H2Tech - Q2 2021 - 45

CHEMICAL AND FERTILIZER PRODUCTION
* Time of flight diffraction (TOFD)
* Replication of surfaces
* Positive material identification
(PMI)
* Thermographic temperature
surveys.
Note: These methods are highly dependent on the technique and skill level of
the inspector. False positives and negatives
are possible with improper techniques
and inadequate skill levels. This is why it
is essential to consult qualified inspection
experts to assist in inspection efforts.
One of the most critical ways to mitigate the potential for HTHA is for plant
engineering to review plant processes,
U1 materials of construction and operating conditions to identify potential
HTHA risks with H2-containing equipment. An important component of this
includes conducting an engineering review of pressure, H2 partial pressure and
temperature. Operating with safety margins-e.g., 10°C (50°F) below the API
RP 941 Nelson curve-also can provide
additional assurance. Engineering should
establish integrity operating limits for all
vulnerable equipment. Materials control
programs are essential, including guards
against uncontrolled materials substitutions, and active positive materials identification programs for incoming materials
and field/retro components.
If feasible, aging plants should consider reviewing potential replacement of
equipment with higher-alloy material that
is less susceptible to HTHA, according to
the API RP 941 Nelson curve for desired
operating conditions. This review should
include determining whether welded
equipment or piping was post-weld heattreated, and if not known, assume nonpost-weld heat-treated and operate at
lower temperature and pressure. Installing temperature indicators at critical locations to monitor actual temperatures and
performing regular thermography measurements can help ensure that operating
windows and limits are not exceeded, or
identify those that need to be addressed.
Environmentally assisted stress corrosion cracking (SCC). SCC occurs when

a susceptible material is exposed to a specific environment and tensile stress. The
combination of these three factors leads to
brittle fracture of normally ductile materials at stress levels within the normal operating range. Multiple types of environmen-

tally assisted SCC exist, including chloride
SCC, amine SCC and ammonia SCC.
Nitrate SCC may come from exposure
to nearby nitric acid, ammonium nitrate, or
urea emissions that hydrolyze to form nitrate. During stress corrosion cracking, the
material may not show signs of wall loss or
pitting, but fine cracks will form within the
material or on the surface. This process has
serious implications on the utility of the
material because the applicable safe stress
levels are drastically reduced in the corrosive medium. Some environments of concern are potash, nitrates and sulfides.7,13,14,15
The most common cracking mechanism detected in ammonia storage tanks,
spheres and other process equipment is
ammonia SCC (NH3 SCC), which is a
function of ammonia exposure in conjunction with an O2 source-typically O2 or
CO2. NH3 SCC can occur at atmospheric
liquid ammonia conditions of -33°C
(-27°F), but at faster rates at higher temperatures under pressurized ammonia conditions. Tanks used for transporting liquid
NH3 , like rail tank cars, tank cars, barges
and vessels, are also susceptible to SCC.
For ammonia equipment, susceptible
materials to NH3 SCC include carbon
steel, low-alloy steels and stainless steels
exposed to high O2 levels or chlorides.
Residual stress within parts increases the
potential for SCC; therefore, post-weld
heat treatment (PWHT) of carbon steels
can lower the probability of SCC. PWHT
is often used to mitigate NH3 SCC in pressurized equipment. NH3 storage tanks are
also particularly susceptible to NH3 SCC,
as they may lack PWHT and be more susceptible to O2 contamination in the presence of NH3 . High hardness can be a concern, as higher hardness material is more
susceptible, with the heat-affected zone
(HAZ) and areas with localized stress being the most vulnerable. Vapor spaces may
be preferentially attacked due to higher
temperatures and the higher presence of
O2 that is not protected by water in the
NH3 liquid phase.
For NH3 equipment, chlorides may be
present in insulation or cooling waters,
both of which can result in SCC of stainless steels. Affected NH3 process units
include susceptible materials in the convection unit, such as headers exposed to
chlorides in insulation; primary reformer
inlet pigtails; the synthetic loop startup
heater coils, which may be exposed to atmospheric SCC promoters; and exchang-

ers with seawater or cooling water with
high chloride content.
Several inspection methods can be
used to identify SCC cracks:
* Wet fluorescent magnetic particle
testing (WFMT)
* Angled beam ultrasonic (UT) at
weld HAZs
* Hydrostatic testing
* Acoustic emissions testing (AET)
NH3 SCC is mitigated by avoiding
air, O2 and CO2 sources, and by the addition of small amounts of water (0.2%)
where O2 may be present. The design and
operation of atmospheric tanks should
avoid vacuum conditions that would pull
air into the vapor space. Vapor spaces are
more susceptible due to higher temperatures and the higher presence of O2 that
is not protected by water in the ammonia.
Especially where PWHT is not practical,
such as atmospheric storage tanks, materials are specified with both minimum and
maximum hardness, and suitable weld
materials are chosen. For chloride SCC,
keeping steam temperatures below 60°C
(140°F) can prevent cracking in stainless
steel heat exchangers.
Brittle fracture. The loss of ductility de-

termines whether a brittle fracture could
occur. The conditions, mechanisms and/
or degradations that cause the loss of ductility must be considered. Brittle fracture
occurs when a material breaks with little
to no plastic deformation. Typically, a
fracture occurs rapidly, with no warning
and with less energy needed than a ductile
fracture. FIG. 3 shows a brittle fracture that
occurred suddenly in an NH3 plant.17,18
Most metals undergo a ductile-tobrittle transition in fracture toughness
with decreasing temperature. Carbon
steel and low-alloy steel materials will
undergo a transformation from ductileto-brittle behavior as the material tem-

FIG. 3. Brittle fracture of a pressure vessel
in an ammonia plant.
H2Tech | Q2 2021

H2Tech - Q2 2021

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