If you are a purchase manager or a mechanical engineer, an instrument named flow meter you will always meet, but not expert on it. The flow meter is used in mining, oil and gas, chemical, and power plants. The main purpose of the flow meter is used to measure the quantity of material flowing in the pipelines, it means to calculate the flow rate of a substance through a specified flow section.
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Here&#;s everything you will learn in this guide:
- What is Flow Metering
- What are the Different Types of Flow Meters
- What Affects Flow Meter
- How is Flow Meter Measured
- What is Flow Metering
- Flow Meter Installation
- Why do We Measure Flow
- Which Type of Flowmeter is Most Accurate
The rate of flow across a specified flow segment is the primary metric for flow metering. In order to maximize the effectiveness of automated manufacturing, flow metering must be incorporated into every technical process flow chart.
Each kind has a unique method for determining the flow rate of a fluid. Today&#;s market offers five primary types of flowmeters, including differential pressure flowmeters, velocity flowmeters, positive displacement flowmeters, mass flowmeters, and open-channel flowmeters.
Existing flowmeters are classified as follows:
Flow meters are based on hydrodynamic methods: variable differential pressure, variable level, streamline flow, vortex method, as well as others.
Flowmeter with continuous motion objects: speed, power, etc.;
Flowmeters are based on various physical phenomena: thermal, electromagnetic, acoustic, optical, etc.
ultrasonic flow meters3
What are the Two Types of Flow Meters?
For Beginners, we just simply first categorize two types of flow meters based on the medium state: gas and fluid or liquid. Fluid flow meters have 5 subcategories: differential pressure flowmeter, velocity flowmeter, positive displacement flowmeter, mass flow flowmeter, and open-channel flowmeter.
What Affects Flow Meter?
Variables like temperature, viscosity, pressure, and conductivity may all impact the accuracy of specific types of flow meters. The quality of the water may also be a factor in determining the flow meter&#;s design. Knowing the properties of the flow being monitored is the next stage in selecting a flow meter.
What are the Different Types of Flow Meters?
In the previous paragraph, we learned that there are 5 main types of flow meters, and now we introduce them one by one.
1. Differential Pressure Flow Meters
Most water and gas flow meters use differential pressure technology. Experience speaks for itself in the form of numerous appearances. Whether it&#;s gas, liquid, or steam, the differential pressure flow meter can detect the number of gases, steam, or liquids that are moving through a system. They find particular utility in applications where extremes of pressure, temperature, or diameter are present. You can find them in the petrochemical, natural gas, power plant, and chemical industries.
Typical Types of Differential Pressure Flow Meters Include:
Orifice Plates
Orifice Plate Flow Meter Systems monitor the change in the flow&#;s pressure from upstream to downstream when the flow is partially obstructed in the pipe.
orifice plates 1
orifice plate flow meter
Flow Nozzles
A Flow Nozzle is an inexpensive and basic design of a venturi meter. The nozzle is fixed between the ends of the material-carrying pipe. A differential pressure sensor monitors pressure decreases in order to calculate the flow rate.
nozzle flow meter
Venturi Tubes
Venturi Flow Meters monitor liquid flow rate and measure the change in pressure caused by a flow path&#;s cross-sectional flow area.
venturi flow meter
Rotameters
Rotameters are variable flow meters that measure the force of gravity acting on a fluid by rotating a float that travels vertically through a tapered tube. They are frequently employed for liquid and gas measurements. Rotameters are simple, affordable, monitor low pressure drops, and have linear output and a large variety of conditions.
variabelt area rotameter
2. Velocity Flow Meter
Flow velocity is measured with velocity flow meters. They generate a reading by monitoring the flow&#;s depth and average air velocity, enabling engineers to maintain the appropriate flow throughout the stream. The range of velocity flow meters is higher than that of differential pressure flow meters. There are pilot tubes, calorimetric, turbines, and electromagnetic velocity flow meters.
pipe velocity
Pilot Tube
Utilized in ventilation and HVAC systems, Pilot Tube Flow Meters measure flow velocity by converting kinetic energy to potential energy.
