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Materials of Construction
Tetra Technologies
General Considerations

When designing, constructing and operating equipment that comes into contact with calcium chloride, the following items should be kept in mind.

Calcium chloride is a chloride brine that is mildly corrosive. However, if handled properly, the equipment will not be damaged or suffer from pitting.

Corrosion is an oxidative process; oxygen is a necessary prerequisite for most forms of corrosion. However, concentrated solutions of calcium chloride have low solubility of oxygen and this is highly advantageous. Indeed, the maximum corrosion rate is encountered at calcium chloride concentrations of about 2 to 6%. Concentrated brines with over 30% CaCl2 are much less corrosive. Allowing air to come into contact with calcium chloride solutions should be avoided whenever possible. Corrosion in a tank containing calcium chloride is often observed at the liquid/air interface, since the brine and air come into contact with each other at this point.

pH will heavily influence the rate of corrosion. At a low pH, the combination of hydronium ion, H+ and chloride, Cl-, ions will cause a very rapid corrosion. During storage, the pH of calcium chloride should always be kept at pH above 6.5. If a low pH is suspected, it should be measured, and a suitable base, i.e., NaOH or Ca(OH)2, should be added to increase the pH. Carbon dioxide can be absorbed from the air and neutralize the small amounts of lime normally present in the calcium chloride, which will cause the pH to drop. Therefore, air should not be allowed to circulate freely over a surface of calcium chloride. Optimum pH (minimum corrosion rate) occurs in the pH range of 8 to 9.5 for mild steel.

Temperature will affect the rate of corrosion; a higher temperature will result in faster corrosion. Hence, keeping the temperature low is beneficial. In applications where calcium chloride is used as a cooling agent, low temperatures will further increase the operating life of equipment.

Some non-metallic materials such as glass, polypropylene, PVC and polyethylene are completely resistant to degradation by calcium chloride.

 
Storage Tanks

Mild steel storage tanks and pipelines are suitable for use with calcium chloride liquid. Special chromium-bearing steels "stainless steels" are not recommended for use with calcium chloride liquid owing to a tendency of pitting. If special grade steels are to be used, it is important that they have a high resistance to corrosion by chlorides.

Titanium and some exotic metal alloys are almost completely inert towards calcium chloride, even at elevated temperatures. However, due to their high price, these materials are used only in special applications. For example, titanium might be used in a plate heat exchanger where hot solutions of calcium chloride are cooled.

Using mild steel in storage tanks might give the stored solution a slightly yellow or brown colour. This discoloration is caused by iron ions being released into the solution. In order to prevent this and any corrosion from occurring, the inside of the tank can be lined or coated with a material such as epoxy.

Any solution of calcium chloride is an electrolyte (conductor of electricity), and galvanic type corrosion can occur when two or more dissimilar metals are in electrical contact. The use of adjacent dissimilar metals should, therefore, be avoided when they will be in contact with calcium chloride solutions.

For smaller tanks, plastic materials can also be used. For example, glass-fibre reinforced polyester is resistant against degradation by calcium chloride.

 
Intake Pipeline

For delivery by road tankers, the intake pipeline must be located within 4.6 metres of a suitable road access. A flange of the correct dimension for connecting to the flexible hose should be fitted at a height of approximately 760 mm above ground level. Immediately behind it should be a 50.8 mm valve joined to a vertical 76.2 mm pipeline which rises above the storage tank before sloping downwards to make connection with the tank top.

Flushing out the intake pipe with water after a tanker has off loaded may be required. A hose pipe should be available at the discharge point for use in dealing with any spillage of liquid.

 
Air Vent

Storage vessels should be completely enclosed to prevent undue circulation of air over the surface of the liquid. A vent pipe should be fitted to the top of the tank, as far away from the intake pipeline connection as is practicable, in order to minimize carry over of spray at completion of discharge by air pressure.
 
Contents Gauge

A float and scale type of gauge is recommended. The scale board should be visible from the discharging point if possible. Alternatively, hydrostatic or pneumercator types of gauges may be used.
 
Feed Pipelines

Feed pipelines are normally fabricated from 50.8 mm bore mild steel pipe using either screwed socket or flanged joints. Screwed joints should be sealed with "Fluon" (P.T.F.E) pipe thread sealing tape. Flanged joints should be made with a rubber joint ring. Feed pipelined should, if possible, follow a continuous slope to the outlet point for ease of draining and they should be fitted with a water connection for flushing out the main at regular intervals.
 
Valves

Cast iron lubricated plug cocks are recommended. Alternatively, rubber diaphragm valves may be used.
 
Pumps

Centrifugal pumps are recommended for use with calcium chloride liquid. They should be constructed from ferrous materials throughout, e.g., cast iron body and impeller and steel shaft.

Positive displacement pumps can be used if necessary.

Additionally, facilities for washing out pumps with water should be provided.





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