PD Meters
ACCURATE ADAPTABLE FLOW METERS - PD Meters
PD Meters (Positive displacement meter) work on the principle similar to the concept of repeatedly filling a beaker with any fluid and then pouring it downstream while keeping a count of the number of the times the beaker is filled so as to determine its volume. It is capable of measuring a very low flow of fluids because its internal components are mass balanced but are not balanced hydraulically. This is the only device that measures the volume of the liquid passing through it directly without using any external power. In order to determine volume, small pockets of the fluid are trapped within a high precision chamber between the rotating components. The rotational velocity of the rotor is directly proportional to the fluid flow rate because it is the flow of fluid which is causing the rotation in the flow meter.
Based on the above principle, the number of times fluid pockets are trapped and released between rotating components indicates the volume flow through the PD meter. The process of trapping takes place by using rotating parts. Its structure is such that it forms a moving seal between each other and the body of flow meter. These parts used for sealing are highly tolerant so as to prevent any leakage of fluid from PD meter without being measured. For more accurate measurement required in some processes, rotating parts are supported by bearings also.
The speed of rotation depends on the amount of liquid flowing. When some rotation is sensed, the signal generated is detected by the transmitter which is processed to determine the fluid flow. A flow meter can either have mechanical registers with a local display or send output pulses as a secondary device to show the total flow rate. PD meters find application in volume measurement of fluid in pipes. In the case of liquids, the accuracy is more in case of more viscous liquids since they are less slippery.
There are several types of geometries of positive displacement flow meters such as oscillatory piston, reciprocating piston, nutating disc (wobble plate) and bi-rotor types.
A properly installed and precisely flow calibrated positive displacement flow meter is generally accepted as transfer standard. This instrument is highly accurate and can be used in a wide spectrum of fluids (liquids and gas) such as corrosive fluids, viscous fluids, and even for dirty fluids. They are best known for custody transfer applications of gasoline and oils which require critically functional meters. Technological innovations have led to incorporation of many advanced features in PD flow meters such as electronic interfacing, high turndown, and temperature compensation.
PD meters work most efficiently with a clean, erosion free, and non-corrosive environment. Some designs can tolerate impurities also. The highest differential pressure (pressure drop) is produced by PD meters with internal wiping seals. High accuracy allows positive displacement meters to be used at residencies for measuring the amount of water or gas used. Other applications of this instrument include fuel measurement, chemical injection, high pressure, precision test stands, hydraulic testing, etc.