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Corrosion Case Study

The following corrosion case was perform on a Monomer Balance Tank of aHDPE Process Plant

Damage Mechanism:   Uniform or Localized Loss of Thickness (Corrosion Under

Insulation)

Description of Damage:

Corrosion of pipping, pressure vessels and structural components resulting

from water trapped under insulation or fireproofing.

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BACKGROUND:

•   During a planned maintenance to a main and major equipments,   it’s considered to

check thickness measurement on FA-100X Monomer Balance Tank.

•   The last check for this equipment was 4 years ago; at that time no problem was

found.

•  Vessel is affected by marine ambient due to the proximity of the facility with the

gulf of mexico (about 1.2 miles).

•   Vessel is affected by a  high humidity  grade because of the  tropical zone where

facility is located.

Corrosion Case Study

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Corrosion Case Study

DEVELOPMENT:

•   This tank was shutdown under normal conditions, purged and isolated and blocked

at main flanges and valves.

•   All insulation around the vessel was removed.

•   A visual inspection was made all around the tank, finding corrosion under insulation

(CUI, designation accordint to API-571) on nozzles, flanges neck, upper and lowersurface walls.

•   An   Ultrasonic Thickness Measurement   was made all over the corrosion areas

founded.

•   A loss of Thickness was localized on several points; this thickness was under the

minimun limiting of retiring thickness for this vessel.

•   Calculations were made to determining the minimun requeired thickness and themaximun allowable work pressure.

•   Recomendations were made to solve the anomalies, according to API-510

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Corrosion Case Study

Undermined area

Thickness found

0.270”  -  0.380”

Undermained area

Thickness found

0.300”  -  0.313”Thickness found

0.270”  -  0.380”

Severe corrosión

area

Diagram of FA-100X showing CUI Points Located by Ultrasonic Inspection

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Corrosion Case Study

PICTURES

Top of

Vessel

Bottom

of Vessel

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Corrosion Case Study

PICTURES

Nozzle

Nozzle

interior

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Corrosion Case Study

PICTURES

Severe

corrosion on

nozzle and cap

Severecorrosion at

vessel’s

support

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Corrosion Case Study

CALCULUS:

Minimun Required Thickness and MAWP (According to ASME VIII- Sec. 1)

t =PR

SE  – 0.6P

FA-100X data:

P= 9.5 Kg/cm2R= 1217.80 mm

S= 1406.14

E= 0.85

t = 6.85 mm (0.270 Inch)

R= 1217.80 mm

S= 1406.14

E= 0.85

P=  SEt

R + 0.6t

Minimun Required Thickness

t= 9.71 mm (0.382 Inch)

P= 6.7 Kg/cm2 (0.66 Mpa)

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Corrosion Case Study

CONCLUTION AND RECOMMENDATIONS:

According to the minimun thickness found during ultrasonic inspection 0.270 Inch,

and due to the minimun thickness for retirement calculated, based on ASME VIII Sect.

1, UG-27 of 0.382 Inch,   it’s recommended to permanent repair the vessel, according

to API-510 paragraph 8.1.5.2 subsection C) .

8.1.5.2 Permanent Repairc. Replacing a section or the component containing the defect.

In case that permanent repair is not possible done, a temporary repair can be donde

following API-510, paragraph 8.1.5.3.1, subsection a).

8.1.5.3.1  The filler metal used for weld repairs to vessel base metal should haveminimum specified tensile strength equal to or greater than the minimum specified

tensile strength of the base metal.

a. The repair thickness shall not be more than 50% of the required thickness of 

the base metal (this excludes corrosion allowance).

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Corrosion Case Study

THE END