  The kinematics of the plasma in 14 PNe is analysed by measuring the expansion
velocities (V$_{exp}$) of different ions as derived from their collisionally
excited lines (CELs) and optical recombination lines (ORLs). V$_{exp}$ are
analysed as a function of the ionisation potential of ions that at first
approximation represents the distance of the ion to the central star. In most
cases the kinematics of ORLs is incompatible with the kinematics of CELs at the
same ionisation potential, specially if CELs and ORLs of the same ion are
considered. In general V$_{exp}$ from ORLs is lower than V$_{exp}$ from CELs
indicating that, if the gas is in ionisation equilibrium, ORLs are emitted by a
gas located closer to the central star. The velocity field derived from CELs
shows a gradient accelerating outwards as predicted from hydrodynamic modelling
of PNe ionisation structures. The velocity field derived from ORLs is
different, in many cases the velocity gradient is flatter or nonexistent and
high and low ionised species present nearly the same V$_{exp}$. In addition,
the FWHM(ORLs) is usually smaller that the FWHM(CELs). Our interpretation is
that ORLs are mainly emitted by a plasma that coexists with the plasma emitting
CELs, but does not fit the ionisation structures predicted by models. Such a
plasma should have been ejected in a different event that the plasma emitting
CELs.
