TLO 3 Fluid FLOW Measurement PDF

Preview

Summary

Warning: TT: undefined function: 32 TLO-3: FLUID FLOW MEASUREMENT MEETINGS: 1-To name different types of flowmeters used in industry. To distinguish the constructional differences between orifice meter and ventury meter. To understand the basic principle of operation of an obstruction type flowmeter...

Description

CE 413: HYDRAULICS TLO-3: FLUID FLOW MEASUREMENT

22 MEETINGS:

3.0. FLUID FLOW MEASUREMENT Accurate measurement of flow rate of fluids is an essential requirement for maintaining the quality of industrial processes. In fact, most of the industrial control loops control the flow rates of incoming fluids in order to achieve the control objective. As a result, accurate measurement of flow rate is very important. Needless to say, there could be diverse requirements of flow measurement, depending upon the situation. It could be volumetric or mass flow rate, the medium could be gas or liquid, the measurement could be intrusive or nonintrusive, and so on. As a result there are different types of flow measuring techniques that are used in industries. Flow is classified into open channel flow and closed conduit flow. Open channel flow occur when the flowing stream has a free or unconstrained surface open to the atmosphere. Flows in canals or vented pipelines - like drain and sewers - which are not flowing full, are typical examples. In open channel flow the force causing the flow the force of gravity on the fluid. A progressive fall or decrease in the water surface elevation occurs as the flow moves downstream. Closed conduit flow occurs when the flow is caused by a pressure difference in the conduit. Flow in water supply pipes or district heating pipes are typical examples. The flow rate depends mainly on the pressure difference between the ends, the distance between the ends, the area of the conduit and the hydraulic properties of the conduit - like the shape, roughness and restrictions like bends. 3.1. FLOW METERING PRINCIPALS 

Obstruction type ( differential pressure or variable area)



Inferential (turbine type)

 

Electromagnetic Positive displacement (integrating)



Fluid dynamic (vortex shedding)



Anenometer



Ultrasonic



Mass flowmeter (Coriolis)

https://www.engineeringtoolbox.com/flow-meters-d_493.html

To name different types of flowmeters used in industry. To distinguish the constructional differences between orifice meter and ventury meter. To understand the basic principle of operation of an obstruction type flowmeter. To explain the basic principles of operation of turbine type flowmeter and electromagnectic flowmeter. To develop a schematic block diagram for signal conditioning circuit for a typcial flowmeter.

1-5

CE 413: HYDRAULICS TLO-3: FLUID FLOW MEASUREMENT

23 MEETINGS:

3.2. VENA-CONTRACTA Consider an orifice is fitted with a tank. The liquid particles, in order to flow out through the orifice, move towards the orifice from all directions. A few of the particles first move downward, then take a turn to enter into the orifice and then finally flow through it.

3.3. HYDRAULIC COEFFICIENTS There are numerous number of devices used to measure the flow of fluids. In any of these devices, the Bernoulli’s Energy Theorem is greatly utilized, and additional knowledge of the characteristics and coefficients of each device is important. In the absence of reliable values and coefficients, a device should be calibrated for teh expected operating conditions. 3.4. COEFFICIENT OF DISCHARGE, Cd The coefficient of discharge is the ratio of the actual discharge through the device to the ideal or theoretical discharge which would occur without losses. It is also referred to as the ratio of mass flow rate at nozzle's discharge edge to the standard nozzle which enlarges an exact working fluid maintained at the similar initial conditions and pressures.

Cd 

actual discharge Q  theoretica l discharge QT

Q AV Q Cd  A 2gh

Cd 

The actual discharge may be accomplished by series of observation, usually by measuring the total amount of fluid passing through the device for a known period. The theoretical value can be accomplished using the Bernoulli’s Theorem neglecting losses.

To name different types of flowmeters used in industry. To distinguish the constructional differences between orifice meter and ventury meter. To understand the basic principle of operation of an obstruction type flowmeter. To explain the basic principles of operation of turbine type flowmeter and electromagnectic flowmeter. To develop a schematic block diagram for signal conditioning circuit for a typcial flowmeter.

