Chemical Engineering May 2021 - 43

Feature Report
Part 2
Specifying Severe-Service
Valves for Urea Applications
To withstand the harsh conditions and potential safety risks encountered in urea
agricultural applications and other severe-service processes, special consideration
must be given to valve construction, performance and installation
S
evere-service valves (SSVs) are
specially designed and manufactured
for critical applications in
which the potential consequences
of valve failure include serious and costly
risk to life, plant operation and property.
SSVs are typically exposed to relatively
high pressures and temperatures, as well
as corrosive or lethal media.
A significant chemical industry application
for SSVs is the production of urea
(Figure 1). Globally, over 90% of the urea
produced is used as an economical form
of nitrogen fertilizer for agriculture (Figure
2). With the world's estimated population
of eight billion people and growing, global
demand for food, and therefore fertilizer, is
strong and resistant to economic cycles.
According to Expert Market Research
(EMR), the global market for urea in 2020
was 187.8 million metric tons. By 2026,
that number is expected to grow 12.61%,
to 211.5 million metric tons.
Another application for urea is the reduction
of air pollution from diesel engines,
which run at high temperatures and produce
high concentrations of nitric oxide.
Urea tanks are now standard equipment in
many new diesel trucks, buses and cars.
Urea synthesis
Urea is produced by the reaction
of ammonia and carbon
dioxide typically at high pressure
(140 bars; 2,031 psi) and
temperature (183°C; 362°F).
There are different processes
that have been developed to
synthesize urea. One of those
involves a urea reactor, stripper,
high-pressure carbamate
condenser and high-pressure
off-gas scrubber (Figure 3). In
the reactor, CO2 and NH3 are
partially converted into urea
and water. The stripper separates
the residual CO2 from the NH3, which
is further processed through the high-pressure
carbamate condenser. The scrubber
separates residual inert gases, such as
oxygen and nitrogen. Note that ammonium
carbamate, an intermediate product,
is highly corrosive.
Severe-service metallurgy
When specifying severe-service valves for
urea service, the most important aspect
to understand is the uniqueness of the required
metallurgy. These processes generally
function best when the oxygen content
is kept to a minimum, yet many alloys require
oxygen to be present in order to form
and maintain corrosion-resistant surfaces.
Over the years, there has been an evolution
of alloys that were selected by the urea
industry for their ability to function in these
processes at increasingly lower levels of
oxygen. Early on, austenitic stainless-steel
alloys, such as 316L-Urea Grade (316L-UG
or UNS S31603), 316L LN (UNS S31653
or EN 1.4429) and 25-22-2 (UNS S31050),
were used with varying degrees of success.
However, as even lower levels of process
oxygen became more vital, Duplex
(DSS) and Super Duplex (SDSS) stainless
steels were introduced, which permitted
Don Bowers
Conval, Inc.
Jennifer Jones
Armour Valve Ltd.
IN BRIEF
UREA SYNTHESIS
SEVERE-SERVICE
METALLURGY
VALVES FOR UREA
HANDLING
FIGURE 1. Urea manufacturing requires that all equipment, including valves, be
able to handle severe conditions and exposure to harsh media
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