pitot tube
Calorimetric Flow Meters
Calorimetric Flow Meters, also known as thermal flow monitors, determine the distinction between continuous and controlled heating using the principles of heat transfer. The first sensor monitors the temperature of the heating element, while the second measures the temperature of the fluid.
calorimetric flowmeter
Turbines
Turbine Flow Meters are used with clean and viscous liquids with an accuracy of 0.5% to measure the turbine&#;s speed. They have a rotor with many blades placed perpendicular to the flow. The output frequency is a sine wave or square wave. Signal conditioners can be put on them, but only on categories that are explosion-proof.
liquid turbine flowmeter
liquid turbine flowmeter2
Electromagnetic Flow Meters
Electromagnetic Flow Meters sometimes referred to as magnetic flow meters, are volumetric devices that employ Faraday&#;s law of electromagnetic induction.
types of electromagnetic flow meter3
Vortex Flow Meters
Vortex Flow Meters assess flow rate by placing a barrier directly in the flow channel, causing liquids or gases to go around the impediment. Flow around the barrier generates two symmetrical vortices on the opposite side, altering the flow&#;s pressure. Between the vortices, a flow sensor records pressure fluctuations.
vortex flow meter
Ultrasonic Flow Meters
ultrasonic flowmetersultrasonic flow meters
Ultrasonic Flow Meters compute flow volume by measuring fluid velocity using ultrasound. There are two varieties of ultrasonic flow meters: in-line and clamp-on. In-line ultrasonic meters consist of two sets of ultrasonic devices oriented in opposition and put into the flow pipe. Clamp-on devices utilize the same two components, but neither device is placed into the pipe.
ultrasonic flow meters1
Hydraulic Flow Meters
Hydraulic Flow Meters are utilized for testing, diagnosing, and maintaining hydraulic systems by measuring the flow rate or volume of a liquid within a hydraulic system. They evaluate the system&#;s efficiency and effectiveness and fix any issues that arise.
hydraulic flowmeter
Air Flow Meters
Air Flow Meters monitor air pressure and velocity. They are suitable for obtaining rapid, consistent measurements of ventilation systems and for process monitoring.
3. Positive Displacement Flow Meter(PD flow meters)
Positive displacement flow meters use rotors as sensors to detect viscous liquid flow. By using vanes, gears, pistons, or diaphragms, airflow is displaced. The rotation of the rotors is proportional to the flow volume. They are utilized for measurements when a straight pipe is unavailable or as a substitute for turbine meters and paddlewheel sensors when the flow is too turbulent.
4. Mass Flow Meters
The volumetric flow rate is calculated by dividing the mass flow rate by the fluid&#;s density using a mass flow meter. It employs the Coriolis Effect, an inertial force that operates on moving objects inside a given reference point. The thermal Mass Flow Meter employs two sensors to monitor temperature, an active heat sensor to measure heat loss in the liquid, and a flow meter to calculate the flow rate.
Mass Gas Flow Meters or Coriolis Flow Meters are based on motion mechanics and the Coriolis Effect. When fluid enters a sensor, it is divided, causing the sensor&#;s tubes to vibrate and produce a sine wave. The mass flow rate is determined by the time delay between the oscillations of the two tubes. They are used for checking for leaks and measuring low flow rates.
5. Open Channel Flow Meters
Digital Flow Meters are any devices with a digital display that measures flow. Power providers utilize a standard digital meter to monitor the flow of energy to your home and to transmit your usage statistics to the electrical company, which uses the information to bill you.
Additional Types of Flow Meters
Slurries, water, and other closed-pipe fluids are measured using water flow meters. Liters or cubic meters are used to express the flow rate.
Fuel flow meters count the volume of liquid being moved. The user may see on a mechanical or digital display how much fuel has been transferred during a transaction.
Peak flow meters evaluate lung capacity and are used to treat breathing problems. By breathing into the mouthpiece, which records the volume of air taken in per minute in liters, the portable, low-cost gadget assesses lung capacity.