1-5

CE 413: HYDRAULICS TLO-3: FLUID FLOW MEASUREMENT

24 MEETINGS:

It has no dimensions and depends directly on the rate of flow and velocity of working fluid. It is symbolized by Cd and its value is different for each fluid depending on the kind of measurement of flow. In nozzle flow measurement, the efficiency of Cd is higher when compared to the flow measurement at the orifice. The discharge coefficient is raised by increasing the overall pressure ratio and reducing the convergence semi angle. Also, the range of Cd is commonly superior in supercritical series. 3.4. PROPERTIES OF COEFFICIENT OF DISCHARGE • It is a non-dimensional quantity which differs directly as the flow of discharge. • The value of Cd is high in nozzle measurement of flow, whereas its value is small in orifice measurement of flow. • It can be increased by lowering the convergent angle in flow measurement. It can also be increased when the pressure of working fluid is enhanced.

3.5. COEFFICIENT OF VELOCITY, Cv The coefficient of velocity is the ratio of the actual mean velocity (at the vena contracta) to the ideal or theoretical velocity which would occur without any losses.

Cv 

actual velocity at vena contracta v  vT theoretica l velocity of the jet

The difference between the velocities is due to friction of the orifice. The value of Coefficient of velocity varies slightly with the different shapes of the edges of the orifice. This value is very small for sharp-edged orifices. For a sharp edged orifice, the value of increases with the head of water. * The theoretical velocity of jet at vena-contracta is given by the relation,

v  2gh , where h is the head of water at vena contracta.

3.6. COEFFICIENT OF CONTRACTION, Cc The coefficient of contraction is the ratio of the actual area of the contracted section of the stream or jet to the area of the opening through which the fluid flows.

Cc 

area of stream or jet at vena contracta a  area of opening A

To name different types of flowmeters used in industry. To distinguish the constructional differences between orifice meter and ventury meter. To understand the basic principle of operation of an obstruction type flowmeter. To explain the basic principles of operation of turbine type flowmeter and electromagnectic flowmeter. To develop a schematic block diagram for signal conditioning circuit for a typcial flowmeter.

1-5

CE 413: HYDRAULICS TLO-3: FLUID FLOW MEASUREMENT

25 MEETINGS:

3.7. COEFFICIENT OF RESISTANCE, Cr The ratio of loss of head in the orifice to the head of water available at the exit of the orifice is known as coefficient of resistance.

Cr 

Loss of head in the orifice Head of water

The loss of head in the orifice takes place, because the walls of the orifice offer some resistance to the liquid as it comes out. The coefficient of resistance is generally neglected, while solving numerical problems.

3.8. RELATIONSHIP BETWEEN Cd, Cv, AND Cc Actual discharge,

Q  C x QT  discharge coefficien t Q  actual area (actual velocity)  av Incorporating Cv and Cc

C  C c x Cv 3.9. ORIFICE COEFFICIENTS

To name different types of flowmeters used in industry. To distinguish the constructional differences between orifice meter and ventury meter. To understand the basic principle of operation of an obstruction type flowmeter. To explain the basic principles of operation of turbine type flowmeter and electromagnectic flowmeter. To develop a schematic block diagram for signal conditioning circuit for a typcial flowmeter.

1-5

CE 413: HYDRAULICS

26 MEETINGS:

TLO-3: FLUID FLOW MEASUREMENT

3.10. HEAD LOST The head lost through Venturi meters, orifices, tubes and nozzles may be expressed as:

 1    A2  1   HL   2    Cv  1   A1

   

2

 v2  2g 

If the orifice or nozzle takes of directly from a tank where A1 is very much greater than A2, then the velocity of approach is negligible, making the equation:

 1  v2  HL   2   C v  1  2g *v = actual velocity 3.11. VENTURI METER http://www.mechanicalbooster.com/2016/06/venturimeter-principle-construction-working-dia gram.html

Venturi meter is an instrument used in measuring the discharge through pipes. To name different types of flowmeters used in industry. To distinguish the constructional differences between orifice meter and ventury meter. To understand the basic principle of operation of an obstruction type flowmeter. To explain the basic principles of operation of turbine type flowmeter and electromagnectic flowmeter. To develop a schematic block diagram for signal conditioning circuit for a typcial flowmeter.

1-5

CE 413: HYDRAULICS TLO-3: FLUID FLOW MEASUREMENT

27 MEETINGS:

3.12. MAIN PARTS OF A VENTURIMETER

1. A short converging part: It is that portion of the venturi where the fluid gets converges. 2. Throat: It is the portion that lies in between the converging and diverging part of the venturi. The cross section of the throat is much less than the cross section of the converging and diverging parts. As the fluid enters in the throat, its velocity increases and pressure decreases. 3. Diverging part: It is the portion of the venturimeter (venturi) where the fluid gets diverges.

