Types of Questions Asked in the Interview

1. What is the difference between volumetric flow meter and mass flow meter?

2. Why Flow measure in Square-root?3. What type of flow measurement devices is best for slurries?4. What are the different types of primary elements used for

flow measurement?5. What are the different types of orifice plates and state their

uses.6. What is Bernoulli’s theorem and where is it applicable?

ect.

Flow Measurement

Prepared by: Anuradha TandonAssistant Professor,IC Section, EE Dept.,IT, NU

WHAT IS FLOW ?

• Measure of the velocity of a fluid per unit area in a closed conduit; i.e., pipe or duct

– FLOW = VELOCITY (fluid) X Area of Pipe or Duct or Stack

Why Flow Measurement?

• Mainly for the purpose of – cost accounting for the services like water, gas,

steam etc.– plant material balancing– quality control

Types of Fluid

• Fluids whose flow is required to be measured may be

• Clear or opaque,• Clean or dirty,• Wet or dry, or• Erosive or corrosive

How Flow Measurement is Done?• Commonly used Flow Measurement Devices

are:–Differential Pressure (Head) Type

• Orifice Plate - Concentric, Eccentric, Segmental, Quadrant Edge, Integral, Conditioning

• Venturi Tube• Flow Nozzles• Pitot Tube, Averaging Pitot Tube (Annubar)• Elbow• Wedge Meter• V-Cone

–Variable Area Meters• Rotameter• Cylinder and piston type flow meter

–Mass Type – measures the mass flow rate directly.• Thermal

–Velocity Type• Magnetic• Ultrasonic - Transit Time, Doppler• Turbine• Vortex

–Open Channel Type• Weir• Parshall Flume

–Other Types• Positive Displacement• Target

How Flow Measurement is Done?(cont….)

Flow Measurement - Terms• DENSITY (r)

– A Measure Of Mass Per Unit Of Volume (lb/ft3 or kg/M3).

• SPECIFIC GRAVITY – The Ratio Of The Density Of A Material To The Density Of

Water Or Air Depending On Whether It Is A Liquid Or A Gas.

• COMPRESSIBLE FLUID – Fluids (Such As Gasses) Where The Volume Changes With

Respect To Changes In The Pressure. These Fluids Experience Large Changes In Density Due To Changes In Pressure.

• NON-COMPRESSIBLE FLUID – Fluids (Generally Liquids) Which Resist Changes In Volume As

The Pressure Changes. These Fluids Experience Little Change In Density Due To Pressure Changes.

• Rangeability– The ratio of maximum flow to minimum flow, but not zero

flow.

• Repeatability– The ability of a flow meter to indicate the same readings

each time the same flow conditions exist. These readings may or may not be accurate, but will repeat.

• Linear– Transmitter output is directly proportional to the flow

input.

• Square Root– Flow is proportional to the square root of the measured

value.

Flow Measurement – Terms (cont..)

• Beta Ratio (d/D)– Ratio of a differential pressure flow device bore (d) divided

by internal diameter of pipe (D).– A higher Beta ratio means a larger orifice size. A larger

orifice plate bore size means greater flow capacity and a lower permanent pressure loss.

• Pressure Head– The Pressure At A Given Point In A Liquid Measured In

Terms Of The Vertical Height Of A Column Of The Liquid Needed To Produce The Same Pressure.

Flow Measurement – Terms (cont..)

• Flow is measured as a quantity (either volume or mass) per unit time

• Volumetric units– Liquid

• gpm, bbl/day, m3/hr, liters/min, etc.– Gas or Vapor

• ft3/hr, m3/hr, etc.– Mass units (either liquid, gas or vapor)

• lb/hr, kg/hr, etc.

• Flow can be measured in accumulated (totalized) total amounts for a time period– gallons, liters, meters passed in a day, etc

Flow Measurement – Units

Reynolds Number

• The Reynolds number is the ratio of inertial forces (velocity and density that keep the fluid in motion) to viscous forces (frictional forces that slow the fluid down) and is used for determining the dynamic properties of the fluid to allow an equal comparison between different fluids and flows.

• It is used to determine the point at which the flow goes from viscous to the turbulent stage.

Reynolds Number (cont….)• The Reynolds number is the most important

value used in fluid dynamics as it provides a criterion for determining similarity between different fluids, flow rates and piping configurations.

