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INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors Flow sensors This chapter will explain the different types of flow sensors and the theory of operation of each type of sensor. The flow sensor can be classified into 2 types. They are:
Intrusive flow sensor – The sensor disturbs the flow of fluid that it is measuring.
Non-intrusive flow sensor – The sensor can measure the flow of the fluid without disturbing it.
Flow technology Fluid is the term that describes any substance that flows. Liquids such as water or hydraulic fluid and gasses such as oxygen or nitrogen are all considered fluids since they flow. The flow rate of a fluid as it flows through a pipe can be calculated by the formula Q = V x A where Q = liquid flow through a pipe V = average velocity of the flow A = cross-sectional area of the pipe Example : Determine the flow of hydraulic fluid through a 2 in. diameter pipe that has an average velocity of 60 in. per second. Solution: A = π r2 A = 3.14 × (1 in.) A = 3.14 sq in.
2
Q = V×A Q = 60 psi × 3.14 in. Q = 118.4 cu in. / sec From the diagram above we can see that the fluids is swirling as it flows. The swirling actions tend to create opposition to the flow. Calculating Flow from Pressure-Drop measurement Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors
One of the easiest ways to determine the amount of flow is to place an obstruction in the flow such as an orifice plate to create a pressure drop. These types of sensors are categorized as obstructive flow sensors.
Side view Front view of orifice plate
Test port P1
Test port P2
Fluid Flow
Orifice plate Flange plate
The figure above shows a diagram of the orifice plate that is mounted in a pipe. When fluid begins to flow in the pipe, a pressure is created on the side identified as P1. When the fluid flow through the orifice, a lower pressure is created on the opposite side of the orifice plate identified as P2. The simplified formula using the pressure drop is: Q = k Where
P1 − P 2
Q = flow in gallons/minute (gpm) k = is the constant that is determined by the
orifice plate.
P1 = is higher pressure in front of the orifice P2 = is lower pressure behind the orifice
Examples of Flow Meters that utilize a pressure drop 1. Venturi Flow meter
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors A venture is a point in a pipe that has been narrowed so the flow is restricted slightly. In figure below notice that a high pressure port is provided in front of the point where the venture is narrowed down and a low pressure port is provided directly after the point where the pipe is narrowed down. Test port P1 Test port P2
Fluid Flow
The high pressure port is provided to sample the increased fluid pressure where it will increase slightly because of the restriction caused by the venture, and the low pressure port is provided to sample the pressure as it drop after it flows past the restriction caused by the venture. Additional pressure ports are provided on both sides of the restriction in larger venturies to allow an average pressure to be measured. The venture is widely used because it has no moving parts and the small amount of restriction it causes to create the pressure drop does not disturb the fluid flow too much. This means that the venture can handle larger volumes of flow than most other types of flow meters.
2. Flow Tubes The flow tube is similar to the venture in that it places a restriction in the flow that creates a pressure drop. Figure below shows an example of a flow tube that is located in a pipe.
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors Test port P1
Fluid Flow
Test port P2
Flow Nozzle
From this diagram we can see that the flow tubes looks like a short piece of pipe with a thick wall. The front face of the pipe is rounded so that fluid is disturbed as little as possible when it enters the tube. The diameter of the flow tube is approximately half the diameter of the original pipe so that a pressure drop is created as the flow is diverted through the flow tube. A pressure tap is provided prior to the flow tube to measure the higher pressure and directly after the opening of the flow tube to measure the lower pressure. The difference in the pressure must be measured by a pressure differential sensor 3. Pitot Tube The pitot tube is a device that has two tubes that are placed in a fluid flow to sense the impact pressure and the static pressure used to determine the amount of pressure drop.
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors Test port P2
Test port P1
Test port P1
Fluid Flow Static tube
In the first example two tubes are connected side by side. One of the tubes has a hole in it that faces the fluid flow (impact tube), while the other tube has a hole in it that faces away from the fluid flow (static tube). The impact tube measures the higher pressure (impact pressure), and the static tube measures the lower pressure (static pressure) as fluid flows past the pitot tube.
