Chemical Engineering May 2021 - 45

Outokumpu/IMOA
FIGURE 4. This electron microscope image of Duplex stainless steel
shows the two-phase microstructure of austenite and ferrite grains
plicable quality/engineering standards
from international orgnaizations,
such as: ASME; Canadian
Registration Number (CRN) certification;
European Conformity (CE)
certification; International Organization
for Standardization (ISO);
the German institute for standardization
(DIN); and the Pressure
Equipment Directive (PED).
From a production standpoint,
compared with
other
alloys, machining
DSS and SDSS takes more
time in terms of feeds and speeds.
These alloys may be
welded, but they require
close attention
to interpass temperature
(200 to 300°F
maximum). Care
must be taken to
limit bead thickness.
Good welding metallurgy
must maintain
the proper balance
between the austenite
and ferrite phases.
This can be negatively
affected by cooling too quickly,
having high ferrite content in the
weld metal and having inadequate
shielding and purging gas. A proper,
open-joint design is necessary. Preheating
should be limited to less
than 100°F. Major processes may
be used, such as shielded metalarc
welding (SMAW), tungsten inertgas
welding (GTAW), gas metal-arc
welding (GMAW) and submergedarc
welding (SAW). Standard heliumleak
testing must be performed.
Valves for urea handling
Due to the relative high cost of DSS
and SDSS alloyed valves, piping and
tubing, as well as the complexity and
high cost of welding, it is wise to
specify long-lifecycle valves that are
easily accessible for maintenance,
repairs and renewal.
When using such inline renewable
valves (Figure 6) in urea service, the
troubleshooting and repair process is
much simpler, faster and more economical.
Once the valve is repaired,
no welding is required to return it to
service. Thus, inline valve renewability
often results in enormous savings
in downtime, labor and materials.
Such valves are available in many
configurations, including ball, globe,
check and gate valves. The following
are some additional general considerations
for specifying valves in urea
service applications:
1. Understand the challenges
of the application as it
relates to
the valve capabilities, the process
flow conditions, corro
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   
        
             
     
Measure
Analyze
Connect
Understand
Take Action
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Chemical Engineering May 2021

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