Re=Dv rm

D

v

r

m

DIAMETER (FT)

VELOCITY (FT/SEC)

DENSITY (LB/FT3)

VISCOSITY (cp)

=

==

=

C

C CONSTANT (6.72X10-4 LB/FT SEC cp)=

Types of Fluid Flow in Pipes

• Based on Reynolds Number fluid motion in a pipe can be characterized as one of three types:

1). Stream line or Viscous or Laminar - occurs at low Reynolds number

2). Turbulent - occurs at high Reynolds Number3). Combined viscous or Transitional

0 2000 4000

LAMINAR TRANSITION TURBULENT

1). Stream line or Viscous or Laminar

• Laminar flow , is characterized by concentric layers of fluid moving in parallel down the length of A pipe. The highest velocity (Vmax) is found in the center of the pipe. The lowest velocity (V=0) is found along the pipe wall.

SIDE VIEW END VIEW

VMAX

CONCENTRIC FLUID LAYERSPARABOLIC FLOW PROFILE

Vavg Vmax = 2*Vavg

2). Turbulent Flow• Turbulent flow is characterized by A fluid motion

that has local velocities and pressures that fluctuate randomly. This causes the velocity of the fluid in the pipe to be more uniform across A cross section.

3). Combined Viscous or Transitional

• If the flow is transitional , then both types may be present at different points along the pipeline or the flow may switch between the two.

Differential Pressure (Head) Type• Also called the inferential type of flow meters as the

quantity under measurement is inferred from the measurement of some other quantity.

• They include:

1. Orifice Plate - Concentric, Eccentric, Segmental, Quadrant Edge, Integral, Conditioning

2. Venturi Tube3. Flow Nozzles4. Pitot Tube, Averaging Pitot Tube (Annubar)5. Elbow6. Wedge Meter7. V-Cone

• Produce a pressure difference when fluid flow is maintained through them

• Diff. Pressure proportional to square of flow rate

• Basic operating principle - Bernoulli’s theorem

Differential Pressure (Head) Type(cont….)

Bernoulli’s Theorem

• The Bernoulli equation defines the relationship between fluid velocity (v), fluid pressure (p), and height (h) above some fixed point for a fluid flowing through a pipe of varying cross-section, and is the starting point for understanding the principle of the differential pressure flow meter.

• Bernoulli’s equation states that:

Pressure head + velocity head + elevation head (at a point) =

pressure head + velocity head + elevation head ( at some other point) + loss due to friction

Bernoulli’s Theorem (cont….)

V2 > V1

P2 < P1

V1, P1

P1 + r gh1 r V12 +

21

= P2 + r gh2 r V22 +

21

Pressure Energy

Kinetic Energy Per Unit Volume

Potential Energy Per unit Volume

Energy Per Unit Volume Before = Energy Per Unit Volume After

Bernoulli’s Theorem (cont….)• Bernoulli’s law explains the movement of fluid through an

orifice or nozzle. It shows that the total energy upstream of the nozzle is equal to the energy at the nozzle throat.

• It explains how the potential energy is converted to kinetic energy as the fluid speeds up and the pressure drops at the nozzle throat.

• The basic equations are: V = Velocity of the flowing fluid Q = Volume flow rate W = Mass flow rate A = Cross sectional area of pipe

through which fluid is flowing h = differential head (pressure) across the restriction element ρ = density of the flowing liquid

g = acceleration due to gravity

K =

Bernoulli’s Theorem (cont….)

ghKAW

ghKAQ

ghKV

2

2

2

• K = = a constant

where, C= discharge coefficient β = Diameter ratio d (diameter of the restriction element)

= -----------------------------------------------------D (diameter of the pipe)

21

C

Bernoulli’s Theorem (cont….)

1. Orifice Meter

• The Orifice Plate is a simple device, considered to be a precision instrument.

1. Orifice Meter (cont….)

Orifice PlateOrifice Flanges

• It is simply a piece of flat metal with a flow-restricting bore that is inserted into the pipe between flanges.

