Civil Engineering Reference
In-Depth Information
TABLE 24-5. Materials Suitable for Caustic Soda Systems
Components
Recommended Materials for Use with 50% NaOH up to 140
F
Rigid pipe
Standard-weight black ion
Flexible connections
Rigid pipe with ells or swing joints, stainless steel or rubber
hose
Diluting tees
Type 304 stainless steel
Fittings
Steel
Permanent joints
Welded or screwed fittings
Unions
Screwed steel
Valves—Nonleaking (plug)
Body
Plug
Steel
Type 304 stainless steel
Pumps (centrifugal)
Body
Impeller
Packing
Steel
Ni-resist
Blue asbestos
Storage tanks
Steel
Sodium Hypochlorite
On-site generation of sodium hypochlorite is gaining in pop-
ularity. It is typically produced at 0.8 percent strength and contains 0.07 pounds of
equivalent chlorine per gallon of solution. To generate a pound of chlorine in solution
requires 3.5 pounds of salt (NaCl), 2.5 kW-hrs of electricity, and 15 gallons of water.
Because the sodium hypochlorite is generated on-site, the solution will not degrade
due to the almost immediate use of the material. Also, its strength is lower than the
commercially delivered material, which usually has a strength of 15 percent. In order
to provide the reliability of production, several days' supply of salt is required. A
typical unit is shown in Figure 24-22.
In some plants, sodium hypochlorite is used to disinfect the water. When feeding
this chemical, extreme care must be taken in designing the piping system. The follow-
ing are some of the issues that need careful design consideration:
Scaling
—Because of the high pH of the chemical, it raises the pH of the dilution
water. At pH's at or above 10, the calcium in the dilution water will react to form
calcium carbonate and scale the carrier pipe.
•
Leakage
—The solvent-welded joints of a PVC piping system will leak if they
are not prepared and installed per the manufacturers' recommendations.
•
Gas production
—This chemical loses strength over time and produces a gas in
the process. This gas can accumulate in the pipeline, valves and pumps. This
effect has caused pumps to ''air lock'' and ball valves to explode due to trapped
gas. It has been estimated that a 15 percent solution produces about 1 percent of
gas per day at room temperature.
•
Storage
—The chemical is very aggressive and will attack many materials. The
most reliable we have found is fiber-reinforced plastic or polyethylene. Either
tank may last only from 7 to 10 years.
•
Crystallization
—Place tank outlets 6 to 12 inches above the bottom of the tank
to minimize formed crystals from entering the pipe and finding their way to the
pumps.
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