Plan 03 uses special seal chamber designs such as tapers and vanes to create circulation to the seal chamber. This is a new piping plan number and first appeared in API 682 4th edition although, in practice, it has been used for many years. In Plan 03 there is no throat bushing such as is commonly used in Plan 02.
A problem with Plan 03 is that the circulation rate and effectiveness is unknown. This limits Plan 03 to low duty, chemical service pumps.
Note that the piping schematic alone does not adequately illustrate Plan 03. An additional drawing of the seals and seal chamber is needed.