A bypass line is required to connect the pipeline with related facilities such as a booster station, tank farm, etc. Typical sizes for the bypass line are given in (Table 3), based on fluid velocities for continuous service. However the bypass line is outside the scope of this standard document.
2.4.8.2 Kicker Line / Receiving line
A kicker line is required to connect the major barrel with the bypass line to enable diversion of the fluid through the barrel to launch or receive a pig. For a launcher the kicker line shall be connected to the major barrel as close as possible to the end closure and for a receiver as close as possible to the reducer. Typical sizes for the kicker line are given in (Table 3), based on fluid velocities for intermittent service.
Install barred tees on kicker/ receiving line to avoid broken pigs passing through.
2.4.8.3 Balance Line
A balance line shall be provided on launchers to enable filling and pressurising or depressurising (equalizing) of the barrel on both sides of the pig at the same time.
This is to prevent a pig, which is ready to be launched from moving either forwards (and thereby hitting and possibly damaging the pig trap valve) or backwards (and losing the seal in the reducer). To ensure this, the balance line, branching off from the kicker line, shall be connected to the minor barrel as close as possible to the pig trap valve. Consideration should also be given to the provision of a balance line on receivers to prevent any possible pressure differential across a received pig.
The drain line configuration as depicted in P & ID, Appendix-2, 3 and 4 can also be used as balance line. However it is recommended that the drain lines shall not be used as balance line for new installation.
2.4.8.4 Pressurising Line
The pressure equalising line across the kicker valve shall be provided for operating pressure higher than 1000 psig in oil service. For gas and multiphase service the pressurising line across kicker valve shall be provided for line size 6” @ 600# rating and above. This smaller diameter pressurising line around kicker valves serves to control speed of operation, control of barrel pressurisation and/or to avoid damage to the kicker valve seats or other internals. The pressurising line should be at least 2-inch diameter.
2.4.8.5 Safety Relief Line
A pig trap system shall be protected against overpressure by providing relief valves.
The valve shall be provided at the highest point on the pig trap major barrel and piped to flare/vent system.
Where flare/vent system is not available, the valve shall be piped to a safe location (atmosphere). Relief valves discharging to atmosphere should be located at the maximum practical elevation to keep discharge piping (to safe location) as short as possible. Dispersion analysis shall be performed to determine the safe distances. To keep this discharge pipe free from liquid a small (8 mm dia) weep hole shall be drilled at the lowest point.
2.4.8.6 Drain Line
Drain points shall be provided near end closure and near the pig trap valve at lowest point of the barrel to allow maximum drainage of liquid accumulated in the barrel.
Connection of drain lines to pressurized systems should be avoided to enable the contents of the barrel to be emptied as necessary. In addition, before opening the drain valve(s) the barrel shall be depressurised and vented to minimise vapour release or over pressurisation of the drain system.
Diameters of drain lines are as shown in (Table 3). The barrel drain lines shall be sloped (at least 1:300) towards a closed drain system or a designated open drain.
To provide positive isolation of the barrel from the closed drain system during opening of the pig trap end closure, a double block and bleed valve arrangement shall be installed on the barrel drain line with spectacle blind. The first drain valve shall be located as close as possible to the barrel to avoid accumulation of dirt/debris in the branch that may render the drain valve inoperative.
A back flushing connection on the drain line underneath the vessel should be installed if there is a possibility of blockage from deposits of sludge, sand, etc. in the barrel.
2.4.8.7 Vent/flare/blowdown Lines
A vent line shall be provided near the end closure to vent/purge the barrel and near the pig trap isolation valves to ensure depressurisation behind a pig in the event of it being stuck in the minor barrel. The vent line shall be sized based on liquid content, blow down calculations etc. However the diameter of the vent line(s) shall be at least 50 mm (2 inch).
The vent line shall be equipped with isolation valve with interlock to ensure that it cannot be opened when the end closure is also open. For high-pressure gas and multi phase flow systems consideration should be given to the provision of a blowdown line, incorporating a globe valve or restriction orifice sized (such that over
pressurisation of the vent/flare system is avoided) for controlled depressurisation.
During depressurisation, pig traps in gas service and associated piping may become cold. Protection of personnel should be by means of operating procedure rather than insulation.
Table 3 Typical Diameters of Pipe work [Ref 1]
Pipeline
The orientation of branches on the Pig Trap System shall be as follows:
¾ Drains: bottom of pipe (at 180° position)
¾ Vents, pressure gauges, purge connection, thermal relief and pig signaller:
top quadrant of the pipe (at 0° position)
¾ Kicker line, balance line and by-pass line: side of pipe (at 90° position) The diameters of the branch connections are specified in Table 3.
2.4.8.9 Pressure Indicator Connections
Pressure indicator connections shall be installed at the following locations:
¾ On the major barrel near the end closure.
¾ On the minor barrel near the pig trap valve.
