Ultrasonic Metal Thickness Survey
Description and Type of Inspection:
During the summer of 2008, PEM Companies conducted an Ultrasonic Metal Thickness Survey on the Lake Charles Pipeline in Louisiana and Texas. We collected the data and created isometric (CAD) drawings. These include the inspection locations for all vessels and piping that were inspected. The Ultrasonic Thickness readings were then entered into the Corrosion Monitoring System that PEM Companies maintains on computer, for all locations inspected. This system stores the thickness readings and compares the data with previous survey data for erosion and corrosion rates, and frequency of inspection, based on the API – 570 piping code.
(See Appendix #1 for Example)
Maximum Operating Pressure Determination
(MOP) (Maximum Operating Pressure) was calculated from ASME – B 31.3 pressure calculations for the pig traps and pipeline valve settings that were inspected. Specified minimum yield strength varies depending on pipe specifications of metal inspected. The Design Factor used on this survey was determined to be Location 1, Division 2, with a Design Factor of .72.
(See Appendix #2 for Example)
Critical Thickness Determination
The critical thickness is determined in two ways:
(1) The first way is the percent of wall loss less than the nominal thickness of the location inspected. The nominal thickness is the thickness of a fitting or piping that corresponds with A.S.A. Pipe Schedule Manufactures Chart. There is a 12.5% mill tolerance (higher or lower) from the chart. Light wall loss is determined to be from 12.5% to 19% lower that the Pipe Schedule Chart. Any (TML) found to be lower that the manufactures chart is noted. Even though the thickness is 1% thru 12.5% lower than the manufactures chart, that (TML) is noted and is explained as within mill tolerance in the comment section of this report. Any (TML) found to be from 12.5% thru 19% lower that the chart is noted as light wall loss. Any (TML) found to be 20% thru 39% lower that the chart is noted as moderate wall loss. Any thickness readings found to be 40% or greater of chart is noted to be severe wall loss. Any (TML) noted as 40% or more wall loss is recommended to be replaced, and is reported to management for further evaluation.
(2) The second way is if the (MOP) Maximum Operating Pressure is greater than the B.31.3 pressure calculation that is done for each (TML) Thickness Measurement Location that was inspected. These locations are reported to management for further evaluation. See Appendix for the B.31.3 Calculation of Maximum Allowable Working Pressure.
Determination of TML Inspection Locations:
(TML) Thickness Measurement Locations are determined as per the API 570 section 3-1 inspection procedure. This includes the following:
(1) Injection points – These include fittings such as elbows, tees and reducers. PEM inspects as many fittings as possible unless the locations of these injections points are inaccessible as per API section 3.2.1. (Sometimes these areas are dangerous to access without man lifts or scaffolding installed for that purpose).
(2) Dead legs – The corrosion rate can be accelerated in these areas especially in hot service piping due to convective currents. PEM monitors (TML’s) on the stagnate end and at the connection points of the selected dead legs as per API-570 section 3.2.2.
(3) Corrosion Under Insulation (CUI) – PEM installs 2 ½” access holes in insulated equipment for thickness readings to be gathered. These holes are then plugged with a removable plug for future inspections. The locations are determined as per API-570 section 3.2.3.
(4) Soil To Air Interface – (S/A) interfaces for buried piping without adequate cathotic
protection is included for visual inspection. All areas with coating damage, bare pipe, are then inspected visually and a pit depth is then determined with an external pit gauge if needed.
(5) Localized corrosion – External corrosion under pipe supports, on pipe hangers, valves and fittings, damaged insulation will be visually inspected and a external pit reading will be taken. This reading will be subtracted from the (UT) (Ultrasonic Thickness) taken adjacent to the corrosion.
Calibration of Equipment
The equipment used to conduct the survey is the Krautkramer DMS 2 manufactured by Krautkramer Branson. The equipment is calibrated per the manufacturer’s instructions using a .100” thru .500” calibration step block. Also, a .500” thru 1.00” step block. The equipment is then checked again just prior to the (TML) to be inspected, on the thickness closest to the nominal thickness of the (TML) on the test block standard. Then the location is inspected and recorded.
Mills Per Year Estimates (Wall loss)
The Formula used for the (Mill Per Year) wall loss is based on the ANSI /API 570 Piping Inspection Code. (See Appendix #1 for example of formula) Ultrasonic thickness readings are gathered and imported into the PEM Ultrasonic Thickness Corrosion Monitoring system. The thickness readings are collected at the same place as the previous year location. The thickness measurement locations (TML’s) are marked with a paint marker and a location number that corresponds with a computer spread sheet. An Isometric (CAD) drawing is created for easy location of (TML’s) for the following inspection. Every effort is made to take the thickness readings at the same place for each survey performed. Sometimes equipment is re-painted or modified and (TML) locations are removed and need to be re-done. PEM tries to put the (TML’s) in the same place as the previous years survey. Sometimes the locations might be off from the previous year’s locations. Even if the locations are off just an eight of an inch this could give a higher or lower thickness reading for the preceding survey. Each location mark is started at the weld of a fitting and then every 2 inches in a top to bottom direction (for a vertical inspection) called a V-Band, or left to right direction (for a horizontal inspection) called an H-Band of the fitting inspected. A C-Band inspection is used for a straight run of pipe, and readings are collected around the circumference of the pipe starting at the 12 o’clock position and then at the 3, 6, and 9 o’clock position. On larger pipe this process is done using the 12 o’clock position but has eight readings around the circumference of the pipe. Four readings are taken for piping six inches OD and smaller. Eight readings are taken for piping eight inches OD and larger. This can explain preceding UT readings to be a little smaller, or larger than the previous year readings. Other factors for larger or smaller readings than previous year readings include ambient temperature, pressure of transducer contact on the (TML) to be inspected, paint thickness of (TML).