An operator can see a liquid as it passes through a pipe thanks to flow indicators. They are a component of an industrial process that requires an instantaneous inside view of the flow within a pipe and are sometimes referred to as sight flow indicators. The mass or speed of the flow is not calculated or measured by flow indicators. They are the most basic type of flow meter and have no moving components, they are often referred to as plain sight indicators.
How is Flow Meter Measured?
Different type of meter measure has different working principles, so let&#;s learn how each type of flow meter is measured.
1. Principle of Differential pressure flowmeter/Orifice
orifice flow meter with differential pressure transmitter
The flow is determined in a differential pressure drop device by measuring the pressure drop over blockages inserted in the flow. The differential pressure flow meter is based on the Bernoulli Equation, which states that the pressure drops and subsequently measured signal are functions of the square flow speed.
differential pressure transmitter
DP(&#;P) = ρv2 / 2 (1)
where
DP(&#;P) = Differential Pressure (Pa, psi)
ρ = Fluid Density (kg/m3, slugs/ft3)
v = Flow Velocity (m/s, in/s)
Note that it is common to use &#;head&#; instead of &#;pressure&#;
h = DP / γ (2)
where
h = Head (m, in)
γ = Specific Weight (N/m3, lb/ft3)
pressure drop ratio
2. Principle of Variable Level Flowmeter
A liquid is continuously pumped into the vessel while simultaneously flowing out of a hole in the bottom or a side wall. Variable-level flowmeters are based on the relationship between flow rate and the height of the level in the vessel. The main transformer is a container with an aperture of area S. The liquid level in the vessel&#;s height is what is being measured, and the flow rate, which is the flow rate coefficient, is determined from the relationship and is independent of the liquid&#;s density.
Slit flowmeters, which are tiny weirs in the wall of a vessel into which a liquid is continuously delivered, are frequently used to measure the flow rate of liquid in open channels (trays). The height of the liquid level over the lower edge of the slit determines the flow rate. A flowmeter of this type has the following characteristics, which are determined by the shape of the slit cross-section: The flow rate for a rectangular slit that is b1 wide and b2 high, profiles b1(b2) have been designed on which they are applied and a linear relationship is established.
3. Principle of Rotameter Flow Meter
The primary transformer in streamlined flow flowmeters absorbs the dynamic pressure of the flow and moves under its action by a value proportional to the flow rate. The most common are constant pressure differential flowmeters, in which a streamlined body glides vertically and the weight of the body produces a counterforce.
Among these are Rotameters flowmeters, as well as floating and piston-type (slide valve) flowmeters. The rotameters are manufactured in the shape of a vertical conical (conicity, 0.001-0.01), upwards diverging glass pipe on which the graduations are created, a float travels inside the pipe with inclined ribs on its upper rim.
The float rises and spins in response to the flow, ensuring that the float is centered in the center of the flow. The height z of the float lift determines the volume flow rate of the liquid. It is proportional to the size of the circular gap Sc(z) between the pipe walls and the float and is dependent on the float parameters (volume Vf, midsection area Sf = (/4)df2 and density of the float material (f).
In specific measurement settings, the relationship Sc(z) is virtually linear for tiny pipe taper angles. Where A is determined by an initial calibration Float flowmeters work in the same way. Under the operation of a dynamic head, the piston in a piston-type flowmeter travels in a bush with specially formed windows through which the liquid flows out at a flow rate .
float flowmeter
4. Principle of Vortex Flowmeter
vortex flowmeter
The frequency of fluctuations in pressure or velocity that occur in the cross-flow across a body (cylinder, prism, plate) is calculated and depends on the flow rate in vortex flowmeters. The Strouhal number Sr = du-1f relates the frequency f to the mean streamlining velocity u and the body size d. The flow rate for a flow section area S = (π/4)D2 is determined by the relation
vortex flow meter
Sr = const ensures proportionality between and f, which is attained when the cylinder is streamlined across the range 104 &#; Ro &#; 2 × 105 (Ro = ωd/u is the Rossby number). This guarantees that the observed flow rate has a wide range.
vortex flowmeter schematic
However, is constrained by the circumstances of sustained vortex formation (for example, water u > 0.2 m/s). Typically, primary transformers with d/D values between 0.15 and 0.2 are utilized (D is the diameter of the pipeline). The pressure pulsations are converted into an electric signal using piezo ceramic pressure pickups. Flow rate measurement error is estimated to be 0.5-1.5%.
flow meter installation 1flow meter installation2flow meter installation3
5. Principle of Turbine Flowmeter
Tachometric flowmeters have a rotary element whose measuring velocity is proportional to the volume flow rate.
