Positive Displacement Measuring Principle
Positive Displacement (PD) Flowmeters are volumetric flow meters that measure flow by passing discreet parcels of fluid in precisely known volumes. PD flow meters are precision instruments whose internal moving components are mass-balanced yet remain hydraulically imbalanced. The result is that the flowmeter can measure very low flows of both liquids and gases without using external power. The PD flow meter derives the power necessary to work from the energy contained in the flow process. The positive displacement flow meter has an advantage in that it requires no external power, and unlike other flow technologies, requires no fully developed flow profile for accurate flow measurement which can be accurate up to 0.1%.
As illustrated in the picture below, each moving chamber of fluid is separated from the next chamber by a capillary seal, the integrity of which is a function of the precision to which the meter is manufactured. This high precision allows these flow meters to be almost universally accepted as transfer standards when properly installed and flow-calibrated. The close tolerances of the internal metering components require the positive displacement flow meter be used only with clean fluids. The metallurgy of the meter dictates the type of process for which it can be used. Generally, these flow meters are used in dedicated unidirectional flow applications, such as fuel flow meter oil dispensing, or as natural gas flowmeters or water flowmeters.
Good instrumentation practice requires a filtering mechanism and a capacitance vessel for air removal for liquids or a coalescing filter for gases to be an integral part of the flowmeter installation. Advances in technology allow the flow meters to be temperature-compensated and interfaced electronically with central control systems. They can also be easily configured to be the integral part of a truck-mounted flowmeter delivery system. Individual manufacturers' specifications will help guide the user to select the correct flowmeter for the application. Examples of the more common Positive Displacement meters are oscillating piston, nutating disc, oval gear, roots, vane, rotor, and multi-piston.
Oscillating Piston Flow Meters
Liquid enters a precision-machined chamber containing an oscillating (rotating) piston. The position of the piston divides the chamber into compartments containing an exact volume. Liquid pressure drives the piston to oscillate and rotate on its center hub. The movements of the hub are sensed through the flowmeter wall by a follower magnet. Each revolution of the piston hub is equivalent to a fixed volume of fluid, which is indicated as flow by an indicator/totalizer. Close clearances between the piston and the chamber ensure minimum liquid slip for highly accurate and repeatable flow measurement of each volume cycle. The maximum viscosity allowable is 4,000 centipoise.
Nutating Disc Flow Meters
Liquid enters a precision-machined chamber containing a disc which nutates (wobbles). The position of the disc divides the chamber into compartments containing an exact volume. Liquid pressure drives the disc to wobble and a roller cam causes the nutating disc to make a complete cycle. This motion is translated into rotary motion by means of a ball and shaft, which is attached to the disc. The movements of the disc are transmitted by gear train to an indicator/totalizer or pulse transmitter. There are inherently more leakage paths in this design and it tends to be used where longer flow meter life is required rather than high performance; however, close clearances between the disc and chamber ensure minimum leakage for accurate and repeatable measure of each volume cycle. The maximum viscosity allowed for this type of flow meter is 11,000 centipoise.
Oval Gear Flow Meters
Two identical oval rotors mesh together by means of slots around the gear perimeter. The oval shaped gears are used to sweep out an exact volume of the liquid passing through the flow measurement chamber during each rotation. The flow rate can be calculated by measuring the rotation speed. Close tolerances ensure that leakage is minimized. In contrast to nutating disc meters, the calibration factor does not vary with viscosity. Though claims for high performance are made, oval gear flowmeters are generally not as precise as the sliding vane design. Another disadvantage is that pulsations are introduced into the flow by the meter. Oval gear flow meters are typically used in the flow measurement of solvents and 'dry' liquids. The maximum viscosity allowed for this flow meter is 1,000 centipoise.
Roots Flow Meters
The roots flow meter is similar in many respects to the oval gear flow meter. A design is shown where two-lobed impellers rotate in opposite directions to each other within the body housing. These peanut-shaped gears sweep out an exact volume of liquid passing through the flow measurement chamber during each rotation. The flow measurement can be calculated by measuring the rotation speed. In contrast to nutating disc meters, the calibration factor does not vary with viscosity. The maximum viscosity allowed for this flow meter is 5,000 centipoise.
Multi-Piston Flow Meters
Piston Flowmeters of either single or multiple designs find widespread use in fuel flow meter dispensing and the low flow measurement of light hydrocarbons. In the multiple piston design shown below, the pistons are arranged in opposing pairs and connected through a series of cranks to the register mechanism. This arrangement ensures that when one cylinder is ported to the inlet, the opposing cylinder is ported to the outlet so that fluid has to flow through the flow measuring chambers with minimum leakage. This design introduces significant pulsations into the flow, which are generally not suitable for flow rates above 100 l/min.
Bi-Rotor Flow Meters
Bi-rotor flowmeter features two precisely machined rotating members known as helical rotors which rotate and mesh within the meter's interior housing in order to form a flow measuring chamber of known volume which may be used to accurately determine flow measurement as a function of the rotors' velocity. The helical rotors' motion is transmitted to the flow transmitter display via a sealed coupling and drive system that enables the flow transmitter display to provide accurate data for both flow rate and total accumulated flow. The unique helical rotor design provides a number of advantages over traditional gear-type positive displacement flow meter including reduced pressure drop, the virtual elimination of down-stream pulsations, enhanced particle tolerance, and reduced maintenance. The advantages provided by the helical rotor make the Positive Displacement flow meter an ideal choice for many applications including fuel flow meter, oil-in-water media and fluids with entrained solids providing strainer or filters are used before fluids enters the flow meter.