Why back pressure is necessary?

When measuring liquids, back pressure in the flow line should be sufficient to prevent cavitation. Cavitation occurs due to pressure drop at the obstruction in the flow line. Due to pressure drop, bubbles are formed and are driven to the further locations in the flow line where they break and erode the parts of flow line. To avoid such cavitations, the back pressure in the line must be always greater than the vapor pressure of the fluid.

To maintain back pressure in the line, larger pumping force is required. In case of water and liquids of similar vapor pressure, required back pressure is approximately 1 kg/cm² at velocity of 6 meter/second. When there is no back pressure in the pipeline, it can be created by placing a partially closed valve on the downstream side of the flow meter.