Aims. We present low resolution spectroscopic observations for a sample of 53 planetary nebulae (PNe) located in the southern sky between Vela and Norma constellations and pertaining to the Galactic disk with expected Galactocentric distance range of 5 to 10 kpc. Methods. We derive nebular chemical composition and plasma parameters with the classical empirical method. For most of the observed objects, this has been done for the first time. We compare our results to published data for PNe of the Galactic bulge and PNe in the inner-disk region with expected typical Galactocentric distance of about 3 kpc. We use the spectra to search for emission-line central stars in the observed sample. Results. The distributions of the chemical abundances of the observed disk sample are generally indistinguishable from Galactic bulge and inner-disk PNe populations. The exceptions are possible differences in the He/H distribution, as compared to bulge PNe and Ne/Ar, as compared to the inner-disk PNe sample. The derived O/H ratios for the observed disk PNe fit to the concept of flattening of the chemical gradient in the inner parts of the Milky Way. Investigating the spectra, we found six new emission-line central stars comprising examples of all known types: WEL, VL, and [WR]. We confirm that these types represent three evolutionary unconnected forms of enhanced mass-loss in the central stars of PNe. We note on the problem of high ionisation PNe with nebular C?IV emission that can mimic the presence of WEL central stars in 1D spectra.
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