To name different types of flowmeters used in industry. To distinguish the constructional differences between orifice meter and ventury meter. To understand the basic principle of operation of an obstruction type flowmeter. To explain the basic principles of operation of turbine type flowmeter and electromagnectic flowmeter. To develop a schematic block diagram for signal conditioning circuit for a typcial flowmeter.

1-5

CE 413: HYDRAULICS TLO-3: FLUID FLOW MEASUREMENT

28 MEETINGS:

3.13. VENTURIMETER CONSTRUCTION

1. Short converging part: It is a tapered portion whose radius decreases as we move forward. 2. Throat: It is middle portion of the venturi. Here the velocity of the fluid increases and pressure decreases. It possesses the least cross section area. 3. Diverging part: In this portion the fluid diverges. 3.14. WORKING OF A VENTURIMETER The venturimeter is used to measure the rate of flow of a fluid flowing through the pipes. Lets understand how it does this measurement step by step.  Here we have considered two cross section, first at the inlet and the second one is at the throat. The difference in the pressure heads of these two sections is used to calculate the rate of flow through venturimeter.  As the water enters at the inlet section i.e. in the converging part it converges and reaches to the throat.  The throat has the uniform cross section area and least cross section area in the venturimeter. As the water enters in the throat its velocity gets increases and due to increase in the velocity the pressure drops to the minimum.  Now there is a pressure difference of the fluid at the two sections. At the section 1(i.e. at the inlet) the pressure of the fluid is maximum and the velocity is minimum. And at the section 2 (at the throat) the velocity of the fluid is maximum and the pressure is minimum.  The pressure difference at the two section can be seen in the manometer attached at both the section.  This pressure difference is used to calculate the rate flow of a fluid flowing through a pipe.

To name different types of flowmeters used in industry. To distinguish the constructional differences between orifice meter and ventury meter. To understand the basic principle of operation of an obstruction type flowmeter. To explain the basic principles of operation of turbine type flowmeter and electromagnectic flowmeter. To develop a schematic block diagram for signal conditioning circuit for a typcial flowmeter.

1-5

CE 413: HYDRAULICS

29

TLO-3: FLUID FLOW MEASUREMENT

MEETINGS:

3.15. EXPRESSION FOR THE RATE OF FLOW THROUGH VENTURIMETER

Q act  C d

a1a2 a12  a22

2gh

3.16. Exercise. VENTURI METER 1. A horizontal 150mm diameter pipe gradually reduces its section to 50mm diameter, subsequently enlarging into 150mm section. The pressure in the 150-mm pipe at a point just before entering the reducing section is 140kPa and in the 50mm section at the end of the reducer, the pressure is 70kPa. If 600mm of the head is lost between the points where the pressures are known, compute the rate of flow of water through the pipe. 2. A 150mm diameter horizontal Venturi meter is installed in a 450mm diameter water main. The deflection of mercury in the differential manometer connected from the inlet to the throat is 375mm. a) Determine the discharge neglecting head lost b) Compute the discharge if the head lost from the inlet to the throat is 300mm of water, and c) What is the meter coefficient? 3. A vertical Venturi meter, 150mm in diameter is connected to a 300-mm diameter pipe. The vertical distance from the inlet to the throat being 750mm. If the deflection of mercury in the differential manometer connected from the inlet to the throat is 360mm, determine the flow of water through the meter if the meter coefficient is 0.68. Determine also the head lost from the inlet to the throat. 4. Neglecting losses, calculate the discharge through the Venturi meter shown:

To name different types of flowmeters used in industry. To distinguish the constructional differences between orifice meter and ventury meter. To understand the basic principle of operation of an obstruction type flowmeter. To explain the basic principles of operation of turbine type flowmeter and electromagnectic flowmeter. To develop a schematic block diagram for signal conditioning circuit for a typcial flowmeter.

1-5

CE 413: HYDRAULICS

30

TLO-3: FLUID FLOW MEASUREMENT

MEETINGS:

3.17. NOZZLE A nozzle is a device designed to control the direction or characteristics of a fluid flow (especially to increase velocity) as it exits (or enters) an enclosed chamber or pipe. A nozzle is often a pipe or tube of varying cross sectional area, and it can be used to direct or modify the flow of a fluid (liquid or gas). Nozzles are frequently used to control the rate of flow, speed, direction, mass, shape, and/or the pressure of the stream that emerges from them. In a nozzle, the velocity of fluid increases at the expense of its pressure energy.

