MIL-HDBK-1003/7
Brackish Water Service. Brackish water is defined as any water with chlorides
7.3.3.2
in the range of 1500 mg/L to 12000 mg/L and associated high concentrations of total
dissolved solids. Brackish water also refers to the recirculating systems with
freshwater makeup where the cycles of concentration produce high chlorides and total
dissolved solids.
In spite of overall excellent corrosion resistance, stainless steels have
not been used extensively in brackish or seawater. Type 316 stainless has been used
successfully in a few instances and where special care was taken to keep the tubes free
of fouling. Because of stringent preventive maintenance requirements and procedures,
Type 316 stainless steel is not considered the best tube material for use in brackish
water applications. For condenser cooling water with high chloride concentration,
increased attention is being given to newly developed austenitic and ferritic stainless
steels. It is generally accepted that for austenitic stainless steel to resist
corrosion, the molybdenum content should be 6 percent with a chromium content of 19 to
20 percent. For ferritic steels, the molybdenum content should be at least 3 percent
and the chromium content should probably be 25 percent or more.
Copper alloys, including aluminum brass, aluminum bronze, and copper-nickel
have been used extensively in brackish water applications. Because of overall failure
rate experience, 90-10 copper-nickel is the recommended tube material for use with
brackish water in the main body of tubes with 70-30 copper-nickel in the air removal
sections. When inlet end infringement and erosion attack due to water flow is a
potential problem, 85-15 copper-nickel should be considered. However, if the brackish
cooling water is also characterized by high sulfide concentrations, consideration for
use of the "super" stainless steels is recommended.
Seawater Service. Seawater materials are considered wherever the chlorides in
7.3.3.3
the cooling water are greater than 15,000 ppm. Seawater also includes cooling tower
systems where brackish water is concentrated and high chlorides and total dissolved
solids result. As long as the seawater is relatively clean and free of pollution, the
recommendations for brackish water materials are applicable.
Titanium tubes are being used with increasing frequency for seawater
application. Titanium is essentially resistant to all oxidizing media by virtue of the
stable, protective oxide film. The major problems with titanium tubes include its high
fouling rate in low water velocity systems, its susceptibility to hydrogen
embrittlement, and its low modulus of elasticity. Where scale formation or micro-
biological slimes can possibly occur, an on-line mechanical tube cleaning system is
required to maintain a high tube cleanliness factor. Careful attention must also be
given to support plate spacing to avoid vibration when using thin walled titanium tubes
and extra support plates are needed.
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