  Central molecular outflows in spiral galaxies are assumed to modulate their
host galaxy's star formation rate by removing gas from the inner region of the
galaxy. Outflows consisting of different gas phases appear to be a common
feature in local galaxies, yet, little is known about the frequency of
molecular outflows in main sequence galaxies in the nearby universe. We develop
a rigorous set of selection criteria, which allow the reliable identification
of outflows in large samples of galaxies. Our criteria make use of central
spectra, position-velocity diagrams and velocity-integrated intensity maps
(line-wing maps). We use this method on high-angular resolution CO(2-1)
observations from the PHANGS-ALMA survey, which provides observations of the
molecular gas for a homogeneous sample of 90 nearby main sequence galaxies at a
resolution of ${\sim}100\,$pc. We find correlations between the assigned
outflow confidence and stellar mass or global star formation rate (SFR). We
determine the frequency of central molecular outflows to be $25\pm2$%
considering all outflow candidates, or $20\pm2$% for secure outflows only. Our
resulting outflow candidate sample of $16{-}20$ galaxies shows an overall
enhanced fraction of active galactic nuclei (AGN) (50%) and bars (89%) compared
to the full sample (galaxies with AGN: 24%, with bar: 61%). We extend the trend
between mass outflow rates and SFR known for high outflow rates down to lower
values ($\log_{10}{\dot{\rm M}_{\rm
out}}\,[\mathrm{M}_\odot~\mathrm{yr}^{-1}]<0$). Mass loading factors are of
order unity, indicating that these outflows are not efficient in quenching the
SFR in main sequence galaxies.