Flowmeters in the shape of a miniature turbine have found wide applications, with the rotational speed of the turbine being determined by the frequency meter&#;s measurement of the number of electric pulses per unit of time. Turbine flowmeters are built as either an axial small turbine (Turbine flowmeter with rotation in the direction of flow) with variable helix angle propeller blades or a tangent small turbine (Turbine flowmeter with rotation normal to the direction of flow) with flat, radially placed blades. As the moving element of ball-type flowmeters, a ball is moved around the circuit by the swirling flow with the assistance of a propeller guide.
Power flowmeters measure the value of a parameter that describes how a force acts on the flow, with the force&#;s impact being proportional to the mass flow rate. The force increases the flow&#;s speed. The flowmeters are divided into three categories based on the type of acceleration: turbo-power flowmeters, Coriolis flowmeters, and gyroscopic flowmeters.
- The turbo-power flowmeters either use a stationary auger or an external action (such as a rotor with an electric motor) to swirl the flow.
- Coriolis flowmeters experience Coriolis acceleration as a result of force.
- Gyroscopic flowmeters that measure the gyroscopic moment.
The torque on the rotating shaft is monitored and inversely proportional to the mass flow rate in the flowmeter depicted in the Turbo-power flowmeter.
typical installation of turbine flowmeter
6. Principle of Thermal Flowmeter
Thermal flowmeters are based on the flow rate of the amount of heat received by a liquid flowing from a heater. A heater (typically an electric one) is injected into the flow on the pipeline portion, and its power W is recorded, as well as the difference in flow temperatures ΔT = Tout &#; Tin at the upstream and downstream. The mass flow rate is hence proportional to the heating power W with ΔT held constant. Because the coefficient K is affected by heat losses into the surroundings, the nonuniform distribution of velocity throughout the pipeline cross-section, and other factors, a preliminary calibration was done. When the flowmeter is thoroughly made and calibrated, it can provide an accurate flow rate measurement of ±(0.3-0.5)% and can be used as a reference for inspecting and calibrating other flowmeters. The thermoanemometric technique determines local velocity by measuring the temperature of a hot wire or hot film supplied with a continuous current. Using existing relationships, this local velocity may then be connected to the mean velocity.
7. Principle of Electromagnetic Flowmeter
electromagnetic flowmeter3
When measuring the flow rate of a liquid, electromagnetic flowmeters typically require a conductance of at least 103 Ohm/m. Their operation depends on the interplay of the transverse magnetic field and the moving current-conducting liquid. In this instance, the magnetic induction of the transverse field B, the liquid flow rate V, and the distance D between the electrodes all contribute to the electromagnetic force E that is produced in the liquid (located along the normal both to the velocity vector of the liquid and to the vector of the magnetic field intensity). D is the same as the inner diameter of the pipe.
The source of the current (E) is indicated by
electromagnetic flowmeter schematic
The benefits of the electromagnetic flowmeter include its ability to measure abrasive, extremely viscous, and aggressive liquids as well as its independence from the viscosity and density of the material, lack of pressure loss, scale linearity, and high-speed response. But, It is unable to measure the flow rates of gases, vapors, or dielectrics.
types of electromagnetic flow meter
types of electromagnetic flow meter2
types of electromagnetic flow meter3
Flow Meter Installation
Flow measurement is a continual responsibility for each sector, which is why flow meters have grown so important. A flow meter must be properly placed in order to provide accurate and trustworthy data.
flow meter installation 4
The following are some of the most important measures to take while installing a flow meter:
- Understand the location where it will be installed.
- It cannot be installed in areas with high vibrations or magnetic fields.
- Understand the flow&#;s direction.
- Avoid downward flow in liquid operations.
