Positive Displacement (PD) flowmeters are types volumetric flow meters. PD flowmeters work in a cyclic process, in each cyclic precisely defined (“positive”) quantity of fluid is entrapped in its chamber using a mechanical rotating system in the chamber and releases the entrapped fluid back into the stream. Thus the number of cycles will be proportional to the flow rate of the flow stream.
PD flowmeter doesn’t need any external power supply. PD flowmeters are very suitable to measure flow rates of viscous fluid.
PD flowmeters are usually used for automatic batching and accounting applications. There are different types of PD flowmeters named after the rotating systems in the chambers. These are the major types of PD flowmeters:
Reciprocating piston meter:
Reciprocating piston or oscillating piston flowmeter consists of a measuring chamber and piston rotating in it. There are single and multiple-piston type reciprocating piston meters. The selection depends upon the range of flow rates.
The piston in the chamber oscillates within the chamber to open the inlet at one position and let the fluid pass in for an instant and let this valve to flow out through the outlet at another instance closing the inlet and opening the outlet.
The volumetric flow measuring technic is shown above. The meter has two oval-shaped close fitted gears. The rotation of the gear system blocks a particular amount of fluid within the gap of the system. By monitoring the number of shaft rotations, one can calculate liquid flow rate.
The nutatinng-disc consists of a round disc with a globe at the centre fixed in a chamber. The disc is allowed to rotate in the chamber. Fluid is entered through one end and blocked in the chamber because of the change in disc position and open to flow through the other flow outlet. They are ideal for pressure ranges around 150-psi with an upper limit of 300 psi.
The Rotating-vane meter consists of more than two impeller blade. The fluid flow allows the blades to rotate. These impellers divides the chamber into two or more compartments, and the number of rotations of the impeller are counted and recorded in volumetric units. These types of meters are frequently employed in the petroleum industry. Based upon the construction material, maximum pressure and maximum temperature limits of rotary vane meters are 350°F and 1,000 psi respectively.
Advantages of PD flowmeters:
No upstream/downstream requirements
Can measure very low flows and viscous flows
High-quality, high accuracy, a wide range, and are very reliable, insensitive to inlet flow profile distortions, low pressure drop across the meter.
Until the introduction of electronic correctors and flow controls on other types of meters, PD meters were most widely used in batch loading and dispensing applications. All mechanical units can be installed in remote locations.
Moving parts can create large pressure drop
Maintenance is necessary
Measuring discrete fluid flow insted of actual fluid flow
May takeup lot of space
Have high inertia of the moving parts; a sudden change in the flow rate can damage the meter.
Only for limited ranges of pressure and temperature
Most PD meters require a good maintenance schedule and are high repair and maintenance meters.