Plan 13 is similar to Plan 11 except that the flow comes out of the seal chamber and goes back to the pump suction.   This is why Plan 13 is sometimes called a “reverse flush”.  Typically Plan 13 is used on vertical pumps since the vertical pump has the discharge at the top of the pump where the seal is.  This also helps the seal to vent gas out of the seal chamber.  Plan 13 is often combined with other piping plans, especially with dual seals, for example Plan 13/52.

Whereas Plan 11 tends to increase seal chamber pressure, Plan 13 tends to reduce seal chamber pressure.

Because the seal chamber pressure in simple overhung process pumps is typically near pump suction pressure, the restriction orifice is sometimes omitted for Plan 13.  Omitting the restriction orifice should be done only with care and checking pressures and flows.