A water booster pump circuit which has suction, discharge and bypass valves (all 24"). The water is mixed upstream of this pump with caustic due to process requirement (pH = 8-8.5, Sp. Gravity = 0.97, Pressure at suction is 60 psi and discharge is 100 psi, Temp. is around 90 C and flow through these valves is around 1700 l/s)> The pump is installed inside a building but all the valves are installed outside and ambient temperature range for the valves is 40 to -50 C. Water also carries 0.5% suspended clay/sand in it.
I would look at a lined Butterfly valve for this application.  For water use an EPDM liner, disc could be cast iron.  There is no packing in a lined butterfly-the lining is an interference fit with the shaft where it passes through the body.  Install it with the shaft horizontal and with the actuator on the left-hand side when you are looking downsteam.  The bottom of the vane will then open toward the upstream direction and push the layer of sediment away from the seat when the valve opens. Also, the bottom bearing will not tend to accumulate grit.   The liner flexes when the valve cycles and will crack off the deposits that form.  Some vendors to consult are Tyco/Keystone, Bray, Demco, Crane, but there are many others.  
As pointed out earlier, knife-gate valves are really only suitable for heavy slurries such as paper stock and sewage because knifegate valves slice off the stringy components in the fluid when they close.  Dragging a wide, contaminated blade 24 inches through a big wad of packing is just asking for leaks.

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