- It must be mounted on a straight pipe.
- Verify that it is totally filled with fluid.
- Inspect liquids and gas lines for the presence of vapors or air and droplets, respectively.
- A filter should be installed upstream of the meter to trap particulates if at all practicable.
- Allow for pipe expansion.
- Maintain a bypass line in case a repair is required.
Why do We Measure Flow?
The volume of fluid that travels through a certain cross-sectional area per unit of time is defined as flow rate. Accurate flow rate measurement with a suitable flowmeter is critical to ensure fluid control systems work smoothly, safely, and economically.
Which Type of Flowmeter is Most Accurate?
The most repeatably accurate flow meters now available are thought to be ultra-high accuracy flow meters like Coriolis flow meters. When great precision is essential for maintaining a continuous level of product quality, safety, and profitability, Coriolis meters are the best choice. in general cost more than any other flow technology. A flow meter with an accuracy of 5% which is much less expensive than another flowmeter with an accuracy of 0.2% could provide sufficient results for your process to function smoothly and give large cost savings. Understanding the precise accuracy requirements for your application and accuracy vs budget concerns might be challenging at times. You can get free assistance from our sales engineers in locating the ideal solution for your application.
Final Thoughts
The flow meter isn&#;t that complicated. You just need to learn basic terminology, category, and work principles. Go deep and learn if you needed from the real selection or use the application as much as you, you will quickly handle or expert soon.
Looking to learn more about industrial valves? Reading the following articles will help you expand your automation process knowledge.
In this article, you will learn the flow transmitters questions and answers related to flow measurement in industrial instrumentation.
Flow Transmitters Questions and Answers
What is CFM, SCFM, ACFM, Am³/hr, and N m³/hr?
- CFM is a unit of flow rate (cubic feet per minute)
- SCFM is a unit of flow rate (Standard Cubic Feet per Minute) at STP condition i.e. standard temperature 60º F and standard pressure 14.69 A PSI.
- ACFM (Actual Cubic Feet per Minute) is the actual flow at the operating temperature and pressure conditions.
- Am³/hr (Actual Cubic Meter per hour) is the actual flow at the operating temperature and pressure conditions.
- Nm³/hr (Normal Cubic Meter per hour) is normalized value of flow at NTP conditions i.e. flow at Normal Temperature 0º C and Normal Pressure 101.32 kPa condition.
Note : STP/NTP conditions are decided by different organizations/authorities as per their specifications.
Why back pressure is necessary?
When measuring liquids, back pressure in the flow line should be sufficient to prevent cavitation.
Cavitation occurs due to pressure drop at the obstruction in the flow line. Due to pressure drop, bubbles are formed and are driven to the further locations in the flow line where they break and erode the parts of flow line.
To avoid such cavitations, the back pressure in the line must be always greater than the vapor pressure of the fluid. To maintain back pressure in the line, larger pumping force is required.
In case of water and liquids of similar vapor pressure, required back pressure is approximately 1 kg/cm² at velocity of 6 meter/second.
When there is no back pressure in the pipeline, it can be created by placing a partially closed valve on the downstream side of the flow meter.
What is the Vortex Flow meter Principle?
Whenever an obstacle is placed in a flowing fluid, vortices are generated alternately on both sides of the obstacle.
The frequency of vortex generation is directly proportional to the velocity of the fluid and is independent of all other parameters.
Thus,
F α V
Frequency is directly proportional to velocity.
F = KV
Frequency gives inference of velocity.
Where
F = Frequency of Vortex Generation
V = Velocity of Fluid
K = Proportionality Constant
Fig : KARMAN VORTEX STREET IN PIPELINE
Flow Rate (M3/Hr) = Area of Pipe in M2 x Velocity of Fluid in M/sec x seconds
Why upstream and downstream straight length is required?
Flow Meter accuracy is guaranteed for a fully developed flow profile in the pipeline at the installed location.
Normally from flow disturbance point, flow is developed after a distance of about 20D or more.
Hence minimum upstream straight length should be 20D for normal pipelines.
Minimum downstream straight length should be 5D from the flow meter to avoid disturbances in the flow.
