Pig Trap Design And Assessment Considerations - T.D. Williamson ...

More documents

Recommendations

Info

material thicknesses calculated using B31.3tend to be more conservative or thicker thanthose calculated using B31.4 or B31.8. PipeWall Thickness Formula: wt = PD/2(SE+PY)(wt=wall thickness; P=design pressure;D=pipe diameter(OD); S=allowable stress;E=quality factor; Y=coefficient) ; based uponthe tensile strength of the material. Weldingstandard is per ASME Section IX.ASME Section VIII Division 1 – ASMEBoiler & Pressure Vessel Code Rule forConstruction of Pressure Vessels. Allowablematerials are restricted to ASME Section II“listed” materials – usually SA105, SA350LF2, SA182 F316. Some commonly usedhigh-yield materials (i.e. A694 & A707)are not permitted under this Code. Designmaximum allowable stress values are foundin ASME Section II as “tabulated” values.Overall, material thickness calculated usingASME Section VIII Div. 1 tend to be moreconservative or thicker than those calculatedusing B31.3. Pipe Wall Thickness Formula:wt = PR/2(SE-0.6P) (wt=wall thickness;P=design pressure; R=pipe radius(inside radius);S=allowable stress [lesser of (Su(tensilestrength) divided by 3.5) or (2/3 multipliedby Sy(yield stress))]; E=quality factor); basedupon the tensile strength of the material. UStamp – the official Code U Symbol for stamp(marking) on the vessel or closure to denoteinspection and testing in compliance withCode requirements.All of the codes discussed can be usedto design pig traps. Inspection criteria varyamong these different codes. Generally, giventhe same pipeline design parameters, a pig trapdesigned from ASME B31.3 will tend to becostlier than one designed from ASME B31.4or B31.8 because of the difference in materialgrades and thicknesses. In addition, a pig trapdesigned from ASME Section VIII Div.1 willtend to be more expensive than one designedfrom ASME B31.3 for the same reasons.Pig Trap/PiggingSystem AssessmentsLike any other piece of equipment connectedto the pipeline, pig traps should be assessedat regular intervals for various reasons. Thepipe and fittings that make up a pig trap willage and deteriorate over time. Pipeline companiesmay acquire existing pipelines withpig traps and it will be important to know thecondition of these used assets. Some existingpipelines undergo a change in use whether ithas to do with a reversal of flow direction or achange in product being transported. Existingpig traps can change the way they are used. Apig trap sized for cleaning pigs may need to bemodified so that it can accommodate inspectionpigs. A launcher may need to be changedfrom a manually operated pigging system to anautomated pigging system.A receiver may need to be modified sothat it can receive more than one pig at atime. The initial design of the pig trap may beinappropriate for the current pipeline conditions.At some point in the history of the trap,modifications may have been made that wereinappropriate for the proper performance ofthe pigging system. These are all reasons forperforming an assessment on a pig trap orpigging system.The time it takes to perform a trap assessmentdepends on the level or levels of assessmentrequired, the experience of the trapassessment team, the working conditionswhere the trap is located, the remoteness of thetrap to be assessed (onshore and offshore) andthe operational circumstances of the pipelinesection at the time of assessment.There is a difference between trap assessmentsand piggability assessments. A trapassessment is an evaluation on the pieceof pipeline equipment used for introducingor retrieving a pipeline tool from the pipelinewithout interruption of pipeline flow.This assessment deals with the evaluationof an existing piece of pigging equipmentor pigging system used for launching andreceiving pigs or pipeline tools. In contrast,a piggability assessment is an evaluationof an existing pipeline for the purpose ofdetermining the necessary modificationsin order to make a pipeline section capableof safely launching, running and receivingpipeline tools.Levels Of Trap AssessmentA trap assessment can range in complexityand sophistication. There are many levels of38 Pipeline & Gas Journal / January 2011 / www.pgjonline.com
40 Pipeline & Gas Journal / January 2011 / www.pgjonline.comtrap assessments that can be conducted. Letus look at a description of each level of trapassessment. It may be noted that each level ofassessment increases in complexity.The first level — or Level 1 — trap assessmentis called a Research Assessment. Thislevel of assessment explores the history anddesign parameters of the pig trap. It notes anymodifications that have been made to the pigtrap from the time of installation to the present,mechanical design limitations of the pigtrap, the physical location or designation of thetrap or pipeline section, trap dimensions, thelocation and use of each nozzle on the pig trap.The Level 2 trap assessment is called anOperational Assessment. This level of assessmentis concerned with the current-day operationalcondition of the pig trap and aspects ofthe overall pigging system. This assessmentobserves whether the closure opens, closes orseals properly, whether the pressure warningdevice on the closure operates correctly, whetherthe various valves on the pigging system sealadequately, whether the pig signal and pressuregauge work as they should and whether thekicker or bypass nozzles are sized properly.The Level 3 trap assessment is called theFunctionality Assessment. This level of trapassessment determines if the pigging systemis performing to its peak effectiveness. If it isdesirable to launch or receive inspection tools,is the trap or the surrounding area dimensionallycapable of handling this operation? Arethere physical obstructions (handrails, stairs,piping, etc.) that make it difficult to safely loador unload pigs? Are piping modifications neededto allow for inspection pigging? Should anequalization line be added across the trap reducerfor safety or to improve pigging operations?Should additional vent and pressure gaugenozzles be added to the line size pipe for safety?Should additional drain nozzles be added to thepig trap? These are all questions that this levelof trap assessment seeks to answer.The Level 4 trap assessment is called theCorrosion Assessment. This level of assessmentis the most complex and typically willrequire an NDE technician or engineer to beon the trap assessment team. This evaluationseeks to determine the existence of corrosionagents acting on the components of the pig trapand whether or not these corrosion agents haveaffected the structural integrity of the carbonsteel enough to cause the trap to be unsafe tooperate. Are liquid or debris samples beingtaken? If so, are the samples analyzed for corrosionagents? Are there sampling ports forobtaining liquid or debris samples on the piggingsystem? Have any inhibitors or biocidesbeen applied to this pipeline section? If so, whatwas the name of the inhibitor or biocide usedand what was the interval of application? Didthe inhibitor or biocide have a positive effect ornot? Have in-line inspections been conductedon this pipeline section? Has any form ofNDE been conducted on the trap since it wasinstalled? If so, has the data been analyzed toassess minimum pipe wall thicknesses? Is NDEperformed regularly on the traps to monitorcorrosion growth rates? If previous NDE datais provided, corrosion growth rates can be usedto estimate the time when repair or replacementof the trap would be necessary.
Page 1: Pig Trap Design AndAssessment Consi

Pig Trap Design And Assessment Considerations - T.D. Williamson ...

Create successful ePaper yourself

Delete template?

Save as template?