Case Study | May 17, 2001

Case study: Discflo pumps ease through difficult lime slurry applications

Source: Discflo Corporation
Case study: Discflo pumps ease through difficult lime slurry applications
Pumping lime slurry is no picnic. "Quick Lime" powder is added to water at thousands of treatment plants for stabilization and water softening. The reaction of the quick lime in water causes a "plating out," and all surfaces that come in contact with the slurry are coated and caked with lime; this includes the pumps, too.

By Stephen Gerber, application engineer, Gerber Pumps International, and Jeannie Smith, Discflo Corporation

Contents
South Central Regional Wastewater Treatment Board searches for a solution
Discflo pumps offered solution to difficult application problems
Pump operating system and simple physical design make lime slurry applications easy
Discpac maintenance quick and easy
Successful installation leads to more lime slurry projects in Florida

The South Central Regional Wastewater Treatment Board of Delray Beach, Florida was using centrifugal trash pumps for their lime slurry application, however the plating out was creating problems for their pumps. Since the impeller of a centrifugal pump must be balanced on the shaft, and as the lime builds up on the impeller, balance is lost and the impeller and shaft wobble. The imbalance destroys the seal and creates a condition that often destroys the motor as well.

In most lime slurry applications there is additional abrasive solid content that complicates things even further. At Delray, grit and pebbles quickly wore down the centrifugal trash pumps, even with their wider clearances. In the end, the pumps were too costly and labor intensive to keep running so the city switched to a diaphragm pump.

Diaphragm pumps have unique check valve/sealing arrangements. As the lime plates out, it coats the check valves so that they can't close completely. These check valves must seal completely or the slurry is pushed backward into the inlet or suction side. This is called "blow by" and it causes excessive pump wear. In addition, grit and pebbles that accumulated in the diaphragm-pumping zone during off times caused excessive stress and ruptured the diaphragms. Caked with lime and battered with grit and rock, the pumps didn't hold up well. They required maintenance about every other day to keep them running.

Consequently, these two very different pumps produced the very same result – exorbitant maintenance requirements and high operating costs.

South Central Regional Wastewater Treatment Board searches for a solution
Gerber Pumps International, Inc., based in Wintersprings, Florida is recognized for their expertise in difficult water treatment applications. They have built a strong reputation for getting results with hundreds of successful pump installations in treatment plants throughout the southeast. In March 2000, Bob Hagle, the executive director of South Central Regional Wastewater Treatment Board of Delray Beach contacted Gerber Pumps. He was looking for a solution.

Gerber Pumps recommended a pump from El Cajon, California-based Discflo Corporation. Working closely with South Central Regional Technical Specialist Dane Anderson, maintenance chief of the South Central Regional Wastewater Board, a system was designed that finally ended the maintenance nightmare and high costs of the lime slurry application. (Return to top)

Discflo pumps offered solution to difficult application problems
There are broadly two categories of pumps: centrifugal and positive displacement. Discflo pumps fit neither category. In a Discflo pump, fluid is "pulled" into the pump without impingement. This is what makes it different from other conventional-type pumps. This difference has significant advantages that provide a real option for end-users with difficult or shear-sensitive applications.

At the heart of the Disc pump is a series of parallel discs, called the Discpac. As a fluid enters the pump, its molecules adhere to the disc surfaces and form a protective boundary layer that remains stationary, relative to the rotating discs. As the discs rotate, energy is transferred to successive layers of molecules in the fluid between the discs through viscous drag. This generates velocity and pressure gradients between the discs. The combination of boundary layer and viscous drag does two things, 1) it protects the moving parts of the pump, and 2) it creates a force that pulls the fluid into the pump in a smooth, pulsation-free flow.

Because of its unique non-impingement pumping, the Discflo pump suffers little or no wear, even under the harshest abrasive conditions and has almost no impact on shear-sensitive fluids and delicate products. (Return to top)

Pump operating system and simple physical design make lime slurry applications easy
The plating out reaction in the slurry causes lime buildup on the surfaces of the Discflo pump as it does in other pumps. For this pump, however, this was not an issue. Lime buildup forms in a smooth even layer on the surface of the Discpac and in effect, becomes part of the "boundary-layer/viscous-drag" operation. Buildup can reduce the overall capacity of the pump at a given RPM by a small amount, but it has no affect on the performance of the pump. The Discpac and rotating element remain balanced, seals are protected, and the pump maintains a smooth, laminar flow.

Abrasive solids in the slurry present little threat to the Discpac because the boundary layer coats and protects it. In fact, in this application, abrasives solids can help by preventing excess lime build up on the Discpac surfaces. The various pressure gradients and hydraulic shearing action between the discs create an equilibrium of the plated lime so there is no excess accumulation on the discs and the boundary layer maintains an effective operating thickness. (Return to top)

Discpac maintenance quick and easy
Stephen Gerber says they have never experienced the problem of excessive plating out buildup on the Discpac. However, he said, "If in fact the pump capacity ever suffered because of extreme lime buildup, the solution is quick and easy."

The back pullout design makes removing the Discpac for cleaning simple. Unlike centrifugal pumps, there are no exact tolerances to worry about. Any buildup could be easily "sliced" away from the Discpac's smooth surfaces returning them to their original condition. The unit is serviced easily without "miking" or feeler gauges.

The lime slurry pump at Delray has been in operation since April 2000 and has required no maintenance. The complete elimination of maintenance has saved the plant thousands of dollars in just this first year of operation. This application represents the typical Discflo pump performance in lime slurry and sludge applications, according to the manufacturer. It is in stark contrast to the near-constant maintenance that is required to run conventional-type pumps under the same conditions. (Return to top)

Successful installation leads to more lime slurry projects in Florida
Discflo's unique fluid handling capabilities combined with the expertise and problem – solving focus of Gerber Pumps has brought unheard of economy to hard-to-pump lime slurry and sludge operations. Engineers and treatment plant operators are taking notice; Bert Gerber reports that there has been much interest, and it has resulted in several current lime slurry projects in Florida alone.

Coral Springs Improvement District recently installed three Discflo pumps for lime slurry. The City of Cocoa has seven Discflo pumps in their Lime Sludge operations. (Several of these were installed in the early ‘90s and continue to operate virtually maintenance-free.) Two new units are being installed in their lime slurry operation and are expected to be online this month. The City of Moorehaven, Florida has placed an order for one lime slurry pump that is currently in production, and the City of Gainesville has designed an expansion project that will include six Discflo lime slurry pumps.

In multiple installations in difficult lime treatment applications, Discflo pumps have outlasted conventional pumps and consistently have delivered lower life cycle costs. In many cases, the life cycle costs have been reduced by 90% or more. Discflo pumps come in over 20 models and can be configured for a wide variety of operating ranges and fluid conditions. (Return to top)

For more information about Discflo Corporation's products and services, visit their Water Online storefront. Some of Discflo Corporation's product offerings include: ANSI Pump, Disc Pump and Disc Mixer.

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