Pig
Launchers and Pig Receivers have been discussed many times on
"Cheresources". However, I understood that there exists a confusion
regarding the design codes to be applied for pig launchers / receivers. Today's
blog entry 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:
1. ASME B31.3: which governs the design of
process piping (code) (USA)
2. ASME B31.4 which governs the design of liquid
pipelines(code) (USA)
3. ASME B31.8 which governs the design of gas
pipelines (code) (USA)
4. ISO 13623:2009: Petroleum and natural gas
industries - Pipeline transportation systems (standard)
5. BS EN 14161:2011: Petroleum and natural gas
industries - Pipeline transportation systems (MODIFIED version of ISO
13623:2009) (standard) (European)
6. ISO 13703:2000: Petroleum and natural gas
industries - Design and installation of piping systems on offshore production
platforms (standard)
7. PD 8010-1:2015: Steel Pipeline on Land (code)
(European)
8. PD 8010-2:2015: Subsea Pipelines (code)
(European)
9. 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.