  The commonly used West and Yennie integral formula for the relative phase
between the Coulomb and elastic hadronic amplitudes requires for the phase of
the elastic hadronic amplitude to be constant at all kinematically allowed
values of t. More general interference formula based on the eikonal model
approach does not exhibit such limitation. The corresponding differences will
be demonstrated and some predictions of different phenomenological models for
elastic pp scattering at energy of 14 TeV at the LHC will be given. Special
attention will be devoted to determination of luminosity from elastic
scattering data; it will be shown that the systematic error might reach till 5
% if the luminosity is derived from the values in the center of the
interference region with the help of West and Yennie formula.