Test port P2
Test port P1
Test port P1 Static tube Fluid Flow
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors The second example shows the tubes that are mounted one inside the other. The inside tube measures the impact pressure, and the outside tube measures the static pressure. In both examples the ends of the pitot tube that are outside the pipe are connected with plastic tubing to a sensor or instrument that can measure a very small pressure differential. The difference between the impact pressure and the static pressure is very small so a nanometer or ultralow differential pressure sensor must be used to measure the pressure differential. The manometer can be used if only a visual indication is needed, and a differential pressure sensor or transducer is required if the pressure drop is converted to an electrical signal.
4. Elbow Meters Another to create a pressure drop in a fluid system is to use an existing elbow in the piping system. It has been determined that the pressure of fluid will show a slight pressure differential as it passes through an elbow. The fluid that flows near the inside radius of the elbow will have a slightly lower pressure than the fluid that flows on the outer radius of the elbow. Figure below shows one of the examples of this type of meter.
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors
Test port P1
Fluid Flow
Test port P2
Two low pressure ports are provided on the inner side of the elbow to provide the lower pressure reading, and two ports are provided on the outer side of the elbow to provided higher pressure reading so that a better average of each pressure is determined. The amount of pressure difference from this type of sensor is very small and a manometer or ultralow differential pressure sensor must be used to sense it. The pressure difference is used in a calculation to determine fluid flow. The amount of pressure difference will increase when the fluid flow increases, and the pressure difference will decrease when the fluid flow decreases.
5. Weirs
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors Another way to create a pressure difference in a flow of fluid is to place a weir directly in the flow. A weir is a narrowing of an open channel to create an obstruction that is placed in the flow to cause a pressure drop as fluid flow through it. Weir
P1
P2
Fluid Flow
P1
P2 Weir
When the fluid flow increases, the pressure drop through the weir will increase. One example of this type of application would be a weir placed in a canal used to move cooling water to a pond. Many industries use ponds for cooling water or to store waste water while it is being treated. It is important to get an estimate of the amount of water that is flowing into the pond if water-treatment chemicals are added, so a weir is used to measure flow. The pressure of the water when it is obstructed by the weir will cause the velocity of flow to change slightly, which will cause a slight pressure increase. When the water exits the weir, its pressure will decrease slightly. This small amount of pressure difference can be used to calculate flow. Weirs are also used to extensively in irrigation ditches to measure the amount of water flow. In these applications the amount of flow is totalized so the water can be sold.
Velocity Flow Meters Velocity flow meters use the change in velocity that occurs when fluid flow changes to measure the amount of flow. Examples of the velocity flow meter are the paddlewheel flow meter, the turbine flow meter, the vortex flow meter, the electromagnetic flow meter, and the ultrasonic (Doppler) flow meter.
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors 1. Paddlewheel Flow Meters A paddlewheel flow meter uses a completely different way to determine the amount of fluid flow than the pressure differential flow meter. This type of sensor has a paddlewheel that is placed directly in the fluid flow so that it can rotate freely as fluid passes it. The fastest the fluid flows past it, the faster the wheel rotate. Each paddle or web of paddlewheel has a magnet mounted in it so that a sinusoidal waveform is produced when it passes the detector mounted in the head of the sensor. A frequency-to-voltage converter is used to convert the sinusoidal signal to a variable-voltage signal.
Figure above shows the paddlewheel flow meter mounted in a pipe. It is important to understand that the paddlewheel flow meter is mounted in a straight run of piping where the flow will be laminar. The paddlewheel is a low-cost sensor used in applications that do not require a high degree of accuracy.
2. Turbine Flow Meters Turbine flow meters are very similar to paddlewheel flow meter except the turbine flow meter is much more accurate. From the diagram below we can see that the turbine wheel is mounted so that it is directly in the flow. Each of the vanes of the turbine wheel has a Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors magnet mounted in it so that when the vane spins under the magnetic pickup, an electric pulse is generated. When fluid starts to flow, the wheel will begin to rotate. When the flow increases, the wheel will rotate more quickly and the number of electric pulses will increase. The electric pulses can be averaged over time to provide a flow rate, or they can be totalized to determine the total flow.
