Every pump with a rotating shaft has to seal where the shaft crosses the pressure boundary. A packed gland is cheap, tolerant and leaks by design, which is acceptable on water and unacceptable on almost anything else. A mechanical seal runs two lapped faces together and does not leak visibly, at the cost of being far less tolerant of dry running and abrasives.

The faces and elastomers are chosen by the liquid. Temperature sets what the elastomer can survive; chemical attack sets what both the elastomer and the faces can survive; abrasives set how quickly the faces wear regardless of chemistry. A seal specification that names only the seal type has answered none of these.

Where no leak at all is acceptable, the answer is to remove the dynamic seal entirely. A magnetic drive pump transmits torque through a containment shell, so there is no rotating shaft crossing the pressure boundary and nothing that wears into a leak path. That is why chemical duty buys magnetic drive and water duty does not.

Practically, send the liquid, its temperature and whether it carries solids before asking about seal options. Those three answer the question faster than any list of seal types can.