Chemical Engineering June 2017 - 56

CGIS
Flowserve
Flowserve
FIGURE 2. The iron disc of this seated butterfly
valve was dissolved by sulfuric acid after the PVDF
coating was removed by flowing acid
bleaching agent - chlorine dioxide.
Chlorine dioxide can be made
by reducing sodium chlorate in a
strong acid (sulfuric or hydrochloric
acid) solution and a reducing agent
like methanol, hydrogen peroxide or
sulfur dioxide. The basic production
route is: chlorate + acid + reducing
agent ➞ chlorine dioxide + byproducts.
A commercially important production
route uses methanol as the
reducing agent and sulfuric acid for
the acidity. Advantages of not using
chloride-based processes are twofold;
the formation of elemental chlorine
is eliminated, and sodium sulfate,
a valuable chemical for the pulp
mill, is a side-product.
Lessons learned
Decades worth of experience working
with SSVs in the CPI have allowed
the author and other industry
experts to glean many important lessons.
The following are three that are
helpful to consider when selecting
your next valve.
Lesson 1: Consider the whole application.
The photograph in Figure
2 is a testament to the idiom that
" you get what you pay for, " and it
shows the importance of considering
the application holistically to
avoid adverse outcomes. In 1995, a
new copper mine was built in Chile.
It was designed to use 85 g/L of
weak sulfuric acid to irrigate a heap
of copper oxide ore. The acid was
sprayed over a huge pile of crushed
ore, which leached the copper into
solution, where it was later electrowon
out into pure copper cathodes.
This type of mine and process plant
is known as a heap leach-SXEW
(solvent extraction, electro-winning).
The original process designers
56
FIGURES 3 AND 4. Proper heat-treatment of metal valve components can have a crucial effect on valve
performance. The micrograph on the left shows a non-heat-treated metal structure at 200-times magification,
while the right image shows the metal after heat-treatment (same magnification)
selected titanium as their corrosionresistant
material of construction for
the valves used to direct and isolate
the weak acid. These 24-in. valves
cost upwards of $60,000 each originally.
When a small brownfield project
later arose, the local project team
balked at the price and decided
to investigate alternatives. They
checked the corrosion charts and
saw that a Buna-N (nitrile rubber)
resilient seated ductile iron butterfly
valve with a PVDF (polyvinylidene
fluoride)-coated ductile iron disc was
rated " A " for the temperature and the
85 g/L concentration of sulfuric acid.
These valves cost under $6,000
each, far lower and much more attractive
for the buyer.
The problem with this type of valve
is that they only lasted a few months
because they were not used only as
full open-or-closed isolation valves.
Besides being used to isolate the
centrifugal pumps for maintenance,
the valves were also used on the
discharge of the pumps to assist
the pump during startup to develop
head pressure. That required them
to be placed in the near closed position.
Unfortunately, the velocity of the
acid that was developed while nearly
closed was so high that it physically
removed the thin PVDF coating
covering and protecting the ductile
iron disc. This exposed the iron disc
and the acid simply dissolved it. Of
note, the originally supplied titanium
valves are still in service 22 years
later. Sometimes buying cheap costs
more in the longterm.
Lesson 2: Use properly processed
materials. At a recent valve conference
in Düsseldorf, the global audience
heard a presentation from the
quality assurance (QA) manager of a
major global chemical company. The
QA manager identified a significant
number (more than 35%) of the material
test reports (MTRs) for valves
and fittings purchased by his company
during the year were incorrect,
missing, contained obvious errors
and, in some cases, were fraudulent.
If this is a common occurrence,
then the confidence in the valves
used in severe service applications
is surely mistaken.
The information in the presentation
was reminiscent of a situation
the author experienced at a plant in
Australia in 2006. A distraught client
reported a number of valve issues related
to the leakage of sulfuric acid in
the plant.
An investigation of the situation
focused attention on the knife gate
valves that had been provided by
the author's company. The manufacturer
used castings to make the
valve bodies and wrought plate to
make the blade. For the casting
material, the material selected had
been ASTM A890 Gr 5a, a superduplex
stainless steel well suited for
the 70-80°F temperatures of the 40
wt.% sulfuric acid in the countercurrent
decantation circuit of this hydrometallurgical
facility.
For several years, these valves
worked flawlessly. The client became
very comfortable and decided to try
them in an upstream, more challenging
process - one in which the temperatures
reached 200°F. The higher
temperatures resulted in a significant
problem. From the isocorrosion chart
for sulfuric acid, it can be observed
that this acid creates some interesting
variations based upon the weight
percent and temperature. At the
higher temperature, the material had
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JUNE 2017

http://WWW.CHEMENGONLINE.COM

Chemical Engineering June 2017

Table of Contents for the Digital Edition of Chemical Engineering June 2017