"where'er you walk" is in simple ternary form because the a section returns completely at the end of the piece and the b section is harmonically independent from the a section.
the a section is an asymmetrical parallel period. the first phrase ends in meausre 7 in a half cadence (the V chord is tonicized--look at all those e naturals!) and the second phrase ends in measure 16 in a PAC in the original key of b flat, making this section tonally closed. there is kind of a cadential extension for 3 measures that ends in yet another PAC in measure 19.
the b section begins in measure 20. it changes key to the relative minor (g minor), but ends in a PAC in the key of c minor right before the return of the a section. the section feels more tonally open than the first, which is probably why the a section is revisited. this section also does not give the listener any particular structural feeling, which is why i think it can be called a phrase group.