atmospheric pressure is
-34°C (-29°F). There are, however, a
number of instances and situations where
brittle fracture failures are a concern in
the NH3 industry. The use of carbon or
low-alloy steel, thick-walled loop equipment is likely to have increased MDMT.
Auto-refrigeration can reduce temperatures below the MDMT.16,17,18
Pressure vessels using steels pre-1987
materials of construction and older
equipment produced prior to ASME code
revisions recognizing plane strain impacts
on DBTT can be more susceptible to
brittle fracture.19 Use of inferior, vintagegrade steels such as SA212, SA285 and
SA225-which are prone to brittle fracture under certain conditions-are more
vulnerable. Carbon steels and low-alloy

74
59
DBTT

0
-100

-50

100

0
150

Temperature, °C
FIG. 4. Charpy impact curves showing the transition from a ductile behavior to a more brittle
behavior as the temperature decreases.17

Q2 2021 | H2-Tech.com

Energy, ft-lbs

perature drops, resulting in low toughness at low temperatures and increasing
the hazard of brittle fracture. Increasing
the thickness of a material can result in
higher ductile-to-brittle fracture temperatures. A typical impact energy curve, or
Charpy curve, is shown in FIG. 4.8,16
Brittle fracture can occur in the presence of a small flaw in a low-toughness
material when stresses are sufficiently
high. It is a concern in NH3 equipment
since NH3 's boiling point of -34°C
(-29°F) is slightly less than the common
carbon steel minimum design metal temperature (MDMT) of -29°C (-20°F).
Refrigerated services typically require
material specifications and/or Charpy
impact testing to avoid introducing brittle
fracture hazards to the plant.
A limited portion of the NH3 process
sees these temperatures in normal conditions. NH3 process units where brittle
fracture is a concern include the synthesis
loop, syngas compression, refrigeration,
molsieves, NH3 wash and cryogenic purification. Equipment such as chillers, flash
drums and NH3 product separators may
be susceptible to reaching temperatures
below the ductile brittle transition temper-

steels with low toughness and an existing
flaw or crack, and materials that contain
residual stresses due to improper stress
relief or that do not undergo PWHT, are
more susceptible. Normalized steel is less
susceptible to brittle fracture than steels
having undergone other treatment.
Operationally and from a design standpoint, pressurized ammonia systems that
normally operate above -29°C (-20°F)
can quickly decrease in temperature below the minimum allowable when the system is depressurized. Additionally, thicker wall vessels created for high pressure,
such as those found in the syngas compression, synthesis loop and purge units,
are more susceptible. These vessels can
have significantly higher minimum design temperatures for the same materials
due to plane strain, and are more likely to
fall below their minimum allowable temperatures at ambient conditions. Liquid
nitrogen exposures (including nitrogen
purge equipment and process equipment
under poor purge controls) are a hazard,
and liquid nitrogen supply contractors
may introduce a brittle fracture hazard to
the site if using improper equipment.
To prevent brittle fracture, inspection
methods should focus on the initiation
of cracks. After initiation, ensure that the
crack size is stable to avoid fast and unstable fracture. Methods using API 5791/ASME Fitness for Service, Part 9 and
other specifications can be used for this
assessment.20,21 Typical inspection methods to search for cracks include:
* Ultrasonic testing (UT) for cracks,
particularly in welds
* Penetrant testing (PT)
* Magnetic particle testing (MT)
Options to mitigate brittle fracture include following up-to-date ASME codes
for carbon steel pressure vessel materials
that are subject to temperatures of less
than -29°C (-20°F). Selecting proper
materials for equipment and piping based
on establishing the appropriate MDMT,
including the use of fine-grained carbon
steels with proven toughness by impact
testing, nickel-containing low-alloy steels
with 9% Ni steel at temperatures as low
as -196°C (-320°F), austenitic stainless
steels (i.e., 300 series), and aluminum
alloys (because they have no ductile-tobrittle transition when cooled), are essential to preventing brittle fracture.
Performing PWHT to relieve stress can
be used to improve the DBTT of the equip-

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