Pig trap accidents caused by operator error can be prevented using Smith Flow Control’s (SFC) key interlock systems.
Mechanical key safety interlocking is the only technology that can meet the universally accepted safety requirement for loading and unloading pig traps by ensuring that the vessel is vented before it is possible to open the vessel closure. Operators cannot bypass the safety system and this eliminates the risk of ‘operator error’.
SFC interlocks can be specified to ensure the minimum safety arrangement of interlocking the vent valve with the pig trap closure; this guarantees that the vent is open and depressurised before the operator is permitted to open the closure door.operator is permitted to open the closure door.
Pig trap with SFC interlocks (in yellow)
Motor operated valves (MOVs) are frequently used and interlocked as part of the pig trapping procedure because valves are too large and cumbersome for manual operations. When power actuated valves need to be interlocked with manually operated valves and the vessel closure, the key interlock system verifies the valve’s position (open and/or closed), independent of any on-board instrument indication. In the event of a power failure during these procedures, the integrity of the key sequence is maintained even if the valve is to be operated manually. This is achieved by inserting a coded key into the interlock fitted to the manual override hand-wheel assembly which engages the drive mechanism; other coded keys in the sequence are only released once the valve has completed its full stroke.
Sometimes operating sequences on launcher/receiver systems can include complex variations. The range of process applications to which SFC interlocks can be applied is unlimited and tailored systems can be designed for specific client process requirements. For example, the simplest interlock system is linear but, on occasion, non-linear sequences are necessary to incorporate the control of complex Nitrogen purging, pressure equalisation, venting and draining. For this SFC has designed a Sequence Control Unit (SCU) which is a mechanical key issuing device located close to the valve. It differs from the traditional key exchange units in that, upon inserting a permit key, a selector knob is rotated to a fixed position before the next key is issued. In this way, complex sequences can be accommodated, using the same safety principles.
Sequence Control Unit for complex sequences
The final, major part of the sequence involves interlocking the vessel door. SFC’s DL3 interlock device is customised to suit any make or type of enclosure door and completes the safety interlock solution for the safe operation of a pig launcher/receiver system. The DL3 interlock ensures total isolation before the vessel door can be opened. When the DL3 is incorporated into vessel access safety procedures, total equipment and personnel safety is assured.
coded key (green) allows the vessel closure to be opened