saturation property of nuclear forces ? and its relation binding energy per nucleon constantcy? Please explain why binding energy per nucleon is constant(pratically) for atomic number, A,  larger than 30 and less than 170, and  explain the saturation property of nuclear forces with analogy that is easy to understand?
 A: You have to realized that the combined  forces that bind the protons and neutrons together are a complex interplay between two forces: 
a)The electromagnetic one, where the charge of a proton repels the charge of another proton and no binding could occur
b) the strong force , the force that binds the quarks into the protons and neutrons, and spills over around each proton and neutron and is an attractive one. 
From this you can understand that the number of particles that can be "bound" depends on the interplay of the repulsive and attractive forces and is a many body problem not solvable analytically, but with various nuclear models. These models are fairly successful in describing the behavior of the nuclei and the way the energy is distributed ( binding energy).
A third  process that enters the problem is that neutrons are not stable, if they are not bound within a collective nuclear potential they decay  ( beta decays of isotopes).
Qualitatively you can think that after a certain mass number  (A) , the assumption of average density will be fairly good  Too many protons would spoil the broth by repulsion, so the mass number is limited at the high end,   too many neutrons would have outer energy  level neutrons decay and this is also a limit. On the lower end of mass number, the number of nucleons in  the nucleus  is too small  and the statistical arguments of density can no longer be a good approximation.
