GOVERNMENT ENGINEERING COLLEGE, BHARUCH (014) Chemical Engineering department SEM-iii Sub: fluid flow operation topic: orifice meter & rotAmeter Guid by: N. SolaNKI

GROUP NO:- 4 ( ) Enrollment No. Student’s Name Mithaiwala Chintan Morasiya Jenish Panchal Priyansh Parmar Dhaval Parmar Gaurav Parmar Jignesh

Content: Our presentation is about “flow measurement instruments”. There are many instruments that are used for measurement of discharge/flow rate of fluid through the pipe. We have included 2 instruments from them & they are: 1.Orifice meter 2.Rota meter

Orificemeter Principle:  The basic principle on which the orifice meter works is that by reducing the cross sectional area of the flow passage the pressure difference created & the pressure difference enables the determination of discharge/flow rate of the fluid through the pipes.

Introduction  It is a variable head meter used for measuring the discharge through a pipe.  In this meter, the fluid is accelerated by causing it to flow through a sudden constriction (orifice), the kinetic energy of the fluid increases & the pressure energy is therefore decreases.  The orifice meter is relatively cheap & reliable instrument and its installation requires small length as compared to the venturi meter because of this where the space is limited, the orifice meter may be used for the measurement of discharge through pipes.

construction  An orifice meter consists of a flat circular plate with an accurately machined & drilled circular hole called an orifice.  The orifice is concentric with the pipe axis & drilled with sharp edges.  The thickness of the plate is less than or equal to 0.05 times the diameter of the pipe.  From the upstream of the plate, the edge of the orifice is made flat for a thickness less than or equal to 0.02 times the diameter of the pipe & the remaining portion of the plate is beveled with the bevel angle of 30 to 45°.  The plate is usually made of stainless steel to resist corrosion.

 The orifice plate is clamped between two pipe flanges with the bevelled surface facing the downstream.  The orifice diameter may vary from 0.2 to 0.85 times the pipe diameter, but generally it is 0.5 times of the pipe diameter.  Pressure taps are also provided, one each on the upstream side & downstream side of the orifice plate.  On the downstream side of the orifice plate, the pressure tap is provided at the section where a converging jet of the fluid has a minimum cross sectional are resulting in almost a maximum velocity & consequently a minimum pressure at this section.  Hence, a maximum possible pressure difference exits between the section 1 & section 2.  This pressure difference is measured by connecting a manometer or other pressure measuring device, e.g., pressure guages.

Flow pattern in orifice meter  The flow lines run parallel & widely spaced on the upstream side of the plate but as the flow approaches the orifice plate, the flow lines slowly converge at the orifice and then diverge out to pipe smoothly.  Since the lines slowly converge & then diverge, the flow area becomes minimum at a short distance on the downstream side of the orifice plate which is known as the vena-contracta.  The jet of fluid issuing from the orifice plate gradually expands from the vena-contracta to again fill the entire cross section of the pipe.

 Working  An orifice meter of known coefficient of discharge is installed in the pipeline & pressure taps are connected to a pressure measuring device.  Air pockets, if any, are removed from the tubing, measuring device ( manometer, pressure gauges) after starting the flow of the fluid through the pipeline in which it is installed for the flow measurement.  In the meter, the fluid is first accelerated & then retarded so that a pressure drop across the meter is created.  Once the steady state is attained, the pressure drop across the meter is noted.  This pressure drop is then used to calculate the volumetric flow rate using a mathematical flow equation for the meter.

Rotameter  In the orifice meter, the area of constriction/ area of flow is constant & the differential pressure/ pressure drop across the meter varies with the flow rate/ discharge, while in the variable area meter, the pressure drop across the meter is constant & the flow rate is a function of the area of constriction/area of flow.  Thus, any change in the flow rate can be measured in terms of the change in the area of flow.  The most important area meter is ROTAMETER.

Principle  Rotameter operates on the principle that there is a different constriction/ orifice area for each flow rate & the pressure drop across the meter is constant.  In other words, the pressure drop across the meter is constant & the area through which the fluid flows varies with the flow rate.  This area is related, through a proper calibration, to the flow rate.

Construction  Rotameter consists of a tapered glass tube mounted vertically in a frame with the large end up.  The tube contains a freely moving solid float.  The diameter of the float is smaller than the diameter of the bottom of the tapered tube.  The density of the float material is higher than that of the liquid.  A perforated plate is provided at both the ends of the tapered tube for arresting the float in the tube. A guide is provided for the float so it always remains at the centre in the tube along the axis of the tube.  Float is usually made from non-corrosive materials such as aluminium, bronze, SS, nickel etc.

 A nearly linear flow scale is marked in the glass tube it is mounted close to the tube so that the position of the float can be marked & the flow rate is then obtained from a calibration curve or a direct scale of the flow rate can also be provided over the tapered tube or near the tube.  The float is the indicating element and the reading edge of the float is taken at the largest cross-section of the float.  The either ends of the meter may be screwed or flanged.  Rotameters do not require straight runs of pipe before & after the point of installation.

Working  In rotameter as the flow varies, the float rises or falls, thus changing the area of the annular space between the float & the wall of the tube.  The area available for flow is the annular space between the float & the wall of the tube.  When no fluid flows through the meter, the rotameter float rests at the bottom of the tube.  But as the fluid begins to flow from the lower side of the tube, the float rises until the differential pressure just balance the weight of the float & the fluid flows through the meter through annular space.  As the flow increases, the float rises further in the tube, thus, increasing the area available for flow keeping differential pressure across it constant.  On the other hand, as the flow rate decreases, the float falls in the tube, thus decreasing the area with constant pressure drop across it.

 At a given flow rate, float stabilies at a certain fixed position in the tube and at steady state it is recorded as the rotameter reading on the scale provided.  The variation of the flow area with flow rate can be measured in terms of change in the float position.  A rotating motion of the float helps to keeps it steady. Application  Rotameters are widely used in the chemical industry for the measurement of flow rates of compressible as well as incompressible fluids.  They provide a directing reading of the flow rate.

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