Page 528 -

CHEMICALS AND CHEMICAL HANDLING 15.33

Materials for fabrication of pump liners, body, impeller, and shaft depend on the cor-
rosive and abrasive nature of the material pumped. In some instances, temperature can be
a factor if nonmetallic parts are involved. When one is ordering a pump for chemical ser-
vice, it is best to consult with the pump supplier and check with the chemical manufac-
turer. Total reliance on published tables of corrosion- or abrasion-resistant materials for
various chemicals can be misleading.
In itself, speed is not a major design factor where true solutions are being handled.
However, slurries, viscous liquids, and polymer solutions require special handling and use
of low-rpm pumps. In no case should a centrifugal-type slurry pump operate in excess of
1,750 rpm. Liquid, undiluted polymers may be transferred using a 1,750-rpm pump; how-
ever, once dilution and aging have taken place, the product is subject to molecular shear-
ing if it is transferred in a centrifugal pump operating at any speed. Screw-type progres-
sive cavity pumps are recommended for this purpose.
Generally, pumps designed to use flushing water on shaft seals should not be used for
chemical transfer. Chemical dilution, scaling in lime slurries, and the expense of furnish-
ing a non-cross-connected seal water system are some of the problems encountered. Pump-
ing of carbon slurry is one of the few exceptions.
Modern technology has made available a variety of dry-mechanical-shaft seals to han-
dle most types of liquids and slurries. In addition, indirect magnetic drives are available
for smaller pumps.
Day Tanks. Day tanks generally minimize the amount of hazardous chemicals that
may be stored within a chemical feed area or minimize the amount of chemicals that will
be lost if there is a rupture in the suction lines between the storage tank and the chemi-
cal feeders. The use of day tanks is primarily a safety issue and applies to hazardous chem-
icals such as acids or caustic soda.
In addition, day tanks locate stored chemicals closer to pulsating diaphragm metering
pumps to minimize losses in suction piping, provide a near-constant suction head, and al-
low for proper operation of the metering pump. Locating day tanks close to pumps is use-
ful when bulk storage tanks are relatively remote because of chemical delivery and un-
loading constraints. Day tanks are typically sized to provide 24 h of chemical storage
based on the maximum chemical volumes metered to the application points. Smaller day
tanks can limit the volume of hazardous chemicals and subsequently prevent the area from
receiving a more stringent hazardous classification under building and fire codes.
The filling mechanism between the bulk storage tank and the day tank should be care-
fully designed to prevent overfilling the day tank. Controls include automatic shutdown
of transfer pumps or valves controlling the flow between bulk storage tanks and the day
tank, and alarms on high levels in the day tanks. Visual and electronic level measure-
ments are recommended, and secondary containment of day tanks should also be pro-
vided. Some systems require an operator to hold a safety switch to continue the transfer
operation.
Day tanks can be mounted on scales to monitor feed rates and calibrate chemical me-
tering equipment. Materials used for day tanks are generally the same materials used for
bulk storage.
Piping and Conduits. Design of piping systems to convey chemicals must include
provisions for redundancy, isolation, and maintenance. These provisions are essential when
one considers problems that can occur from deposits and precipitates when eductors and
positive displacement pumps are used to convey slurries. Eductors and piping that han-
dle slurries should be sized to maintain minimum velocities to minimize solids deposi-
tion, especially for powdered activated carbon slurries. Flushing provisions are desirable
for systems with slurries or polymer solutions to evacuate chemicals in the piping and
prevent deposition or solidification of chemical solids when the system is off for any
length of time.