3. Vortex Flow Meter The vortex flow meter is also a velocity-type flow meter that places a flow element in the flow stream that is not streamline. When the flow stream strikes the flow element, a series of vortices is produced (shedded). For this reason, this type of flow meter is sometimes called a vortex shedding flow meter. When a vortex is produced, it causes the fluid to create a swirling motion as it moves. A very sensitive electronic detector can detect the presence of the vortices. The number of vortices that are produced is directly proportional to the flow rate. This type of sensor is very accurate and it can also be used when the fluids has suspended solids (slurries) moving in the flow. The number of vortices that are detected can be averaged to produce a flow rate, or they can be added to produce a total flow. A frequency-to-voltage circuit and amplifier must be used to increase the small-pulse signal from the electronic detector. It is also important to understand that the vortex type flow meter work best in very turbulence flow. Figure below shows the location of the flow element in a pipe. Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors
vortex Flow element
Flow
4. Electromagnetic Flow Meter Another type of flow meter uses the operating principle of creating an electrical field in the fluid and then measures the strength of the field. This type of flow meter takes advantage of the principle of electromagnetic induction. As you know, a voltage can be induced in a conductor when it passed through a magnetic field. In this case the fluid must electrically conductive so that it will accept the magnetic field and act as conductor.
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors
Figure above shows an electromagnetic flow meter. The meter has two major parts. The first part is a set of coils that creates the magnetic field. When the fluid flows past these coils, it will act like an electrical conductor and will hold and electrical charge. The second part of the flow meter is a set of electrodes that is used as the detector. The detector acts like a voltmeter and measures the intensity of the electrical charge. The stronger the fluid flow, the stronger the electrical charge. This type of flow meter is extremely useful in that it does not need to place any disturbance in the fluid flow, and it is also useful to measure the flow of very corrosive fluids. Early versions of this type of meter consumed large amounts of electrical power, but newer versions use pulse technology to limit the current supplied to the magnetizing coils so that the power consumption is reduced. 5. Ultrasonic Flow Meters
The ultrasonic flow meter uses Doppler meters to measure the shift of a frequency signal that is sent into the liquid flowing through a pipe. From the diagram in Figure above we can see that the transmitting element injects a signal with a given frequency into the fluid flow. Bubbles in the fluid or any suspended solids in the fluid reflect the signal back to a receiver. The receiving element called Doppler meter is placed a short distance downstream and it detects the frequency as the fluid flows past it. A special circuit called a time-of-travel meter, measure the time delay or shift in the frequency that is caused by the fluid flow. The faster the fluid flows, the more the frequency is shifted. This type of flow meter is useful because it can measure the flow without creating any obstruction. It is important to understand that some amount of suspended solids or bubbles must be present in the flow to get the best reflection of the signal. This type of meter is also used extensively as a portable flow meter. The portable flow meter is used in troubleshooting to detect flow in complex hydraulic systems. When a hydraulic systems start to fail, it generally leaks fluid past an inoperable valve back to the reservoir. Since the hydraulic piping is made of metal or hoses, it is difficult to tell how much fluid flow is being bypassed, so an ultrasonic flow meter can be temporarily placed around
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors each segment of the hydraulic piping and the amount of flow in the main lines and the return lines to the reservoir can be determined.
Positive Displacement Flow Meters Another category of flow meters is called the positive displacement flow meter. This type of flow meter is the most accurate in that the flow is broken into segments and each segment is measured as the flow is moved. One example of a positive displacement flow meter uses a piston pump. The volume of the piston is known, and all of the fluid flows through the piston pump. This allows the total volume of fluid to be measured by counting the stroke of the piston. In example, if a piston has a volume of 0.1gal, and it makes 200 strokes per minute, the flow would be 20gpm. Another type of positive displacement flow meter uses a set of oval gears (lobes) that pump a specific volume of fluid each time the gears mesh. The faster the flow moves, the faster the gears rotate. The numbers of rotations are counted and the flow rate can be calculated because the volume of fluid that is pumped during each revolution of the lobes is known. Fig. 1067 shows a positive displacement flow meter. The nutating disk is another type of positive displacement flow meter. The nutating disk is a movable disk that is offset so that it makes a concentric circular motion each time it rotates. The housing for the disk is perfectly round, so that each time the disk rotates, its oval path will trap a specific amount of fluid. Since the volume of trapped fluid is known, the number of revolutions the disks makes can be counted and the flow rate can be calculated.
