Why did API 510 11th Edition Introduce “Sweating Service” for CUI Inspections?

To maintain the guidelines for the integrity of industrial assets, the API standards undergo frequent and regular updates. This article discusses a slight alteration introduced in the latest edition of API 510. The term “Sweating Service” is newly introduced in two different sections of the standard. It is worth discussing sections 5.5.6.2 and section 5.5.6.3.

API 510 section 5.5.6.2 Susceptible Locations for CUI on Equipment

With carbon and low-alloy steels, CUI usually causes localised corrosion. However, vessels in Sweating Service may have general corrosion with localised corrosion occurring at locations with coating failure.

API 510. 5.5.6.3 Insulation Removal (note point I)

Considerations on the need for insulation removal are not limited to but include:

• Consequences of CUI leakage
• History of CUI for the vessel or comparable equipment
• Visual condition of the external covering and insulation
• Evidence of fluid leakage (e.g. stains)
• Equipment in intermittent service
• Condition/age of the vessel coating under insulation, if applicable
• Potential for the type of insulation to absorb/hold more water (e.g. calcium silicate vs cellular glass)
• Ability to apply specialised NDE that can effectively locate CUI without insulation removal
• Potential for damage due to vessels in sweating service

Section 5.5.6.3 presents various considerations that offer examples for characterising the risk of Corrosion Under Insulation (CUI). As per the API 580 definition, the risk is determined by multiplying the Consequence of Failure by the Probability of Failure, with “Failure” referring to the loss of containment.

While the first item in the list focuses on the consequences of CUI leakage, addressing the aspect of the risk related to the consequences of failure, the subsequent points delve into the probability of failure. Consequently, these points warrant careful attention during the assessment of the probability of failure due to CUI.

To enhance clarity and facilitate better comprehension for the readers, it is beneficial to restructure these points more intuitively. This can be achieved by emphasising the sources of information to assess the probability or by highlighting the various factors that should be considered during such assessments, as illustrated below:

Risk Assessment on whether to remove insulation or not

Information sources for probability assessment

Considerations on the need for insulation removal are not limited to but include:

Similar Service

• History of CUI for the vessel or comparable equipment

Current inspection data

• Visual condition of the external covering and insulation
• Evidence of fluid leakage (e.g. stains)
• Condition of the vessel coating under insulation, if applicable

Operating Conditions

• Equipment in intermittent service
• Potential for damage due to vessels in sweating service

Design

• Age of the vessel coating under insulation, if applicable
• Potential for the type of insulation to absorb/hold more water (e.g. calcium silicate vs cellular glass)

Reduce Probability

• Ability to apply specialized NDE that can effectively locate CUII without insulation removal

Risk assessment on whether to remove insulation or not:

Factors to be considered in Probability Assessment

Considerations on the need for insulation removal are not limited to but include:

Estimate the probability of CUI based on lessons learned and similar service

• History of CUI for the vessel or comparable equipment

Estimate the probability of external water ingress

• Visual condition of the external covering and insulation
• Evidence of fluid leakage (e.g. stains)

Estimate the probability if water ingress exists, CUI will be more aggressive

• Condition/age of the vessel coating under insulation, if applicable
• Potential for the type of insulation to absorb/hold more water (e.g. calcium silicate vs cellular glass)

Estimate the probability that operating conditions promote the presence of liquid water phase

• Equipment in intermittent service
• Potential for damage due to vessels in sweating service

Estimate the effectiveness of available NDT

• Ability to apply specialised NDE that can effectively locate CUI without insulation removal

What is ‘Sweating Service’?

Within API 510, there is no formally defined explanation for this term; however, it conventionally refers to systems operating below the typical atmospheric dew point.

To elaborate, the dew point represents the air temperature at which water begins to condensate, causing the formation of water droplets on the inside of a car window during cold weather or the morning dew that dampens grass. Likewise, condensation occurs on the outer surface of a cold glass of water. The introduction of this term in the latest edition of API 510 is not unexpected. Understanding the definition of sweating service is crucial, as it signifies the potential for water to condense on insulated steel surfaces. As such, this service fosters Corrosion Under Insulation (CUI) and warrants due consideration during the preparation of inspection plans or assessments concerning the necessity for insulation removal.

To summarise

• Sweating Service is related to vessels with wall temperature below the air dew point
• If the wall temperature is below the dew point then water condenses
• Water condensation means the presence of liquid water that leads to a higher probability of CUI
• This situation can be found in Cryogenic service (cold duty temperature)
• Insulation for Cryogenic service needs to be vapour sealed
• It is not advisable to remove cold-duty insulation for inspection unless it is deemed necessary
• CUI on sweating service seems to be of generalised nature than of localised nature