LIQUID FLOW MEASUREMENT

PREPARED BY: MANDAR SUMANT

INTRODUCTION

Measuring the flow of liquids is a critical need in many industrial plants. In some operations, the ability to conduct accurate flow measurements is so important that it can make the difference between making a profit or taking a loss. In other cases, inaccurate flow measurements or failure to take measurements can cause serious (or even disastrous) results.

With most liquid flow measurement instruments, the flow rate is determined inferentially by measuring the liquid's velocity or the change in kinetic energy. Velocity depends on the pressure differential that is forcing the liquid through a pipe or conduit. Because the pipe's cross-sectional area is known and remains constant, the average velocity is an indication of the flow rate.

THE BASIC RELATIONSHIP FOR DETERMINING THE LIQUID'S FLOW RATE IN SUCH CASES IS:

Q = V x A where

Q = liquid flow through the pipe V = average velocity of the flow A = cross-sectional area of the pipe

Other factors that affect liquid flow rate include the liquid's viscosity and density, and the friction of the liquid in contact with the pipe.

Other factors that affect liquid flow rate include the liquid's viscosity and density, and the friction of the liquid in contact with the pipe.

Direct measurements of liquid flows can be made with positive-displacement flow meters. These units divide the liquid into specific increments and move it on. The total flow is an accumulation of the measured increments, which can be counted by mechanical or electronic techniques.

REYNOLDS NUMBERS

The performance of flowmeters is also influenced by a dimensionless unit called the Reynolds Number. It is defined as the ratio of the liquid's inertial forces to its drag forces.

Laminar and turbulent flow are two types normally encountered in liquid flow Measurement operations. Most applications involve turbulent flow, with R values above 3000. Viscous liquids usually exhibit laminar flow, with R values below 2000. The transition zone between the two levels may be either laminar or turbulent.

The equation is:

R = 3160 x Q x Gt x D x µ where: R = Reynolds number Q = liquid's flow rate, gpm Gt = liquid's specific gravity D = inside pipe diameter, in.µ = liquid's viscosity, cp

LAMINAR FLOW

At very low velocities or high viscosities, R is low, and the liquid flows in smooth layers with the highest velocity at the center of the pipe and low velocities at the pipe wall where the viscous forces restrain it. This type of flow is called laminar flow.

R values are below approximately 2000. A characteristic of laminar flow is the parabolic shape of its velocity profile

TURBULENT FLOW

However, most applications involve turbulent flow, with R values above 3000. Turbulent flow occurs at high velocities or low viscosities. The flow breaks up into turbulent eddies that flow through the pipe with the same average velocity.

Fluid velocity is less significant, and the velocity profile is much more uniform in shape. A transition zone exists between turbulent and laminar flows.

Depending on the piping configuration and other installation conditions, the flow may be either turbulent or laminar in this zone.

TYPES OF FLOW METERS

Numerous types of flow meters are available for closed-piping systems. In general, the equipment can be classified as differential pressure, positive displacement, velocity, and mass meters.

Differential pressure devices (also known as head meters) include orifices, venturi tubes, flow tubes, flow nozzles, pitot tubes, elbow-tap meters, target meters, and variable-area meters

DIFFERENTIAL PRESSURE DEVICES

The use of differential pressure as an inferred measurement of a liquid's rate of flow is well known. Differential pressure flow meters are, by far, the most common units in use today. Estimates are that over 50 percent of all liquid flow measurement applications use this type of unit.

The basic operating principle of differential pressure flow meters is based on the premise that the pressure drop across the meter is proportional to the square of the flow rate. The flow rate is obtained by measuring the pressure differential and extracting the square root.

ORIFICE

The orifice plate is installed in the pipe between two flanges. Acting as the primary device, the orifice constricts the flow of liquid to produce a differential pressure across the plate. Pressure taps on either side of the plate are used to detect the difference. Major advantages of orifices are that they have no moving parts and their cost does not increase significantly with pipe size.

FLOW NOZZLE At high velocities, can handle approximately 60 percent greater liquid flow than orifice plates having the same pressure drop.

