DISCHARGE MEASUREMENT

SUBMITTED BY BINOY.K.P ROLL NO:5 MS5(EVE)

SSMPTC TIRUR

CONTENT

INTRODUCTION TYPES OF FLOWMETERS ADVANTAGES AND DISADVANTAGES

APPLICATIONS CONCLUSION REFERENCE

INTRODUCTION Flow measurement is the quantification of bulk fluid movement.

Flow can be measured in a variety of ways.

Positive-displacement flow meters accumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow. Other flow measurement methods rely on forces produced by the flowing

stream as it overcomes a known constriction, to indirectly calculate flow.

Flow may be measured by measuring the velocity of fluid over a known area. Both gas and liquid flow can be measured in volumetric or mass flow rates,

such as liters per second or kilograms per second.

TYPES OF FLOW METER

Differential Pressure Flow meters Velocity Flow meters Positive Displacement Flow meters Mass Flow meters Open Channel Flow meters

Differential Pressure Flow meters In a differential pressure drop device the flow is calculated by measuring

the pressure drop over an obstructions inserted in the flow. Based on the Bernoullis Equation, where the pressure drop and the further

measured signal is a function of the square flow speed Bernoullis Equation Pressure head + velocity head + elevation head (at a point) =

pressure head + velocity head + elevation head ( at some other point) +

loss due to friction

The most common types of differential pressure flow meters are: Orifice Plates Flow Nozzles

Venturi Tubes Variable Area Rotameters

Orifice Plate

Head type meter used for large & medium pipes Orifice plate- inserted to pipe to create a partial restriction

to flow Pressure before orifice plate rises and pressure after it

reduces but velocity increases The orifice plates are simple, cheap and can be delivered

for almost any application in any material. Their accuracy are poor at low flow rates.

With an orifice plate, the fluid flow is measured through the difference in pressure from the upstream side to the downstream side of a partially obstructed pipe. The plate obstructing the flow offers a precisely measured obstruction that narrows the pipe and forces the flowing fluid to constrict

Venturi Tube

Consists of 3 sections 1. Converging conical section at up stream 2. Cylindrical throat- provides a panel for measurement- pressure decreased-

flow rate steady 3. Diverging recovery outlet

Throat to diameter ratio 0.25 to 0.75 Discharge co-efficient 0.9 to 1.0

Made of cast iron, gun metal, stainless steel May be circular, square or rectangular Calculation similar to orifice plate Better than orifice meter Initial cost is high

Flow Nozzles

Flow nozzles are often used as measuring elements for air and gas flow in industrial applications. The flow nozzle is relative simple and cheap, and available for many

applications in many materials. Principle- Bernoullis theorem High accuracy is attainable. Used in higher velocities

difficult to maintain Compared to orifice plate, it is costlier.

Variable Area Flowmeter or Rotameter The rotameter consists of a vertically oriented glass (or plastic) tube

with a larger end at the top, and a metering float which is free to move within the tube. Fluid flow causes the float to rise in the tube as the upward pressure differential and buoyancy of the fluid overcome the effect of gravity. The float rises until the annular area between the float and tube

increases sufficiently to allow a state of dynamic equilibrium between the upward differential pressure and buoyancy factors, and downward gravity factors. The height of the float is an indication of the flow rate. The tube can be

calibrated and graduated in appropriate flow units..

2.Velocity Flowmeters

In a velocity flowmeter the flow is calculated by measuring the speed in one or more points in the flow, and integrating the flow speed over the flow area.

Consists of o Pitot tube o Calorimeteric flow meter o Turbine Flowmeter o Vortex Flow Meter o Electromagnetic Flowmeter o Ultrasonic Doppler Flowmeter

Pitot Tubes

The pitot tube are one the most used (and cheapest) ways to measure fluid flow, especially in air applications as ventilation, even used in airplanes for the speed measurent. The pitot tube measures the fluid flow velocity by converting the kinetic

energy of the flow into potential energy. Static pressure tapping is taken from pipe & connected to low pr. Side of U-

tube manometer & high pr. Side to the pitot tube. Differential pressure indicated is proportional to the velocity of flow.

Calorimetric Flowmeter

The calorimetric principle for fluid flow measurement is based on two temperature sensors in close contact with the fluid but thermal insulated from each other. One of the two sensors is constantly heated and the cooling effect of

the flowing fluid is used to monitor the flowrate. In a stationary (no flow) fluid condition there is a constant temperature difference between the two temperature sensors. When the fluid flow increases, heat energy is drawn from the heated sensor and the temperature difference between the sensors are reduced. The reduction is proportional to the flow rate of the fluid.

The calorimetric flowmeter can achieve relatively high accuracy at low flow rates.

Turbine Flowmeter There is many different manufacturing design of turbine flow meters, but

in general they are all based on the same simple principle: If a fluid moves through a pipe and acts on the vanes of a turbine, the

turbine will start to spin and rotate. The rate of spin is measured to calculate the flow. if the turbine meter is calibrated for a single fluid and used at constant

conditions. Accuracy may be better than +/-0.1%.

