In addition to the fresh oil feed and
hydrogen, additional upward liquid
flow for the ebullition is provided by a
specially designed ebullating pump
which takes reactor liquid and recycles
it to the reactor bottom plenum. The
pump has a relatively low head but
large flowrate. Ebullating pumps have
proven to be very reliable.

http://www.eptq.com/Pages/Articles/Summer1998/Summer1998_PDF/PTQ98305.pdf