FLOW MEASUREMENT

Application of flow measurement:

1-Flow measurement is essential for getting information about the proportions & amount the materials which is flowing in process.

2-The quality of product in continuous processing plant mainly depends upon the correct flow rate of raw materials.

3-The flow rates of utilities like air, water & steam are required for cost accounting of plant.

Flow measuring methods:

There are two types of flow measurement

1-Rate of flow

2-Total flow/Quantity flow

Rate of flow:

It is the amount of fluid that flows Past a given point at any given instant.

Total flow:

It is the amount of fluid that flows Past a given point in a definite time period.

Flow Measuring Instrument

Rate of Flow Measuring Instruments:

1-Variable Head or Differential Pressure Type

2-Magnetic Flow Meters

3-Variable area meters

4-Turbine Meters

5-Target Meters

6-Vortex meters

7-Ultrasonic flow meters

 Variable head or Differential Pressure type meters:

It is based upon following Principles

1-Continuity Equation

2-Bernoulli’s Equation

Continuity Equation:

Bernoulli Theorem:

It state that in steady flow of fluid the sum of potential energy ,kinetic Energy & pressure energy remains const. at any section  

P1/ρ + V12 /2g + Z1 = P2/ρ + V22 /2g + Z2   = Constt.

Parts of differential flow meter

1-Primery Element:  

The parts of the meter which produce a differential pressure known as Primary Element . For Example-Orifice Plate, Flow Nozzle, Venturi tube, Pitot tube etc.

2-Secondary element:

Which measures the differential pressure produce by primery element as like Manometer, Bellows diffrential  pressure gauge, etc.

Orifice Plate:

This is a circular plate having hole in it. This plate inserted into a pipe line by means of Flanges. So that it acts as the restriction to fluid flow.

Types of Orifice Plate: This is Four types according to holes.

1-Concentric

2-Eccentric

3-Segmental

4-Quadrant Edge

Concentric

These are most commonly used for flow measurement. This has special features such as simple structures, high accuracy, and ease of installation & replacement. The orifice plates are correctly finished to the dimensions, surface roughness, and flatness to the applicable standard. These plates are recommended for clean liquids, gases & steam flow, when the Reynold number 7 ranges from 10000 to 10 .

Eccentric

For liquids containing solid particles that are likely to sediment or for vapors likely to deposit water condensate, this orifice plate is used with its eccentric bore bottom flush with the bottom of the piping inside surface so that the sedimentation of such inclusions are avoided. Likewise, for gases or vapors, it may be installed with its eccentric bore top flush with the ID of the piping to avoid stay of gas or vapor in its vicinity.

Segmental Orifice

Segmental orifice plates are most useful where there are substantial entrained water or air and also if there are suspension in the fluids. This avoids build up in front of the orifice plate. The orifice hole is placed at the bottom for gas service and top for liquids.

Quadrant Edge

The inlet edge of the bore of this orifice plate is rounded to a quarter circle. This orifice plate is usually used for viscous fluids & Reynolds number between 2000 to 10000

Reynolds Number & Pipe Flow

The Reynolds ( Re ) number is a quantity which engineers use to estimate if a fluid flow is laminar or turbulent. This is important, because increased mixing and shearing occur in turbulent flow. This results in increased viscous losses which affects the efficiency of hydraulic machines. A good example of laminar and turbulent flow is the rising smoke from a cigarette. The smoke initially travels in smooth, straight lines (laminar flow) then starts to “wave” back and forth (transition flow) and finally seems to randomly mix (turbulent flow). These ranges are discussed below.

RANGE 1: Laminar Flow (see diagram below) Generally, a fluid flow is laminar from Re = 0 to some critical value at which transition flow begins.

RANGE 2: Transition Flow (see diagram below) Flows in this range may fluctuate between laminar and turbulent flow. The fluid flow is on the verge of becoming turbulent.

RANGE 3: Turbulent Flow (see diagram below) The fluid flow has become unstable. In turbulent flow, there is increased mixing that results in viscous losses which are generally much higher than in those in laminar flow. NOTE: The Re at which turbulent flow begins depends on the geometry of the fluid flow. The value is different for pipe flow and external flow (i.e. over/outside and object). Since we are studying fluid flow in hydraulic systems, WE WILL CONSIDER ONLY INTERNAL FLOWS (PIPE FLOWS). Streamlines In Laminar, Transition, and Turbulent Flow Regimes In Pipe Flow:

 < –What Streamline Looks Like In Different Types Of Flows –>

What is vena contracta?

