Description: BKGD IMG-Pigging.webp
There exists a confusion regarding the design codes to be applied for pig launchers / receivers. here I tries to explain the design codes related to them and some rationale for them.
Older designs of pig launchers / receivers were based on the ASME Section VIII Div. 1 pressure vessel code. While these designs still exist and work, the modern design approach is to design them according to the connected pipeline or pipe code.
For fresh engineers new to piping / pipeline standards and codes, the following piping / pipeline codes and standards are followed almost universally. Some countries have developed their own standards which are also mentioned below:
- ASME B31.3: which governs the design of process piping (code) (USA)
- ASME B31.4 which governs the design of liquid pipelines(code) (USA)
- ASME B31.8 which governs the design of gas pipelines (code) (USA)
- ISO 13623:2009: Petroleum and natural gas industries – Pipeline transportation systems (standard)
- BS EN 14161:2011: Petroleum and natural gas industries – Pipeline transportation systems (MODIFIED version of ISO 13623:2009) (standard) (European)
- ISO 13703:2000: Petroleum and natural gas industries – Design and installation of piping systems on offshore production platforms (standard)
- PD 8010-1:2015: Steel Pipeline on Land (code) (European)
- PD 8010-2:2015: Subsea Pipelines (code) (European)
- CSA Z662:2015: Oil and gas pipeline systems (standard) (Canadian)
API RP 14E: Recommended Practice for Design and Installation of Offshore Production Platform Piping Systems is a 1991 recommended practice and has not undergone a revision since 1991.
Among the aforementioned codes / standards the ASME codes are quite popular for pipeline / piping design.
Hence a pipeline designed as per ASME B31.4 should have a launcher / receiver designed as per the same code and a pipeline designed as per ASME B31.8 would have launcher / receiver as per the same code. In a rare case when a pipeline is designed as per ASME B31.3, this would apply for the launcher / receiver as well.
Designing a pig launcher / receiver as per PV code ASME Section VIII Div.1 does not make economic sense. Generally, given the same pipeline design parameters, a pig trap designed from ASME B31.3 will tend to be costlier than for one designed from ASME B31.4 or B31.8 because of the difference in material grades and thicknesses. In addition, a pig trap designed from ASME Section VIII Div.1 will tend to be more expensive than one designed from ASME B31.3 for the same reasons.
Normally a pig launcher / receiver for “liquid pipelines” which is designed as per pipeline code is provided a thermal expansion relief valve (TRV). This is to ensure its protection from overpressure due to thermal expansion of trapped liquid. For gas or two-phase fluid pipelines TRVs are not required.
Designing the launcher / receiver relief valve for external fire case is impractical although theoretically possible. In practice pigging is an intermittent operation and well planned and monitored. Before and after pigging operations it is ensured that the launcher and receiver are drained / vented to prevent any residual fluid inside. If a fire effects an empty launcher / receiver, the launcher / receiver will rupture even before the PSV designed for fire case pops at the set pressure. So practically it does not make a sense to have a PSV for fire case. Refer the link below for a very enlightening discussion on PSV on launcher / receiver. Specifically the post by don1980
One of the reasons that engineers consider a PSV for fire case is because the pig launcher / receiver is designed as per pressure vessel code. If it were to be designed as per pipeline code then at the most for a liquid system launcher / receiver a TRV (for thermal expansion) may be provided which does not require any sizing calculations and providing a conventional 3/4 x 1″ or 1″x 1-1/2″ PSV would suffice. For single-phase gas or 2-phase fluid no PSV would be required.