1. Orifice Meter (Cont…)• It is inserted into the pipe to create a partial

restriction to flow• Pressure before orifice plate rises and

pressure after it reduces but velocity increases

1. Orifice Meter (Cont…)

• Position where velocity is maximum & static pressure is minimum is known as vena contracta

1. Orifice Meter (Cont...)• Differential Pressure Calculation:

• Calculate pressure using the basic formula or use the flow processor/computer

1. Orifice Meter (Cont….)

• Usually pressure tapping is at a distance D & D/2 for up stream & down stream

• D- diameter of pipe• For pipe size 0.05m or more• Orifice to pipe diameter is 0.6

1. Orifice Meter (Cont….)

• Flow measurement using orifice meter:

1. Orifice Meter (Cont…)

• Normal thickness -0.015 m for pipe with diameter 0.15 m & 0.003 for larger

• Materials used bronze, stainless steel, phosphor bronze, gun metal etc.

• It is rugged and inexpensive. • It’s accuracy under ideal conditions is in the

range of 0.75 to 1.5 percent.

Orifice plate & flanges

Types of orifice plates

•If fluid contains suspended materials , particles tend to settle down in orifice,•To avoid this segmented or eccentric type is used

Quadrant Edge Type Orifice Plate• The quadrant, quadrant edge or quarter-circle orifice is recommended for measurement of fluids with high viscosity which have pipe Reynolds Numbers below 10,000. •The orifice incorporates a rounded edge of definite radius which is a particular function of the orifice diameter.

1. Orifice Meter

Advantages: Simple to use Low cost, especially on large sizes No need for recalibration Precision instrumentDisadvantages: Maintenance problem Long installations (20D to 30D) Poor accuracy

Orifice Selection Consideration

• Quadrant Edge Orifice Plate can be considered if Reynolds number is too low.

• Orifice plate must be specified with proper flange rating to account for proper bolt circle.

• Typical acceptable beta ratio is .25 to .7 for non commerce meter, .3 to .6 for accounting meter but also check specifications.

2. Venturi Meter

2. Venturi Meter (cont….)• In a Venturi tube, the fluid is accelerated through a converging

cone, inducing a local pressure drop. An expanding section of the meter then returns the flow to near its original pressure. These instruments are often selected where it is important not to create a significant pressure drop and where good accuracy is required.

• Used when higher velocity and pressure recovery is required.

• May be used when a small, constant percentage of solids is present.

2. Venturi Meter (cont….)

2. Venturi Meter (Cont….)

• 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

2. Venturi Meter (Cont…)Advantages:• Due to smooth design of the Venturi tube means that

it is less sensitive to erosion than the orifice plate.• Thus more suitable for use with dirty gases or liquids. • The Venturi tube is also less sensitive to upstream

disturbances, and therefore needs shorter lengths of straight pipe work upstream of the meter than the equivalent orifice plate or nozzle.

Disadvantages:• Not useful below 76.2 mm pipe size• Costly

3. Flow Nozzles

3. Flow Nozzles (Cont….)• Used when higher velocity & pressure recovery are

required• Better suited for gas service than for liquid• Made from materials such as stainless steel or chrome

- moly steel• Can be used at Reynolds number of 50,000 or above• Discharge co-efficient is 0.99 or greater• Beta ratio of 0.2 to 0.8

3. Flow Nozzles (cont…)

Advantages:• Permanent pressure loss is less than that in orifice

meter• Widely accepted for high-pressure and temperature

steam flow.

Disadvantages:• Costly than orifice meter• Limited to moderate pipe sizes• Requires more maintenance

4. Pitot Tube

4. Pitot Tube (Cont…)

• Operating principle is:“When a solid body is kept centrally and

stationary in a pipe line with a fluid streaming down, the velocity of the fluid starts diminishing due to the presence of the body, till it reduces to zero directly in front of the body. This point is known as the stagnation point. “

• As the kinetic head (pressure) is lost by the liquid, it gains a static head and by measuring the difference pressure, the fluid velocity can be measured.

4. Pitot Tube (Cont….)

• Not used widely due to less accuracy, i. e., in the range of ± 0.5 % to ±5 %.

4. Pitot Tube (Cont…)

Advantages• No process loss• Economical to install

Disadvantages• Accuracy is very poor• Cannot be used with dirty or sticky fluids

Other Type of DP Meters• There are many other types of differential pressure

flowmeter, but they are not very common• the segmental wedge, V-cone, and elbow. • Each of these has advantages over the orifice plate, Venturi

tube, and nozzle for specific applications. • For example, the segmental wedge can be used with flows

having a low Reynolds number, • Dall tube has a lower permanent pressure loss than a

Venturi tube. • However, none of these instruments are yet covered by

international standards and, thus, calibration is needed to determine their accuracy.

