Positive Displacement Flow Meters

Positive displacement flow meters act like a hydraulic pump in reverse. The moving parts inside the flow body are designed so that a finite amount of liquid is allowed to fill a chamber and then is expelled, before it is then filled again with the same amount. The volume per revolution of the internal components is a known value per revolution and is a true volumetric measurement. The precise motion of the rotating element against the uniform measuring chamber creates the direct volumetric measurement.

There are many different internal configurations for positive displacement flow meters. Regardless of the different parts that are moving internally in the meter, they all operate on this same principle. A great simple example would be to compare the principle to taking a cone cup measuring cup, allowing it to fill up and then dumping it out, over and over and over again. In essence, each rotation is “batching” a specific amount from the inlet side to the outlet side of the meter. As a result, positive displacement flow meters are well suited for applications with intermittent flow as the mechanism move when there is flow and stop when there is none.

Positive displacement technology is well proven and is the standard technology for certain applications and excel in measuring viscous media or media with lubricating properties. This media distinction helps to protect the gears.

When properly installed and calibrated, they are accepted universally as the flow meter technology of choice for applications like custody transfer, due to the exact volumetric measurement.

Positive displacement flow meters are also used in residential water consumption, again because of the exact nature of cost per volume being charged to the end user. However, in general, water is not typically an ideal media for positive displacement flow meters and other technologies typically offer a better solution for water measurement. Special considerations have to be made for such a low viscosity media as water and most PD meters used for this purpose are specifically built for this purpose with mechanical counters that can be read on-site by the utility staff.

There are many positive displacement flow meter types.

• Oval Gear
• Helical Gear
• Spur Gear
• Screw Gear
• Reciprocating/Oscillating Piston
• Multi-piston
• Nutating Disc/Wobble Plate
• Rotary/Sliding Vane
• Bi-rotor and Tri-rotor

Gear flow meters are the types of positive displacement flowmeters that utilize gears to create the exact volumetric measurement. The types of gear meters are Oval Gear Flow Meters, Helical Gear Flow Meters, Spur Gear Flow Meters, and Screw Gear Flow Meters. Oval gear flow meters are probably the most common gear meter. They utilize two intermeshed gear wheels within a specifically shaped chamber that rotate upon liquid flow. Obviously, gears do best when lubricated so for this type of positive displacement flow meter technology it is essential to use high viscosity or lubricating media.

Oval gear flow meters utilize the shape that they are named after, the gears are oval. Being one of the most common positive displacement flow technology types, they are in use all over the world.

Oval gear flow meters offer many different types of outputs and displays. Because no power is required, mechanical counter displays are a popular choice. An example of that would be the older-type gas pumps at the gas station with the rolling number wheels.

Some oval gear flow meters also offer digital displays. These can display the rate of flow electronically and sometimes offer batching and totalizing capabilities.

Common example application areas for positive displacement flow meters are as follows.

• Oil and Gas
• Water and Wastewater
• Chemical Industry and Chemical Injection
• Power Generation
• Pharmaceuticals
• Food and Beverage
• Pulp and Paper
• Metal and Mining
• Aerospace
• Hydraulic Testing

While there are many suitable media types for positive displacement flow meters, here are some examples.

• Detergents
• Deoderizers
• Caustics
• Acids
• Fuels, Diesel, Kerosene
• Oils
• Honey
• Molasses
• Anti-icing Agents
• Brake Fluid
• Waxes
• Adhesive Coatings
• Polyurethane Foams
• Combustion Modifiers
• Corrosion Inhibitors
• Inks
• Pastes
• Resins
• Glues
• Creams

Positive displacement flow meters offer many advantages that recommend it over other flow meter technology types for certain applications.

• Can process a wide variety of viscosities, especially high viscosity media
• Next to no maintenance
• High pressure and hydraulic capabilities
• Can measure intermittent flow and low flow rates
• High turndown ratio
• Accurate, repeatable, linear, and reliable
• High resolution
• No power needed for purely mechanical models without electronics
• Doesn’t require a well-developed flow profile
• No straight run requirements, not affected by turbulent flow
• Unaffected by noise and vibration
• Accuracy not affected by viscosity or temperature changes over a wide range
• Less expensive than some other technologies suitable for similar applications
• Can measure non-conductive liquids

Certain elements of positive displacement flow meter technologies do not recommend them for certain application types.

• Certain types require viscous media or media with lubricating properties to protect the gears
• Only for clean media
• Usually only uni-directional flow measurement
• Not for applications with air pockets
• Heavy, if made from metal, as flow bodies tend to be thick
• Usually require filtration upstream of the meter
• Do have pressure drop that has to be accounted for
• Must be installed in the correct orientation for that meter

Positive displacement flow meters are sometimes referred to as “PD Meters”. They offer great accuracy due to the exact volumetric measurement principle. PD meters deliver one of the highest accuracies of all the flow meter technologies and the best of any of any of the mechanical flow meter types at 0.1% to 2.5% and is model dependent. The low range of that accuracy capability is in line with Coriolis flow meters which is the most accurate flow meter technology type. To learn more about the accuracy of all flow meter technology types, visit our article.

Positive displacement flow meter pricing varies greatly. The larger the line size and the higher the flow and pressure capabilities, the more expensive the flow meter will be. Other considerations are the material build. There are some lower capacity positive displacement flow meters made out of plastic that will obviously be cheaper than other models, which are usually made out of metal. Thus, there is a wide variety in positive displacement flow meter technology. To view our large line of positive displacement flow meters, visit our online offering.