Flow Measurment

8/10/2019 Flow Measurment

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FLOW MEASUREMENT


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INTRODUCTION

For example, the orifice plate flowmeter, which was firstused commercially in the early 1900s and has a typicalaccuracy of 2% of reading, is still the only flowmeterapproved by most countries for the fiscal measurement

of natural gas. Although newer techniques such as Coriolis flowmeters

have become increasingly popular in recent years, theflow measurement industry is by nature conservative andstill dominated by traditional measurement techniques.

Fluid motion in a pipe can be characterized as one ofthree types: laminar, transitional, or turbulent.


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Principles of Fluid Flow in Pipes

In laminar flow , the fluid travels as parallel layers(known as streamlines) that do not mix as they move inthe direction of the flow.

If the flow is turbulent, the fluid does not travel in parallellayers, but moves in a haphazard manner with only theaverage motion of the fluid being parallel to the axis ofthe pipe.

If the flow is transitional , then both types may be presentat different points along the pipeline or the flow mayswitch between the two.

In 1883, Osborne Reynolds performed a classic set ofexperiments that showed that the flow characteristic canbe predicted using a dimensionless number, now knownas the Reynolds number.


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The Reynolds number Re is the ratio of the inertiaforces in the flow to the viscous forces in the flow andcan be calculated using:

If Re < 2000, the flow will be laminar.

If Re > 4000, the flow will be turbulent.

If 2000


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The Bernoulli equationdefines the relationshipbetween fluid velocity (v),fluid pressure (p), andheight (h) above some

fixed point for a fluidflowing through a pipe ofvarying cross-section,and is the starting pointfor understanding the

principle of the differentialpressure flowmeter.

Bernoullis equationstates that:


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Bernoullis equation can be used to measure flow rate.

Consider the pipe section shown in figure below. Since the pipe is

horizontal, h 1 = h 2, and the equation reduces to:


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The conservation of mass principle requires that:


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Flow meter Selection

Choosing which flowmeter is best for a particularapplication can be very difficult.

The main factors that influence this choice are therequired performance, the properties of the fluid to be

metered, the installation requirements, the environmentin which the instrument is to be used, and, of course,cost.

There are two standards that can be used to help selecta flowmeter: BS 1042: Section 1.4, which is a guide to

the use of the standard differential pressure flowmeters

BS 7405, which is concerned with the wider principles offlowmeter selection


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Differential Pressure FlowmetersThe Orifice Plate

The orifice plate is the simplest and cheapest. Itis simply a plate with a hole of specified size andposition cut in it, which can then clampedbetween flanges in a pipeline

The increase that occurs in the velocity of a fluidas it passes through the hole in the plate resultsin a pressure drop being developed across theplate.

After passing through this restriction, the fluidflow jet continues to contract until a minimumdiameter known as the vena contracta isreached.


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The Orifice Plate


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The Orifice Plate

The orifice plate is the simplest and cheapest.

The increase that occurs in the velocity of a fluid as it passesthrough the hole in the plate results in a pressure drop beingdeveloped across the plate. After passing through this restriction,the fluid flow jet continues to contract until a minimum diameter

known as the vena contracta is reached. The equation to calculate the flow must be modified to


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Rotameter

Rotameter consists of a conical

transparent vertical glass tube

containing a bob.

The flow rate is proportional to theheight of the bob.

The rotameter is characterized by:

Simple and robust construction

High reliability

Low pressure drop


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Rotameter

Applicable to a wide variety of gases and liquids

Flow range 0.04 L/h to 150 m3/h for water

Flow range 0.5 L/h to 3000 m3/h for air

Uncertainty 0.4% to 4% of maximum flow Insensitivity to nonuniformity in the inflow (noupstream straight piping needed)

Typical maximum temperature 400C

Typical maximum pressure 4 MPa (40 bar) Low investment cost

Low installation cost


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Installation

Correct installation is essential for successful use of aDP flowmeter because the assumption of a steady flow,with a fully developed turbulent velocity profile, ispassing through the flowmeter.

Standards contain detailed recommendations for theminimum straight lengths of pipe required before andafter the flowmeter, in order to ensure a fully developedflow profile.

Straight lengths of pipe are required after the flowmeterbecause disturbances caused by a valve or bend can

travel upstream and thus also affect the installedflowmeter.

If it is not possible to fit the recommended lengths ofstraight pipe before and after the flowmeter, then theflowmeter must be calibrated once it has been installed.


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Installation

The other problem one faces during installation is the

presence of a rotating flow or swirl. This condition distorts the flow velocity profile in a veryunpredictable way, and is obviously not desirable.

Situations that create swirl, such as two 90 bends indifferent planes, should preferably be avoided.

However, if this is not possible, then swirl can beremoved by placing a flow conditioner (also known as aflow straightener) between the source of the swirl andthe flowmeter.

There many flow conditioner designs which can be used

to both remove swirl and correct a distorted velocityprofile.

Because they obstruct the flow, all flow conditionersproduce an unrecoverable pressure loss,

which in general increases with their capability (and

complexity).


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Summary

For pipe flows, variable area flowmeters are suitable forlow flow rates of gases or liquids at moderatetemperatures and pressures.

Advantage rugged construction, high reliability, low

pressure drop, easy installation, and low cost. Disadvantages measurement uncertainty of 1% or more,

limited range (10:1), slow response, and restrictions onthe meter orientation.

Variable area flowmeters in open-channel flows haveapplications for flow measurements in waste waterplants, waterworks, rivers and streams, irrigation, anddrainage canals.

Flow Measurment

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