Here is my opinion:
Identifiable face materials compatible with the fluid to be sealed and any cleaners or solvents put through the lines.
Materials able to handle the full temperature range of the product you are sealing. Viton compatible with water. Hard faces that are not sensitive to temperate change or caustic cleaners. Unfilled carbon graphite seal faces. No elastomers with shelf life. No stainless steel springs or bellows.
The seal should shut with both spring and system hydraulic pressure. Hydraulically balanced designs for low heat generation. Two way balance in dual seal designs. Built in pumping ring for cartridge dual seals. Tandem configuration in dual seal designs. No rotating 'back to back' designs. Stationary configuration for non-cartridge applications. Self aligning design for stationary cartridge versions. Springs located out of the fluid.
The elastomer should move to a clean surface as the faces wear. No spring loaded elastomers. Non fretting designs. Independent of shaft tolerance and finish.Static elastomer located away from the seal face.Cartridge sleeve sealed at wet end. Vibration damping of the seal face. Seal should be located close to bearing support. No elastomer in the seal face. Faces in compression.
Wide operating range. Low hysteresis. Equal & opposite clamping of stationary face. Sealing fluid located at the outside diameter of the seal faces. Leak detection capability. Independent of shaft finish and tolerance. Compensate for thermal expansion and adjustments. Meet fugitive emission standards.
Simple installation. Eliminate all elastomers if possible. Short length leaving room for a shaft support bushing. Finite element analysis of all components. A method of supporting the shaft in the event of a bearing failure. Trapped gaskets.
Packaging to survive a one meter drop. Back up sealing. Built in seal face vent for vertical applications. No glued elastomers in split seal configurations.
BEST PUMP TECHNOLOGY
Low shaft diameter to length ratio. L3/D4 less than 60 (2 in metric). Large operating window
C or D frame adapter to simplify driver to pump alignment. Centerline design for thermal expansion. Oversize stuffing box. Suction recirculation with just a few exceptions. Adequate bearing retention (no snap rings). Positive bearing sealing. Oil level indication. Oil cooling availability.
Double volute to prevent shaft deflection.Suction specific speed number below 8500. Dynamically balanced rotating assembly. Impeller specific speed number selected for the application. Duplex metal impeller. Impeller investment cast. Adjust impeller from the wet end to prevent mechanical seal face load change.