Three Way Valve Interlocking.

In addition to our earlier discussion on the subject, I verified with operations people and that an additional reasoning based on their practice in dual safety valve, is that the situation is, the PSV is gone for maintenance or inline/cutoff from the system. And the same status is to be known at the key cabinet. So if the PSV-2 outlet LO key is in cabinet it is understood that the PSV is in maintenance. if it is in the inlet valve of the PSV-2 along with the second key which is used to unlock and close, it is to be understood as that it is only a cutoff.  Also during maintenance the pipe spool is placed to replace the PSV and incase of any leak in the closed inlet valve, the pressure build-up is avoided by keeping the outlet LO.

As per the operation person it is important to know the PSV status in cabinet to verify the maintenance records.

So LO of outlets has to be interlocked with inlet and not independently LO as a GEP.

In addition I understand Netherlocks has recently comeup with new locking devices in which some valves which are not reachable as in your case, can be controlled by the flexible metal  wire covered in plastic sheath like an electrical cable and can be located anywhere and can operate a raising stem valve. And the key can be removed or inserted using the extension rods. It looks quit neat and without any disturbance to any piping.

Smithflow guys say as per thier experience they do not have a better solution than the same wire cable for such changeover valves.

One more news is that all big oil and gas vendors are presenting themselves in Dubai trade fair. So any one in the region could benefit.

With regards,
Kannan.