Vortex Flow Meter An obstruction in a fluid flow creates vortices in a downstream flow.

Every obstruction has a critical fluid flow speed at which vortex shedding occurs. Vortex shedding is the instance where alternating low pressure zones are generated in the downstream. These alternating low pressure zones cause the obstruction to move

towards the low pressure zone. With sensors gauging the vortices the strength of the flow can be measured.

Electromagnetic Flowmeter

operate on Faraday's law of electromagnetic induction that states that a voltage will be induced when a conductor moves through a magnetic field. The liquid serves as the conductor and the magnetic field is created by energized coils outside the flow tube. The voltage produced is directly proportional to the flow rate. Two

electrodes mounted in the pipe wall detect the voltage which is measured by a secondary element. Electromagnetic flowmeters can measure difficult and corrosive liquids and

slurries, and they can measure flow in both directions with equal accuracy. Electromagnetic flowmeters have a relatively high power consumption and

can only be used for electrical conductive fluids as water.

Ultrasonic Doppler Flowmeter Principle- when a sound wave is transmitted through a fluid in the direction

of fluid flow, its velocity is increased &when transmitted against the fluid flow, its velocity is decreased. Consists of 2 transducers mounted in the pipelines pulses are transmitted

from one to the other at an angle through the fluid flow. By knowing angle, fluid flow & speed of propagation volumetric rate of

flow can be calculated. Depends on clean fluid without bubbles. Accuracy & repeatability are better.

Accuracy at very low flow rate is low.

3.Positive Displacement Flowmeter The positive displacement flowmeter measures process fluid flow by

precision-fitted rotors as flow measuring elements. Known and fixed volumes are displaced between the rotors. The rotation of the rotors are proportional to the volume of the fluid being displaced. The number of rotations of the rotor is counted by an integral

electronic pulse transmitter and converted to volume and flow rate. The positive displacement flowmeter may be used for all relatively

nonabrasive fluids such as heating oils, lubrication oils, polymer additives, animal and vegetable fat, printing ink, freon, and many more.

4.Mass Flowmeters

Mass meters measure the mass flow rate directly. Thermal Flowmeter The thermal mass flowmeter operates independent of density, pressure, and

viscosity. Thermal meters use a heated sensing element isolated from the fluid flow

path where the flow stream conducts heat from the sensing element. The conducted heat is directly proportional to the mass flow rate and the

temperature difference is calculated to mass flow. The accuracy of the thermal mass flow device depends on the calibrations

reliability of the actual process and variations in the temperature, pressure, flow rate, heat capacity and viscosity of the fluid.

Coriolis Flowmeter Direct mass measurement sets Coriolis flowmeters apart from other

technologies. Mass measurement is not sensitive to changes in pressure, temperature, viscosity and density. With the ability to measure liquids, slurries and gases, Coriolis flowmeters are universal meters. Coriolis Mass Flowmeter uses the Coriolis effect to measure the

amount of mass moving through the element. The fluid to be measured runs through a U-shaped tube that is caused to

vibrate in an angular harmonic oscillation. Due to the Coriolis forces, the tubes will deform and an additional

vibration component will be added to the oscillation. This additional component causes a phase shift on some places of the tubes which can be measured with sensors. The Coriolis flow meters are in general very accurate. The Coriolis

meter can also be used to measure the fluids density.

5.Open Channel Flowmeters A common method of measuring flow through an open channel is to

measure the height of the liquid as it passes over an obstruction as a flume or weir in the channel. Measurement weirs, also called spillways or overflow weirs, are backwater

constructions (dam-like structures) that the water flows over. These constructions change the channel bottoms run in such a way as to cause backwater in the upper stream. The level of the water is measured at a designated point behind weir or in

flume a hydraulic structure using various secondary devices (bubblers, ultrasonic, float, and differential pressure are common methods). weirs are used on flow streams with low solids (typically surface waters),

while flumes are used on flows containing low or high solids contents.

APPLICATION

The following are common users of flow monitors for measurement

purposes:

Cities and Municipalities - Water works engineers and municipal managers understand the hydraulic loadings on their sewer systems and

manage them. It helps them understand the capacity of their sewers in

wet weather conditions

Wastewater Treatment Plants - For control of aeration, chemical feed, chlorination etc. based on amount of flow entering the plant.

Capacity Analysis - In areas where municipal and industrial growth is planned.

Billing - Direct flow measurement is the only true way to bill according to wastewater treated.

CONCLUSION

Flow measurements by the differential pressure have some disadvantages of non linearity and high energy consumption. Newly developed flow meter attempt to overcome the

problems inherent in D.P meters. They offer much better range and linearity.

REFERENCE http://www.engineering toolbox.com /flow meter-d 493html

http://en.wikipedia.org/wiki/flow measurement Text book of mechanical and industrial measurements Er. R.K. Jain

Text of experimental Methods for Engineers J.P.Holman