Static head exerted by point ‘a and b' over the reference line are equal.

H₁ ρ_h = X ρ_L + (Y + C) ρ_h

Similarly, H₂ = (X + Y + C)

 

The difference between overflow and under flow head

ΔH = H₂ - H₁

In case if heavy phase is water

ρ_h =1.0    ΔH = (1 - ρ_L) X