Liquids with suspended solids can also be metered. However, use of the units is not recommended for highly viscous liquids or those containing large amounts of sticky solids.

VENTURI METER

Venturi tubes have the advantage of being able to handle large flow volumes at low pressure drops. A venturi tube is essentially a section of pipe with a tapered entrance and a straight throat. As liquid passes through the throat, its velocity increases, causing a pressure differential between the inlet and outlet regions.

The flow meters have no moving parts. They can be installed in large diameter pipes using flanged, welded or threaded-end fittings. Four or more pressure taps are usually installed with the unit to average the measured pressure. Venturi tubes can be used with most liquids, including those having a high solids content.

PITOT TUBE

It senses two pressures simultaneously, impact and static. The impact unit consists of a tube with one end bent at right angles toward the flow direction. The static tube's end is closed, but a small slot is located in the side of the unit. The tubes can be mounted separately in a pipe or combined in a single casing.

Pitot tubes are generally installed by welding a coupling on a pipe and inserting the probe through the coupling. Use of most pitot tubes is limited to single point measurements. The units are susceptible to plugging by foreign material in the liquid. Advantages of pitot tubes are low cost, absence of moving parts, easy installation, and minimum pressure drop.

ELBOW TAPE METER When liquid travels in a circular path,

centrifugal force is exerted along the outer edges. Thus, when liquid flows through a pipe elbow, the force on the elbow's interior surface is proportional to the density of the liquid times the square of its velocity. In addition, the force is inversely proportional to the elbow's radius.

Any 90 deg. pipe elbow can serve as a liquid flow meter. All that is required is the placement of two small holes in the elbow's midpoint (45 deg. point) for piezometer taps. Pressure-sensing lines can be attached to the taps by using any convenient method.

TARGET METER

Target meters sense and measure forces caused by liquid impacting on a target or drag-disk suspended in the liquid stream. A direct indication of the liquid flow rate is achieved by measuring the force exerted on the target.

In its simplest form, the meter consists only of a hinged, swinging plate that moves outward, along with the liquid stream. In such cases, the device serves as a flow indicator.

VARIABLE AREA FLOW METER (ROTAMETER)

Variable-area meters, often called rotameters, consist essentially of a tapered tube and a float.

When there is no liquid flow, the float rests freely at the bottom of the tube. As liquid enters the bottom of the tube, the float begins to rise.

The position of the float varies directly with the flow rate. Its exact position is at the point where the differential pressure between the upper and lower surfaces balance the weight of the float.

Because the flow rate can be read directly on

a scale mounted next to the tube, no secondary flow-reading devices are necessary. If desired, automatic sensing devices can be used to sense the float's level and transmit a flow signal.

Rotameter tubes are manufactured from glass, metal, or plastic. Tube diameters vary from 1/4 to greater than 6 in.

POSITIVE DISPLACEMENT PUMP

Operation of these units consists of separating liquids into accurately measured increments and moving them on. Each segment is counted by a connecting register.

Because every increment represents a discrete volume, positive-displacement units are popular for automatic batching and accounting applications.

Positive-displacement meters are good candidates for measuring the flows of viscous liquids or for use where a simple mechanical meter system is needed.

RECIPROCATING PISTON METER These are of the single and

multiple-piston types. The specific choice depends on the range of flow rates required in the particular application.

Piston meters can be used to handle a wide variety of liquids. A magnetically driven, oscillating piston meter is shown in Fig.

Liquid never comes in contact with gears or other parts that might clog or corrode.

OVAL GEAR METER

Oval-gear meters have two rotating, oval-shaped gears with synchronized, close fitting teeth.

A fixed quantity of liquid passes through the meter for each revolution. Shaft rotation can be monitored to obtain specific flow rates.

NUTAING DISC METER

Moveable disk mounted on a concentric sphere located in a spherical side-walled chamber. The pressure of the liquid passing through the measuring chamber causes the disk to rock in a circulating path without rotating about its own axis. It is the only moving part in the measuring chamber.

A pin extending perpendicularly from the disk is connected to a mechanical counter that monitors the disk's rocking motions. Each cycle is proportional to a specific quantity of flow.