The discharge through a nozzle can be calculated using the equation.

Q  CAn 2gH Where:

H An

= =

total head at base of nozzle area at the nozzle tip

To name different types of flowmeters used in industry. To distinguish the constructional differences between orifice meter and ventury meter. To understand the basic principle of operation of an obstruction type flowmeter. To explain the basic principles of operation of turbine type flowmeter and electromagnectic flowmeter. To develop a schematic block diagram for signal conditioning circuit for a typcial flowmeter.

1-5

CE 413: HYDRAULICS TLO-3: FLUID FLOW MEASUREMENT

3.18.

31 MEETINGS:

Exercises. NOZZLES

1. A 60mm fire hose discharges water through a nozzle having a jet diameter of 25mm. The lost head in the nozzle is 4% of the velocity head in the jet. If the gage pressure at base of nozzle is 400kPa : a) Compute the discharge b) What is the maximum horizontal range to which the stream can be thrown neglecting air resistance? (besavilla #182) 2. A 60mm fire hose discharges a 30mm jet. If the head lost in the nozzle is 2m, what gage pressure must be maintained at the base of a nozzle to throw a stream to a vertical height of 30m, neglecting air resistance? (besavilla #183) 3. The pipeline shown takes water from a reservoir and terminates in a nozzle having a jet diameter of 50mm. With the pipe losses shown in the figure, and assuming a nozzle loss of 8% of the velocity head in the jet, compute the discharge and make a table showing the elevation head, velocity head, pressure head, and total head at each of the six stations. (besavilla #189)

3.19. PITOT TUBE Although the pitot tube is one of the simplest flow sensors, it is used in a wide range of flow measurement applications such as air speed in racing cars and Air Force fighter jets. In industrial applications, pitot tubes are used to measure air flow in pipes, ducts, and stacks, and liquid flow in pipes, weirs, and open channels. While accuracy and rangeability are relatively low, pitot tubes are simple, reliable, inexpensive, and suited for a variety of environmental conditions, including extremely high temperatures and a wide range of pressures.

To name different types of flowmeters used in industry. To distinguish the constructional differences between orifice meter and ventury meter. To understand the basic principle of operation of an obstruction type flowmeter. To explain the basic principles of operation of turbine type flowmeter and electromagnectic flowmeter. To develop a schematic block diagram for signal conditioning circuit for a typcial flowmeter.

1-5

CE 413: HYDRAULICS TLO-3: FLUID FLOW MEASUREMENT

32 MEETINGS:

3.20. HOW DOES A PITOT TUBE WORK Pitot tubes were invented by Henri Pitot in 1732 to measure the flowing liquid or air velocity. Basically a differential pressure (d/p) flowmeter, a pitot tube measures two pressures: the static and the total impact pressure. The static pressure is the operating pressure in the pipe, duct, or the environment, upstream to the pitot tube. It is measured at right angles to the flow direction, preferably in a low turbulence location.

3.21. FORMULA FOR VELOCITY As a particle approaches the point of stagnation, its velocity is gradually retarded to zero.

v  2gh

To name different types of flowmeters used in industry. To distinguish the constructional differences between orifice meter and ventury meter. To understand the basic principle of operation of an obstruction type flowmeter. To explain the basic principles of operation of turbine type flowmeter and electromagnectic flowmeter. To develop a schematic block diagram for signal conditioning circuit for a typcial flowmeter.

1-5

CE 413: HYDRAULICS TLO-3: FLUID FLOW MEASUREMENT

33 MEETINGS:

3.22. Exercises. PITOT TUBE 1. A glass tube with a 90o bend is open at both ends. It is inserted into a flowing stream of oil (sp gr = 0.90) so that one of the opening is directed upstream and the other is directed upward. If the oil inside the tube is 50mm higher than the surface outside, determine the velocity measured by the tube. 2. A Pitot tube (C=1.0) is used to measure air speeds. With water in the differential manometer and gage difference of 75mm, calculate the air speed using ρair = 1.16 kg/m 3. 3. Air (ω=12N/m3) is flowing through a system shown. If oil (sp gr. = 0.827) shows a deflection of 80mm, calculate the flow rate neglecting head lost.

3.23. GATES A gate is an opening in a dam or other hydraulic structure to control the passage of water. It ha...