Does anyone out there know what is the largest valve ever built in terms of orifice diameter? It's not an application specific question just something that came up in a discussion.

In my opinion a valve is:

'A device for repeated opening and closing of a fluid stream moving inside (valve inline) or in or out (end of line) of a pipeline or open or closed vessel.'

Valves ca be a lot of things for a large amount of applications, from car tyre inlets and human implant purposes, to industrial and municipal fluid distribution, water, oil, gas, air etc, etc  

Above attampt at a definition is to try to exclude what in my mind is not common type 'valve' eg cosntructions such as dam ports, sluices, penstocs and similar devices.

In English language 'valve' includes also butterfly valves and knife-gate valves.

If we go to for instance German a 'ventil' (similar concept to valve) will not include butterfly valve, this will in German be an 'absperrklappe'(roghly meaning a closing flap or clap) and a knifegate valve or gate valve will be a 'schieber' (roghhly meaning a devise to be pushed.)

Above only to illustrate that other languages might not include as much as the English under the 'valve' headline.

Largest valve (English concept) will after this probably after all be found in some of the hydroelectrical plants in China as an inlet valve??? , or at least as some 'special built valves' somewhere in the world for the same purpose?

About 5 meter diameter or more (?????), I have seen something lately, but cannot find back to it at the moment.....

You will probably need to better define what qualifies as a valve for the purpose of your discussion.

Does a sluice gate (on a dam) count? If so, maybe go check out the Three Gorges Dam in China - they seem to have some big ones.

If you mean something like a ball valve, with steel body, internal cavity, etc., the answer will be different.

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