Flow Measurement

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FLOW MEASUREMENT

BY—

ASHISH SINGH 2

AVINASH CHAUDHARY

Definitions Of Flow Meter  

A gauge indicating the velocity of wastewater moving through a treatment plant or of any liquid moving through various industrial processes.

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A device for measuring the rate of fluid flow, usually in pipes.

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A device for monitoring and measuring the flow of a substance (typically fluid or gas). 3

Definitions Of Flow Meter  

An instrument for measuring the flow of liquids.

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Any device that measures the flow of a specific gas or gas mixture passing through.

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General definition of flowmeters.  A flowmeter is a device that meters movement of fluid in a conduit or an open space. This fluid could be water,chemicals, air, gas , steam or solids 

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Selection of flowmeters 

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Accuracy requirements in the particular application

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Repeatabililty requirements in the particular application

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Price /economic constraints

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Personal or company preference 5

Selection of flowmeters  

Brand preference

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Robustness or ability to work in harsh or hazardous type conditions

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Size

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Ease of installation 6

Selection of flowmeters  

Longevity

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Response time

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Special requirements

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Flow Measurement 

Flow Rate Mass Flow rate  Volume Flow rate  Velocity 

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Types of flow – Stream Line - parabolic velocity profile  Turbulent - vortices 

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Methods of measurement Direct: positive displacement (batch sensors,metering pumps)  Indirect: measurement of velocity or kinetic energy 

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Classification 

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Volume Flow Rate

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Mass Flow Rate

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Velocity

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Flow Sensors 

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Volume Flow Rate Sensors :

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Rotameters (float)  Batch/Oval gear 

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Mass flow rate sensors :

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Thermal  Coriolis Force 

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Flow Sensors 

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Velocity Sensors : 

Turbine, paddle wheel  Vortices  Electromagnetic  Ultrasonic  With moving marks  Obstruction devices 

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Calibration Of Flowmeters 

Source of error in flowmeter :

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 Variation

in fluid property -

Density  Viscosity  Temprature  Pressure 

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 Orientation

of the meter  Flow disturbance.  

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Calibration Of Flowmeters (cont…) 

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Step required for calibration of a flowmeter : Running operation before test.  Start the preliminary fill.  End of pre-fill, start weighing cycle.  Weighing cycle in operation.  End of weighing cycle.  Empty for recycling. 

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Calibration Of Flowmeters (cont…) 

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Gravimetric Calibration of Liquid Flowmeters Standing-start-and-finish method  Flying-start-and-finish method 

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Volumetric Calibration of Liquid Flowmeters

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Calibration of Gas Flowmeters

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Turbine meter  

Turbine type flowmetering devices are applied worldwide to the measurement and control of liquid products in the industrial, chemical and petroleum marketplaces.

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The unit consists of a multiple-bladed rotor mounted with a pipe, perpendicular to the liquid flow. The rotor spins as the liquid passes through the blades.

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Turbine meter (cont…)  

The rotational speed is a direct function of flow rate and can be sensed by magnetic pick-up, photoelectric cell, or gears. Electrical pulses can be counted and totalized

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The number of electrical pulses counted for a given period of time is directly proportional to flow volume.

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Turbine meter (cont…)  

A tachometer can be added to measure the turbine's rotational speed and to determine the liquid flow rate.

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Turbine meters, when properly specified and installed, have good accuracy, particularly with low-viscosity liquids.

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Turbine meter (cont…)  

A major concern with turbine meters is bearing wear.

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A "bearingless" design has been developed to avoid this problem.

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Liquid entering the meter travels through the spiraling vanes of a stator that imparts rotation to the liquid stream.

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Turbine meter (cont…)  

This stream acts on a sphere, causing it to orbit in the space between the first stator and a similarly spiraled second stator.

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The orbiting movement of the sphere is detected electronically. The frequency of the resulting pulse output is proportional to flow rate.

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Turbine meter (cont…) 

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Advantages : Good accuracy (± 0.25 to ± 0.5%).  Fairly low pressure drop.  Easy to install.  Good temperature and pressure rating.  Can be compensated for viscosity variation.  Exellent repeatability and range. 

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Turbine meter (cont…) 

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Disadvantages : High cost.  Limited use for slurry applications.  Problems caused by non-lubricating fluids. 

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Turbine meter (cont…) 

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Applications : Military application.  In blending system in the petroleum industry.  Airborne application for energy fuel and cryogenic flow measurement. 

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Turbine Flowmeter

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Vortex Meter  

Vortex meters make use of a natural phenomenon that occurs when a liquid flows around a bluff object.

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The frequency of the vortex shedding is directly proportional to the velocity of the liquid flowing through the meter.

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Vortex Meter (cont…) 

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The three major components of the flowmeter are a bluff body  a sensor  a signal amplification and conditioning transmitter 

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The meter is equally suitable for flow rate or flow totalization measurements. Use for slurries or high viscosity liquids is not recommended

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Vortex Meter

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Electromagnetic Flowmeter  

Electromagnetic meters can handle most liquids and slurries, providing that the material being metered is electrically conductive.

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Major components are the flow tube (primary element. The flow tube mounts directly in the pipe.

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Electromagnetic Flowmeter (cont…)  

Pressure drop across the meter is the same as it is through an equivalent length of pipe because there are no moving parts or obstructions to the flow.

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The voltmeter can be attached directly to the flow tube or can be mounted remotely and connected to it by a shielded cable.

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Electromagnetic Flowmeter (cont…)   

The operating principle of magnetic flowmeters is based upon Faraday's Law of electromagnetic induction

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“ It states that a voltage will be induced in a conductor moving through a magnetic field.” 

