H2Tech - Q2 2021 - 44

CHEMICAL AND FERTILIZER PRODUCTION
environments containing carbonic, acid
and chlorides. However, these materials
are not impervious to corrosion.
Copper alloys are highly susceptible
to corrosion attack in ammonia environments and should be avoided for equipment in the pressure boundary or in
direct contact with process fluids in the
ammonia industry.
Three metallurgical damage mechanisms commonly affect ammonia equipment and are discussed in relation to
process variables, process equipment and
materials of construction:
* High-temperature hydrogen attack
(HTHA)
* Environmentally assisted stress
corrosion cracking (SCC)
* Brittle fracture.
Awareness of mechanisms that can
damage equipment is essential in developing solutions to properly inspect the equipment, mitigate damage and prevent failure. The pertinent damage mechanisms
are also input for evaluations regarding
fitness-for-service as the mechanism and
rate of attack need to be understood to
determine the remaining life. For a proper
risk-based inspection (RBI) program or
during a hazards analysis, the appropriate mechanisms are identified so that the
probability of failure can be determined in
addressing reliability issues.5
High-temperature H2 attack (HTHA).
HTHA is a form of degradation caused
by H2 reacting with carbon to form methane in a high-temperature environment.
When steel is exposed to H2 at elevated
temperatures, H2 will diffuse into the alloy and combine with carbon to form
small pockets of methane. The meth-

FIG. 2. H2 damage observed in the carbon
steel line at the heat-affected zone (HAZ).
Decarburization and fissuring region caused
by H2 depleting the iron carbides. Nital etch.
(Original magnification: 200 times).6,12

Q2 2021 | H2-Tech.com

ane is trapped at grain boundaries and
in voids and does not diffuse out of the
metal. Once accumulated, the methane
expands, forming voids that can lead to
initiated cracks in the steel. An example of
such fractures, viewed in the microstructure of a carbon steel pipe weld, is shown
in FIG. 2.6 High-strength, low-alloy steels
are particularly susceptible to this mechanism, which leads to embrittlement of the
bulk parent metal. The embrittlement in
the material can result in a catastrophic
brittle fracture of the asset.7-10
Susceptible materials include highstrength, low-alloy steels (legacy C-½Mo
steels), plain carbon steels, non-post-weld
heat-treated (PWHT) welds, and copper
alloys. Alloy steels such as 1.25Cr-0.5Mo
provide resistance for milder HTHA conditions, but the alloy must be matched
properly to the process conditions that
the metal sees. API RP 941 provides guidance to aid in materials selection for fixed
equipment operating in environments
with H2 partial pressures at elevated temperatures and pressures.11 This guidance
also can be useful to materials engineers
and process engineers alike, as knowledge
of both process conditions and the materials of construction will provide information on an asset's susceptibility to this
particular damage mechanism.
The most obvious equipment concerns are any equipment exceeding normal operating temperatures or operating
window limits, specifically carbon and
low-alloy steel vessels and piping operating at temperatures that are above the API
RP 941 Nelson curve values. Aging plants
should be mindful of API RP 941 Nelson curve changes and should determine
whether process changes or HTHA mitigation strategies may be implemented.
HTHA is not a concern in stainless steel
vessels; however, stainless steel-lined vessels with the possibility of high H2 partial
pressure behind the liner are a concern.
H2 content is high in the ammonia
process streams-up to 67% on a volume
basis-and it is important to evaluate for
HTHA potential where the temperatures
rise above 204°C (400°F) for carbon steel
materials. The H2 content should be considered on a wet gas basis, which may reduce the risk susceptibility for equipment
prior to condensation of water vapor occurring after the shift unit.
Typical concerns start with the shift
unit and equipment through the ammo-

nia synthesis loop, where temperatures
are above 204°C (400°F). However,
HTHA may be present in other areas,
such as secondary unit pressure shells
due to refractory failure, which can lead
to high temperatures on steel pressure
shells. Referring to FIG. 1, HTHA also can
be a concern throughout the ammonia
process, including equipment in the shift
units (both high and low shift), methanation and synthesis loop.
Synthesis loop equipment, particularly
converters without furnace stress relief
or operating at high temperatures on the
pressure shells, and startup heater coils
are also vulnerable points for attack. Additionally, hot spots from refractory failures
can occur in the primary reformer outlet,
in secondary reformer and waste heat
boilers, molsieves and pressure envelopes.
Thermal imaging inspections or other
means are used to monitor hotspots on
refractory lined pressure shells of HTHA
(or creep) in susceptible materials.
Hester and Benac provide details regarding an investigation conducted into
a carbon steel effluent cooler header piping rupture, installed in an ammonia converter and synthesis loop, that occurred 5
yr after a change in operating conditions.
The process temperature was increased
from 232°C (450°F) to 510°C (490°F),
and the operating pressure was decreased
from 29 MPa (4,200 psig) [14.5 MPa
(2,100 psig) H2 partial pressure] to 23.4
MPa (3,400 psig) [11.7 MPa (1,700 psig)
H2 partial pressure]. This process change
placed the carbon steel pipe above the API
RP 941 Nelson curve temperature for carbon steel at the corresponding H2 partial
pressure. The piping rupture was found to
have a brittle fracture appearance.
Failure analysis revealed that HTHA
was the damage mechanism that caused
the pipe rupture. This example case demonstrates the vulnerability of this portion
of the ammonia process if material limits
are exceeded and how process changes
can create the potential for eventual failure.6,12 Uncontrolled materials substitutions can also lead to failures.
Several inspection methods may be
successfully used to identify HTHA:
* Visual inspection
* Advanced ultrasonic backscattering
techniques (AUBT)
* Advanced phase array
* High-sensitivity wet fluorescent
magnetic particle testing (WFMT)

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H2Tech - Q2 2021

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