The answer is simple.
Imagine if someone mistakenly closes the PSV outlet block valve, what would be the scenario. To avoid that, the interlock should be there between the inlet side and the outlet side of the dual PSVs.
The sequence goes like this....the outlet valve of PSV-1 which is in locked open using first key and the key is removable only if the valve is in open position and for closing that valve the first key is required and the inlet valve of PSV-1 which is LC to be unlocked and with that first key and is opened and LO using second key and this key will be removed after locking and will be used to unlock the inlet valve of PSV-2 and the valve is closed and locked using third key and this key is removed and inserted to lock open the outlet valve of PSV-2 and this means the third key stays there in LO and that the outlet valve cannot be closed.
The core purpose is that both the outlet valves are open irrespective of the respective inlet open/close positions and that it can never be closed and to enable one stream to be opened before closing the other stream. So interlocking will be required either in three valve system or two way valve system.
Even your first initial PID is also incorrect. It should have the PSV outlet side valves also interlocked.
Additionally, my recommendation is that never use a three way valve for a safety valve as the reason for two valves and two streams is for safety reasons that even if one stream is having a problem the other one is available. It is not a matter of cost saving but a safety matter.
With regards,
Kannan
| "Bagesh Kumar" <bagesh.kmr@gmail.com> Sent by: piping_valves@yahoogroups.com 12/09/2008 03:18
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Dear All,
Please suggest me why interlocking is necessary in Dual relieve Safety Valve, My issue is like this,
In a Russian project the Licensor PID specified the single relief valve diagram. After detailing our process team found that Dual relief Valves are required, As the detailing went on they generated the scheme for Dual relief Valves with Two Gate Valve in inlet and Two in Outlet by single operation scheme, i.e. One Lock Open and one Lock Closed, Process team wanted to purchase an interlock system between both Gate Valves, Then it came for material parts, I suggested them to use two Three way Valves in dual relief operation, because I had used it earlier in a plant of Zimmer licensed, and the operation of these valves are similar to my project requirement, after that our PID with both diagrams went for an approval to PMC, now PMC wanted to use this Change-over Valve system, but interlocking to be done between Inlet three way Valve and Outlet three way Valve, I remember previously I have not used any interlocking between these.
My question remains,
Why Interlocking is necessary between these two changeover valves in Dual relief Safety Valve operation, Although Vendor is ready to provide interlocks between these.
I request member suggestion.
Attached is the scheme suggested by me, after that PMC added interlocking provision.
Earlier it was like this.
Regards
Kumar Bagesh
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