This article frames mechanical seal selection as one chain from pump to medium: the pump sets the stuffing box, then pressure sets single or double, temperature sets the elastomer and balanced face, the fluid sets materials, speed sets peripheral velocity, and the flush plan is chosen with the seal. It covers practical points such as ANSI B73 / Sulzer APP, API 682 arrangements 1/2/3, SiC and TC pairings, and Plan 11/21/32/52/53/54. The takeaway is that no single number picks the seal; the final limits live on the data sheet for the exact model.