Positive Displacement Flow Meters

Positive displacement flow meter measurement uses a fixed small volume to repeatedly measure the volume of fluid passing through the flow meter.

Therefore, there must be a space that constitutes a standard volume inside the positive displacement flow meter, which is usually called the “measurement space” or “measurement chamber” of the positive displacement flow meter. This space is composed of the inner wall of the instrument housing and the rotating parts of the flow meter.

The working principle of Positive displacement flow meter is: when the fluid passes through the flow meter, a certain pressure difference will be generated between the inlet and outlet of the flow meter. The rotating part of the flow meter (referred to as the rotor) rotates under the action of this pressure difference and discharges the fluid from the inlet to the outlet. In this process, the fluid fills the “metering space” of the flow meter again and again, and then is continuously sent to the outlet.

Under given flow meter conditions, the volume of the metering space is determined as long as the number of rotations of the rotor is measured. The cumulative value of the fluid volume passing through the flow meter can be obtained.

Advantages of positive displacement flow meter:

• High measurement accuracy.
• The installation pipeline conditions have no impact on the measurement accuracy, and there is no need for straight pipe sections.
• Can be used for measuring high viscosity liquids.
• Wide range.
• The direct-reading instrument can directly obtain the cumulative total without external energy, which is clear and easy to operate.
• Can be configured with an electronic display that supports pulse, current 4-20mA, RS485 and other signal outputs.
• Can be customized for ultra-high and low-temperature applications.

Disadvantages of positive displacement flow meter:

• The structure is complex and the volume is huge.
• The type, diameter, and working status of the medium to be measured have great limitations.
• Most meters are only suitable for clean single-phase fluids.
• Produce noise and vibration;
• Not suitable for liquids with foreign matter and low viscosity (above 1CP);
• Bearings are prone to wear and require regular inspection.

Positive displacement flow meters can be classified according to measuring components.

Common types include the following:

1. Oval gear
2. Helical gear
3. Spur gear
4. Reciprocating/oscillating piston
5. Multi-piston
6. Nutating plate/disk
7. Rotary vane/sliding vane
8. Bi-rotor and tri-rotor