Can Your Pipe Pass the Acid Test?

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.