Resolving conflicts with pipeline codes and pressure vessel codes
- Pig launcher with rapid opening closure
The problem for designers is that the pipeline and launcher / receiver are usually specified with different design codes or design factors. This usually leads to the minor barrel wall thickness having to be thicker than the pipeline wall thickness it joins to. In some cases, making the minor barrel internal diameter significantly smaller.
Example:-
Pipeline designed to a pipeline code such as ASME B31.8 or B31.4 with a high design factor being used, normally 0.72 or even 0.8.
Launcher / receiver may be considered to be a pressure vessel as such a Pressure Vessel Code ASME VIII Div 1 applied.
For 16” pipeline at 900# rating of material grade API 5L X60 required thickness is 13.5mm thick using ASME B31.8, a 0.72 design factor and 3mm corrosion allowance.
For a mating 16” pig launcher/ pig receiver minor barrel under the same parameters the required thickness is 24mm using the vessel code ASME VIII Div 1
It can be seen that it this situation providing a matching transition from pipeline to minor barrel in line with pipeline pigging tolerances presents a design problem.
Solutions:-
Customer to amend launcher / receiver design code to more closely correspond to or equal the pipeline code.
In large diameters manufacture special oversize pipe from a rolled seam welded plate.
In smaller diameters produce a special forged pipe section.
For minor barrel internal diameter differences not greater than 97.5% of nominal pipeline internal diameter, taper the inlet/outlet connection flange.
In addition, generally pig launcher / pig receiver manufacturers like Pipeline Engineering, only require small quantities of pipe for the major and minor barrels and are restricted to the purchase of such material from pipe stockists. Usually therefore only pipe corresponding to API pipe schedule thickness is available to us. Whereas when pipe is manufactured for a pipeline project the diameter and thickness produced in many cases varies from the normal API schedule standards. As such, non-standard pipe is not available to the launcher / receiver manufacturer it again creates a potential mismatch in internal diameters that the launcher / receiver manufacturer must address with specially made material.
Project/Pipeline Engineers should consider the above when producing pig launcher / pig receiver design specifications.
July 8th, 2009 at 10:07 pm
Here in Australia our pipeline standard (AS2885) defines launchers/receivers as “Pipeline Assemblies” designed to the same standard as the pipeline. Our usual practice is not to purchase a proprietary trap but to purchase the closure, high strength reducer and full bore trap valve and free issue these, together with a length of suitable line pipe, to a fabricator for assembly.
July 9th, 2009 at 2:58 pm
Chris, many thanks for your comment. It interesting to know that the AS standards are aware of such issues. Pipeline Engineering has in the past provided Pig Launcher/Receivers to the AS2885 specification and hence have experience of its workings. A contract we undertook to supply a 42″x46″ 900# Launcher/Receiver for the LNG4 pipeline was originally specified to the Australian Unfired Pressure Vessel Code AS1210 but later amended to the AS2885 pipeline specification. The only real issue we found with this change was that the pipeline standard required a great many more Weld Procedure Qualifications than the AS Pressure Vessel Specification.