VELOCITY METER

These instruments operate linearly with respect to the volume flow rate. Because there is no square-root relationship (as with differential pressure devices), their rangeability is greater.

Velocity meters have minimum sensitivity to viscosity changes when used at Reynolds numbers above 10,000. Most velocity-type meter housings are equipped with flanges or fittings to permit them to be connected directly into pipelines.

TURBINE METER

It has found widespread use for accurate liquid measurement applications.

The rotor spins as the liquid passes through the blades.

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

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

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

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

A major concern with turbine meters is bearing wear. A "bearingless" design has been developed to avoid this problem.

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

The frequency of the resulting pulse output is proportional to flow rate.

VORTEX METER Vortex meters

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

Eddies or vortices are shed alternately downstream of the object. The frequency of the vortex shedding is directly proportional to the velocity of the liquid flowing through the meter

The three major components of the flow meter are a bluff body strut-mounted across the flow meter bore, a sensor to detect the presence of the vortex and to generate an electrical impulse, and a signal amplification and conditioning transmitter whose output is proportional to the flow rate.

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

ELECTROMAGNETIC METER The operation of magnetic

flow meters is based on Faraday's law of electromagnetic induction.

Magmeters can detect the flow of conductive fluids only. Early magmeter designs required a minimum fluidic conductivity of 1-5 microsiemens per centimeter for their operation.

The newer designs have reduced that requirement a hundredfold to between 0.05 and 0.1.

The magnetic flow meter consists of a non-magnetic pipe lined with an insulating material. A pair of magnetic coils is situated as shown in Figure , and a pair of electrodes penetrates the pipe and its lining.

If a conductive fluid flows through a pipe of diameter (D) through a magnetic field density (B) generated by the coils, the amount of voltage (E) developed across the electrodes--as predicted by,

Faraday's law--will be proportional to the velocity (V) of the liquid. Because the magnetic field density and the pipe diameter are fixed values, they can be combined into a calibration factor (K) and the equation reduces to:

E = KV

Manufacturers determine each magmeter's K factor by water calibration of each flow tube. The K value thus obtained is valid for any other conductive liquid and is linear over the entire flow meter range.

For this reason, flow tubes are usually calibrated at only one velocity. Magmeters can measure flow in both directions, as reversing direction will change the polarity but not the magnitude of the signal.

The K value obtained by water testing might not be valid for non-Newtonian fluids (with velocity-dependent viscosity) or magnetic slurries (those containing magnetic particles). These types of fluids can affect the density of the magnetic field in the tube. In-line calibration and special compensating designs should be considered for both of these fluids.

RECENT DEVELOPMENT IN MAGMETER

When a magnetic flow meter is provided with a capacitance level sensor embedded in the liner, it can also measure the flow in partially full pipes. In this design, the magmeter electrodes are located at the bottom of the tube (at approximately 1/10 the pipe diameter) in order to remain covered by the fluid. Compensation is provided for wave action and calibration is provided for full pipe, no flow (static level), and partially filled pipe operation.

Another recent development is a magnetic flow meter with an unlined carbon steel flow tube. In this design, the measuring electrodes mount externally to the unlined flow tube and the magnetic coils generate a field 15 times stronger than in a conventional tube.

This magnetic field penetrates deep into the process fluid (not just around the electrode as with standard magmeter probes). The main advantage is low initial and replacement costs, since only the sensors need be replaced.

SELECTION AND SIZING

Magnetic flow meters can detect the flow of clean, multi-phase, dirty, corrosive, erosive, or viscous liquids and slurries as long as their conductivity exceeds the minimum required for the particular design. The expected inaccuracy and rangeability of the better designs are from 0.2-1% of rate, over a range of 10:1 to 30:1, if the flow velocity exceeds 1 ft/sec. At slower flow velocities (even below 0.1 ft/s), measurement error increases, but the readings remain repeatable.

