Corrosion under insulation (CUI) is defined as the external corrosion of piping and vessels that occurs when water gets trapped beneath insulation. CUI damage takes the form of localized external corrosion in carbon and low alloy steels. The factors that affect the amount of CUI damage under insulation include:
a) Duration of the exposure to moisture,
b) Frequency of the exposure to moisture,
c) Corrosivity of the aqueous environment, and
d) Condition of protective barriers (e.g. coating and jacketing),
e) Equipment design issues,
f) Service exposure temperature,
g) Insulation type,
h) Condition of weather barriers and caulking.
i) Type of climate,
j) Site maintenance practices, and
k) Leaking steam tracing systems.
CUI in carbon and low alloy steels is a form of oxygen corrosion. CUI damage is characterized by either general metal wastage or pitting due to the localized breakdown of passivity. For damage to occur, water should be either absorbed by the insulation or collected beneath the insulation due to breaks in the insulation or cladding (jacketing). Water ingress may occur from numerous sources such as rainwater, a deluge system, spillage from process operations, or condensation on the metal surface in humid environments. Contaminants in the insulation such as chlorides and sulfides may contribute to the corrosivity of the environment.
CUI damage occurs on carbon and low alloy steel when exposed to moisture and oxygen. This occurs when moisture is allowed to penetrate the insulation and contact exposed steel at metal temperatures between 32°F (0°C) and 212°F (100°C). When determining CUI susceptibility, a much broader operating temperature range should be considered, typically from 10°F to 350°F (-12 C to 175 C) due to fluctuations in operating temperature, ineffective insulation maintenance, temperature gradients within the equipment considered (long pipe runs, fractionation columns, heat exchangers, etc.), and various operating modes.
In some instances, these differences arise because users have reported actual metal temperature for CUI incidents, other users have reported actual process temperature in reports of CUI damage, and some have introduced a margin of safety. This has led to an expanding of the range where CUI damage may occur. CUI damage is most severe at metal temperatures between the 170°F and 230°F (77°C and 110°C),
All operating conditions should be considered, including the out-of-service state, for equipment which is offline at ambient temperatures for significant periods of time. Equipment which cycles in and out of the CUI range during regeneration cycles, or is frequently out-of-service at ambient conditions, can experience aggressive CUI damage even though when in normal operation it is outside the CUI temperature range.
CUI Damage Below 32°F (0°C) and Above 212°F (100°C)
The user will sometimes note that the temperature range quoted for CUI varies from one document to another depending upon field experiences. Normally, liquid water would not be predicted at temperatures below 32°F (0°C) and above 212°F (100°C). In some instances, this is because users report damage based on the process operating temperature rather than the actual metal surface temperature. Typically, the metal surface temperature of an insulated component will be close to or at the process operating temperature. There are situations where the metal surface temperature will vary somewhat from the process operating temperature.
In some instances, this is because users report damage based on the process operating temperature rather than the actual metal surface temperature. The key factor for CUI damage to occur is that a corrosive aqueous layer be present on the insulated metal surface during any operating period or during downtime.
One possible situation is where water breaches the insulation on piping with the metal surface temperature between 212°F to 300°F (100°C to 149°C). CUI damage could be occurring as the result of continual flashing of water at the hot metal surface. Even at surface metal temperatures up to 600°F (316°C), CUI could occur during operation if water reaches the metal surface during a shutdown period and flashes off during startup. Another instance where CUI can occur at process temperatures above 212°F (100°C) is where deposits in a deadleg reduce the surface metal temperature sufficiently to allow CUI to take place. CUI damage may also occur at process temperatures below 32°F (0°C) as the result of cyclic exposure conditions above 32°F (0°C), or frequent unit shutdown. It is more important to determine whether water is breaching the insulation system rather than dwelling on what the exact temperature of the insulated metal surface during normal operation. It should be noted that it is very difficult for insulation jacketing/cladding systems to be leak tight. Refer to API RP 571 for further information on CUI inspection practices.
a) Duration of the exposure to moisture,
b) Frequency of the exposure to moisture,
c) Corrosivity of the aqueous environment, and
d) Condition of protective barriers (e.g. coating and jacketing),
e) Equipment design issues,
f) Service exposure temperature,
g) Insulation type,
h) Condition of weather barriers and caulking.
i) Type of climate,
j) Site maintenance practices, and
k) Leaking steam tracing systems.
CUI in carbon and low alloy steels is a form of oxygen corrosion. CUI damage is characterized by either general metal wastage or pitting due to the localized breakdown of passivity. For damage to occur, water should be either absorbed by the insulation or collected beneath the insulation due to breaks in the insulation or cladding (jacketing). Water ingress may occur from numerous sources such as rainwater, a deluge system, spillage from process operations, or condensation on the metal surface in humid environments. Contaminants in the insulation such as chlorides and sulfides may contribute to the corrosivity of the environment.
CUI damage occurs on carbon and low alloy steel when exposed to moisture and oxygen. This occurs when moisture is allowed to penetrate the insulation and contact exposed steel at metal temperatures between 32°F (0°C) and 212°F (100°C). When determining CUI susceptibility, a much broader operating temperature range should be considered, typically from 10°F to 350°F (-12 C to 175 C) due to fluctuations in operating temperature, ineffective insulation maintenance, temperature gradients within the equipment considered (long pipe runs, fractionation columns, heat exchangers, etc.), and various operating modes.
In some instances, these differences arise because users have reported actual metal temperature for CUI incidents, other users have reported actual process temperature in reports of CUI damage, and some have introduced a margin of safety. This has led to an expanding of the range where CUI damage may occur. CUI damage is most severe at metal temperatures between the 170°F and 230°F (77°C and 110°C),
All operating conditions should be considered, including the out-of-service state, for equipment which is offline at ambient temperatures for significant periods of time. Equipment which cycles in and out of the CUI range during regeneration cycles, or is frequently out-of-service at ambient conditions, can experience aggressive CUI damage even though when in normal operation it is outside the CUI temperature range.
CUI Damage Below 32°F (0°C) and Above 212°F (100°C)
The user will sometimes note that the temperature range quoted for CUI varies from one document to another depending upon field experiences. Normally, liquid water would not be predicted at temperatures below 32°F (0°C) and above 212°F (100°C). In some instances, this is because users report damage based on the process operating temperature rather than the actual metal surface temperature. Typically, the metal surface temperature of an insulated component will be close to or at the process operating temperature. There are situations where the metal surface temperature will vary somewhat from the process operating temperature.
In some instances, this is because users report damage based on the process operating temperature rather than the actual metal surface temperature. The key factor for CUI damage to occur is that a corrosive aqueous layer be present on the insulated metal surface during any operating period or during downtime.
One possible situation is where water breaches the insulation on piping with the metal surface temperature between 212°F to 300°F (100°C to 149°C). CUI damage could be occurring as the result of continual flashing of water at the hot metal surface. Even at surface metal temperatures up to 600°F (316°C), CUI could occur during operation if water reaches the metal surface during a shutdown period and flashes off during startup. Another instance where CUI can occur at process temperatures above 212°F (100°C) is where deposits in a deadleg reduce the surface metal temperature sufficiently to allow CUI to take place. CUI damage may also occur at process temperatures below 32°F (0°C) as the result of cyclic exposure conditions above 32°F (0°C), or frequent unit shutdown. It is more important to determine whether water is breaching the insulation system rather than dwelling on what the exact temperature of the insulated metal surface during normal operation. It should be noted that it is very difficult for insulation jacketing/cladding systems to be leak tight. Refer to API RP 571 for further information on CUI inspection practices.
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