Measurement: Flow meters - technical background: Positive displacement (PD) meters
This Section is intended as an introduction to the types of volumetric meters and their methods of operation. We do not approve particular types of meter but do state the standards they must meet in Notice 179.
Positive Displacement meters (PD) are the meters commonly used in the oil industry and are incorporated in the product line. The interior of the meter bowl is divided into calibrated measuring chambers by a number of blades. As product flows and enters the inlet section it occupies each chamber and causes the blades to rotate. As each chamber passes the outlet section of the meter bowl it is emptied into the delivery line. Either the rotating central spindle or the rotor is linked to a drive shaft which is connected at the other end through gearing to the meter register.
The drive shaft may be very short, with the register bolted directly to the meter bowl, or it may be several feet in length if the register is situated on the platform of the loading gantry.
A PD meter uses energy derived from the moving oil it is measuring, not only to drive its own mechanism but also those of any ancillary equipment fitted to it. The moving parts are subject to friction, so any addition, removal or replacement of ancillary equipment may affect meter accuracy. It is normal for the meter manufacturer to specify a range of rates of flow within which compensation for the effects of friction and inertia has been made in the design of a meter. Reliable measurements are only possible if there are no sudden fluctuations. A flow regulator is often included in the pipeline upstream of the meter to prevent surges or excessive variations of flow rates.
The rate of product flow governs the frequency of rotation of the blades and drive shaft, and this is transmitted to the register by means of a variable gear mechanism, known as the calibrator. The calibrator should never be altered other than during meter proving. Since the viscosity of the oil being measured influences the inertia effect of the mechanism of a meter, a meter is calibrated to measure oil of a limited viscosity range. The meter would be inaccurate for products outside that range without re-calibration.
For further information on the most common types of meters and what affect them see HCOTEG174280
For information on other types of flow meters see HCOTEG174290