Chemical Handling System Change Cuts Costs

Switching from a manual system utilizing granular hydrated lime to maintain an alkaline pH in the treatment plant handling approximately 24 million gallons per day, to an automated, instrumented system based on neutralizing with 25 percent sodium hydroxide, has resulted in a number of cost saving and environmental benefits for the Harpeth Valley Water Treatment Municipal facility in West Nashville, Tenn.

The Harpeth Utilities District is the largest in the state. It is a nonprofit public utility chartered to meet the water and wastewater needs of more than 11,000 water and over 8,000 sewer service customers. It pumps more than 4 billion gallons of clean water and 2 billion gallons of wastewater per year, has 300+ miles of water and 140+ miles of sewer lines in place. Twenty-four wastewater lift stations are in active service. Water from this facility is wholesaled to seven other utility Districts and Municipalities.

According to Plant Superintendent, Ronnie Pannan, the decision to invest in this new system was made late in 1997. The installation was completed about a year later, and the operation has been up and running trouble-free ever since. A critical part of the system involves two sealless, magnetically driven thermoplastic pumps handling corrosive caustic from the 12,500 gallon double wall storage tanks to a 500-gallon double wall day tank.

These pumps are tied into a completely computerized system that has eliminated arduous manual labor, reduced operating and reporting time, provided more accurate and timely operational information and records, and materially improved the workplace environment. The use of double wall tanks and double wall piping has eliminated concerns about chemical leakage and made it unnecessary to build and maintain concrete containment basins normally required as a protection against tank leakage.

A critical decision in the changeover from the hydrated lime system previously used to neutralize the clarified water involved the choice of the pumps that would be required to transfer the caustic liquid from the main storage tanks to the day tank, and to accomplish it without concern for leakage or system interruption. A review of commercial pumps available led to specifying that the pumps be of sealless design, be magnetically coupled, have no components in contact with the fluid, and be constructed to withstand the adverse conditions with minimum maintenance. After an extensive study of available products, the consultant engineers specified a thermoplastic design meeting ANSI process pump specification B73. 1.

Here are the specifics on the unique mag drive pumps that automatically provide dependable transfer of the liquid caustic as the level in the day tank reaches the preset low point:

Model Designation:

• Vanton Chem-Gard CGM-PY
• Meeting ANSI process pump standards.

Description:

• 3?x 2? polypropylene flanged pumps with 6? keyed impellers.
• Dual nonmetallic containment can assemblies.
• High performance permanent rare earth magnets.
• No metal components in contact with the fluid.

Materials of Construction:

• Casing, casing cover and impeller: Precision-molded, homogeneous polypropylene.
• Inner can (in contact with the fluid): Polytetrafluoroethylene(PTFE). Outer can: Fiberglass reinforced plastic (FRP).
• Teflon sleeve and cap to isolate the stainless steel shaft from fluid.

Bearings:

• Extra-large stationary bearings of silicon carbide.
• Rotating sleeve/thrust bearings of ultrapure alumina ceramic.
• Bearings to be locked in place with anti-rotation keys.

Impeller: