9 Flow Measurements

Vinay Gupta

    Learning Objectives

In this module we will study about flow measurement.

• Measurement of flow of gases and liquids falls into the category of non-electrical quantities.
• Here we will study about 4 different types of flow meters
• They are Turbine Flow meter, Electromagnetic Flow meter, Hot Wire Anemometer and Ultrasonic flow meter.

    Introduction

Flow measurement is the process of measuring fluid flow through pipes and duct in a plant or an industry. In day-to-day application flow measurements data is collected from water or gas service meters or in gasoline pumping stations. There is wide range of flow meters that work on various operating principle, suited to gas, liquid or slurry media and designed according to the containment such as pipes or open ducts. A measurement can be performed via positive- displacement flow meters. They accumulate a fixed volume of liquid and then calculate the number of times the fixed volume is filled to measure flow. Other methods rely on the forces produced when flow stream passes through known constriction and flow rate may also be estimated by measuring the velocity of fluid over a known area.

In this session we will study about most common methods used for flow measurements.

Turbine Flow Meters

The Turbine Flow Meters are most highly developed non-friction displacement type of flow meter. Here an axially mounted turbine wheel (freely rotating motor) is placed in the path of the fluid stream. Here flowing fluid impart force on to the blade surface and rotor is set into motion at an angular velocity proportional to flow rate of the fluid. The design of the turbine flow meter is shown in figure 1.

Figure 1. Turbine Flow Meter

By reducing the bearing friction, the device is designed to give a linear output with minimum losses. The speed is measured with great accuracy by counting the rate at which blade pass a fixed point with help of magnetic proximity pickup that generates voltage pulses. By feeding the pulses generated to an electronic pulse rate meter, the total flow rate can be estimated by counting total number of pulses in fixed time intervals. With digital meter such measurements can be performed with great accuracy and for analog signals the pulses can be fed to frequency to voltage converter.

Turbine flow meters are most commonly used types of flow meter. They provide direct means to measure both liquid and gas flow rates. They are used in remote monitoring and aircraft applications. The linearity of such a device is very good i.e. +-0.25% of the full scale. The dynamic response of the device also good and has a typical time constant of approximately 10 ms. Full scale flow rates from 1 x 10-6 to 1.2 m3/s can be achieved with liquids and gases.

The main drawback of such system is damage with particles suspended in the fluid. When blade gets damaged, the system requires recalibration. Length of the pipe upstream should 15 times of that diameter. This helps to maintain required flow pattern for accurate estimation of flow rate. These devices are expensive and are useful for fluids of limited viscosity. The system has to be recalibrated with any change in the viscosity or variation in temperature of the fluid.

Turbine flow meters have poor accuracy at low flow rates due rotor or bearing drag that slows the turbine wheel. They should not be operated at high velocities, that can lead to premature bearing wear or damage can occur. Caution is to be taken when measuring non-lubricating fluids because bear wearing can lead to inaccuracies in the measurement. Abrupt switching from gas flow to liquid should be avoided. This can put considerable mechanical stress on the system and damage the flowmeter. These conditions occur when pipes are being filled and under slug flow conditions. Slug flow conditions exists when large bubbles of gas are formed in the fluid flowing through pipes. These gas bubbles behave like lumps of particle moving with fluid stream.

Electromagnetic Flow-meters

The operating principle of electromagnetic flow meters is based on Faraday’s law of electromagnetic induction. It states that an EMF of e volts gets induced in conductor of length l meters when it moves at a transverse velocity of v m/s across the magnetic field of B Wb/m2. The induce EMF in the conductor is given by the following expression

e = Blv volts ———- (1)

The induced voltage depends on the rate at which conductor moves through the magnetic field. The magnitude of the induced voltage acts as an indicator of the flow rate of liquid.

Figure 2. Electromagnetic Flow Meter

   The schematic of the electromagnetic flow meter is shown in the figure 2. Its tube is made up of non-conducting material with 2 electrodes mounted opposite each other on the tube wall. Outside of the tube there are magnets with its magnetic field right angle to the electrodes. The fluid flowing through the tube should have conductive at least of 10-3 mho(siemens) per meter. When a conductive fluid flows through the insulated tube via magnetic field, voltage gets induced across the electrodes.

The induced voltage is directly proportional to (i) the magnetic field strength, B (ii) the distance between the electrodes, d and (iii) average velocity of conductive fluid flowing through non-conducting tube. Both DC and AC magnetic fields can be used for such devices but with DC fields have danger of electrolytic polarization at the electrodes. This can lead to fluid velocity profile distortion. The magnetic field used here is usually of 50 Hz and magnitude of 75 X 104 A/m. The pipes used for flow measurements should be of non-magnetic materials in order to allow magnetic field to penetrate the fluid and also prevent short circuit due to induced voltage.

The main advantage electromagnetic flow meter is that they do not obstruct flow that may lead to drop in pressure. This type of flow meters have no moving parts that cause friction and insensitive to viscosity, density and flow disturbances. It has a wide linear range is of 10:1 and is independent of the properties of the fluid except for the electrical conductivity. The only condition for measurement is that pipe should always be full. These devices are suitable for bi-directional measurements and are well suited for slurries, corrosive and solid contaminated liquids.

Hot-Wire Anemometer

An anemometer is device that is used for measuring of speed of wind. They measure average velocity, velocity fluctuations and are used in aerodynamic research and allied fields. The Hot Wire Anemometer are very sensitive and are responsive towards high frequency variations in fluid flow rate.

A Hot Wire Anemometer has a small length of very fine heated metal wire supported on a probe. The basic design of the Hot Wire Anemometer is shown in figure 3. This probe is exposed to fluid whose flow-rate measurements are to be conducted.

Figure 3. Hot Wire Anemometer

The wire is heated by passing electric current through it and during the flow measurement heat gets dissipated through convection. This dissipation of heat is in addition to losses through radiation and conduction along the wire support. This results in loss in temperature along with the wire resistance. Equilibrium temperature of the wire is reached when the heat produced on the current flow i.e. I2R becomes equal to convection heat loss from the surface of the wire. Any losses due to radiation and conduction are negligible. There are types of anemometer one operates on the principle of constant current and other constant temperature type.

Platinum wire of diameter ranging from 0.005 to 0.3 mm and length of 7 to 10 mm is used as thermo-element. Its dimensions depend on the diameter of the flow channel. For a small diameter one gets large resistance per unit length with lesser inertia, but it is also less capable to sustaining large fluid pressure. Length of the wire is half of the diameter of the pipe. A large l/d ratio makes sure that the wire is an endless cylindrical body.

The sensitive of the device depends on the temperature difference between the hot wire and the fluid. It increases with increase in temperature but higher limit of temperature is fixed by material characteristics. Under normal circumstances the higher limit of temperature is 400-500 OC on account of increased radiation losses. In constant current type system, the heating wire element is connected to bridge circuit in order to maintain a constant level of heat. In this system, wire temperature gets adjusted by itself due to convection loss until equilibrium is obtained. As the convection film coefficient is a function of velocity, the wire temperature at equilibrium is a measure of velocity. The rheostat used in the bridge circuit is of the resistance of the order of 2 kΩ . The resistance in arms of the bridge is equivalent to that of a heater wire.

In constant temperature type, current is adjusted to keep the wire temperature constant. Here current required to maintain wire temperature constant is the measure of flow velocity. The current flowing through the wire is measured by voltage drop across the standard resistor connected in series with heater wire element.

For measurement of steady or average velocities, the constant current mode is usually preferred.

Ultrasonic or Acoustic Flow Meters

This type of flow meters consists of two piezoelectric crystals in liquid or gas separated by a distance. One of the crystal works as a transmitter and other acts as a receiver. There are two types of flow meters; first one measures the phase shift between the oscillations directed downstream and upstream alternatively. The second one makes measurement of separation of frequency of oscillations directed downstream and upstream simultaneously. The schematic of ultrasonic flow meter is shown in the figure 4.

Figure 4. Schematic Diagram of Ultrasonic Flow-meter Operating on Doppler Effect.

    The ultrasonic flow meter works on the principle of Doppler effect. The two piezo-crystal A and B work as transmitter and receiver alternatively. Ultrasonic signals get transmitted between them through the liquid. Crystals are connected to oscillator via switch S that alternates the supply between the two crystal i.e. using them as transistor or receiver alternately. The detector is phase sensitive device connected to switch S, that measures the transit time from upstream to downstream or vice-versa.

Velocity of sound propagation C in a medium is in m/s, v is the linear velocity of the fluid flow in m/s & d is the distance in meter between the 2 piezo-crystals A and B. The transit time in the direction of flow i.e. from upstream to downstream is given by

as the fluid flow velocity << C, the acoustic velocity.

The linearity of the system is good within 2% and on the lower side linear flow velocity upto 1 mm/s can be easily measured. Dynamic response of the flow meter is limited by the switching frequency. Though, uncertainty in the value of C can be a source of an error. Flow channel walls are designed to not permit any acoustic transmission. To this purposeplastic based materials are preferred to construct the channel walls. For different groups of fluids, probes of different frequencies are employed. The normal range of frequencies used is between 200 to 5,000 kHz.

Summary

In this module we studied about measurement of non-electrical quantities like fluid flow. Here we looked into various types of flow meters. We started with purely mechanical designs that were used in Turbine Flow meters. Thereafter we learned about electromagnetic and hot wire anemometers. Most recent ones are ultrasonic flow-meters whose applications are limited to industrial and large scale plants only.

    References :-

1. Electrical and Electronic Measurements and Instrumentation, Sawhney A. K., Dhanpat Rai & Sons, Reprint 1985
2. Measurements and Instrumentation, Bakshi U.A., Bakshi A.V., Technical Publications, 2009
3. Principles of instrumental analysis, Skoog, Douglas A., F. James Holler, and Stanley R. Crouc,. Cengage learning, Edition 2017
4. Instrumentation, measurement and analysis. Nakra, B.C. and Chaudhry, K.K., Tata McGraw-Hill Education, 2003.
5. Measurement and instrumentation: theory and application, Morris, A. S., & Langari, R. , Academic Press, 2012.