Positive Displacement (PD) Flow meters are volumetric flow instruments that measure flow by passing a precise volume of fluid with each revolution. PD flow meters are precision instruments whose internal moving components are hydraulically locked in tandem with the volume of fluid moving through the flow meter. The meters can measure intermittent flows, very low flow rates, and liquids of almost any viscosity. When fluid flows, PD meters instantly move. And when the fluid stops, the meter stops.
Moving in lock step with the fluid also means that measurements are not affected by the liquid’s viscosity, density or turbulence in the pipe. Each measured volume of fluid is uniquely separated from the next by a frictionless liquid seal. The sealing effectiveness is a function of manufacturing precision and the ease of rotation of the meter’s components. The same close clearances that make the precision parts work also mean that relatively clean fluids must be measured. Materials of construction vary widely and allow the family of PD meters to measure a wide variety of process liquids. Their high precision has allowed PD flow meters to be almost universally accepted for billing and transfer standard applications, such as dispensing gasoline, fuel oil, and natural gas or water measurement.
There are a variety of methods to provide output signals and readouts. PD meters have the ability to offer simple once per revolution counting to high resolution encoder equivalent signals which can take more than 4000 measurements per revolution. Because these measurements are not sensitive to changes in pressure, temperature, or viscosity, the meter’s K-Factor typically does not need to be compensated for during use.
Accuracy and operating range, or turndown ratio, varies widely by type and manufacturer, but many meters fall in the range of 0.1% to 1.0% of reading, with turndown ranging from 10:1 to 200:1 or more. Due to their high accuracy, PD meters are the most widely used meters for custody transfer applications.
With such a large variety and diverse operating range, prices of positive displacement meters can vary widely. A simple water meter for your home may cost less than $100, while prices for the most sophisticated and largest meters can run up to several thousand dollars.
PD meters are an established technology that covers the spectrum of measurement devices from some of the most widespread, easiest to use, and most affordable, like rotary piston or nutating disc meters, to some of the highest precision, highest accuracy, highest resolution meters available today, like reciprocating piston meters. New materials, like composites, new manufacturing techniques, like additive manufacturing, and new electronic sensing technology, like high resolution hall effect sensing, continue to keep the basic idea of measuring a known volume of fluid at the forefront of the fluid measurement industry.
There are many types of positive displacement flow meters, including; reciprocating piston, oscillating or rotary piston, bi-rotor types (spur gear, oval gear, helical gear, rotary vane), and nutating disc (wobble plate). Some examples are listed below:
Piston Flow Meters
Piston flow meters are of single and multiple-piston types. They function by rotating, oscillating, or reciprocating. For any motion, the pistons displace fluid in the same way that a syringe operates. Each piston displacement captures the same amount of fluid.
Oscillating Piston Meters
Typically seen in residential water service and to measure relatively viscous fuels, these meters are a single piston design that oscillates between the inlet and outlet ports as fluid flows. Measurement output is usually achieved by having a magnet installed on the piston and a transmitter picks up the change in the magnetic field as the piston moves.
Reciprocating Piston Meters
Typically linked via a crankshaft and opening and closing ports in the cylinder, reciprocating piston meters act similarly to piston engines that power most automobiles and trucks. The sensor output is usually driven from the rotating crankshaft and can be mechanical, electrical, optical, or magnetic. These meters are mainly used to measure very low flow rates, down to 0.005 cc/min.
Gear Flow Meters
Gear flow meters use two round gears that are mounted in overlapping compartments. The measured fluid is trapped in the voids of the gear teeth and transported from the inlet port to the outlet port as the fluid flow causes the gears to rotate.
Oval-gear meters have two rotating, oval-shaped gears with synchronized, close fitting teeth. A fixed quantity of liquid passes through the meter for each revolution.
Helical [Gear] Flow Meters use two screw-shaped rotors to slice the fluid stream into fixed displacement volumes. The rotors’ orientation is in-line with the fluid flow path. These meters rotate with very low pressure drop, and can turn at high rpm’s making them accurate over wide flow ranges and compatible with very high viscosity fluid applications.
Other PD Flow Meters
Rotary vane meters consist of equally divided, rotating impellers, in two or more compartments, inside the meter’s housings. The impellers are in continuous contact with the casing. A fixed volume of liquid is swept to the meter’s outlet from each compartment as the impeller rotates.
Nutating disk meters have a moveable disk mounted on a concentric sphere located in spherical side-walled chambers. The pressure of the liquid passing through the measuring chamber causes the disk to rock (wobble) in a circulating path without rotating on its axis. The disk/sphere is the only moving part in the measuring chamber.