Restricting the flow path means that piping joints with different diameters will result in pressure loss.
Turbulent flow occurs in bent sections, making it difficult for fluids to flow. When this happens, laminar flow changes to turbulent flow and energy loss occurs. Energy loss leads to lowered pressure.
If ball valves and other such valves are left fully opened, then pressure loss should not be a concern. However, with valves that change the flow path inside the valve, pressure loss will occur even if fully opened.
When fully opened, it forms a free-flowing structure, so there is no pressure loss.
Even if fully opened, the flow path is changed, resulting in pressure loss.
As stated in the operating principles, differential pressure flow meters (diaphragm types) create pressure loss from flow restricting plates to measure flow. Additionally, Karman vortex flow meters restrict and accelerate the flow path in order to provide stable vibrations for the piezo element. Also, with paddle wheel flow meters, the flow path is restricted in order to generate enough thrust to spin the paddles when there is small flow. For these types of flow meters, pressure loss occurs easily. Conversely, with electromagnetic, non-insertion thermal, and ultrasonic flow meters, there is no need to restrict the flow path for detection, which stands as a sizeable advantage in regard to pressure loss.