In API Plan 03, the circulation of the medium between the pump and the seal chamber takes place through a specially designed conical chamber without a throttle bushing. This arrangement improves seal cooling through the natural flow of the pumped medium and allows vapour or gas to be vented. The flushing port remains plugged, but can be used in the future – e.g. for additional venting or external circulation in vertical pumps. API Plan 03 is particularly suitable for applications where standard cylindrical chambers could retain particulates, but where the seal does not generate much heat. This plan is not recommended for high-pressure, high-temperature chambers or when using cylindrical stuffing boxes.