Variable Area Meters• The size of the restriction is adjusted by an

amount necessary to keep the pressure differential constant when the flow rate changes as opposed to that in the DP meters where the size of the restriction was fixed and the pressure differential across it changes as the flow rate changes.

• The amount by which the area is changed to keep the pressure constant is proportional to flow

• Includes1. Rotameter, and2. Cylinder and piston type flow meters

1. Rotameter• Rotameter is a variable area device.• The float moves up and down in

proportion to the fluid flow rate and the annular area between the float and the tube wall.

• As the float rises, the size of the annular opening increases. As this area increases, the differential pressure across the float decreases.

• The float reaches a stable position when the upward force exerted by the flowing fluid equals the weight of the float.

• Every float position corresponds to a particular flow rate for a particular fluid's density and viscosity.

1. Rotameter (Cont….)Advantages:• Relatively low cost• Good rangeability• Suitable for liquid as well as gas flow measurements• Handles wide variety of corrosive fluids• Can be easily equipped with alarm switches or

transmitting devices

Disadvantages:• Glass tube subject to breakage• Must be mounted vetically

2. Cylinder and Piston Type Flow Meter

• Most commonly used for measuring flow of fuel oils, tar, chemical liquors and other such highly viscous fluids

Advantages:• Can handle slurries and greasy materials• Can handle corrosive materials• Is totally obstructionless and therefore very low pressure

drop• Available in large pipe sizes and capabilities• Capable of handling extremely low flows and very high

volume flow rate • Can be used as bi-directional meter• Measurement is unaffected by viscosity, density, temperature

and pressure

2. Cylinder and Piston Type Flow Meter (Cont…)

Disadvantages:• Relatively expensive• Heavy• Must be full at all times• Must be explosion proof when istalled in hazardous

electrical areas

2. Cylinder and Piston Type Flow Meter

Mass Flow Meters

• Popular for the measurement of unsteady flow of gases

• They are of two types:1. Heat Transfer flow meter2. Hot-wire flow meter

1. Heat Transfer Flow Meter

• Measures the rise in the temperature of the fluid after a known amount of heat has been added to it.

• Mass flow rate is calculated as

Where, W= mass flow rate Cp = Specific heat of fluid

T1 = initial temperature of the fluid after heat has been transferred

T2 = final temperature after heating the fluid

)( 12 TTC

QW

p

• Uses an electric immersion heater• Two thermocouples are used measure the

temperature T1 and T2 placed on each side of the heater

• The problem is that the heater is directly placed in the fluid and therefore can be easily damaged by corrosion and erosion.

• Large input power is required if flow rate is very high.

1. Heat Transfer Flow Meter (Cont…)

2. Hot Wire Flow Meter

• The difference in temperature is measured by measuring the resistance of the resistors as all resistors have some known temperature coefficient

• A Wheatstone bridge arrangement can be used to detect changes in resistance

Velocity Type Flow Meters

1. Magnetic2. Ultrasonic - Transit Time, Doppler3. Turbine4. Vortex

Magnetic Type Flow Meter

1. Magnetic Type Flow Meter (Cont…)

• A magnetic flow meter is a volumetric flow meter which does not have any moving parts and is ideal for wastewater applications or any dirty liquid which is conductive or water based.

• Magnetic flowmeters will generally not work with hydrocarbons, distilled water and many non-aqueous solutions).

• Magnetic flowmeters are also ideal for applications where low pressure drop and low maintenance are required.

1. Magnetic Type Flow Meter (Cont…)

• The operation of a magnetic flow meter is based upon Faraday's Law, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor.

1. Magnetic Type Flow Meter (Cont…)

• The characteristics are:Recommended Service: Clean, Dirty & Viscous

Conductive Liquids & SlurriesRangeability: 40 to 1Pressure Loss: NoneAccuracy: 0.5%Straight Run Required: 5D Upstream, 2D DownstreamViscosity Effect: NoneRelative Cost: HighSizes: 1” to 120”Connections: FlangedType of Output: Linear

2. Ultrasonic - Transit Time, Doppler

2. Ultrasonic - Transit Time, Doppler (cont…)

• Employ two transducers located upstream and downstream of each other.

