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HomeHomeDiscussionsDiscussionsGeneral Corrosi...General Corrosi...Inspection method for pipe with lamination defectsInspection method for pipe with lamination defects
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8/26/2008 11:00 PM
 
We have a low temp CS pipeline carrying liquid ammonia at minus 33 Deg, these pipes were found with "lamination defects" during construction stage which were later considered to be acceptable after thorough analysis from design view point. These defects were marked on the pipelines and need to be monitored for their growth on regular intervals form outside. We had initially welded some probles along these defects and had measured the growth using LCPD technique. It seems the LCPD doesnt work and we need to resort to some other technique, all other NDT methods such UT or MPI are not suitable as these require insulation removal which is not possible beacuse of sub zero temperature of medium. Does any one know about a suitable NDT method that can be employed to monitor these defects. Secondly your experince on pipes with lamination defects and their behaviour overtime regards
 
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8/28/2008 11:00 PM
 
UT monitoring will work and U can create suitable insulation windows at the marked zone which will be properly aealed during operation and can be opened short time during UT measurement. Otherwise U can also install ACFM or Acoustic emission sensors by some expert team. Also consult your designer. regards >We have a low temp CS pipeline carrying liquid ammonia at >minus 33 Deg, these pipes were found with "lamination >defects" during construction stage which were later >considered to be acceptable after thorough analysis from >design view point. These defects were marked on the >pipelines and need to be monitored for their growth on >regular intervals form outside. We had initially welded some >probles along these defects and had measured the growth >using LCPD technique. It seems the LCPD doesnt work and we >need to resort to some other technique, all other NDT >methods such UT or MPI are not suitable as these require >insulation removal which is not possible beacuse of sub zero >temperature of medium. > >Does any one know about a suitable NDT method that can be >employed to monitor these defects. Secondly your experince >on pipes with lamination defects and their behaviour >overtime > >regards
 
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8/28/2008 11:00 PM
 
Bigpond There will be limitations with NDT testing and monitoring in a cold sub-zero - 33 deg cent liquid ammonia pipeline i.e., sub-zero cold insulated pipeline from monitoring of laminations "on-stream" or with insulation not removed. No technique will give good results. UT Flaw check needs "icing to be remove totally for any fruitful examination" with either cut windows or insulation removed each time you would like to know if the lamination has a trend to propagate. MPT will not be useful for lamination studies. Your design check had cleared - as you say. In that case, lamination is sand-witched and may not be harmful to continue your operation so long as "hoop stresses" are within limits. The best way to check if lamination is posing a problem from operating the line is to remove insulation, de-ice the line and check with UT during a shut down. From your description it looks that lamination may not pose any real problem if "hoop stresses" in the pipeline is within limits. Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India August 30, 2008 E-mail: nishi@vsnl.com >We have a low temp CS pipeline carrying liquid ammonia at >minus 33 Deg, these pipes were found with "lamination >defects" during construction stage which were later >considered to be acceptable after thorough analysis from >design view point. These defects were marked on the >pipelines and need to be monitored for their growth on >regular intervals form outside. We had initially welded some >probles along these defects and had measured the growth >using LCPD technique. It seems the LCPD doesnt work and we >need to resort to some other technique, all other NDT >methods such UT or MPI are not suitable as these require >insulation removal which is not possible beacuse of sub zero >temperature of medium. > >Does any one know about a suitable NDT method that can be >employed to monitor these defects. Secondly your experince >on pipes with lamination defects and their behaviour >overtime > >regards
 
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8/29/2008 11:00 PM
 
Thanks for replies, UT is considered to be the best techniques as rightly indicated by CVSrinivasan but requries complete ice removal and bringing the line to ambient. Since the line is 6 KM long and not recommended for frequent shutdown due to excessive thermal expansion and contractions, which make UTI non practical. We had welded some cables during construction across these defects on all along the pipeline which was recommended by ACFM guys, now after three years they find out that these cables are not suitable as for this type of defect, the current requirement are too smalll which require very thin cable which is not again not practicle for outdoor installations and on insulated pipes. ACPD is another technique which we were told, can be used, does anyone has any experience with this? Fundamantly if laminations are within safe limit, one should not worry about their growth, we are sure about the hoop stresses as system is protected againt surge and overpressure, but not sure about the lognitudinal stress which may be caused during thermal cycle (temperature varies from -33 deg Cto -26 deg C during ship loading) Does any one has experince with laminated pipe and their behaviour, are they acceptable phenonmenon found in pipes? regards
 
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8/29/2008 11:00 PM
 
Bigpond If your pipe is protected against surges (due to pumping fluid or due to vapor pressure developed in line during time of idling when shipped fluid is pumped i.ed during transfer time from idling till full transfer is done )+ with pipe protected against over pressures (with TSV/PSV in this long line) there is no big risk to operate the line with known lamination. Usually in such long transfer line, expansion stresses are taken care with both sliding and fixed loops in such long 6 Km transfer piping. If sliding or fixed support gives problem and your laminated locations is at the straight loop and with known pitting (due to icing and de-cicing at the transfer periods) you have to be careful in monitoring. Differential temp of 7 to 10 deg cent ( from -33 to minus 26 /25 deg cent) in such long pipeline is normal. Longitudinal stress developed will not cause any major problem even with lamination. In fact, the worst problem for such pipeline is the external pitting due to icing and de-icing and with gaps in insulation or loss of insulation properties. With pitting and lamination the pipe may have to be monitored carefully at such locations. The best way is to open insulation during non-shipping times and thoroughly do UT and RT at such locations. Other than this, pipe will behave normally with even known lamination pattern which you have already established. If you suspect that the length of lamination in pipeline is sand-witched at different straight length areas and each laminated portion is quite lengthy, the best course is replace such segments . NDT techniques will not be fool proof for "on-stream" monitoring or do NDT by removing insulation during de-icing time to do careful evaluation by UT/RT. Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India August 31 E-mail: nishi@vsnl.com >Thanks for replies, UT is considered to be the best >techniques as rightly indicated by CVSrinivasan but >requries complete ice removal and bringing the line to >ambient. Since the line is 6 KM long and not recommended for >frequent shutdown due to excessive thermal expansion and >contractions, which make UTI non practical. We had welded >some cables during construction across these defects on all >along the pipeline which was recommended by ACFM guys, now >after three years they find out that these cables are not >suitable as for this type of defect, the current >requirement are too smalll which require very thin cable >which is not again not practicle for outdoor installations >and on insulated pipes. ACPD is another technique which we >were told, can be used, does anyone has any experience with >this? > >Fundamantly if laminations are within safe limit, one should >not worry about their growth, we are sure about the hoop >stresses as system is protected againt surge and >overpressure, but not sure about the lognitudinal stress >which may be caused during thermal cycle (temperature varies >from -33 deg Cto -26 deg C during ship loading) > >Does any one has experince with laminated pipe and their >behaviour, are they acceptable phenonmenon found in pipes? >regards
 
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