A special variety of the rotating tube flow meter is the oval – geared metering elements.
These oval-geared meters are generally used on very viscous liquid, which is difficult to measure using other flow meters. In this design, as shown in Fig, a precise volume of liquid is captured by the gap formed between housing and the gear.
In position A, uniform forces are applied equally on the top and bottom of oval gear B, so that the gear does not rotate. Rotor A has entrapped a known volume of liquid between the rotor and the meter body, and there is a balanced force on the bottom of the gear. However, there is a force on the bottom of gear A, causing it to rotate clockwise (CW).
This causes gear B to rotate in a counter clock wise (CCW) direction to position B. In position B, fluid enters the space between gear B and the meter body, as the fluid that was entrapped between gear A and the body simultaneously leave the area of entrapment.
The higher upstream pressure oppose the lower downstream pressure at the ends of gear A and gear B, which makes gear A and gear B continue to rotate in CW and CCW directions respectively, to position C.
In position C, a known amount of fluid has been entrapped between gear B and the meter body. This operation is then repeated, with each revolution of the gears representing the passage of four times the amount of fluid that fills the space between the gear and the meter body.
Therefore, the fluid flow is directly proportional to the rotational velocity of the gears. If slippage between the oval-gears and the housing is small, and the flow rate and viscosity are high, these flow meters can provide high accuracies. (0.1%).
These flow meters are available in the sizes suitable for 6 mm to 400 mm diameters pipelines. Their materials of construction include brass, carbon steel, and 316 stainless steel. Operating pressures are available up to 100 kg/cm2 and temperatures up to 300°C.