Flow throttles are for restricting flow between cavities under different levels of pressure, and/or for pressure reduction (pressure down-stream from the throttle is lower) or pressure build-up (pressure upstream from the throttle is higher).

The throttling effect is produced by increased flow losses at the inlet/outlet to parts of the line with different cross sections and/or by frictional losses in relatively long and narrow pipelines or gaps (see table).

Examples of throttle designs in mechanical seals