¾ On the bypass line on the pipeline side of the bypass valve.
¾ On the main line downstream of the ESD, if an ESD valve is installed in the main line, or
¾ On the bypass line upstream of the ESD valve, if an ESD valve is installed in the bypass line.
The diameter of these connections shall be 2-inch, with 2-inch double block and bleed valve with ½” NPT outlet for instrument connection. All pressure indicators shall be visible to those operating the closure mechanism.
2.4.8.10 Pressure Transmitter
A Pressure Transmitter shall be installed on the Pig trap for the Control Room to know the status of pig trap during stand-by condition, wherever facility to transfer instrumentation signals exists.
2.4.8.11 Audible Local High Pressure Alarm
A local audible high pressure alarm for receivers in gas service including the trunk lines shall be provided so that the Operator is alerted well in advance of the pressure rise to take necessary action. This requirement applies to stations where power or pneumatic supply is available.
2.4.8.12 Purge Connections
A 2” flanged purge connection with two isolation valves and check valve shall be provided for all systems. It should be located near to the pig trap valve or near the end closure to allow purging and/or flushing the full length of the barrel with nitrogen before opening the end closure. The adjacent vent valve should remain closed during purging.
2.4.8.13 Chemical Injection Connection
A flanged off, 2-inch connection with two isolation valves and check valve shall be provided for launchers on the bypass line.
2.4.8.14 Spectacle Blinds
A provision of positive isolation facility like spectacle blinds is not normally required in pig trap systems where the work duration is considerably small. Hence the risk of swinging and re-swinging the spectacle would be greater and time consuming than the actual work with only valved isolation. Secondly for hazardous service if a spectacle blind is fitted with the spade in position, then the flange bolts shall not be loosened for swinging the spade unless it is certain that the valves are not passing.
As per SHELL guidelines the spectacle blind may be provided for following systems:
¾ For all flashing liquids and toxic systems in ASME class 600 and below.
¾ For all systems in service of ASME class 900 and above.
It is recommended to provide spectacle blind on Kicker line and drain / vent lines when connected with to other facilities with the spacer in place during normal operations. Since the spectacle blinds are required for isolation of the trap from the pipeline, these shall be located on the pig trap side of the valves.
The spectacle blind on the main line is not recommended as it may interfere with pigging.
2.4.8.15 Sample Connection
A suitable sample connection shall be installed in the pig receiver area on the drain line, primarily for sampling sludge for any bacterial growth & effectiveness of chemical injection. The orientation of the sample tapping shall be from bottom of pipeline or horizontal for liquid service, horizontal for multi phase flow and from top of pipeline or horizontal in gas service. The sample points shall be designed for safe operations. There shall be no spill-over or sour gas venting during sampling. The venting and draining philosophy as outlined for vent and drain shall be applicable for sample point drain and vent if any. The minimum size of sample tap shall be 2”, reducing further to suit sample connection.
2.4.8.16 Water Inlet Connection
Hydrogen Sulphide (H2S) reacts with iron and forms iron sulphide (pyrophoric scale/
dust). Pyrophoric scale may accumulate in the debris collected by the pig. In a dry condition this material may smoulder i.e. when exposed to atmosphere. The debris must be kept wet and disposed off in a suitable area by burning. Hence a water inlet connection shall be provided on launchers and receivers in gas service containing H2S in order to soak the pyrophoric dust and douse the pig in the traps before opening the end closure.
2.4.8.17 Branch Connections
The configuration of the branch connections between the various lines and ancillary items should be as shown in (Table 4), based on the typical pipe diameters listed in (Table 3).
The diameter of all branch connections shall be at least 50 mm (2 inch).
Table 4 Branch connection configurations [Ref 1]
Branch Connection Location
Sizes mm (inches)
Configurations
Bypass line on main line
75 on 100 (3 on 4) Tee Size 4” & larger Barred tee Kicker line/ receiving
line on major barrel
Size 4” & larger Welded branch
connection (see note 1) Balance line on
connection (see note 1)
Drain on minor and major barrel
Kicker line/ receiving line on bypass line
Size 4” and above Tee Pressuring line to
Kicker line/ receiving line
50 (2) and larger Welded branch
connection (see note 1) Small items
(e.g. vents and gauges)
50 (2) Weldolet
Notes:
1) "Welded branch connections" include tees, extruded outlets/sweepolets and weldolets as well as fabricated items. In all cases they shall conform to the design codes (ASME B31.4 and B31.8).
2) The distance between branch connections should be addressed to ensure that it does not coincide with pig cup/disc separation as this may result in pig stoppage.
Barred tees shall be installed on all branches larger than 50% of the pipeline diameter. However it is advisable to install barred tees on kicker/ receiving line to avoid broken pigs passing through the kicker/ receiving line. The reducing barred tees shall minimum satisfy the requirements of Figure-3 (Refer Appendix – 8)
2.4.9 Pig Signaller