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- 3.4Flow measurement with Differential pressure
- PRIMARY DEVICES ORIFICE PLATE
| Meshed Oval gear In this meter two meshed oval gears are used. They seal the inlet from the outlet flow and also keep the oval impellers in the correct relative positions. As the mesh gears rotate, they trap precise quantities of fluid in the gaps between the gears and the measuring chamber. The total volume of fluid passing through the meter each time the two gears make one revolution is four times the volume in the gap between the gear and the measuring chamber. These meters are used to measure liquid flow like oil, water, and gasoline. 58 Field Instrumentation GEN Rev) Meshed Oval gear 3.4Flow measurement with Differential pressure Flow measuring system (flow meter) consists of primary and secondary devices. Primary device or restriction in the flow line creates a change in fluid velocity that is sensed as a differential pressure (head. Secondary device measures this differential pressure and this measurement is related to flow rate. Head type flow measuring system depends on Bernoulli theorem the total energy at a point in a pipeline is equal to the total energy at a second point if friction between the points is neglected. PRIMARY DEVICES ORIFICE PLATE Orifice plate is used for all clean fluids, but not used for fluids containing solids in suspension. A conventional orifice plate consists of a thin circular plate containing a concentric hole. It is normally made of stainless steel or Monel metal. Most common orifice plate is sharp-edged, concentric type. 59 Field Instrumentation GEN Rev) Other two types of orifice plates are eccentric and segmental, to accommodate limited amounts of suspended solids. ORIFICE PLATE Within the orifice plate, the change in cross sectional area between the pipe and the orifice produces a change inflow velocity. The velocity head at the throat increases, causing a corresponding decrease in static head. Therefore there is ahead difference between a point immediately ahead of the restriction and a point within the restriction or downstream from it. The resulting differential head or pressure is a function of velocity that can then be related to flow. Most of the primary devices operate satisfactorily at high flow rates due to the change inflow coefficient that occurs when the flow rate changes from high to very low. If the orifice is not very smooth and flat, the flow measurement will be affected. In some orifices a small drain hole (weep hole) is provided to permit condensate or vapors) to pass through the plate without interfering with the flow measurement. For reversible flow measurement orifice plates are beveled on both faces, and they are installed in fittings where the pressure taps are the same distance upstream and downstream from the plate. |