If exist some rule in the selection of the wedge type (solid, flexible, parallel slide, double disc, etc) of the gate valve.
In the valve specifications that I have, the solid wedge is used for all diameters in 150# and for diameters lesser than 2" for 300# to 2500#; from 300# until 900# the flexible wedge is used for diameters bigger than 2"; for 1500# and 2500#, the parallel slide or double disc is used for diameters bigger than 2".All valve selections, in my opinion, starts with a complete description of application, full fluid description with flow, pressure, temperature , operation mode and cyclus, certifications, material requirement,etc, etc..
Secondly: within theese parameters: what is available commercially, to what deliverytime, price, operational cost including maintenance and life expectancy at local conditions. Survey to include (idealistically) all valve constructions suitable.
Third: selection for each single (group?) of valves, idealistically again best value cost over required lifetime.
Reason: the market and product construction changes. True values ten or fifteen years ago need not necessarily be true today.
Apart from this a 'rule of tumbs' and experience over time has its value, but if this is refitting or valves for a new project I would try to 'open up' for market and product input before deciding or setting up 'locked' data sheets.

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