It is important that the conductivity of the process fluid be uniform. If two fluids are mixed and the conductivity of one additive is significantly different from that of the other process fluid, it is important that they be completely intermixed before the blend reaches the magmeter. If the blend is not uniform, the output signal will be noisy. To prevent that, pockets of varying conductivity can be eliminated by installing a static mixer upstream of the magmeter.

Magmeter size is determined by capacity tables or charts published by the manufacturer. It provides a flow capacity nomograph for line sizes from 0.1 in. to 96 in.

For most applications, flow velocities should fall between 3 ft/sec and 15 ft/sec. For corrosive fluids, the normal velocity range should be 3-6 ft/sec.

If the flow tube is continuously operated below 3 ft/sec, metering accuracy will deteriorate, while continuous operation exceeding the upper limit of the normal velocity range will shorten the life of the meter.

The obstructionless nature of the magmeter lowers the likelihood of plugging and limits the unrecovered head loss to that of an equivalent length of straight pipe.

The low pressure drop is desirable because it lowers pumping costs and aids gravity feed systems.

Water type electromagnetic flow meter is light weight, compact and can be easily installed between existing pipe flanges. The non-moving part instrument has negligible pressure drop and can handle numerous liquids and slurries provided they are conductive

ULTRASONIC FLOW METER

The speed at which sound propagates in a fluid is dependent on the fluid's density. If the density is constant, however, one can use the time of ultrasonic passage (or reflection) to determine the velocity of a flowing fluid.

Some manufacturers produce transducer systems that operate in the shear-mode, sending a single pulse and receiving a single pulse in return.

Narrow-beam systems are commonly subject to walk-away (the signal completely missing the downstream transducer). Wide-beam systems overcome beam refraction and work better in changing liquid density and temperature.

With the advent of digital signal processing, it has become possible to apply digital signal coding to the transmitted signal. This can eliminate many of the problems associated with noise and variations in liquid chemistry.

ULTRASONIC FLOW METER

TRANSIT METER DOPPLER METER

DOPPLER METER Two ultrasonic transducers are employed in the

system. One transmits a continuous ultrasonic wave into the flow. Another one is used to receive the ultrasonic wave back scattered from suspending particles (or targets). The received wave has a frequency shift comparing with the transmitted one.

This shift is the so-called Doppler frequency shift, proportional to the flow velocity. Therefore, by detecting the Doppler frequency, we are able to derive the flow velocity. The flow rate of a pipe flow is then obtained by computing the product of the velocity and the cross-section area of the pipe.

In 1842, Christian Doppler discovered that the wavelength of sound perceived by a stationary observer appears shorter when the source is approaching and longer when the source is moving away. This shift in frequency is the basis upon which all Doppler-shift ultrasonic flow meters work.

Doppler flow meter transducers operate at 0.640 MHz (in clamp-on designs) and at 1.2 MHz in wetted sensor designs. The transducer sends an ultrasonic pulse or beam into the flowing stream. The sound waves are reflected back by such acoustical discontinuities as particles, entrained gas bubbles, or even by turbulence vortices

The meter detects the velocity of the discontinuities, rather than the velocity of the fluid, in calculating the flow rate. The flow velocity (V) can be determined by:

V = (f0 - f1)Ct/2f0 cos(a)

Where Ct is the velocity of sound inside the transducer, f0 is the transmission frequency, f1 is the reflected frequency, and a is the angle of the transmitter and receiver crystals with respect to the pipe axis.

Because Ct /2f0cos(a) is a constant (K),

V = (f0 - f1)K

Thus, flow velocity V (ft/sec) is directly proportional to the change in frequency. The flow (Q in gpm) in a pipe having a certain inside diameter (ID in inches) can be obtained by:

Q = 2.45V(ID)2 = 2.45[(f0 - f1)K](ID)2

The presence of acoustical discontinuities is essential for the proper operation of the Doppler flow meter. The generally accepted rule of thumb is that for proper signal reflection there be a minimum of 80-100 mg/l of solids with a particle size of +200 mesh (+75 micron). In the case of bubbles, 100-200 mg/l with diameters between +75 and +150 microns is desirable. If either the size or the concentration of the discontinuities changes, the amplitude of the reflected signal will shift, introducing errors.