Mass Flow Meter
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors A mass flow meter is the most accurate type of flow meter designed to measure the flow of gases as well as other fluids. Two types of mass flow meters are commonly used. The Coriolis mass flow meter is so named because it uses the Coriolis phenomenon to measure mass flow instead of volumetric flow, and the other is called a thermal mass flow meter. The coriolis mass flow meter uses a U-shaped tube that is designed to vibrate up and down at its natural frequency while all of the fluid flows through it. A strong magnet is used to make the U-tube vibrate. Fig. 10-68 shows an example of the Coriolis mass flow meter in three distinct stages. When the U-tube vibrates it will naturally move up and down. When fluid is flowing through the tube, it will oppose the up and down movement, which will cause the U-tube to twist. The amount of twist will be directly proportional to the amount of flow. Sensors are located near the tube to detect the amount of twist and convert it to a usable variable-voltage signal. This type of mass flow meter is useful to measure fluids whose viscosity continually changes because they do not need to have pressure or temperature compensation. The thermal mass flow meter is usable for measuring the mass flow of gases. This type of mass flow meter uses a thermal element whose temperature changes as fluids flows past it. The amount of heat loss is directly proportional to the fluid flow. The thermal element is mounted close to the fluid flow but it does not come directly into contact with the fluid. This allows this type of mass flow meter to be used in virtually all types of applications where the density, pressure, and viscosity may change. The flow meter uses an electronic package that contains a flow analyzer, temperature compensator, and a signal conditioner to provide a linear output signal. Some Consideration of selecting Flowmeter : 1. Is the fluid a gas or liquid ? 2. Is the fluid a gas or Corrosive ? 3. Is the fluid electrically conductive or not ? 4. Does the fluid contain slurry or large solids ? 5. What is the fluid viscosity ? 6. What is the need for accuracy and repeatability ? 7. What is the cost ? 8. 9.
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors
Handout
SENSORS Chapter_Flow_Sensor
10
Intrusive flow sensor – The sensor disturbs the flow of fluid that it is measuring.
Non-intrusive flow sensor – The sensor can measure the flow of the fluid without disturbing it.
Flow technology Fluid is the term that describes any substance that flows. Liquids such as water or hydraulic fluid and gasses such as oxygen or nitrogen are all considered fluids since they flow. The flow rate of a fluid as it flows through a pipe can be calculated by the formula Q = V x A where Q = liquid flow through a pipe V = average velocity of the flow A = cross-sectional area of the pipe Example : Determine the flow of hydraulic fluid through a 2 in. diameter pipe that has an average velocity of 60 in. per second. Solution: A = π r2 A = 3.14 × (1 in.) A = 3.14 sq in.
2
Q = V×A Q = 60 psi × 3.14 in. Q = 118.4 cu in. / sec From the diagram above we can see that the fluids is swirling as it flows. The swirling actions tend to create opposition to the flow. Calculating Flow from Pressure-Drop measurement Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors
One of the easiest ways to determine the amount of flow is to place an obstruction in the flow such as an orifice plate to create a pressure drop. These types of sensors are categorized as obstructive flow sensors.
Side view Front view of orifice plate
Test port P1
Test port P2
Fluid Flow
Orifice plate Flange plate
The figure above shows a diagram of the orifice plate that is mounted in a pipe. When fluid begins to flow in the pipe, a pressure is created on the side identified as P1. When the fluid flow through the orifice, a lower pressure is created on the opposite side of the orifice plate identified as P2. The simplified formula using the pressure drop is: Q = k Where
P1 − P 2
Q = flow in gallons/minute (gpm) k = is the constant that is determined by the
orifice plate.
