DN = Diameter Nominal. The term Diameter Nominal refers to the internal diameter of a pipe. Together with the nominal pressure rating and the materials class, all dimensions of a piping line, e. g. flange dimensions, are defined by indicating the nominal diameter. Steel is often not specified, but assumed as material.
You can compare the strength of the hydrogen bonding by comparing the electronegativities of the other elements. All the four elements, Br, F, Cl and I belong to the same group in the periodic table: group 17 named halogens. Then the kind of bonding they form is similar: polar covalent. You must know the trend of the electronegativities in the periodic table. Electronegativity decrease when you down across a group. Then the electronegativity of F is the higher of the group (indeed, it is the highest of all the 118 elements) . The higher the electronegativity the stronger the attraction that the halogen attracts the electrons and the stronger the hydorgen bonding. Then, the conclusion is that HF has the strongest hydrogen bonding.
Hydrogen bonds are a stronger intermolecular force than either Dispersion forces or dipole-dipole interactions since the hydrogen nucleus is extremely small and positively charged and fluorine, oxygen and nitrogen being very electronegative so that the electron on the hydrogen atom is strongly attracted to the fluorine, oxygen or nitrogen atom, leaving a highly localised positive charge on the hydrogen atom and highly negative localised charge on the fluorine, oxygen or nitrogen atom.