steam driven cargo oil pump

[marengineer]

in tanker ships there are cargo oil pumps driven by the steam to discharge crude oil.steam pressure generated by the boiler is apprmx. 15 bar and is entering cargo oil pump.but at the steam chest gauge, pressure is just about 7-8 bar when pumps is working its normal load.
my question what is the reason of this pressure drop?where the steam is decrease?
can it be for superheat?but how?

thanks