  We study the active flow around isolated defects and the self-propulsion
velocity of $+1/2$ defects in an active nematic film with both viscous
dissipation (with viscosity $\eta$) and frictional damping $\Gamma$ with a
substrate. The interplay between these two dissipation mechanisms is controlled
by the hydrodynamic dissipation length $\ell_d=\sqrt{\eta/\Gamma}$ that screens
the flows. For an isolated defect, in the absence of screening from other
defects, the size of the vortical flows around the defect is controlled by the
system size $R$. In the presence of friction that leads to a finite value of
$\ell_d$, the vorticity field decays to zero on the lengthscales larger than
$\ell_d$. We show that the self-propulsion velocity of $+1/2$ defects grows
with $R$ in small systems where $R<\ell_d$, while in the infinite system limit
or when $R\gg \ell_d$, it approaches a constant value determined by $\ell_d$.
