INTRODUCTION The measurement of fluid flow is important in applications ranging from measurements of blood-flow rates in human artery to the measurement of liquid oxygen in a rocket. The selection of the proper instrument for a particular application is governed by many variables, including cost. Flow-rate-measurement devices frequently require accurate pressure and temperature measurements in order to calculate the output of the instrument. The most widely used flow metering principle involves placing a fixed area flow restriction of some type in the pipe or duct carrying the fluid. This flow restriction causes a pressure drop that varies with the flow rate. Thus, measurement of the pressure drop by means of a suitable differential-pressure pick up allows flow rate measurement. Each of the flow measurement devices inherently has its own advantages and disadvantages. Some of those instruments are: • The Venturi Meter In the venturi meter (shown in figure (1) below) the fluid is accelerated through a converging cone of angle 15-20° and the pressure difference between the upstream side of the cone and the throat is measured and provides the signal for the rate of flow.
Upstream pressure tap
Downstream pressure tap
Upstream Flow Figure (1) : The Venturi meter operation.
The fluid slows down in a cone with smaller angle (5-7°) where most of the kinetic energy is converted back to pressure energy. Because of the cone and the gradual reduction in the area there is no "vena contracta". The flow area is at minimum at the throat. High pressure and energy recovery makes the venturi meter suitable where only small pressure heads are available. A discharge coefficient Cv- of 0.975 may be taken as standard, but the value varies noticeably at low values of the Reynolds' number. The pressure recovery is much better for the venturi meter than for the orifice plate. The venturi tube is suitable for clean, dirty and viscous liquid and some slurry services. Pressure loss is low. Typical accuracy percent is ±i of full range. Required upstream pipe length 5 to 20 diameters. Viscosity effect is high Relative cost is medium • The Orifice Plate
The orifice meter shown in figure (2) below, consists of a flat orifice plate with a circular hole drilled in it. There is a pressure tap upstream from the orifice plate and another just downstream. There are in general three methods of placing the taps. The coefficient of the meter depends upon the position`n of taps. Flange location - Tap location 1 inch upstream and 1 inch downstream from face of orifice. Vena contracta location - Tap location 1 pipe diameter (actual inside) upstream and 0.3 to 0.8 pipe diameter downstream from face of orifice. Pipe location - Tap location 2.5 times nominal pipe diameter upstream and 8 times nominal pipe diameter downstream from face of orifice.
Downstream Flow Upstream Flow
Inlet Pipe Pressure Manometer
Figure (2) : The Orifice Plate operation. The discharge coefficient - Co - varies considerably with changes in area ratio and the Reynolds' number. A discharge coefficient - Co - of 0.60 may be taken as standard, but the value varies noticeably at low values of the Reynolds number. The pressure recovery is limited for an orifice plate and the permanent pressure loss depends primarily on the area ratio. For an area ratio of 0.5, the head loss is about 70 -75% of the orifice differential. ♦ ♦ ♦ ♦ ♦ ♦ The orifice meter is recommended for clean and dirty liquids and some...