Page 200 - Water and wastewater engineering

CHEMICAL HANDLING AND STORAGE 5-3

Unloading may be accomplished by pneumatic equipment (blower or vacuum), screw conveyors,
or bucket elevators.
Pneumatic truck unloading is through a pipe conveying system. The trucks are equipped with
air compressors to off-load the chemical. The compressors are capable of providing air flow rates
3
of up to 20 m /min. The design of the conveying system includes a truck inlet panel, piping to
the storage silo, a safety release valve, and a dust collector located on top of the silo. The piping
diameter is generally standardized at 100 mm with bends having a minimum radius of 1.2 m. The
maximum length of piping depends on the material. Pebble lime may be blown 30 m vertically if
the total length of pipe is less than 50 m. Powder can be transported over 90 m over a combined
vertical and horizontal distance (Anderson, 2005).

Liquid Chemicals. For delivery by tank truck, the plant design must provide labeled fill-pipe

connections with protective caps. To avoid adverse chemical reactions with residue in the pipe,
separate pipes are provided for each chemical. The pipe connection should be surrounded with a
concrete drip sump that has a chemically resistant coating.
To prevent accidental overflows, level indicators and high level audible alarms are provided
on the storage tank. The alarm should be mounted at the unloading station to alert the vehicle
operator.
For smaller deliveries of liquid chemicals in drums or carboys, loading dock and staging are
elements to consider for the delivery system. For very large plants, railcar delivery may need to
be considered.

Liquified Gases. Gases such as chlorine and ammonia are shipped as pressurized liquids.
Chlorine is shipped in containers of the following sizes: 70 kg cylinders, 900 kg cylinders, *
and railroad tank cars. It is important to note that the mass designation only refers to the mass
of chemical contained in the cylinders and does not reflect the additional mass of the container
itself. In all of the containers, liquid chlorine occupies a maximum of about 85 percent of the
volume when it is delivered. The 15 percent free space is to allow the chlorine to expand if the
container becomes warm.
The 70 kg cylinders are physically moved into the plant by a hand truck. The 900 kg cylin-
ders are moved by an overhead crane.
GLUMRB (2003) specifies that weighing scales shall be provided for weighing the chlorine
gas as it is used.

Storage
Suggested chemical storage requirements are listed in Table 5-1 .

Dry Chemicals. Bins and silos can be designed with rectangular, square, hexagonal, or circu-
lar cross sections: the first three make optimum use of plant space, but the circular silo is less
susceptible to sidewall hang-ups that occur in bins and silos of other shapes (Anderson, 2005).
This is particularly true of chemicals, like lime, that are hygroscopic. Hopper bottoms should
have a slope of at least 60 from the horizontal; for the storage of lime, an even greater slope is

* In the United States, these cylinders are commonly referred to as “one-ton” cylinders because the contents weigh 1 short ton
in U.S. Customary units.