orifice meter and its application_ppt

G.H.PATEL COLLEGE OF ENGINEERING AND TECHNOLOGYCHEMICAL DEPARTMENT

 TOPIC NAME: ORIFICE METER SUBJECT: FLUID FLOW OPERATION(2130502) NAME: 

VYOM PATODIYA (150110105033)KRISHNA PESHIVADIYA (150110105034)HARDIK PIPALIYA (150110105035)BHAVIN POSHIYA (150110105036).

 FACULTY NAME:

PROF. VINAY B. PATEL

INTRODUCTION

An orifice plate is a device used for measuring flow rate, for reducing pressure or for restricting flow. An orifice plate is a thin plate with a hole in it, which is usually placed in a pipe. When a fluid (whether liquid or gaseous) passes through the orifice, its pressure builds up slightly upstream of the orifice.  but as the fluid is forced to converge to pass through the hole, the velocity increases and the fluid pressure decreases.

A little downstream of the orifice the flow reaches its point of maximum convergence, the vena contracta (see drawing to the right) where the velocity reaches its maximum and the pressure reaches its minimum. Beyond that, the flow expands, the velocity falls and the pressure increases. By measuring the difference in fluid pressure across tappings upstream and downstream of the plate, the flow rate can be obtained from Bernoulli's equation using coefficients established from extensive research.

PRINCIPAL OF ORIFIC METER

The principle of the orifice meter is identical with that of the venturi meter. The reduction of the cross section of the flowing stream in passing through the orifice increases the velocity head at the expense of the pressure head, and the reduction in pressure between the taps is measured by a manometer. Bernoulli's equation provides a basis for correlating the increase in velocity head with the decrease in pressure head.

One important complication appears in the orifice meter that is not found in the venturi. Because of the sharpness of the orifice, the fluid stream separates from the downstream side of the orifice plate and forms a free-flowing jet in the downstream fluid. A vena contracta forms, as shown in figure.

The jet is not under the control of solid walls, as is the case in the venture and the area of the jet varies from that of the opening in the orifice to that of the vena contracta. The area at any given point, for example at the downstream tap, is not easily determinable and the velocity of the jet at the downstream tap is not easily related to the diameter of the orifice. Orifice coefficients are smaller and more variable than those for the venturi and the quantitative treatment of the orifice meter is modified accordingly.

The basic equation for the orifice meter is obtained by writing the Bernouli equation for the incompressible fluids across the upstream cone. Va and Vb are the average upstream and downstream velocities respectively and ρ is the density of the fluid.

ὰbVb2 - ὰaVa

2 = --------------(1) The continuity relation can be written, Va = 2Vb ----------------(2)Where Da=diameter of pipe, Db=diameter of throat of meter, β4=diameter ratio Db/Da

If Va is eliminated from Eq.(1) and (2), the result is Vb =) -------------------------(3) Eq.(3) applies strictly to the frictionless flow of noncompressible fluids. To account for the small

friction loss between location a and b, Eq.(3) is corrected by introducing an empirical factor Co and writing Vb = () -------------------------(4)

The small effect of the kinetic energy factors ὰa and ὰb are also taken into account in the definition of Co. The coefficient Co is determined experimentally. It is called the venturi coefficient, velocity of approach not included.

The volumetric flow rate is calculated by using below equation, q = Vb * Sb Where q=Volumetric flow rate, Sb=area of throat

IT’S DESIGN

Orifice meter is type of variable head meter. In this meter the obstruction to the flow consist of an engineering constriction in the metered fluid which causes a reduction in the flow pressure.  An orifice meter is a conduit and a restriction to create a pressure drop.

It uses the same principle as a Venturi nozzle, namely Bernoulli's principle which states that there is a relationship between the pressure of the fluid and the velocity of the fluid. When the velocity increases, the pressure decreases and vice versa.

An orifice plate is a thin plate with a hole in the center of the plate. It is usually placed in a pipe in such a way that fluid passed through the hole. When the fluid reaches the orifice plate, with the hole in the middle, the fluid is forced to converge to go through the small hole; the point of maximum convergence actually occurs shortly downstream of the physical orifice, at the so-called vena contracta .

Vena contracta is a point where the velocity and the pressure changes. Beyond the vena contracta, the fluid expands and the velocity and pressure change once again. By measuring the difference in fluid pressure between the normal pipe section and at the vena contracta, the volumetric and mass flow rates can be obtained from Bernoulli's equation. There are different types of orifice plate that are namely Concentric, Segmental, Eccentric and    Quadrant Edge and Conic Edge.

In order to use any of these devices for measurement it is necessary to empirically calibrate them. That is, pass a known volume through the meter and note the reading in order to provide a standard for measuring other quantities. Due to the ease of duplicating and the simple construction, the thin sharp edged orifice has been adopted as a standard and extensive calibration work has been done so that it is widely accepted as a standard means of measuring fluids. Provided the standard mechanics of construction are followed no further calibration is required. An orifice in a pipeline with a manometer for measuring the drop in pressure (differential) as the fluid passes through the orifice. The minimum cross sectional area of the jet is known as the “vena contracta” where the velocity is maximum and static pressure is minimum. It is observed at some distance from orifice because of inertia effects persisting in flow direction. Materials used for orifice plate are mild steel, stainless steel, phosphor bronze and gun metal.

Orifice plates can be a surprisingly good way of measuring small gas flows. To be able to use orifice plates of very small orifice diameter in an existing installation in a declining gas field may be much more economical than to replace the metering or shut the field.

From the beginning of the flow measurement to its conclusion a 4″ orifice meter with different orifice plates (including those described in this appendix) might measure a range of around 3000:1 in terms of mass flow rate.

The orifice fitting, which enables the operator to change or remove an orifice plate easily.

APPLICATION

Orifice plates are most commonly used to measure flow rates in pipes, when the fluid is single-phase (rather than being a mixture of gases and liquids, or of liquids and solids) and well-mixed, the flow is continuous rather than pulsating, the fluid occupies the entire pipe (precluding silt or trapped gas), the flow profile is even and well-developed and the fluid and flow rate meet certain other conditions.

Under these circumstances and when the orifice plate is constructed and installed according to appropriate standards, the flow rate can easily be determined using published formulae based on substantial research and published in industry, national and international standards.

Plates are commonly made with sharp-edged circular orifices and installed concentric with the pipe and with pressure tappings at one of three standard pairs of distances upstream and downstream of the plate; these types are covered by ISO 5167 and other major standards. There are many other possibilities

The edges may be rounded or conical, the plate may have an orifice the same size as the pipe except for a segment at top or bottom which is obstructed, the orifice may be installed eccentric to the pipe, and the pressure tappings may be at other positions. Variations on these possibilities are covered in various standards and handbooks. Each combination gives rise to different coefficients of discharge which can be predicted so long as various conditions are met, conditions which differ from one type to another.

Once the orifice plate is designed and installed, the flow rate can often be indicated with an acceptably low uncertainty simply by taking the square root of the differential pressure across the orifice's pressure tappings and applying an appropriate constant. Even compressible flows of gases that vary in pressure and temperature may be measured with acceptable uncertainty by merely taking the square roots of the absolute pressure and/or temperature, depending on the purpose of the measurement and the costs of ancillary instrumentation.

Orifice plates are also used to reduce pressure or restrict flow, in which case they are often called restriction plates.

REFERENCES

  McCABE SMITH HARRIOTT (McGraw- HILL ) Wikipedia