  We consider the effective mass model of spinless electrons in single wall
carbon nanotubes that is equivalent to the Dirac equation for massless
fermions. Within this framework we derive all possible energy independent hard
wall boundary conditions that are applicable to metallic tubes. The boundary
conditions are classified in terms of their symmetry properties and we
demonstrate that the use of different boundary conditions will result in
varying degrees of valley degeneracy breaking of the single particle energy
spectrum.
