Three Way Valve Interlocking.

Sorry I overlooked the three way valve you had mentioned.

Here I would like to draw a similarity, though the valve constr. and design are different, in ethylene cracking a changeover valve is used to switch from cracking to decoking mode. I happed to observe that flow pressure condition during the change over is significantly affected by the changeover valve and the downstream valves were required to achieve the right condition. Similarly, if you observe the various scenarios of the inlet flow condition under which a safety valve is designed for, the functioning will be affected, like chattering and the availability of the 3% margin.

But even with the two way valves at a transition point both the valves are 100% open and both the PSVs are available and will start chattering, but you have the advantage that the 100% relieving is available at any point of time. But in a changeover valve the 100% relieving is not there as at 50 % open condition the flow is split at 50%, compounded with a situation where one safety valve is malfunctioning, you are at more risk. But you have a bargain that the transition is shorter and easier operation than a two way valve. But if the upstream vessel is of large volume and you expect the PSV size to be in the range of 1" to 3", the 3way could be used.

But again, the outlet and inlet has to be interlocked even in case of the 3way valve, if the PSV-1 inlet side is open, PSV-2 inlet side closed and the outlet side of PSV-1 is closed and PSV-2 outlet side is open, which is possible without an interlock.

Most of the vendors who manufacture such changeover three way valves do guarantee the flow area remaining constant during the transition but in case one PSV has failed the vessel is in high risk for the short duration of the 3way valve operation.

With regards,
Kannan

"Bagesh Kumar" <bagesh.kmr@gmail.com> Sent by: piping_valves@yahoogroups.com 12/09/2008 10:45 Please respond to piping_valves@yahoogroups.com

To <piping_valves@yahoogroups.com> cc Subject RE: [piping_valves] Three Way Valve Interlocking.

But the Valve is not simple Three way, It’s a change over Valve, I agree with your answer for interlock, but the change over valve working principle explains clearly that It is not possible to block the flow, i.e if one closes other side opens simultaneously and in any case the inlet side is always open.

Kumar Bagesh_._,___