P1 = is higher pressure in front of the orifice P2 = is lower pressure behind the orifice
Examples of Flow Meters that utilize a pressure drop 1. Venturi Flow meter
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors A venture is a point in a pipe that has been narrowed so the flow is restricted slightly. In figure below notice that a high pressure port is provided in front of the point where the venture is narrowed down and a low pressure port is provided directly after the point where the pipe is narrowed down. Test port P1 Test port P2
Fluid Flow
The high pressure port is provided to sample the increased fluid pressure where it will increase slightly because of the restriction caused by the venture, and the low pressure port is provided to sample the pressure as it drop after it flows past the restriction caused by the venture. Additional pressure ports are provided on both sides of the restriction in larger venturies to allow an average pressure to be measured. The venture is widely used because it has no moving parts and the small amount of restriction it causes to create the pressure drop does not disturb the fluid flow too much. This means that the venture can handle larger volumes of flow than most other types of flow meters.
2. Flow Tubes The flow tube is similar to the venture in that it places a restriction in the flow that creates a pressure drop. Figure below shows an example of a flow tube that is located in a pipe.
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors Test port P1
Fluid Flow
Test port P2
Flow Nozzle
From this diagram we can see that the flow tubes looks like a short piece of pipe with a thick wall. The front face of the pipe is rounded so that fluid is disturbed as little as possible when it enters the tube. The diameter of the flow tube is approximately half the diameter of the original pipe so that a pressure drop is created as the flow is diverted through the flow tube. A pressure tap is provided prior to the flow tube to measure the higher pressure and directly after the opening of the flow tube to measure the lower pressure. The difference in the pressure must be measured by a pressure differential sensor 3. Pitot Tube The pitot tube is a device that has two tubes that are placed in a fluid flow to sense the impact pressure and the static pressure used to determine the amount of pressure drop.
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors Test port P2
Test port P1
Test port P1
Fluid Flow Static tube
In the first example two tubes are connected side by side. One of the tubes has a hole in it that faces the fluid flow (impact tube), while the other tube has a hole in it that faces away from the fluid flow (static tube). The impact tube measures the higher pressure (impact pressure), and the static tube measures the lower pressure (static pressure) as fluid flows past the pitot tube.
Test port P2
Test port P1
Test port P1 Static tube Fluid Flow
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors The second example shows the tubes that are mounted one inside the other. The inside tube measures the impact pressure, and the outside tube measures the static pressure. In both examples the ends of the pitot tube that are outside the pipe are connected with plastic tubing to a sensor or instrument that can measure a very small pressure differential. The difference between the impact pressure and the static pressure is very small so a nanometer or ultralow differential pressure sensor must be used to measure the pressure differential. The manometer can be used if only a visual indication is needed, and a differential pressure sensor or transducer is required if the pressure drop is converted to an electrical signal.
4. Elbow Meters Another to create a pressure drop in a fluid system is to use an existing elbow in the piping system. It has been determined that the pressure of fluid will show a slight pressure differential as it passes through an elbow. The fluid that flows near the inside radius of the elbow will have a slightly lower pressure than the fluid that flows on the outer radius of the elbow. Figure below shows one of the examples of this type of meter.
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors
Test port P1
Fluid Flow
Test port P2
Two low pressure ports are provided on the inner side of the elbow to provide the lower pressure reading, and two ports are provided on the outer side of the elbow to provided higher pressure reading so that a better average of each pressure is determined. The amount of pressure difference from this type of sensor is very small and a manometer or ultralow differential pressure sensor must be used to sense it. The pressure difference is used in a calculation to determine fluid flow. The amount of pressure difference will increase when the fluid flow increases, and the pressure difference will decrease when the fluid flow decreases.
5. Weirs
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors Another way to create a pressure difference in a flow of fluid is to place a weir directly in the flow. A weir is a narrowing of an open channel to create an obstruction that is placed in the flow to cause a pressure drop as fluid flow through it. Weir
P1
P2
Fluid Flow
P1
P2 Weir
When the fluid flow increases, the pressure drop through the weir will increase. One example of this type of application would be a weir placed in a canal used to move cooling water to a pond. Many industries use ponds for cooling water or to store waste water while it is being treated. It is important to get an estimate of the amount of water that is flowing into the pond if water-treatment chemicals are added, so a weir is used to measure flow. The pressure of the water when it is obstructed by the weir will cause the velocity of flow to change slightly, which will cause a slight pressure increase. When the water exits the weir, its pressure will decrease slightly. This small amount of pressure difference can be used to calculate flow. Weirs are also used to extensively in irrigation ditches to measure the amount of water flow. In these applications the amount of flow is totalized so the water can be sold.
