Positive displacement meter

A positive displacement meter is a type of flow meter that requires the fluid being measured to mechanically displace components in the meter in order for any fluid flow to occur. Positive displacement (PD) flowmeters make volumetric flow measurements taking finite increments or volumes of the fluid. A basic analogy would be holding a bucket below a tap, filling it to a set level, then quickly replacing it with another bucket and timing the rate at which the buckets are filled (or the total number of buckets for the “totalized” flow). With appropriate pressure and temperature compensation, the mass flow rate can be accurately determined. The meters come in several forms, including:

• Reciprocating piston / Oscillating piston – each piston is mechanically or magnetically operated to fill a cylinder with the fluid and then discharge the fluid. Each stroke represents a finite measurement of the fluid.
• Oval gear – two rotating oval gears with synchronized teeth “squeeze” a finite amount of fluid through the meter for each revolution.
• Nutating disk – a disk mounted on a sphere is “wobbled” about an axis by the fluid flow and each rotation represents a finite amount of fluid transferred.
• Rotary vane – A rotating impeller containing two or more vanes divides the spaces between the vanes into discrete volumes and each rotation (or vane passing) is counted.
• Diaphragm – Fluid is drawn into the inlet side of an oscillating diaphragm and then dispelled to the outlet. The diaphragm oscillating cycles are counted to determine the flow rate.

Positive displacement flowmeters are very accurate and have high turndown. They can be used in very viscous, dirty and corrosive fluids and essentially require no straight runs of pipe for fluid flow stream conditioning. They are widely used in custody transfer of oils and liquid fluids (gasoline) and are applied on residential home natural gas and water metering. A diaphragm meter, with which most homes are equipped, is an example of a positive displacement meter. This type of meter is appealing in certain custody transfer flow applications where it is critical that the metering be functional in order for any flow to take place.

PD flowmeters are relatively expensive and produce the highest differential pressure (and subsequently greatest pressure drop head loss) of all the flowmeter types.

Positive displacement flow meters measure the volume or flow rate of a moving fluid or gas by dividing the media into fixed, metered volumes. These devices consist of a chamber that obstructs the media flow and a rotating or reciprocating mechanism that allows the passage of fixed-volume amounts. The number of parcels that pass through the chamber determines the media volume. The rate of revolution or reciprocation determines the flow rate. There are two basic types of positive displacement flow meters. Sensor-only systems or transducers are switch-like devices that provide outputs for processors, controllers, or data acquisition systems. Complete sensor systems provide additional capabilities such as an integral display and/or user interface. For both types of positive displacement flow meters, performance specifications include operating pressure, temperature range, maximum material density, connection size, and percent accuracy. Suppliers indicate whether devices are designed to move fluid or gas.

Design of Positive Displacement Flow Meter Positive displacement flowmeters repeatedly fill and empty compartments of known volume with the liquid or gas from the flowstream. Flowrate is calculated from the rate these compartments are filled and emptied. Types include nutating disc, oval gear, and helical gear.

Nutating disc flowmeters get their name from the idea of nutation, which means nodding or rocking. A nutating disc meter has a round disc mounted on a spindle in a cylindrical chamber. By tracking the movements of the spindle, the flowmeter determines the number of times the chamber traps and empties fluid. This information is used to determine flowrate.

With oval gear flowmeters, two oval gears or rotors are mounted inside a cylinder. As the fluid flows through the cylinder, the pressure of the fluid causes the rotors to rotate. As flowrate increases, so does the rotational speed of the rotors.

Helical gear flowmeters get their name from the shape of their gears or rotors. These rotors resemble the shape of a helix, which is a spiral-shaped structure. As the fluid flows through the meter, it enters the compartments in the rotors, causing the rotors to rotate. Flowrate is calculated from the speed of rotation.

There are a number of other types of positive-displacement flowmeters, including oscillating piston, rotating crescent, rotating paddle, sliding vane, and others. These chiefly differ according to the way they trap the liquid in compartments with different shapes.

Positive-displacement (PD) meters can measure both liquids and gases. Like turbine meters, PD flowmeters work best with clean, non-corrosive, and non-erosive liquids and gases, although some models will tolerate some impurities. Because of their high accuracy, PD meters are widely used at residences to measure the amount of gas or water used. Commercial/Industrial applications include: chemical injection, fuel measurement, precision test stands, high pressure, hydraulic testing and many other precision applications.

Some designs require that only lubricating fluid be measured, because the rotors are exposed to the fluid. PD meters differ from turbine meters in that they handle medium and high-viscosity liquids well. For this reason, they are often used to measure the flow of hydraulic fluids. PD meters above 10 in. tend to be heavy, large, and relatively expensive. PD meters require very little upstream piping, and can easily handle low flows. Pressure drop can be an issue.

See also

• Water meter
• Oil meter
• Gas meter
• Flow measurement
• Displacement Volumetric Meter