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Valves and fittings (irregular insulation surfaces).

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4.3.3.5 Appearance or Morphology of Damage

a) Carbon and low alloy steels are subject to localized pitting corrosion and or

localized loss in thickness.

b) 300 Series SS are also subject to Stress Corrosion Cracking (SCC) if

chlorides are present, while the duplex SS are less susceptible.

c) 300 Series SS and duplex SS are subject to pitting and localized corrosion.

For 300 Series SS, specifically in older calcium silicate insulation (known to

contain chlorides), localized pitting and chloride stress corrosion cracking

can occur.

d) After insulation is removed from carbon and low alloy steels, CUI damage

often appears as loose, flaky scale covering the corroded component.

Damage may be highly localized (Figure 4-73 to 4-79).

e) In some localized cases, the corrosion can appear to be carbuncle type

pitting (usually found under a failed paint/coating system).

f) Tell tale signs of insulation and paint/coating damage often accompany

CUI.

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300 Series SS are also subject to Stress Corrosion

Cracking (SCC) if chlorides are present, while the

duplex SS are less susceptible

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4.3.3.6 Prevention / Mitigation

a) Since the majority of construction materials used in plants are susceptible

to CUI degradation, mitigation is best achieved by using appropriate

paints/coatings and maintaining the insulation/sealing/vapor barriers to

prevent moisture ingress.

b) Flame-sprayed aluminum coatings have been used on carbon steels. The

coating corrodes preferentially by galvanic action, thereby protecting the

base metal.

c) High quality non-metallic coatings, properly applied to the surfaces to be

insulated can provide long term protection.

d) Thin aluminum foil wrapped on stainless steel piping and equipment has

been used on stainless steels as an effective barrier under insulation.

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e) Careful selection of insulating materials is important. Closed-cell foam

glass materials will hold less water against the vessel/pipe wall than

mineral wool and potentially be less corrosive.

f) Low chloride insulation should be used on 300 Series SS to minimize the

potential for pitting and chloride SCC.

g) It is not usually possible to modify operating conditions. However,consideration should be given to removing the insulation on equipment

where heat conservation is not as important.

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Closed-cell foam glass materials will

hold less water against the vessel/pipe

wall than mineral wool and potentiallybe less corrosive

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4.3.3.7 Inspection and Monitoring

a) An inspection plan for corrosion under insulation should be a structured and

systematic approach starting with prediction/analysis, then looking at the

more invasive procedures. The inspection plan should consider operating

temperature; type and age/condition of coating; and type and age/condition

of insulation material. Additional prioritization can be added from a physical

inspection of the equipment, looking for evidence of insulation, mastic

and/or sealant damage, signs of water penetration and rust in gravity drain

areas around the equipment.

b) Although external insulation may appear to be in good condition, CUIdamage may still be occurring. CUI inspection may require removal of some

or all insulation. If external coverings are in good condition and there is no

reason to suspect damage behind them, it may not be necessary to remove

them for inspection of the vessel. ()

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c) Considerations for insulation removal are not limited to but include:

1. History of CUI for the vessel or comparable equipment.

2. Visual condition of the external covering and insulation.

3. Evidence of fluid leakage, e.g. stains.

4. Equipment in intermittent service.

5. Condition/age of the external coating, if applicable.

d) Common areas of concern in process units are high moisture areas such

as those down-wind from cooling towers, near steam vents, deluge

systems, acid vapors, or near supplemental cooling with water spray.

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Common areas of concern in process units are high moisture areas

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e) When developing the inspection plan for CUI inspection, the inspector

should consider

1. Areas that are most susceptible to CUI. If CUI damage is found, the

inspector should inspect other susceptible areas on the vessel.

2. Utilize multiple inspection techniques to produce the most cost effective

approach, including:

Partial and/or full stripping of insulation for visual examination.

UT for thickness verification.

Real-time profile x-ray (for small bore piping).

Neutron backscatter techniques for identifying wet insulation. Deep penetrating eddy-current inspection (can be automated with a

robotic crawler).

IR thermography looking for wet insulation and/or damaged and missing

insulation under the jacket. Guided wave UT.

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4.3.3.8 Related Mechanisms

Atmospheric corrosion (see 4.3.2), oxidation (see 4.4.1) and chloride SCC (see

4.5.1).

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Guided wave UT

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Guided wave UT

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UT for thickness verification

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http://www.impresub.com/en/services/ndt-inspection.html

UT for thickness verification

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UT for thickness verification

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Figure 4-73 Tee in a 1000 psig ethylene line before insulation removal.