Selecting a Mechanical Seal and Seal Flush Plans per API 682

I seek to know the rule of thumb for selection mechanical seal flush plans, per API 682. Generally, if the fluid vapourizes, high temperature, hazardous, abrassive, pump running dry, high viscous fluid, etc, then there is need for either single or double flushed seal. Otherwise, Single seal will do in the absence of all of those critical conditions. But what further criteria there are in selecting API 682 flush plans? A reference guidelines will be much appreciated. Thank you.