Velocity Flow Meters Velocity flow meters use the change in velocity that occurs when fluid flow changes to measure the amount of flow. Examples of the velocity flow meter are the paddlewheel flow meter, the turbine flow meter, the vortex flow meter, the electromagnetic flow meter, and the ultrasonic (Doppler) flow meter.
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors 1. Paddlewheel Flow Meters A paddlewheel flow meter uses a completely different way to determine the amount of fluid flow than the pressure differential flow meter. This type of sensor has a paddlewheel that is placed directly in the fluid flow so that it can rotate freely as fluid passes it. The fastest the fluid flows past it, the faster the wheel rotate. Each paddle or web of paddlewheel has a magnet mounted in it so that a sinusoidal waveform is produced when it passes the detector mounted in the head of the sensor. A frequency-to-voltage converter is used to convert the sinusoidal signal to a variable-voltage signal.
Figure above shows the paddlewheel flow meter mounted in a pipe. It is important to understand that the paddlewheel flow meter is mounted in a straight run of piping where the flow will be laminar. The paddlewheel is a low-cost sensor used in applications that do not require a high degree of accuracy.
2. Turbine Flow Meters Turbine flow meters are very similar to paddlewheel flow meter except the turbine flow meter is much more accurate. From the diagram below we can see that the turbine wheel is mounted so that it is directly in the flow. Each of the vanes of the turbine wheel has a Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors magnet mounted in it so that when the vane spins under the magnetic pickup, an electric pulse is generated. When fluid starts to flow, the wheel will begin to rotate. When the flow increases, the wheel will rotate more quickly and the number of electric pulses will increase. The electric pulses can be averaged over time to provide a flow rate, or they can be totalized to determine the total flow.
3. Vortex Flow Meter The vortex flow meter is also a velocity-type flow meter that places a flow element in the flow stream that is not streamline. When the flow stream strikes the flow element, a series of vortices is produced (shedded). For this reason, this type of flow meter is sometimes called a vortex shedding flow meter. When a vortex is produced, it causes the fluid to create a swirling motion as it moves. A very sensitive electronic detector can detect the presence of the vortices. The number of vortices that are produced is directly proportional to the flow rate. This type of sensor is very accurate and it can also be used when the fluids has suspended solids (slurries) moving in the flow. The number of vortices that are detected can be averaged to produce a flow rate, or they can be added to produce a total flow. A frequency-to-voltage circuit and amplifier must be used to increase the small-pulse signal from the electronic detector. It is also important to understand that the vortex type flow meter work best in very turbulence flow. Figure below shows the location of the flow element in a pipe. Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors
vortex Flow element
Flow
4. Electromagnetic Flow Meter Another type of flow meter uses the operating principle of creating an electrical field in the fluid and then measures the strength of the field. This type of flow meter takes advantage of the principle of electromagnetic induction. As you know, a voltage can be induced in a conductor when it passed through a magnetic field. In this case the fluid must electrically conductive so that it will accept the magnetic field and act as conductor.
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors
Figure above shows an electromagnetic flow meter. The meter has two major parts. The first part is a set of coils that creates the magnetic field. When the fluid flows past these coils, it will act like an electrical conductor and will hold and electrical charge. The second part of the flow meter is a set of electrodes that is used as the detector. The detector acts like a voltmeter and measures the intensity of the electrical charge. The stronger the fluid flow, the stronger the electrical charge. This type of flow meter is extremely useful in that it does not need to place any disturbance in the fluid flow, and it is also useful to measure the flow of very corrosive fluids. Early versions of this type of meter consumed large amounts of electrical power, but newer versions use pulse technology to limit the current supplied to the magnetizing coils so that the power consumption is reduced. 5. Ultrasonic Flow Meters
The ultrasonic flow meter uses Doppler meters to measure the shift of a frequency signal that is sent into the liquid flowing through a pipe. From the diagram in Figure above we can see that the transmitting element injects a signal with a given frequency into the fluid flow. Bubbles in the fluid or any suspended solids in the fluid reflect the signal back to a receiver. The receiving element called Doppler meter is placed a short distance downstream and it detects the frequency as the fluid flows past it. A special circuit called a time-of-travel meter, measure the time delay or shift in the frequency that is caused by the fluid flow. The faster the fluid flows, the more the frequency is shifted. This type of flow meter is useful because it can measure the flow without creating any obstruction. It is important to understand that some amount of suspended solids or bubbles must be present in the flow to get the best reflection of the signal. This type of meter is also used extensively as a portable flow meter. The portable flow meter is used in troubleshooting to detect flow in complex hydraulic systems. When a hydraulic systems start to fail, it generally leaks fluid past an inoperable valve back to the reservoir. Since the hydraulic piping is made of metal or hoses, it is difficult to tell how much fluid flow is being bypassed, so an ultrasonic flow meter can be temporarily placed around
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors each segment of the hydraulic piping and the amount of flow in the main lines and the return lines to the reservoir can be determined.
