When the valve is just starting to open, force from the spring helps keep the orifice covered, but this
is augmented by hydraulic force produced by the pressure drop between the valve inlet and outlet. This
hydraulic force is proportional to the square of orifice size, since the area of pressure imbalance is
equal to pi times the orifice radius squared. This is why opening valves with greater flow often requires
a disproportionately larger coil and lifting force.