Arizona WTP Standardizes With PVC Pumps
Val Vista is Arizonas largest water treatment facility, serving over a million people in two major communities - Mesa and Phoenix. It currently treats 245 mgd. The finished water storage capacity is 80 million gallons. The main raw water source is the northern Arizona watershed, which distributes to the plant via Salt River Project canals.
To assure economical production and stay ahead of increasing Environmental Protection Agency requirements, the Val Vista water treatment facility has added new chemical feed systems for enhanced coagulation and a complete used water recovery system to prevent discharge of backwash and blow-down water back to the canal system. This used water recovery system recycles 15 mgd.
The chemicals required by these systems are aggressive and difficult to deal with. They include 93 percent sulfuric acid, 50 percent sodium hydroxide alum, polymer solutions, carbon slurries and a corrosion inhibitor.
To keep downtime low and maintenance costs at a minimum, it was decided to use thermoplastic pumps and standardize on a single pump design so that spare part inventories would be reduced and maintenance personnel would be familiar with the equipment. Vantons vertical centrifugal SGK series chemical sump was chosen.
The System
All pump components in contact with the fluids being handled are made of solid PVC (polyvinyl chloride). The pump is designed to handle corrosive, hazardous, toxic or ultrapure fluids. Other corrosion-resistant materials are available on request, such as chlorinated polyvinyl chloride (CVPC), polypropylene (PP), and polyvinylidene fluoride (PVDF).
The System
With no liquid chemical-to-metal contact, the fully cantilever shaft pumps are designed to operate in applications where dry running may be necessary for extended periods of time. They are rated for flows to 1000 gpm, at heads to 240 ft. and for temperatures to 275 degrees F, with the upper range determined by the thermoplastics selected. They are available for pump depths to 6 ft. and for greater sump depths with tail pipes.
The System
The impellers are molded with an imbedded stainless steel insert. The thrust bearing is external to the sump to allow for impeller clearance adjustment without removing the pump from the tank. Sleeve bearings consist of pure ceramic inner sleeves and reinforced Teflon(r) outer sleeves, lubricated and cooled by the pumped liquid. The pump shaft is stainless steel, completely encased in a PVC sleeve to isolate it from the fluid. The pumps have structural reinforcing ribs to provide extra strength and rigidity to the thermoplastic column.
The System
The pumps motors are high-efficiency, totally enclosed fan cooled (TEFC), solid-shaft vertical ball bearing motors, suitable for operation on 460-volt, 3-phase, 60 Hz AC. The motors are operated at speeds up to 1800 rpm.
The System
Nine vertical centrifugal pumps were installed in all (eight regular pumps and one standby pump - one handling the alum, one handling the sulfuric acid, three handling the caustic soda, two handling polymers of varying viscosities, and one handling the abrasive carbon slurries.
The System
Some of the pumps are used to clear out various containment areas where spilled chemicals and rainwater collect. The chemical retainment areas include those outside the plant and exposed to the elements. The area experiences significant temperature changes during the year - as high as 150 degrees F in direct sunlight and as low as 35 degrees F in the winter months.
The System
Although the Mesa doesnt receive large quantities of rain, these retention areas are exposed to contaminated rainwater, which often demands rapid treatment before disposal. By feeding this rainwater into the headworks and controlling the flow as required, the operators have eliminated all the headaches of chemical disposal.
The System
The thermoplastic pumping system has operated without problems since its installation in May 1994. The average cost to treat one million gallons is $135, with a chemical cost of $15 per million gallons.
The System
About the Author: Greg R. Froehling is the Treatment Facilities Maintenance Supervisor at Val Vista Water Treatment Plant in Arizona.