The valve authority states how much the control valve's own pressure drop is in relation to the pressure drop that exists in the part of the system it controls. To ensure that the control valve is properly regulated, the authority should be at least 50%.
The pressure drop as in the system after the control valve. That is, the pressure drop that occurs over pipes, appliances etc. in the part of the system that the control valve is supposed to regulate.
Pressure over control valve
The pressure drop which is over the control valve itself.