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Electromagnetic Flowmeter (cont…) 

Faraday's Law :

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E=kBDV 

Where

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E = Induced Voltage, B = Strength of the magentic field, D = Conductor Width, V = Velocity of the conductor  32

Electromagnetic Flowmeter (cont…)  

The liquid serves as the conductor; the magnetic field is created by energized coils outside the flow tube.

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The amount of voltage produced is directly proportional to the flow rate.

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Two electrodes mounted in the pipe wall detect the voltage, which is measured by the secondary element.

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Electromagnetic Flowmeter (cont…)  

Pulse-type excitation techniques have reduced power consumption, because excitation occurs only half the time in the unit.

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Zero settings are no longer required.

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Electromagnetic Flowmeter (cont…) 

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Disadvantages of earlier designs were: high power consumption.  the need to obtain a full pipe.  no flow to initially set the meter to zero. 

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Recent improvements have eliminated these problems.

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Electromagnetic Flowmeter (cont…) 

Where to use High percentage of solids  Sludges, slurries, minerals, paper, sewage - flows with high levels of solids which cannot be measured other types of meters.  Obstructionless measurement  Nothing projects into the flow stream, no head loss, no parts to maintain.  Very corrosive liquids  Acids, caustics and corrosive chemical additives are isolated from the meter by inert linings and electrodes.  Conductive Liquids  Liquids where conductivity is at sufficient levels to induce measurable voltage 

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Electromagnetic Flowmeter (cont…) 

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Advantages : Can handle corrosive material.  Can handle slurries and greasy material.  Obstructionless.  Available in large pipe sizes.  Can handle low flows and very high volume flow rate.  Have very less pressure drop. 

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Electromagnetic Flowmeter (cont…) 

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Disadvantages : Expensive  Heavy (in larger size)  Must be full all times 

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Electromagnetic Flowmeter

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Ultrasonic Flowmeter  

Ultrasonic flowmeters can be divided into Doppler meters and time-of-travel (or transit) meters.

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Doppler meters measure the frequency shifts caused by liquid flow. Two transducers are mounted in a case attached to one side of the pipe.

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Ultrasonic Flowmeter (cont…)  

A signal of known frequency is sent into the liquid to be measured. Solids, bubbles, or any discontinuity in the liquid, cause the pulse to be reflected to the receiver element.

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Because the liquid causing the reflection is moving, the frequency of the returned pulse is shifted. The frequency shift is proportional to the liquid's velocity. 42

Ultrasonic Flowmeter (cont…)  

A portable Doppler meter capable of being operated on AC power or from a rechargeable power pack has recently been developed.

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The sensing heads are simply clamped to the outside of the pipe, and the instrument is ready to be used.

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Ultrasonic Flowmeter (cont…)  

Total weight, including the case, is 22 lb (approx 10kg) .

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A set of 4 to 20 millampere output terminals permits the unit to be connected to a strip chart recorder or other remote device.

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Time-of-travel meters have transducers mounted on each side of the pipe.

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Ultrasonic Flowmeter (cont…)  

The configuration is such that the sound waves traveling between the devices are at a 45 deg. angle to the direction of liquid flow. The speed of the signal traveling between the transducers increases or decreases with the direction of transmission and the velocity of the liquid being measured.

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Ultrasonic Flowmeter (cont…)  

A time-differential relationship proportional to the flow can be obtained by transmitting the signal alternately in both directions.

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A limitation of time-of-travel meters is that the liquids being measured must be relatively free of entrained gas or solids to minimize signal scattering and absorption.

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Ultrasonic Flowmeter

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Rotameter  

A Rotameter is a device that measures the flow rate of liquid or gas in a closed tube. It is occasionally misspelled as "rotometer.“

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It measure flow rate by allowing the crosssectional area the fluid travels through to vary, causing some measurable effect.

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Rotameter (cont…)  

A rotameter consists of a tapered tube, typically made of glass, with a float inside that is pushed up by flow and pulled down by gravity.

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At a higher flow rate more area (between the float and the tube) is needed to accommodate the flow, so the float rises.

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Floats are made in many different shapes, with spheres and ellipsoids being the most common.

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Rotameter (cont…)  

The float is shaped so that it rotates axially as the fluid passes. This allows you to tell if the float is stuck since it will only rotate if it is free.

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Readings are usually taken from the top of the float.

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float has to have a higher density than the fluid, otherwise it will float to the top even if there is no flow.

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Rotameter (cont…) 

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Advantages : A rotameter requires no external power or fuel, it uses only the inherent properties of the fluid, along with gravity, to measure flow rate.  A rotameter is also a relatively simple device that can be mass manufactured out of cheap materials, allowing for its widespread use. 

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Rotameter (cont…) 

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Disadvantages : Due to its use of gravity, a rotameter must always be vertically oriented and right way up, with the fluid flowing upward.  Due to its reliance on the ability of the fluid or gas to displace the float, graduations on a given rotameter will only be accurate for a given substance. The main property of importance is the density of the fluid; however, viscosity may also be significant. Floats are ideally designed to be insensitive to viscosity; however, this is seldom verifiable from manufacturers specs. Either separate rotameters for different densities and viscosities may be used, or multiple scales on the same rotameter can be used. 

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Rotameter (cont…) 

Disadvantages (cont…) : Rotameters normally require the use of glass (or other transparent material), otherwise the user cannot see the float. This limits their use in many industries to benign fluids, such as water.  Rotameters are not easily adapted for reading by machine; although magnetic floats that drive a follower outside the tube are available. 

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Rotameter (cont…)

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Thanking  You 56

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