Positive Displacement Technology
Positive Displacement (PD) Flowmeters are volumetric flow metering instruments that measure flow by passing discreet parcels of fluid in precisely known volumes. PD meters are precision instruments whose internal moving components are mass-balanced yet remain hydraulically imbalanced. The result is that the meter can measure very low flows of both liquids and gases without using external power. The PD meter derives the power necessary to work from the energy contained in the flow process.
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 meters to be almost universally accepted as transfer standards when properly installed and flow-calibrated. The close tolerances of the internal metering components requires the PD 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 meters are used in dedicated unidirectional flow applications, such as fuel oil dispensing, natural gas or water metering.
Good instrumentation practice requires a filtering mechanism and a capacitance vessel for air removal for liquids or a coalescing filter for gases be an integral part of the metering installation. Advances in technology allow the meters to be temperature-compensated and interfaced electronically with central control systems as easily as they can be the integral part of a truck-mounted delivery system. Individual manufacturers' specifications will help guide the user to select the correct meter for the application. Examples of the more common PD meters are oscillating piston, nutating disc, oval gear, roots, vane, rotor, and multi-piston.
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 meter 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 measurement of each volume cycle. Maximum viscosity allowed: 4,000 centipoise.
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 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. Maximum viscosity allowed: 11,000 centipoise.
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 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 meters are typically used in the measurement of solvents and 'dry' liquids. Maximum viscosity allowed: 1,000 centipoise.
The roots meter is similar in many respects to the oval gear 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 measurement chamber during each rotation. The flow rate can be calculated by measuring the rotation speed. In contrast to nutating disc meters, the calibration factor does not vary with viscosity. Maximum viscosity allowed: 5,000 centipoise.
Piston Flowmeters of either single or multiple design find widespread use in fuel dispensing and the low flow measurement of light hydrocarbons. Though complex in design, they are sold in large quantities. In the multiple piston design shown below, the pistons are arranged in opposing pairs and connected through a series if 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 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.
