API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

RISK-BASED INSPECTION METHODOLOGY, PART 2, ANNEX 2.B—DETERMINATION OF CORROSION RATES 2.B-77
more resistant materials [e.g. copper or nickel alloys; titanium; super-austenitic stainless steels (such as 6-
Mo stainless steels); plastic; fiberglass reinforced plastic (FRP) or plastic-lined steel; concrete, etc.].
2.B.11.1.4
Closed Recirculated Systems
Recirculated cooling water systems are applied where water is in short supply or when the water chemistry
must be rigorously controlled. Closed recirculated systems may be treated either by rendering them sterile
and anaerobic or by use of inhibitors. The cost of treatment is minimal, including softening and pH control, if
required or desirable. The heat is removed either in air-cooled heat exchangers or water-to-water
exchangers cooled by an external cooling water system. Bactericidal treatment may be required, using nonoxidizing
biocide, such as hexamethylene biguanide. If no treatment is applied, sulfate-reducing bacteria
(SRB) would otherwise be a potential problem.
A successfully used strategy for preventing problems in the closed cooling water systems is to charge the
system with condensate quality water and then add an effective corrosion inhibitor to prevent corrosion.
Given the controlled environment, the material of construction is usually carbon steel, unless otherwise
required for process reasons.
2.B.11.1.5
Open Recirculated Systems
These types of cooling systems involve constant air saturation as well as some concentration of water-borne
solids in the circulating water. Such systems are corrosive to steel (unless suitably inhibited) and potentially
scaling unless the hardness, pH and alkalinity are also controlled.
In a cooling tower system, the total amount of water actually used is limited to that lost by evaporation plus
the blowdown established to limit the buildup of salts and solids in the system. The extent of soluble salt
concentration is expressed as cycles of concentration, which is the ratio of hardness in the blowdown to that
in the makeup. Water treatment chemicals need only be replaced in accordance with the blowdown rate. In
most systems (water chemistry permitting), the optimum balance between water savings and salt
concentration is effected at four to six cycles of concentration. The additional water savings from a higher
number of cycles are usually offset by the increasing difficulty of coping with higher dissolved salt and
hardness concentrations.
Because of the warm temperature and constant air scrubbing in the tower, the water is not only corrosive but
also a breeding ground for slime and algae introduced from air-borne spores. Unless corrosion-resistant
materials are used, open recirculated systems must usually be corrosion inhibited, treated with biocides to
control biological growths, and chemically treated to control scale and deposits. The cost of such treatment
must be balanced against the obvious savings in water consumption. The choice between corrosion
inhibition vs resistant materials of construction is one of economics and pollution abatement considerations.
Typical corrosion-resistant materials for components in fresh-water type open recirculated systems are
stainless steel, copper alloys, nickel alloys, titanium, cement lined carbon steel, FRP lined or coated steel
(for vessels), etc.
2.B.11.1.6
Factors Affecting Corrosion Rate
There are several factors that affect the rate of corrosion in cooling systems. Depending on the type of
system and water source, the main concerns are related to the following.
a) Impurities in Makeup Water—Calcium, chlorides, alkalinity, silica, etc.
b) Scale Formation—When the process side temperature is greater than 140 °F (60 °C) a scaling potential
exists. The scales are minerals formed by high concentration of specific impurities, e.g. calcite (CaCO 3 )
formed by calcium salts and dissolved CO 2 species. Temperature, pH, and alkalinity influence the
solubility limit of most minerals found in cooling water systems. Unfortunately, high temperature reduces
the solubility of many important minerals, causing scale most often to appear on the hottest surfaces in
the entire cooling system, which are the heat exchangers. Phosphates and zinc are two of the most