Doppler meter use sound pulse reflection principal to measure liquid flow rates. Solid or bubbles in suspension in the liquid reflect the sound back in the receiving transducer element.

TRANSIENT TIME METER

In this design, the time of flight of the ultrasonic signal is measured between two transducers--one upstream and one downstream.

The difference in elapsed time going with or against the flow determines the fluid velocity.

When the flow is zero, the time for the signal T1 to get to T2 is the same as that required to get from T2 to T1. When there is flow, the effect is to boost the speed of the signal in the downstream direction, while decreasing it in the upstream direction.

The flowing velocity (Vf) can be determined by the following equation:

Vf = Kdt/TL

where K is a calibration factor for the volume and time units used, dt is the time differential between upstream and downstream transit times, and TL is the zero-flow transit time.

Theoretically, transit-time ultrasonic meters can be very accurate (inaccuracy of ±0.1% of reading is sometimes claimed).

Yet the error in these measurements is limited by both the ability of the signal processing electronics to determine the transit time and by the degree to which the sonic velocity (C) is constant.

The speed of sound in the fluid is a function of both density and temperature. Therefore, both have to be compensated for.

In addition, the change in sonic velocity can change the refraction angle, which in turn will affect the distance the signal has to travel. In extreme cases, the signal might completely miss the downstream receiver. Again, this type of failure is known as walk-away.

VARIATION OF K-FACTOR WITH REYNOLDS NUMBER

SELECTION OF ULTRASONIC FLOW METER

•TRANSIENT TIME V/S•DOPPLET

BASED ON

PRICIPLE

•HANDELED V/S•WALL MOUNT

BASED ON

PROBABILITY

SELECTION OF ULTRASONIC FLOW METER

•CLAMP ON V/S•WETTED(INSERTION AND FLOW CELL)

BASED ON TRANSDUCE

R INSTALLMEN

T

•SINGLE PATH V/S•MULTI PATH

BASED ON TRANSDU

CER SCHEME

INDUSTRIAL USES OF ULTRASONIC METER

Handheld Ultrasonic Flow Meter.(STUFF-200H),Shenitech.

High accuracy Rechargeable battery Built –in data logger Easy to operate Ideal for flow survey Non-intrusive. Easy installation. No pressure drop. No pipe disturbance. No moving parts

Wall mount clamp-on ultrasonic flow meter(STUF-300F-B),shenitech

High accuracy Easy and economical

installation. No pipe cutting, no hole

drilling Suitable for all commonly used

pipes Suitable for pure liquids and

liquids with minor particles. No dependency on conductivity

Easy to use and set up. Self-explanatory menu-driving programming

APPLICATION AND PERFORMANCE

Doppler flow meters are not recommended for clean fluid applications.

Transit-time flow meters, on the other hand, are often used to measure the flow of crude oils and simple fractions in the petroleum industry.

They also work well with viscous liquids, provided that the Reynolds number at minimum flow is either less than 4,000 (laminar flow) or above 10,000 (turbulent flow).

Serious non-linearities are present in the transition region

Transit-time flow meters are the standard for measuring cryogenic liquids down to -300°C and are also used in molten metal flow metering.

Measurement of liquid argon, liquid nitrogen, liquid helium and molten sulfur have often been reported.

Spool-section type flow meters are most often used for these applications, especially the axial and co-axial designs.

Raw wastewater applications usually have too few acoustic discontinuities for Doppler flow meters.

On the other hand, raw wastewater is not clean enough all the time for transit-time measurement. Other wastewater-related applications are equally problematic, as the solids concentration can be too high for either transit-time or Doppler flow meters to work properly.

The use of multi-path flow meters in raw wastewater and storm water applications is common, while Doppler or cross-correlation hybrid designs are most often used to measure activated sludge and digested sludge flows.

For mining slurries, Doppler flow meters typically work well. Among the few problem applications are those in HDPE pipe, because the pipe wall flexes enough to change the diameter of the measurement area. This affects the accuracy of the meter.

Multi-path, transit-time flow meters also measure stack gas flows in power-plant scrubbers, even in very large diameter stacks.

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