Page 212 - Wastewater Solids Incineration Systems

178 Wastewater Solids Incineration Systems

In this method, ash also can be discharged to a well, sluicing water added, and the
slurry discharged through a drain pipe system to a lagoon.

3.1.1.3 Ash Pumps
Pumps are used to pump ash slurry and thickened ash from gravity thickeners.
Slurry pumps are generally centrifugal units with end suction and suitable for
pumping a 2 to 6% mixture of ash and water. Pump passageways are large enough to
pass the product of the ash grinder. A horizontal-shaft pump located so that ash
slurry will flow by gravity into the pump suction is preferred in some installations;
however, vertical pumps and submersible pumps can also be used. Frequently, ash
slurry pumps are of the recessed impeller type, or the rubber-lined volute design. In
some installations, “chopper”-type pumps have worked well. If the recessed impeller
design is used, then the pump impellers and volutes need to be made of hard metal
wear-resistant alloys.
Care must be used in design of the slurry pumping system to make sure that net
positive suction head conditions are met. The high temperature of ash slurry can
sometimes cause unexpected cavitation in pump suction. Also, other parts of the
pump, such as the seals and bearings, need to be considered because of the abrasive
nature of the slurry and the higher fluid temperature being handled.
To match pump capacity to the ash sluicing rate or to the discharge condition,
varying pump speed may be necessary. When pumping ash to a lagoon, the dis-
charge condition—including pipe elevation and length—may change as the
lagoon is filled.
Ash from a gravity thickener may be pumped with a mechanical or air actuated
diaphragm pump. The thickened ash is typically approximately 10% solids.

3.1.1.4 Ash Pipelines
Ash pipelines are designed to transport the ash slurry at a velocity as low as practical
but high enough to keep the solids suspended. Good hydraulic and engineering
practice suggests velocities from 0.6 to 1.5 m/s (2 to 5 ft/sec). A variety of pipe mate-
rials are available for ash slurry service. Careful consideration must be given to the
pipe material and piping layout. Pipe bends should be long radius and minimized as
much as possible because of the highly abrasive nature of the slurry. A variety of pipe
materials are available for ash slurry, including carbon steel, cast iron, Ni-Hard,
ceramic, and basalt-lined steel pipe.
If space on the plant site allows, then ash piping should be installed abovegrade.
The abovegrade installation allows easier pipe inspection and repair.