Page 188 - CW E-Magazine (10-9-2024)

Special Report

Only ‘buffing’ using soft-back discs the composite reinforcement sleeves repair SCC detected at an investigation
should be carried out on high-pressure prevent failure of the defect through the site, then the length of the replacement
pipelines. Strong consideration should partial transfer of the hoop stress to the should be to ensure the removal of SCC
be given to determining a safe pressure sleeve material and provide restraint of colonies at that specific site.
before applying any crack removal pro- localized bulging in the defect area.
cess. If grinding or buffing results in As a material of construction, car-
a localized metal loss that exceeds the A detailed engineering assessment bon steels are most susceptible to hot
limits specified in the code, a perma- of the defect and sleeve repair must nitrate, hydroxide, and carbonate or
nent repair should be made. precede the installation of the compo- bicarbonate solutions. High-strength
site reinforcement sleeve to ensure that steels are also susceptible to hydrogen
Sleeves the strength of the repair will be equiva- sulphides. Austenitic stainless steel are
This repair can be made with either lent to that of the original carrier pipe. normally susceptible to hot, concen-
a pressure-containment or reinforce- In strict accordance with the manufac- trated chloride solutions and chlorine-
ment sleeve. turer’s specifications, only trained tech- contaminated steam. However, duplex
nicians should install the composite steels, which have a blend of austenitic
Pressure-containment sleeves are reinforcement sleeve. and ferritic metallurgical composition,
designed to retain the pressure of the can withstand higher temperatures
pipeline fluid in the event of a failure Repair by pipe replacement before any SCC initiation occurs. This
of the parent pipe under the sleeve, or Selective pipe replacement is one makes duplex stainless steels an excel-
the parent pipe is “tapped” so that the of many options available in mitigating lent choice for use in high-temperature
sleeve becomes pressure-containing, SCC on a given segment of the pipe- processes when there is risk of SCC.
and the defects no longer grow on the line, or portion thereof, determined to
parent steel. be susceptible to SCC. Pipe replace- Case study: Failure of 316l SS pipe-
ment, although highly effective, is very line in H S environment
2
Applying structural reinforcement costly and requires a service interrup- In a July 2011 incident, a leak was
sleeves for permanently repairing SCC tion for installation. The effectiveness reported from a 2” diameter pipe to a
is restricted to either full compression of a pipe replacement depends on esta- pressure safety valve in a compressor
reinforcing sleeves or after SCC that blishing an adequate replacement dis- plant. Two additional leaks in simi-
has been completely removed through tance to ensure the entire affected pipe lar lines were also reported over the
buffing. If the buffing repair of SCC has been replaced. subsequent two-day period. Figure-4
results in a metal loss in excess of that shows the location of the leaks in two
permitted by the CSA Z662, it can Evaluating potential pipe replace- flare pipes located upstream of the pres-
be permanently repaired by installing a ment locations can be based on two sure safety valve on adjacent compres-
reinforcing sleeve. possible scenarios. The first scenario is sor trains. Figure-5 shows the process
that a company has detected SCC at an schematics of the compressor plant.
The exception is that the reinforce- investigation site and has determined
ment sleeve is restricted to repair the that a pipe replacement is the most suit- Each failure was located at a pipe
metal loss of less than 80%. Reinforce- able means of repair. The second sce- to elbow weld in a horizontal section.
ment sleeves should be used after an nario is that the company has identified The leak was in the top pipe section and
engineering assessment. discrete portions of a pipeline segment within the heat affected zone (HAZ) of
or segments that may be susceptible to
Applying a composite reinforce- SCC in their assessment. Under this
ment sleeve for repairing SCC is scenario, they may have subsequently
restricted to reinforcing SCC that has determined a pipe replacement program
been entirely removed by buffing. For in such areas would be the most effec-
SCC colonies where the buffing repair tive method of addressing the situation.
has resulted in a metal loss in excess of
that permitted by CSA Z662, a perma- Determining the appropriate length
nent repair can be achieved by installing of pipe replacement at a given location
a composite reinforcement sleeve. Simi- depends on the basis for the replace- Fig. 4: Picture showing an incident of stress
lar to the steel reinforcement sleeves, ment. If the pipe replacement is to corrosion cracking in a pipeline

188 Chemical Weekly September 10, 2024

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