Comment Section
The attached survey consists of three sections:
1. The Summary Report - Locations that have reached 40% wall loss or will reach it within the next five- (5) years. This is designed to assist management with planning and budgeting of repairs and or replacement.
2. The Location Report - Contains all of the Ultrasonic Thickness readings for all thickness locations inspected during the annual survey as well as the past readings. PEM recommends a copy of the report be kept at the plant office location.
3. Isometric Drawings - contain isometric sketches of all test points surveyed with a location number that corresponds with the computer read out sheet.
Summary of wall loss - % below nominal thickness of location inspected:
*Areas of light wall loss – 12.5% mill tolerance
>Light wall loss = 1% - 19% loss
>Moderate wall loss 20% - 39% loss
>Severe wall loss = 40% and greater loss
Drawing #9
>Location #2105 – light wall loss – 19%
*Location #2160 – light wall loss – 9% - within mill tolerance.
Drawing #10
>Location #2185 – light wall loss –19%
Drawing #11
>Location #2265 – light wall loss – 15%
*Location #2355 – light wall loss – 11% - within mill tolerance.
Drawing #13
*Location #2520 –light wall loss – 8%
>Location #2546 – light wall loss – 15%
Drawing # 53
*Location #6075 – light wall loss – 9%
*Location #6110 – light wall loss – 9%
Recommendations:
(1) All areas of corrosion or erosion noted in the summary report as light wall loss, (1%-19% wall loss) are still above retirement thickness. These locations, have the 12.5% mill tolerance considered as wall loss. Close monitoring of these areas and a follow-up B-Scan is highly recommended in future surveys.
(2) All areas of corrosion or erosion noted in the summary report as moderate wall loss, (20%-39% wall loss) are still above retirement thickness. Close monitoring and a follow-up B-Scan is highly recommended in future surveys.
(3) There are no areas noted in the summary report as severe wall loss, (39% and above wall loss). Locations of severe wall loss are always recommended to be replaced.
(4) Locations that have lower Calculated MAWP than the Current MAWP are reported to management for further evaluation. There were no thickness locations that had a lower maximum allowable working pressure than the maximum operating pressure of the thickness location that was inspected.
Appendix:
This survey was performed using the ANSI/ API 570 Piping Inspection Code.
Remaining Life Calculations
(Example)
Remaining life (years) = t actual – t minimum
Corrosion rate
(inches per year)
Where:
t actual = the actual minimum thickness, in inches, determined
at the time of inspection.
t minimum = the minimum required thickness in inches
Long Term Calculations
(Example)
The (Long Term) corrosion rate of piping circuits are calculated from the following formula:
corrosion rate (L.T.) = t initial – t minimum
time (years) between last
and initial inspections
corrosion rate (S.T.) = t previous – t last
time (years) between last
and previous inspection
Appendix:
(Example 1)
Calculation of Maximum Allowable Working Pressure – ASME B-31.3
Design pressure/temperature 500 psig/400F
Pipe description NPS 16 standard weight, A 106-B
Outside diameter of pipe, D 16 in.
Allowable stress E 20,000 (varies - depending on material used)
Longitudinal weld efficiency, F 1.0 (Table 841.114A ASME B 31.8 =.72)
Thickness determined from inspection 0.32 in.
Observed corrosion rate 0.01 in./year
Next planned inspection 5 years
Estimated corrosion loss by date of next inspection = 5 x 0.01 = 0.050 in.
MAWP = 2SEt/D
= 2 x 20,000 x 1.0 x (0.32 –(2 x 0.050))/16
= 550psig
Conclusion: OK
(Example 2)
Calculation of frequency of Inspection – ASME B-31.3
Next planned inspection 7 years
Estimated corrosion loss by date of next inspection = 7 x 0.01 = 0.070 in.
MAWP = 2SEt/D
= 2 x 20,000 x 1.0 x (0.32 – (2 x 0.070))/16
= 450 psig
Conclusion: Must reduce inspection interval or determine
that normal operating pressure will not
exceed this new MAWP during the seventh
year, or renew the piping before the seventh
year.
Notes:
- psig = pounds per square inch gauge; psi = pounds per square inch
- The formula for MOP is from ASME B31.8, where t = corroded thickness
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