FLOW EQUATION OF ORIFICE PLATE:

According to Bernoulli’s theoram

P1/ρ + V12 /2g + Z1 = P2/ρ + V22 /2g + Z2                              ———-eq _1

Since the diffrence in elevations is very small due to length of orifice itself is very small so Z1=Z2

(P1-P2)/ ρ=(V22 –V12 )/2g

V22 –V12= 2g (P1-P2)/ ρ=2gh

Where h= (P1-P2)/ρ =Diffrential pressure head

V22 = V12 + 2gh

V2= (V12+ 2gh)1/2                                                                                    ———–eq _ 2

According to continiuty equn.

Q = A1V1 = A2V2                                                                        ——-eq_3

V1 = (A2/A1) V2 -4

Using this value in equn.-2

V2= V22( A2/A1)2 +2gh   ½

V22 (1- A22/A12) = 2gh

V22 = 2gh/ (1- A22/A12)

V2= 2gh/ (1- A22/A12½                                                                                       ———–eq_5

Puting this value in equn.-3

Q = A2V2 =A2 2gh/ (1- A22/A12½                                                          —————–eq_6

A22/A12= Orifice Area2/pipe inside area2

= (π/4 d2)2 / (π/4 D2)2

= ( d / D)4

= ß putting this valve in equn.6

Q = A2 √ 2gh/ (1- ß4)                                                              —————-eq_7 

This is the theoretical  flow rate through restriction

It is always different from actual flow rate because of fallowing reasons

  • 1-some of fluid energy is loosed in overcoming pipe friction
  • 2-Vena contracta does not occur exactly at the orifice but its location depends upon the actual flow rate.

Actual flow rate:

Actual flow rate

Qa=CXTheoratical flow rate

=C A2 √ 2gh/ √(1- ß4

             = K A2 √ 2gh

Where k=C/ √(1- ß4) =flow coefficient

C= Discharge coefficient

Tapping Of Orifice:-

The upstream pressure tap will detect fluid pressure at a point of minimum velocity, and the downstream tap will detect pressure at the vena contracta (maximum velocity). In reality, this ideal is never perfectly achieved. An overview of the most popular tap locations for orifice plates is shown in the following illustration.

1- Flange Taps

These are the predominant type in use and are normally located 1 inch from upstream face of the orifice plate and 1 inch from the downstream face. Flange tapes are not recommended for pipe sizes less than 2 inches since the vena contracta may be less than 1 inch from the orifice plate.

2-Vena contracta taps

These are made at locations, which theoretically take advantage of the highest delta P available at the orifice. The upstream tap is located one pipe diameter firm the face of orifice while the downstream tap is located at the vena contracta, the point of minimum pressure caused by the restriction. However, the point of minimum pressure varies with the d/D ratio, there by introducing an error if the plate bore is changed.

3-Radius taps

These are a close approximation of vena contracta taps, at locations one pipe diameter upstream and 1/2inches pipe diameter downstream from the orifice face.

4-Corner taps

These are located directly at the faces of the orifice plate. They are in common use in Europe and are particularly useful for pipe size less than 2 inches where the venacontracta may occur inside the dimension the standard flange tap. Possible difficulties in using corner taps are small passages are vulnerable to plugging and Tests have indicated pressure instability in the region of the orifice face.

5-Full-flow taps or Pipe taps

Pipe or full flow taps measure the permanent pressure loss across an orifice. The taps are located 2.5 pipe diameters (2.5 D) up stream (a head of the pressure build up near the orifice face) and 8 pipe diameters down stream-where static pressure has reached its maximum following of vena contacta. There is more likelihood of measurement error when pipe tapes are used, and they required longer runs of straight pipe. However, they are some times more economical in that they allow usr of stander flanges (some times all ready existing), and they also have higher flow rate capabilities than flange or vena.

Advantage of orifice:

  • Simple construction
  • Easy installation & replacement
  • Can be used with all types of D.P.T.

Limitation:

  • It has non-linear square rootcharacteristic.
  • It has high permanent pressure loss.

Venturi Tubes

Where high permanent pressure loss is not tolerable, a venturi tube can be used. Because of its gradually curved inlet and outlet cones, almost no permanent pressure drop occurs. This design also minimizes wear and plugging by allowing the flow to sweep suspended solids through without obstruction.

Advantage of venturi tube

1. Causes low permanent pressure loss.

2. Widely used for high flow rate.

3. Available in very large pipe sizes.

4. Has well known characteristics.

5. More accurate over wide flow range than orifice plate or nozzles. 6. Can be used at high and low beta ratios

Disadvantage of venturi tubes:

  1. High cost.
  2. Generally not use full below 76.2 mm pipe size.
  3. More difficult to inspect due to its construction
  4. Limitation of lower Reynolds number of 150000, (some data is however
  5. Available down to Reynolds number of 50000 in some sizes).
  6. Calculated calibration figures are less accurate than for orifice plates.
  7. For greater accuracy, each individual Venturi tube has to be flow calibrated by passing known flows through the Venturi and recording the resulting differential pressures.
  8. The differential pressure generated by a venturi tube is lower than
  9. for an orifice plate and, therefore, a high sensitivity flow transmitter
  10. is needed.
  11. It is more bulky and more expensive.
  12. As a side note; one application of the Venturi tube is the measurement of
  13. flow in the primary heat transport system. Together with the temperature
  14. change across these fuel channels, thermal power of the reactor can be calculated.

Flow Nozzle

A flow nozzle is also called a half venturi.The flow nozzles are used for flow measurement at high fluid velocities & the more rugged & more resistant to erosion than the sharp edged orifice plate

There are two types of flow nozzle

  • 1-Long radius flow Nozzle
  • 2-I.S.A (Internatinal Federation of the national Standardizing Assocition)

The upstream or high pressure tap is located at about 1 pipe Dia from  the entrance to the nozzle while low pressure tap is located in the pipe  directly  apposite to the straight portion of the nozzle as shown in fig.

Working:

  • A fluid enters the inlet section of the Flow nozzle ,it Smoothly converges with corresponding increase in flow velocity & Decrease in static pressure
  • In divergent section flow smoothly diverges with increase in pressure head at the cost of decrease in velocity head.

Advantage

  • Low permanent pressure loss as compared to orifice
  • It can be used for metering fluids at high temp.& Flows
  • For fluids containing solids that settle.
  • Available in numerous materials.

Limitation:

  • Cost is higher than orifice  
  • Low pressure recovery
  • Requires more maintenance (it is necessary to remove a section of pipe to inspect or install it ).
  • Limited to moderate pipe size

Pitot Tubes

Pitot tubes also utilize the principles captured in Bernoulli.s equation, to measure flow. Most pitot tubes actually consist of two tubes. One, the lowpressure tube measures the static pressure in the pipe. The second, the highpressure tube is inserted in the pipe in such a way that the flowing fluid is stopped in the tube. The pressure in the high-pressure tube will be the static pressure in the system plus a pressure dependant on the force required stopping the flow.

Pitot tubes are more common measuring gas flows that liquid flows. They suffer from a couple of problems The pressure differential is usually small and hard to measure. The differing flow velocities across the pipe make the accuracy dependent on the flow profile of the fluid and the position of the pitot in the pipe.

ADVANTAGE OF PITOT TUBES

  1. No process loss.
  2. Economical to install.
  3. Some types can be easily removed from the pipe line.

DISADVANTAGE OF PITOT TUBES

  1. Have poor accuracy.
  2. Unsuitability for dirty or sticky fluids.
  3. Sensitivity to up stream disturbances.

Annubar

An annubar is very similar to a pitot tube. The difference is that there is more than one hole into the pressure measuring chambers. The pressure in the high-pressure chamber represents an average of the velocity across the pipe. Annubars are more accurate than pitots as they are not as position sensitive or as sensitive to the velocity profile of the fluid.

ADVANTAGE OF ANNUBAR TUBES

  1. It is available for a wide range of pipe sizes.
  2. It is simple and economical to install.
  3. It provides negligible pressure drop.
  4. It can be placed in service under pressure.
  5. It can be rotated while in service, for cleaning action.
  6. It provides long term measurement stability

DISADVANTAGE OF ANNUBAR TUBES

  1. Unsuitability for operating dirty or sticky fluids.
  2. Limited operating data.

Magnetic flow meter:

Principle: Electromagnatic flow meter works on the principle of Faraday’s law of electromagnetic induction which states that, when a current carrying conductor moves through stationary transverse magnetic field, then e.m.f. Is induced between the ends of the conductor and this e.m.f. Is proportional to relative velocity between the conductor and the magnetic field.

This e.m.f. Induced is given by

             E = BLV

       Where  E= e.m.f.,   B=Magnetic induction

                  L=Length of the conductor

                  V=velocity of the conductor

Thus,    E $ V

In electromagnetic flowmeter, flowing conductive fluid acts as the conductor

Construction:

The magnetic flow meter consist of flow tube with electrodes and the source of magnetic field.

Flow Tube: The tube is made of non conducting & non magnetic alloy & is insulated by glass lining from flowing liquid to prevent the short circuiting of e.m.f. between the Electrodes.

Electrodes: S.S or Pt. Electrodes are located diametrically apposite to each other with their Axis perpendicular to both the magnatic field & the tube Axis.

Source of Magnatic field: A magnetic field is generated by electromagnets formed by winding to saddle shapped copper coils on the laminated iron core.

Working: Flow of conducting fluides can be considered as a current carring conducter having length equal to inside diameter of the tube or distance beetween the electrodes when the electromagnet produce a steady magnet field around the pipe then by electromagnetic induction e.m.f. is induced between the electrodes this e.m.f. is given by
                            E=BLV
the equn.of continuity to convert a velocity measurment to volumetric flow rate is given as Q=VA,        V=Q/A
Then                   E=BLQ/A
                              =BLQ/πd2/4
                            Q =π d E/4B

Advantage:

  1. It can handle small as well as large flow rates.
  2. Since there are no obstruction in the flow path, so there are no pressure loss.
  3. It can be used for slurries and greasy materials.

Limitations

  1. It can be used for metering conductive liquids only.
  2. It can not be used for metering gases ,steam and petroleum products because they have low electrical conductivity.
  3. For proper functioning, the flow tube should continuously run full with liquid.

Variable area meters:

Principle: The variable area flow meter operates on the same basic principle as the head flow meters laid by the continuity Equn.& the Bernoulli Equn.

    In Variable area  flow meters the size of restriction is varied to maintain the differential pressure across it constant. This change in restriction size with subsequent change in flow area. It is proportional to change in fluid flow rate through the restriction. Thus any change in fluid flow rate can be measured in terms of some quantification of change in restriction size or flow area.

Types of variable area flow meter:

1-Rotameter

2-Piston type or valve type area meter

Rotameter

Construction: It is having taperedmetering tube ,float with scale.

Metering tube: It is Tapered glass or metal tube set vertically in pipe. The metering tube is made of glass for low temp. & Pressure fluids. For high temp. & Pressure Metal tubes are used .In metal tubes the linear motion of the floats is converted into a rotary motion for the movement of a pointer on a calibrated scale. The float movement is converted in to proportional electrical signal also.

Float: Rotameter floats are usually made of metals which have density higher than liquid .

Rotameter scale:  Scale is marked on the glass tube or it is mounted close to the metering tube scale can be direct reading type or percentage scale is used.

Type of rotameter:

1-Direct viewing rotamter :In the Direct viewing rotameter float is viewed directly & its displacement is measured on the scale calibrated in fluid flow rate.This type is used for free flowing & clear liquid.

2-Magnatically coupled rotameter: In this type float movement is magnetically Coupled to the indicating, recording or controlling element. This is also used for electrical or pneumatic transmission of float movement to the remote point.

Turbine type flow meter

The turbine flow meter is used for the measuring of liquid gases &very low flow rate.It works on the principle of the turbine. It consist of a multibladed rotar which is mounted at right angles to the axis of the flowing liquid.The Rotar is supported on ball bearings either at one end or at both the ends. This shaft supporting section also acts as straightening vane that directs the flow to turbine

Working

The flowing  fluid impinges on the  turbine roter blades, imparting a force to the blade surface which causes the rotation of the roter. The speed of  the roter is directly proportional to the fluid velocity,and hence to volumetric flow rate when it is at a steady rotational speed. The speed of rotation is monitored in most of the meters by a magnetic pick coil, which is fitted to the outsideb of the meter housing. As each roter blade passes the magnetic pick coil, it generates a voltage pulse which is a measure of the flow rate and the total number of pulses give a measure of the total flow.

Advantage

  1. Its accuracy is good.
  2. It is easy to install and maintain.
  3. It is used in blending systems for the petroleum industry.

Limitations

  1. Its cost is very high.
  2. Its use is limited for slurries.

Target flow meter

The target flow meter measures flow by measuring the force on a target centered in the pipe at right angles to the direction of fluid flow .the fluid flow develops a force on the target which is proportional to the square of the flow

Working of target meter

A Target meter consists of a target (Disc) which is mounted on a force bar passing through a flexible seal & is positioned in the center of the perpendicular to the flowing stream .The flowing fluid while passing through the pipe develops a force on the target which is proportional to the velocity head. The force bar transmits this force to a force transducer to measure the force which is proportional to the square of the flow .

          Q=k √F

Q=flow rate, k=coefficient, F=Force

Advantage

  1. They are usefull for difficult measurement such as slurries. corrosive mixtures.
  2. Having good accuracy

Limitations

  1. In line mounting required in these flow meters.
  2. They have a limited calibration data.

                  VORTEX METER

Two meters are available that may be classified as vortex meter. One is called a precessing vortex meter or Swirlmeter, and the other uses the term vortex shedding.

1-SWIRL METER  

The swirl meter operates on the principle of vortex precession. It is a digital volumetric device which has no moving part. It gives an output in the form of pulses whose frequencies are proportional to fluid flow rate. It consists of a fix set of swirl blades, unusually made of stainless steel, which introduce a spinning or swirling motion to the fluid at the inlet. At the down stream of a swirl blades there is a venturi – like contraction and expansion of the flow passage. A temperature sensor is placed at the down stream of the blades, which is heated by a constant electric current. At the exit of the meterdeswirl blades are fixed to leaving the meter, from down stream piping effects. As the fluid passes through the fixed set of swirl blades at the inlet, a swirling (spinning) motion is imparted to it. In the area where expansion occurs, the swirling flow processes or oscillates at a frequency proportional to fluid flow rate. This precession of the fluid causes variation in temperature and resistance of the thermistor (sensor). The amount of heat extracted from the thermistor by passing fluid is dependent upon the fluid velocity. Consequently, each high velocity vortex passed the thermistor, changes the resistance and, since a constant current is applied, the resistance changes is converted in to voltage pulses which are amplified, filtered and transformed into constant amplitude high level pulses of square wave form. The frequencies of the pulses are measure by an electronic counter, which gives the flow rate of fluid. The swirl meter has an accuracy of +/- 0.75% within its linear operating range of +/- 1%. Its repeatability is +/- 0.25% and rangability 100:1. It is currently available in meter sizes from 25.4 to 152.4 mm. It is primarily used in gas applications, where a very much lower density results in a significantly lower pressure loss.

2-VORTEX SHEDDING FLOW METER

The operation of vortex shedding flow meter is based on a phenomenon known as vortex shedding which occurs when a gas or liquid flows around a nonstream line (or blunt) object known as sluff body. When a fluid flows past an obstacle, boundary layers of slow moving fluid are formed along the outer surface of obstacle and the flow is unable to flow contours of the obstacle on its down stream side. Thus the flow layers are separated from the surface of the object, and a low pressure area is formed behind the object which causes the separated layers to get detached from the main stream of the fluid and roll them selves into eddies or vortices in the low pressure area. Each eddy on vortex 1st grows and gets detached or shade from alternate sides of the object. The frequencies at which the vortices are form is directly proportional to the fluid velocity. As a vortex is shed from one side of the sluff body the fluid velocity on that side increases and the pressure decreases, and at the same time the velocity on the opposite side decreases pressure increases, thus causing a net pressure change across the sluff body. As the next vortex is shed from the opposite side of the sluff body, the inteire effect is reserved. Therefore the velocity and pressure distribution in the fluid around the sluff body change at the same frequency as the vortex shedding frequency. The changes in pressure of velocity is sensed by a flow sensitive detector which can be either a heat thermistor element or a spherical magnetic shuttle. The vortex shedding flow meters are available in the sizes from 50.8 to 152.4 mm. Its linearity is within +/- ½% and rangeability is 100:1. This meter has also no moving parts.

