Asahi/America, Inc.
35 Green St. * P.O. Box 653 * Malden, MA
02148-0134
Phone 781/321-5409
www.asahi-america.com
Over the course of the past two years, many water treatment
plants in the U.S. have faced troubles with existing sulfuric acid piping
systems. Problems have included pipeline damage, leaks, joint failures,
chemical spillage and down time. These issues are due to a change in acid
supply.
For water treatment plants, the use of reverse osmosis (RO)
membranes is a common method for treating brackish water. The challenge to
operating RO membrane plants is scaling, which is the build up of salts on the
membrane that reduces their functionality and eventually causes failure.
Prevention of scaling is accomplished by dosing incoming
feed water with sulfuric acid in concentrations from 93-97%. A typical
water treatment plant serving 140,000 people will use about 1,600 gallons per
day of acid. Piping materials used to convey and inject the acid are commonly
carbon steel or PVDF (polyvinylidene fluoride), the latter of which is more
common. PVDF is a thermoplastic material with excellent chemical resistance to
most acids.
Switching acids
In regions of the U.S., a reduction in the availability of
the 93% sulfuric acid led to many facilities switching their acid supply from
93 to 98%.
After changing grades from 93 to 98%, facilities using PVDF
piping experienced failures within 3-6 months. The source of the failure
is not poor material or the acid. In concentrations of 98.3% or higher,
sulfuric acid had a natural contaminant known as sulfur trioxide (SO3). This
has led to the new common terminology of 98+% sulfuric acid that attacks both
PVDF and steel pipe.
Many thermoplastic materials will stand up to sulfuric acid
application; however, the presence of SO3 creates stress cracks in the pipe
material. An economical material that resolves all the issues is E-CTFE, a copolymer
of ethylene and chlorotriflouroethylene, commonly known as Halar, which is
manufactured by Asahi/America. E-CTFE has been successfully tested and used in
sulfuric acid applications as well as sulfur trioxide applications.
New water treatment systems are now being built with Halar
pipe and valves for reliability and economical factors. Because the
availability of 93% sulfuric acid has already proven to be in shortage at
times, pipe systems must now be capable of handling all concentrations of acid from
93-98.9%.
Conversely, a plant may be able to reduce acid consumption
by an estimated 4% by using higher concentration acid. The cost of 98% acid is
about 20% less than the 93% acid, depending on supplier. Therefore a typical
plant may expect to save more than $35,000 per year by using a higher
concentration of sulfuric acid.
Halar has been successfully installed in numerous water
treatment RO facilities in the U.S. that use sulfuric acid for the prevention
of scaling on membranes. Halar has superior chemical resistance and could
provide years of trouble-free acid feed service to a facility.
When making the final decision on pipe material, it is critical to consider a material that provides low maintenance and more importantly, chemical resistance to all grades of acid that could be used over the life of facility.
