Specify single seals that can pass fugitive emission standards. Look for built in pumping rings when you specify dual seals. Look for designs with built in environmental controls. There should be a vent in the face of cartridge seals to vent the stuffing box in vertical applications. Be sure the cartridge sleeve is sealed at the inboard end or solids will penetrate between the sleeve and the shaft making removal very difficult.
Stationary versions require some type of a self aligning feature to prevent constant movement.
Use hardened set screws to avoid slippage. The seal probably came with soft corrosion resistant set screws. You will have to change them. Use back up seals to prevent an unexpected shut down. Tandem is the best configuration.