ADVANTAGE OF VORTEX FLOW METERS

  1. Has an excellent range ability.
  2. Handles a wide verity of chemicals, including slurries, liquids with
  3. entrained particles and viscous materials.
  4. Have no moving parts.
  5. Relatively immune to density, temperature, pressure and viscosity variations within the linear range
  6. Very low pressure drop
  7. Has good response speed

DISADVANTAGE OF VORTEX FLOWMETERS

  1. High cost
  2. Not available over 200 mm size
  3. Upper temperature limit is 2040c
  4. In line mounting required.

CORIOLIS MASS FLOW METER

The flow tubes of the coriolis mass flow sensor are driven to vibrate at their natural frequency by a magnet and drive coil attached to the apex of the bent tubes. An ac drive control amplifier circuit in the transmitter reinforces the signal from the sensor,s left velocity pickoff coil to generate the drive coil voltage. The amplitude of this drive coil voltage is continuously adjusted by the circuit to maintain a constant, low amplitude of flow tube displacement, thereby minimizing stress to the tube assembly

Advantage of Coriolis meter

  1. It can direct take measurement of mass flow with high measurement accuracy.
  2. It has a wide range of measurable fluids, including high viscosity fluids, liquid-solid two-phase fluids, liquid-gas two-phase fluids containing trace gases, and medium and high pressure gases of sufficient density.
  3. The vortex flow and non-uniform flow velocity distribution caused by the upstream and downstream pipelines have no influence on the performance of the flow sensor. Generally, it is not required straight pipe lines when installing the sensor.
  4. The change in fluid viscosity has no significant effect on the measured value.
  5. The change in fluid density has little effect on the measured value.
  6. There are multiple outputs, which can simultaneously output instantaneous mass flow or volume flow, fluid density, fluid temperature and other signals. It also has several digital input and output ports, and some models can realize batch control functions.
  7. Bidirectional flow measurement
  8. It can take measurement of high viscosity fluids, such as crude oil, heavy oil, residual oil and other liquids with higher viscosity. Previously, volumetric flowmeters, target flowmeters, etc. were used to measure flow. Now, Coriolis mass flowmeters are used, with good reliability and accurate measurement result.

Disadvantage of Coriolis flow meter

  1. Poor zero stability which affects the flow meter accuracy.
  2. It cannot be used to measure fluids with lower density, such as low pressure or low density gas.
  3. Slightly higher gas content in the liquid may cause a significant increase in measurement error.
  4. It is sensitive to external vibration interference.
  5. It cannot be used for larger diameters. Currently max size we can make is 8 inch Coriolis flow meter.
  6. The pressure loss is large, especially when measuring a liquid with a high saturated vapor pressure, the pressure loss may cause vaporization of the liquid, and cavitation occurs.

ULTRASONIC FLOWMETERS

In ultrasonic flow meters, the measurement flow rate is determined by the variation in parameters of ultrasonic oscillations. There are two types of ultrasonic flow meters currently in use. They Time difference type and Doppler flow meter.

Time difference Ultrasonic Type

This devices measure flow by measuring the time taken ultrasonic wave to transverse a pipe section, both wit and against the flow of liquid within the pipe. It consist of two transducers, A & B, inserted into a Pipe line, and working both as transmitter and receiver. The ultrasonic waves are transmitted from transducer A and transducer B and vice versa. An electronic oscillator is connected to supply ultrasonic waves alternately to A or B which is working as transmitter through a change over switch, when the detector is connected simultaneously to B OR A which is working as receiver. The detector measures the transit time from upstream to downstream transducers and vice versa.

The time T A B for ultrasonic wave to travel from transduser A to transduser B is given by the expression:

TA B = L / (C + V COS θ)

And the time (T B A ) to travel from B to A is given as,

TBA = L / (C – V COS θ )

Where,

L = acoustic path length between A & B C = Velocity of sound in the fluid

θ = angle of path with respect to the pipe axis V = velocity of fluid in pipe

The time difference between T A B and T B A an be calculated as,

∆ θ T = TA B – TBA = ( 2 L V COS θ ) / C

Or V = ∆ T C / ( 2 L COS θ )

Since, this type of flowmeter relies upon an ultrasonic signal traversing across the pipe; the liquid must be relatively free of solid and air bobbles.

