Chemical Engineering May 2021 - 44

FIGURE 2. Granular urea fertilizers are widely used across the globe and their
demand is expect to grow considerably in the next five years
almost zero oxygen to be present
while providing greater strength.
Depending on the age of the facility,
process conditions and the licensor
of the specific process, alloy selection
may vary. It is critical for valve
manufacturers providing urea service
valves to strictly adhere to the
rigorous alloy standards required.
To avoid problems, it is best for end
users to clearly identify and document
the requirements for materials,
procurement and inspection.
The International Molybdenum
Association (IMOA; London, U.K.;
www.imoa.info) provides some of
the following practical guidance
and useful background on DSS [1]:
* DSS alloys have a two-phase
microstructure comprised of grains
of ferritic and austenitic stainless
steel. Figure 4 shows the yellow
austenitic phase as " islands " surrounded
by the blue ferritic phase
* When DSS is melted, it solidifies
from the liquid phase to a
completely ferritic structure
* As the duplex material cools
to room temperature, about half of
the ferritic grains transform to austenitic
grains ('islands'). The result
is a microstructure of roughly 50%
austenite and 50% ferrite
Due to their high strength (around
twice that of typical austenitic or
ferritic stainless steels), improved
corrosion resistance and weldability,
DSS alloys are ideal for valves in
urea applications and other severeservice
processes. These alloys are
strong enough to handle pressures
in the range of 1,500 to 2,500 psi.
Valves for urea service must facili44
tate
self-washing
flow and provide
no cavities where
the build-up of
precipitants can
occur. DSS and
SDSS alloys are
also used in piping
and tubing
because of their
strength and corrosion
resistance.
These chromium-nickel-molybdenum
(Cr-NiMo)
alloys contain
high chrome content, as well as
varying nickel, molybdenum, manganese
and other alloying element
additives. The Huey test (ASTM
A262, Practice C), developed by
ASTM International (West Conshohocken,
Pa.; www.astm.org),
is used extensively to confirm the
ability of alloys to withstand the
type of corrosion present in urea
processes,
and
the
chromiummolybdenum
ratio is a reasonable
(but not perfect) predictor of likely
Huey test performance. The pitting
resistance equivalent number
(PREN) is another useful measure
of corrosion resistance usually relative
to corrosion pitting temperature
(CPT) and corrosion cracking
temperature (CCT), as measured
by the Streicher test (ASTM A262,
Practice B). PREN typically increases
with the chromium, molybdenum
and nitrogen content of the
alloy. For 316L stainless steel, the
PREN is 25. For a typical DSS, the
PREN is around 36. For SDSS with
chrome content of 25% and more,
the PREN can exceed 40. Lastly,
the ferrite content of these alloys is
generally controlled, and in addition
to the Huey test, such test data are
often required on certified material
test reports for urea alloys.
DSS and SDSS alloys are very
expensive to develop and manufacture
(Figure 5). One helpful analogy
would be custom pharmaceuticals.
Some original equipment suppliers
to urea plants have secured access
to custom alloys developed by licensors
and their materials organizations.
Other common high alloys
used in the manufacture of urea
service valves are Ferralium 255,
310 MoLN, Duplex F51, SAF 2507,
Super Duplex F53, Zeron 100
and titanium.
Research and development activities
related to advancing SDSS
alloys' performance are ongoing
in industry. Recent product innovations
include: the Safurex and
Safurex Star grades (developed by
Stamicarbon and Sandvik); Uremium29
(developed by Casale and
Tubacex); and DP28W (developed
by Toyo and Sumitomo).
In most instances, the design
of urea service valves is uniquely
driven by the requirements of
the process and the behavior of
the chemicals involved. However,
valves should conform to the
B16.34 standard developed by the
American Society of Mechanical
Engineers (ASME; New York, N.Y.;
www.asme.org) and all other apFIGURE
3. Typical urea-synthesis processes involve reacting ammonia and carbon dioxide, followed by a
stripping step, carbamate condenser and scrubbing
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM MAY 2021

http://www.astm.org http://www.imoa.info http://www.asme.org http://WWW.CHEMENGONLINE.COM

Chemical Engineering May 2021

Table of Contents for the Digital Edition of Chemical Engineering May 2021