What are Positive Displacement flow meters and how are they used

Positive displacement flowmeters, or PD meters, feature two precisely machined intermeshing gears which rotate within a measuring chamber to determine the volumetric liquid flow rate by counting the number of the gears’ rotations. Positive displacement flow meters function in much the same manner as a positive displacement pump with the exception being that in the Positive Displacement pump the gears drive the fluid media, whereas in the PD flowmeter the fluid media drives the gears. The positive displacement technique is one of the oldest flow measurement technologies in existence and has been in use for over one hundred years. During that time, many different varieties of the PD meter have been developed including versions that feature round gears, oval gears, lobe gears, helical gears and nutating disc rotating elements. These meters are all characterized by high accuracy (up to ±0.1% of reading), good repeatability (up to 0.05% of reading), wide turndown ratios of up to 100:1, and the ability to maintain this level of performance for both high viscosity and changing-viscosity fluid media.

Positive Displacement Flow Meters Key Applications
Positive displacement flow meters flowmeters from SmartMeasurement have been successfully installed in a wide variety of industries and applications including:

• oil & gas
• chemical & petrochemical
• fiscal metering
• fuel transportation
• liquid storage tanks
• food & beverage
• pharmaceutical
• marine & shipping

Why choose a PD flow meter

One common reason for using the positive displacement flow meters is that it can function without power. By outfitting the meter with a mechanical register, totalized flows may be accurately monitored in virtually any engineering unit desired by the end-user without the need to run power to the meter. This is ideal for applications where the meter must be installed in a remote location where power is not available. Positive displacement flow meters meters are available in sizes from ” to 12 inches or more. Operation turndowns can be as high as 100:1, although ranges of 15:1 or lower are much more common. Slippage between the flowmeter components is reduced and metering accuracy is therefore increased as the viscosity of the process fluid increases.  Therefore, positive displacement flow meters are excellent for highly viscous fluids such as oils and fats.

Most flowmeters are sensitive to turbulence, which means that a certain amount of straight, rigid pipe must be installed both upstream and downstream of the meter in order to reduce turbulence and accurately measure flow (Download PDF). Positive Displacement meters do not have this requirement. If necessary, an elbow or reducer bushing may be placed directly at the meter inlet with no impact on the meter’s accuracy.

Positive displacement flow meters work without power. By outfitting the meter with a mechanical register, totalized flows may be accurately monitored in virtually any engineering unit desired by the end-user without the need to run power to the meter. This is ideal for applications where the meter must be installed in a remote location where power is not available. Positive displacement flow meters are available in sizes from ” to 12 inches or more. Operation turndowns can be as high as 100:1, although ranges of 15:1 or lower are much more common. Slippage between the flowmeter components is reduced and metering accuracy is therefore increased as the viscosity of the process fluid increases.  Therefore, Positive displacement flow meters are excellent for highly viscous fluids such as oils and fats.

Positive displacement flow meters operate with small clearances between their precision-machined parts; wear rapidly destroys their accuracy. For this reason, positive displacement flow meters are generally not recommended for measuring slurries or abrasive fluids. However, for clean fluid service, their precision and wide rangeability make them ideal for custody transfer and batching applications. In industrial and petrochemical settings, flow meter are commonly used for batching and mixing control