How exactly would you describe the principle of a valve for flow control? In a globe valve for example the fluid flows through valve seat and generally leaves the valve with the outlet diameter being the same as the inlet diameter. My original assumptions were that Bernoulli's principle had to be somehow related but if you ignore friction losses etc. then surely the energy is conserved and the dynamic pressure at the outlet would equal the inlet dynamic pressure due to the diameters being the same. In this case there would be no change in velocity or static pressure. Energy must be lost somehow for the volumetric flow to change but how?