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14.32 CHAPTER FOURTEEN

• For conventionally reinforced concrete structures in contact with carbon, a minimum
concrete cover of 15 to 40 mm (depending on reinforcement) should be provided over
reinforcing steel. Carbon is extremely corrosive and attacks reinforcing steel through
cracks in concrete.
• Provide a washdown hose site on the vessel deck to facilitate cleaning.
• Provide air release valves on the backwash water supply piping to ensure that air has
been completely purged.
• Provide adsorber-to-waste connections (similar to conventional filter-to-waste) to per-
mit additional removal of GAC fines remaining after backwashing.

Design of Carbon Storage and Transport Facilities. Carbon replacement is a major ex-
pense associated with carbon adsorption systems. Because slurry storage and transport
can cause major attrition losses, facilities must be designed with care. Carbon deliveries
in excess of 9,000 kg (10 tons) usually justify an eductor/carbon slurry transport system.
Design guidelines include the following:
• To minimize carbon-to-carbon abrasion, avoid air-assisted transfer of carbon.
• Transfer all carbon in a water slurry form, designed for a slurry of approximately 1 to
3 lb/gal (0.12 to 0.34 kg/L).
• Minimize the length of carbon slurry transfer pipes through optimum arrangement of
structures and equipment.
• Pipelines delivering carbon slurry to the tanks should discharge below the water sur-
face to minimize attrition and overflows from the dewatering screw. The tank overflow
pipeline should be screened or diverted to a collection point where carbon can be
recovered.
• Provide a metering device to measure carbon slurry flow rate in the piping system.
• Limit velocities in carbon slurry transfer lines to 3 to 5 ft/s (0.9 to 1.5 m/s).
• Avoid the use of throttling valves on carbon slurry lines and on potable water dilution
lines that are ahead of the point of carbon introduction.
• Use a minimum diameter of 50-mm type 316 stainless steel pipe for carbon slurry trans-
fer lines and long sweep bends to minimize pipe erosion and improve hydraulics.
Mitered bends should not be used for piping carbon slurry.
• Minimize the number of movements (valve changes, flow direction changes, pump
starts) to effect a complete carbon transfer.
• Limit the speed of recessed impeller, rubber-lined, centrifugal carbon transfer pumps
to 900 rpm or less.
• Design carbon storage bins to receive the entire wet contents of carbon transferred from
a single carbon vessel, plus at least 20% for expansion.
• Install pressure gauges with diaphragm seals throughout the eduction system to moni-
tor system performance.
• Use double-seated valves for in-line shutoff and isolation throughout, because the sys-
tem may see carbon flow in both directions. Double-seated knife gate valves, plug
valves, or ball valves fabricated from materials suitable for carbon contact can be used.
• Design the carbon slurry system to move slurry through the system continuously until
all carbon has been flushed from piping. The process must not be allowed to stop un-
til all carbon has been moved through the piping, or else carbon will settle and plug
the lines. Provide cleanouts and flushing connections throughout the piping system.