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

2.B-84 API RECOMMENDED PRACTICE 581
2.B.11.3.2
2.B.11.3.2.1
Once Through Cooling Water Systems
Overview
The steps required to determine the corrosion rate are shown in Figure 2.B.11.1. Once through cooling water
systems are much less common in the U.S. refinery industry than recirculation systems. Once through
systems may be economical where there is an abundance of surface water, but the effects of thermal
pollution may be too great to allow their use in some locations. Chemical treatment is normally
environmentally unacceptable and/or uneconomical. These systems rarely use anything but chlorination,
which controls biological growth.
It is assumed in this paragraph that chemical treatment is not applied, and that the material of construction is
carbon steel. In practice, the materials of construction in once through systems may be copper alloys,
titanium alloys, stainless steels, polyvinyl chloride (PVC), cement-lined, or coated (organic) steel, or even
galvanized carbon steel where the supply source is known to be fairly clean fresh water.
The main concern for a once through cooling water system is the quality of the supply water, i.e. type of
water (sea or salt, brackish or fresh water) and level of contaminants (such as solids, phosphates, iron,
ammonia, bugs, and bacteria, etc.). In total there are a large number of factors that ultimately affect damage
to the equipment in a once through cooling water system.
For simplicity of this corrosion model for once through systems, these systems are considered either as
seawater systems or freshwater systems, and the corrosion rates are determined as a function of
temperature, water flow velocity, dissolved oxygen, and content of chlorides.
2.B.11.3.2.2
Fresh Water Once Through System
Fresh water comprises natural surface water with chloride content less than 1000 ppm. The corrosion rate is
estimated in the same manner as an open recirculating system, with no chemical treatment or corrosion
inhibition, except for biocide.
2.B.11.3.2.3
Seawater Once Through Systems
Seawater comprises brackish or seawater with a chloride content of more than 1000 ppm. The salts found in
seawater do not appreciably alter the pH value when dissolved in water. The main type of dissolved salt in
seawater is sodium chloride (NaCl).
Seawater typically contains about 3 % sodium chloride with approximately 19,000 ppm chloride ion (and
11,000 ppm sodium). The chloride ions comprise about 55 % of the total dissolved solids (TDS). Dissolved
oxygen is typically present at normal saturated values of 6 to 8 ppm at 24 °C to 29 °C (75 °F to 85 °F). The
pH of seawater is usually 7.7 to 8.3 in surface waters [29] . Seawater always contains SRB and a level of
sulfate ion concentration (2 to 3000 ppm) conductive to SRB growth under anaerobic conditions. Anaerobic
conditions will exist under deposits (organic or inorganic), in crevices, and under the influence of biological
oxygen demand (BOD) or chemical oxygen demand (COD). One of the most important properties of
seawater is its ratios of concentrations of the major constituents are remarkably constant worldwide,
including the level of chloride and oxygen.
The temperatures encountered in once through seawater systems usually preclude scale formation by
inverse solubility effects, which normally would initiate at about 88 °C (190 °F). Obviously, deposits can occur
under extreme conditions of temperature in condensers, but this is an exceptional circumstance. Deposition
of sand and silt is primarily controlled by establishing a minimum flow and by mechanical cleaning as
required. Biofouling can be caused by soft organisms (slimes, algae, and hydroids) and hard organisms
(barnacles, mussels, oysters, tubeworms, and seasquirts). The degree of fouling depends to some extent on
the material of construction. Metals and alloys that produce toxic salts (e.g. copper, lead, and zinc) are more
resistant. Both fouling and bacterial effects are controlled largely by chlorination.