A single seal is generally used when it is acceptable for the process fluid to leak to atmosphere! As with oil duties, no hydrocarbon can leak to atmoshpere, hence a double seal with higher barrier fluid pressure is used. This way fluid leaks to process and atmos.
Mechanicals sealing does not provide 100% Leak free operation.
It depends. There isn't an easy answer- unless of course the process fluid is toxic... Often times a tandem seal with an unpressurized buffer fluid is enough to keep emissions down.
As Milkboy suggested, a single seal might do the trick. SiC vs Sic could work, so could SiC and Tungsten. There are some old school materials that work well here too.
I'm working on upgrading a pump that is severely abused and am suggesting that a particular kind of dual seal be used to ensure that when the pump is abused that the seal sees a nice clean atmpsphere.