• Each transmits a sound wave to the other, and the time difference between the receipt of the two signals indicates the fluid velocity.

• Transit time meters usually require clean fluids and are used where high rangeability is required.

• Accuracy is within 1% for ideal applications.

2. Ultrasonic - Transit Time, Doppler (cont…)

FLOW t up

Transmitter/ Receiver (T/R)t dn

Frequency pulse

Transit time difference is proportional to mean velocity Vm, therefore Vm can be calculated as follows:

ATransit length L

•Flow is measured by measuring the difference in transit time for two ultrasonic beams transmitted in a fluid both upstream and downstream.

•Ultrasonic Meters are mainly used on large size lines where high rangeability is required.

2. Ultrasonic - Transit Time, Doppler (cont…)

• Ultrasonic flow meters are ideal for wastewater applications or any dirty liquid which is conductive or water based.

• The basic principle of operation employs the frequency shift (Doppler Effect) of an ultrasonic signal when it is reflected by suspended particles or gas bubbles (discontinuities) in motion.

• Current technology requires that the liquid contain at least 100 parts per million (PPM) of 100 micron or larger suspended particles or bubbles.

2. Ultrasonic - Transit Time, Doppler (cont…)

• Characteristics are: Recommended Service: Clean & Viscous Liquids, Natural/Flare

Gas Rangeability: 20 to 1 Pressure Loss: None Accuracy: 0.25% to 5% Straight Run Required: 5 to 30D Upstream Viscosity Effect: None Relative Cost: High Sizes: > ½” Connections: Flanged & Clamp-on Design Type of Output: Linear

3. Turbine Type Flow Meter

3. Turbine Type Flow Meter (Cont….)

• Turbine meter is kept in rotation by the linear velocity of the stream in which it is immersed.

• The number of revolutions the device makes is proportional to the rate of flow.

3. Turbine Type Flow Meter (Cont…)

• Consist of a small vaned wheel (like a propeller) in the flow path

• The propeller spin according to how fast the fluid flows past it

• The rate of rotation will give us an indication of the flow rate

3. Turbine Type Flow Meter (Cont….)

• Characteristics are: Recommended Service: Clean & Viscous Liquids, Clean Gases Rangeability: 20 to 1 Pressure Loss: High Accuracy: 0.25% Straight Run Required: 5 to 10D Upstream Viscosity Effect: High Relative Cost: High Sizes: > ¼” Connections: Flanged Type of Output: Linear

4. Vortex Type of Flow Meter

4. Vortex Type of Flow Meter (Cont..)• Works according to the principle

that an obstruction in a pipe will produce turbulence in a known fashion

• A centrally placed object, known as a bluff body, will shed vortices downstream in a very predictable and linear way

• An ultrasonic beam can be used to detect the vortices

• A pressure system can also be used to detect them

4. Vortex Type of Flow Meter (Cont..)

• Characteristics are: Recommended Service: Clean & Dirty Liquids, Gases Rangeability: 10 to 1 Pressure Loss: Medium Accuracy: 1% Straight Run Required: 10 to 20D Upstream, 5D Downstream Viscosity Effect: Medium Relative Cost: Medium Size: ½” to 12” Connection: Flanged Type of Output: Linear

Positive Displacement Type Flow Meters

• A positive displacement flow meter, commonly called a PD meter, measures the volume flow rate of a continuous flow stream by momentarily entrapping a segment of the fluid into a chamber of known volume and releasing that fluid back into the flow stream on the discharge side of the meter.

• By monitoring the number of entrapments for a known period of time or number of entrapments per unit time, the total volume of flow or the flow rate of the stream can be ascertained.

• The total volume and the flow rate can then be displayed locally or transmitted to a remote monitoring station.

Positive Displacement Type Flow Meters

Sliding-vane type PD meter.

Tri-Rotor Type PD Meter

Birotor PD Meter

Piston Type PD Meter

Oval Gear PD Meter

Positive Displacement Type Flow Meters

• Advantages are:1. Good accuracy at low flow rates2. Good repeatability3. Easy to install and maintain4. Moderate cost

Positive Displacement Type Flow Meters

• Disadvantages are:1. Limited power to drive the mechanical

accessories2. Can be used with clean fluids only