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

2.B-76 API RECOMMENDED PRACTICE 581
2.B.11 Cooling Water Corrosion
2.B.11.1
2.B.11.1.1
Description of Damage
Overview
The objective for this paragraph is to describe a conservative approach for estimating corrosion rates of
carbon steel components in cooling water systems. In fresh water systems, a model uses the Ryznar
Stability Index (RSI), chloride content, temperature, and flow velocity to calculate a corrosion rate. For
seawater systems, a simple correlation of corrosion rate and velocity is used. Many other factors influence
corrosion of the typical modern cooling water system, and this paragraph also describes qualitatively some of
these considerations.
This paragraph does not attempt to account for degradation of alloys other than carbon steel or to quantify
the effectiveness of water treatment. Many other alloys are used in cooling water systems. Some of these
alloys and the threats that they may face are described qualitatively, but corrosion rates of these alloys are
assumed to be very low. While low alloy steels are rarely used in cooling water systems, the corrosion rates
given by this methodology would be reasonably accurate for low alloy steels.
The best way to assess corrosion in cooling water systems is to use a variety of monitoring techniques, but
detailed use of those techniques is not described here. Most cooling water corrosion monitoring begins with
the use of corrosion coupons. Other techniques have been used to monitor cooling water corrosion in situ,
but they are not described. Microbiologically induced corrosion (MIC) is another common degradation
mechanism in cooling water systems, and coupons are not always a reliable way to monitor this corrosion or
other corrosion where there are deposits or stagnant areas. Direct monitoring and control of the
microorganisms is recommended.
2.B.11.1.2
Types of Cooling Water Systems
There are three types of cooling water systems typically found in industrial operations such as refineries, as
follows.
a) Once Through Cooling—Does not reuse the heated water, which normally is pumped from a fresh water
or seawater source.
b) Closed Recirculating Cooling—Continuously reuses water that is captive in the system. No makeup after
the initial water charge (except to replace accidental leakage).
c) Open Recirculating Cooling (Cooling Tower)—Reuses water from which the heat is rejected into a
cooling reservoir (tower or pond), needs continual makeup water to compensate for evaporation,
blowdown, windage, and drift losses, and needs a blowdown facility to limit the concentration of
impurities that are typically introduced with the makeup water.
2.B.11.1.3
Once Through Systems
Usually, the water source must be presumed corrosive since the surface waters are open to the atmosphere
and contain dissolved oxygen. In a general way, corrosion of iron and steel is proportional to the chloride
content when dissolved oxygen is constant, and vice versa. For seawater in particular, the flow velocity has a
significant impact on the corrosion rate.
Because once through cooling water is not reused, the volume of water circulated through such a system
effectively precludes chemical inhibition from a cost-effective standpoint. In some cases, a small amount of
scale inhibitor is added to the influent water. However, chlorination is usually necessary to control biological
growth (e.g. bacteria, slime, marine organisms).
Both fresh and saline waters are sufficiently corrosive to carbon steel that more corrosion-resistant materials
must be employed. In fresh water, galvanized steel is often adequate but brackish or salt water requires