Pumps & Systems - September 2007

thick urethane liner in the hopper of the pump supports it. To date, none of the liners, with installations exceeding four years in operation, have ever required replacement.

A variant of the cake pump with a concentrically rotating auger has a separate variable speed drive dedicated to the auger (see Figure 9). The auger can then be not only used to feed material into the pumping cavity, but it can be also used as a separate shear inducing or mixing device.

This design can be used to add calcium oxide or calcium hydroxide for treating biosolids. Industrially, it has been used to handle pigments discharged from plate and frame filter presses that exceed 75 percent solids and to mix milled grains with hot water to prepare “wort” in breweries and distilleries. It is also being used to handle spent grain discharged from centrifuges in ethanol plants.

Boundary Layer Injection

Another common appurtenance used when pumping dewatered biosolids is a boundary layer injection system, which consists of an injection pump and an injection flange designed to insert a layer of liquid between the pipe wall and the slug of sludge moving through the pipe.

Piston pumps must inherently

generate two to three times the

pressure of a PC pump to produce

the same flow rates per minute or

per hour as that of a PC pump for

the same application.

These systems can typically cut the friction loss associated with pumping sludge cake to one-half or less. In certain cases, especially when a highly diluted solution of polymer flocculent is used as the boundary layer, friction loss can be reduced by as much as 85 percent. These can be used really on any type of cake pump, as it affects the piping system and not the pump. Except as mentioned above, it reduces the differential pressure, which in turn reduces the operating power required and wear and tear on the entire system.

Conclusion

Cake pumps require some type of feed device to push dewatered solids into the pumping cavity of either a piston-type or a PC-type cake pump. The device can help to reduce the apparent viscosity of the cake as well as feed it into the pumping cavity.

The efficiency and characteristics of this feed device can define the character of the cake that is discharged into a storage or transport device. High shear feed devices will produce

Figure 9. A variant of the cake pump with a concentrically rotating auger has a separate variable speed drive dedicated to the auger.

a cake that is temporarily a lower viscosity, which may be aesthetically undesirable. Using equipment that is designed for higher capacities and will operate at lower speeds and shear rates can reduce this effect.

Piston pumps are designed to handle higher pressures and to pump cake over longer distances through smaller diameter pipes than PC pumps. A comparison solely based on pressure capabilities of the two designs is not appropriate, since PC pumps produce a constant output with pressures being one-half to one-third of the pressure spikes that occur as a function of the varying velocities and flows associated with reciprocating pumps.

The feeding systems used on all of these pumps vary a great deal. Some are quite complicated and trade some of the problems associated with highly sheared, low viscosity cake discharges with complicated, maintenance prone feed systems that amount to using two pumps where one pump can be effectively used.

Using appropriate shearing and feeding devices, along with boundary injection systems, pressures can be minimized and cake can be pumped over substantially long distances (exceeding 2,000-ft) with relatively inexpensive, efficient and reliable equipment.

P&S

References

Barrie, D. & Petrik, K., “Real-World Savings: Expanding A Dewatering Facility,” Pumps & Systems (October 2003)

Dillon, M. L., St. Clair, K. A., Kline, P. H., “Predicting Flowrates from Positive-Displacement Rotary Pumps,” Chemical Engineering (July 22, 1985), pp. 57-60

Heald, C. C., Cameron Hydraulic Data, Seventeenth Edition, Second Printing, Ingersoll-Rand, Woodcliff Lake, N.J. (1992)

Hemphill, B., Shanley, R. & Mikowski, T, “Innovative Material Handling at Biosolids Cake Storage Facility,” Public Works (October 2002)

Heywood, N., “How to Reduce Pipe Friction in Slurry Flows”, Proceedings of the 4th One-Day Seminar on Hydraulic Transport in the Mining Industry, Hydraulic Conveying Association of South Africa (April 2003)