THEORY Rotameter The rotameter is a flow meter in which a rotating free float is the indicating element. Basically, a rotameter consists of a transparent tapered vertical tube through which fluid flow upward. Within the tube is placed a freely suspended “float” of pump-bob shape. The rotameter's operation is based on the variable area principle: fluid flow raises a float in a tapered tube, increasing the area for passage of the fluid. The greater the flow, the higher the float is raised. The height of the float is directly proportional to the flowrate. The float rises only a short distance if the rate of flow is small, and vice versa. The points of equilibrium can be noted as a function of flow rate. With a well-calibrated marked glass tube, the level of the float becomes a direct measure of flow rate.
Venturi Meter The venturi meter consists of a venturi tube and a suitable differential pressure gauge. The venturi tube has a converging portion, a throat and a diverging portion as shown in the figure below. The function of the converging portion is to increase the velocity of the fluid and lower its static pressure. A pressure difference between inlet and throat is thus developed, which pressure difference is correlated with the rate of discharge. The diverging cone serves to change the area of the stream back to the entrance area and convert velocity head into pressure head. Venturi flow rate [
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Orifice Meter The orifice for use as a metering device in a pipeline consists of a concentric square-edged circular hole in a thin plate, which is clamped between the flanges of the pipe as shown in the figure below.
Pressure connections for attaching separate pressure gauges are made at holes in the pipe walls on both side of the orifice plate. The downstream pressure tap is placed at the minimum pressure position, which is assumed to be at the vena contracta. The centre of the inlet pressure tap is located between one-half and two pipe diameters from the upstream side of the orifice plate, usually a distance of one pipe diameter is employed. Flowrate of the orifice can be calclulate using equation below.
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900 elbow Energy losses are proportional to the velocity head of the fluid as it flows around an elbow, through an enlargement or contraction of the flow section, or through a valve. Figure below shows flow round a 90o elbow which has a constant circular cross section.
Experimental values for energy losses are usually expressed in terms of a dimensionless loss coefficient K. The value of loss coefficient K is dependent on the ratio of the bend radius, R to the pipe inside diameter D. As this ratio increase, the value of K will fall and vice versa.