Positive Displacement Flow Meters Another category of flow meters is called the positive displacement flow meter. This type of flow meter is the most accurate in that the flow is broken into segments and each segment is measured as the flow is moved. One example of a positive displacement flow meter uses a piston pump. The volume of the piston is known, and all of the fluid flows through the piston pump. This allows the total volume of fluid to be measured by counting the stroke of the piston. In example, if a piston has a volume of 0.1gal, and it makes 200 strokes per minute, the flow would be 20gpm. Another type of positive displacement flow meter uses a set of oval gears (lobes) that pump a specific volume of fluid each time the gears mesh. The faster the flow moves, the faster the gears rotate. The numbers of rotations are counted and the flow rate can be calculated because the volume of fluid that is pumped during each revolution of the lobes is known. Fig. 1067 shows a positive displacement flow meter. The nutating disk is another type of positive displacement flow meter. The nutating disk is a movable disk that is offset so that it makes a concentric circular motion each time it rotates. The housing for the disk is perfectly round, so that each time the disk rotates, its oval path will trap a specific amount of fluid. Since the volume of trapped fluid is known, the number of revolutions the disks makes can be counted and the flow rate can be calculated.
Mass Flow Meter
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors A mass flow meter is the most accurate type of flow meter designed to measure the flow of gases as well as other fluids. Two types of mass flow meters are commonly used. The Coriolis mass flow meter is so named because it uses the Coriolis phenomenon to measure mass flow instead of volumetric flow, and the other is called a thermal mass flow meter. The coriolis mass flow meter uses a U-shaped tube that is designed to vibrate up and down at its natural frequency while all of the fluid flows through it. A strong magnet is used to make the U-tube vibrate. Fig. 10-68 shows an example of the Coriolis mass flow meter in three distinct stages. When the U-tube vibrates it will naturally move up and down. When fluid is flowing through the tube, it will oppose the up and down movement, which will cause the U-tube to twist. The amount of twist will be directly proportional to the amount of flow. Sensors are located near the tube to detect the amount of twist and convert it to a usable variable-voltage signal. This type of mass flow meter is useful to measure fluids whose viscosity continually changes because they do not need to have pressure or temperature compensation. The thermal mass flow meter is usable for measuring the mass flow of gases. This type of mass flow meter uses a thermal element whose temperature changes as fluids flows past it. The amount of heat loss is directly proportional to the fluid flow. The thermal element is mounted close to the fluid flow but it does not come directly into contact with the fluid. This allows this type of mass flow meter to be used in virtually all types of applications where the density, pressure, and viscosity may change. The flow meter uses an electronic package that contains a flow analyzer, temperature compensator, and a signal conditioner to provide a linear output signal. Some Consideration of selecting Flowmeter : 1. Is the fluid a gas or liquid ? 2. Is the fluid a gas or Corrosive ? 3. Is the fluid electrically conductive or not ? 4. Does the fluid contain slurry or large solids ? 5. What is the fluid viscosity ? 6. What is the need for accuracy and repeatability ? 7. What is the cost ? 8. 9.
Handout
SENSORS Chapter_Flow_Sensor
10
INDUSTRIAL AUTOMATION AND NETWORKING SECTION Sensors
Handout
SENSORS Chapter_Flow_Sensor
10
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