API 570 EXAM STUDY GUIDE [MODULE 2 : PART 8] CORROSION UNDER INSULATION - PART 2

CORROSION UNDER INSULATION - PART 2

(API 570 EXAM STUDY GUIDE : MODULE 2 : PART 8)

CUI FROM API 570

 Corrosion under insulation is described in API 570 also. We will go through the same so that entire content on CUI for API 570 inspector exam will be completed.

 External chloride stress corrosion cracking (ECSCC) of austenitic and duplex stainless steel under insulation is also classified as CUI damage. 

 Note: API 571 mentioned that duplex SS is less prone to ECSCC, but it is not mentioned in API 570.

External piping inspections may include CUI inspections.

 * CUI is one of the conditions to be monitored in CML’s.

 * CMLs should be established for areas with continuing CUI

 * More CML's should be selected for corrosive piping systems with higher potential for CUI.

 * Following ultrasonic readings at CML's, proper repair of insulation and insulation weather coating is recommended to reduce the potential for CUI. After inspections, if the CML's are not closed properly CUI will start at these areas.

 * Calipers are useful in determining approximate pit depth from CUI on pipe When piping systems are out of service and have insulation's removed for inspection.

 Note: Digital thickness meter's cannot exactly determine the pit dept due to some inbuilt constraints.

Inspection, Examination, and Pressure Testing Practices: 

CUI : Clause 5.8 of API 570.

 - Inspection for CUI shall be considered for externally-insulated carbon and low alloy piping operating between 10 °F (–12 °C) and 350 °F (175 °C).

 - Non-Intrusive technique such as real time radiography can assess the presence of scale and locate the CUI. Other techniques such as profile radiography, Pulsed Eddy Current and Guided Wave Examination can help to locate damage.

 - CUI damage is often quite insidious in that it can occur in areas where it seems unlikely.

Clause 6.4 Extent of Visual External and CUI Inspections

 As discussed earlier CUI is included in external visual inspection. In external visual inspection, if a line is insulated, the inspector can only assess the condition of insulation. From the condition of insulation or the areas where CUI can occur ( from the experience of the inspector)  he/she does the inspection.

 When it is asked to inspect something, there are 3 questions that comes.

 What is to be inspected?

 When to inspect or what is the interval for inspection?

 How much to inspect?

 Clause 6.4 of API 570 answers to these questions related to the inspection of CUI.

What is to be inspected?

 All piping systems susceptible for CUI shall be inspected along with the external visual inspection set forth by API 570. (As API 570 is a piping inspection code, inspection of equipment susceptible for CUI's not mentioned, in practical, it will also to be inspected)

What is the inspection interval? 

 As the CUI is to be inspected along with the external visual inspection, the interval set forth for external visual inspection is the inspection interval for CUI. The external inspection interval is given in Table 1 of API 570. (We will discuss this in detail while studying API 570)

How much to inspect? 

 This is given in Table 2 of API 570

- NDE inspection for CUI should also be conducted as listed in Table 2 at suspect locations operating between 10 °F (–12 °C) and 350 °F (175 °C) for carbon steel and low alloy steel piping.

 - Pipe class is defined by API 570 for ease of segregating piping for ease of inspection. We will discuss about this in detail in API 570.

 In suspect areas RT, insulation removal or other NDE shall be conducted. If significant CUI is observed in the inspected area, additional areas and if needed 100% of the circuit is to be inspected.

 The extent of the CUI program described in Table 2 should be considered as target levels for piping systems and locations with no CUI inspection experience. Facilities with CUI inspection experience may increase or reduce the CUI inspection targets of Table 2. 

 An exact accounting of the CUI inspection targets is not required.The owner/user may confirm inspection targets with operational history or other documentation.

 Piping systems that are known to have a remaining life of over 10 years or that are adequately protected against external corrosion need not be included for the NDE inspection recommended in Table 2.

6.6.1 Small Bore Piping (SBP)

 Insulated SBP should receive the same inspection practices for CUI as the primary piping or vessels to which it is attached. Insulation stripping and radiography are the preferred inspection methods for insulated SBP. Attention should be paid to insulation system resealing on SBP.

QUESTIONS

API 571 : Corrosion Under Insulation (CUI) by ProProfs

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