Using Mechanical Interlocks on Complex Pig Trap Procedures

Section through a soft-seated centred ISORIA butterfly valve showing the three important sealing points.

Sequence Control Unit for complex sequences

Coded key (green) allows the vessel closure to be opened
Sometimes pig traps can involve complex procedures. Even in these cases, pig trap accidents can be prevented using mechanical key interlocks. Smith Flow Control offers some useful solutions.

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. Sometimes power actuated valves need to be interlocked with manually operated valves and the vessel closure; in this case, 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; this engages the drive mechanism. Other coded keys in the sequence are only released once the valve has completed its full stroke.

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.

The final, major part of a pig trap 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.

Smith Flow Control
Tel: +44 (0)1376 517901

Published in Valve User Magazine Issue 25

Summer 2020 // Issue 53
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