DOPPLER ULTRASONIC FLOWMETERS

In Doppler flow meters, an ultrasonic wave is projected at an angle though the pipe wall into the liquid by a transmitting crystal in a transducer mounted outside the pipe. Part of the ultrasonic wave is reflected by bubbles or particles in the liquid and is returned through the pipe wall to the receiving crystal. Since the reflector (bubbles ) are travelling at the fluid velocity, the frequency of the reflecting wave is shifted according to the Dopple principle. The velocity of the fluid is given by the equation:

V = (∆ f Ct )/ (2 fo COS 􀀘) = ∆ f k

Where

∆ f = difference between transmitted and received frequency

C t = velocity of the sound in the transducer

F o = frequency of the transmission

θ = angle of transmitter and receiver crystal with respect to the pipe axis.

K = constant

ADVANTAGE OF ULTRASONIC FLOWMETERS

  1. Dose not impose additional resistance to the flow or disturb the flow patternas the transducers are inserted in the wall of pipe.
  2. Velocity / output relationship is linear
  3. No moving parts
  4. Repeatability is in the order of 0.01%

OPEN CHANNEL METERS

Weirs are used to measure flow rate primarily in open channels such as water works including irrigation, waste and sewage systems, and in pipes and conduits that are enerally not completely filled with liquid. It is an obstruction in the flowing stream over which the liquid is made to pass. With the use of weir, flow rate can be measured from a few gallons per minute to millions of gallon per day. There are three types of weirs such as Rectangular, V-notch and Cippoletti orTtrapezoida, as shown in figure.

Positive Displacement flow meters

Positive Displacement flow meters are the only flow measuring technology to directly measure the volume of fluid that passes though the flow meter. It achieves this by trapping pockets of fluid between rotating components housed within a high precision chamber. This can be compared to repeatedly filling a beaker with fluid and pouring the contents downstream while counting the number of times the beaker is filled.

Rotor rotational velocity Jy is directly proportional to flow rate, since the flow of fluid is causing the rotation.



In electronic flow meters the rotating components contain magnets that activate various sensor options located outside the fluid chamber. Mechanical positive displacement flow sensors rely on the rotation to drive either a magnetic coupling or a direct gear train connected to the mechanical counter.

PD flow meters do not require a power supply for their operation and do not require straight upstream and downstream pipe runs for their installation. Positive displacement flow meters are available in sizes from in to 12 in and can operate with turndowns as high as 100:1, although ranges of 15:1 or lower are much more common. Slippage between the flow meter components is reduced and metering accuracy is therefore increased as the viscosity of the process fluid increases.

The process fluid must be clean. Particles greater than 100 microns in size must be removed by filtering. Positive displacement meters operate with small clearances between their precision-machined parts; wear rapidly destroys their accuracy. For this reason, PD meters are generally not recommended for measuring slurries or abrasive fluids. In clean fluid services, however, their precision and wide rangeability make them ideal for custody transfer and batch charging. They are most widely used as household water meters. Millions of such units are produced annually. In industrial and petrochemical applications, Positive displacement flow sensors are commonly used for batch charging of both liquids and gases.

Although slippage through the positive displacement flow meter decreases (that is, accuracy increases) as fluid viscosity increases, pressure drop through the meter also rises. Consequently, the maximum (and minimum) flow capacity of the flow meter is decreased as viscosity increases. The higher the viscosity, the less slippage and the lower the measurable flow rate becomes. As viscosity decreases, the low flow.

Rotating Positive Displacement flow MetersOne of the main benefits of using a PD flow meter is the high level of accuracy that they offer, the high precision of internal components means that clearances between sealing faces is kept to a minimum. The smaller these clearances are, relates to how high the accuracy will be. Only fluid that is able to bypass this seal does not get counted, this is known as ‘by-pass’ or ‘slippage’.

Another benefit is the flow meters ability to process a huge range of viscosities and it is not uncommon to experience higher levels of accuracy while processing high viscosity fluids, simply due to the reduction of by-pass. When considering and comparing flow meter accuracy, it is important to be aware of both ‘linearity’ i.e. the positive displacement flow meters ability to accurately measure over the complete turndown ratio, and ‘repeatability’, the ability to remain accurate over a number to cycles. This is another area where positive displacement flow meters excel, repeatability of 0.02% and 0.5% linearity are standard.