Can a differential pressure flowmeter handle turbulent flow?
Yes; though meters are unidirectional a straight run of tubing or pipe is not required.
What are the advantages of using a variable area flowmeter?
Inexpensive somewhat self-cleaning no power required available in different materials for chemical compatibility Low and constant pressure losses.
Suitable for very low flow rates Rangeability 10:1 Capable of measuring fluids of varying density and viscosity (compensation given by float design).
Must a rotameter be mounted vertically?
Generally, rotameters must be mounted vertically, because the float must center itself in the fluid stream.
At high flow rates, the float assumes a position towards the tip of the metering tube and at low flow rates positions itself lower in the tube.
Some of the rotameters have spring loaded floats and therefore may be mounted in any orientation.
Can we use a rotameter in a vacuum application or with back pressure?
Yes, but if you have a valve, it must be placed at the outlet (top of the flowmeter). This is done by inverting the tube inside the frame, and then turning over the frame.
At this position, the tube should read correctly from the original perspective and the valve should be at the outlet, or top of the flowmeter. This allows for proper control of the vacuum.
What is the difference between correlated and direct reading rotameters?
A direct reading flowmeter indicates the flow rate on its scale in specific engineering units (e.g. ml/min or scfh).
Direct reading scales are designed for a specific gas or liquid at a given temperature and pressure. While it is more convenient than a correlated flowmeter, a direct reading flowmeter is less accurate and limited in its applications.
A correlated flowmeter is scaled along either a 65mm or a 150mm length, from which a reading is taken.
The reading is then compared to a correlation table for a specific gas or liquid. This will give the actual flow in engineering units. One correlated flowmeter can be used with a variety of fluids or gases.
What are Multiphase Flow Meters?
Multiphase Flow Meters (MPFM) are devices used to measure the individual oil, water and gas flow rates in a multiphase flow.
The term MPFM is used to define also the metering of wet gas streams (i.e. multiphase flow where the gas content is very high).
A multiphase flow meter is a device used to measure the individual phase flow rates of constituent phases in a given flow (for example in oil and gas industry) where oil, water and gas mixtures are initially co-mingled together during the oil production processes.
Why is a Pressure Transmitter Installed Upstream of a Flow Transmitter?
- Upstream of a flow control valve a pressure transmitter is installed to measure the operating pressure.
- At times it is used for computing the true flow against the designed pressure.
- Downstream of the control valve the pressure changes as the control valve open and closes.
Differential Pressure Flow Meter Accuracy & Rangeability?
Performance of a head-type flowmeter installation is a function of the
- precision of the flow element and
- the accuracy of the d/p cell
On average, flow element precision is expressed in percentage of actual reading terms, whereas d/p cell accuracy is stated as a percentage of calibrated span.
A d/p cell typically provides accuracy of ±0.2% of the calibrated span. With no detrimental effect on accuracy, rangeability of a flowmeter can be extra enhanced by employing several d/p flowmeters in parallel runs.
How Hot Wire Anemometer Works?
The hot-wire anemometer, principally used in gas flow measurement, consists of an electrically
heated, fine platinum wire which is immersed into the flow.
As the fluid velocity increases, the rate of heat flow from the heated wire to the flow stream increases. Thus, a cooling effect on the wire electrode occurs, causing its electrical resistance to change.
In a constant-current anemometer, the fluid velocity is determined from a measurement of the resulting change in wire resistance.
In a constant-resistance anemometer, fluid velocity is determined from the current needed to maintain a constant wire temperature and, thus, the resistance constant
Explain Orifice Performance.
Principally, an orifice plate is a precision instrument. In best circumstances, the inaccuracy of Orifice plates can possibly fall in the range of 0.75-1.5% AR.
However, there are numerous error causing conditions which can terribly affect the accuracy of an Orifice plate.
Factors used to judge the performance of an Orifice plate:
- Precision in the bore calculations
- Quality of the installations
- Condition of the plate itself
- Orifice area ratio
- Physical properties of the fluid flow under measurement
A further class of installation depends upon following factors
- Tap location and circumstance. Generally, there are three ways to position a pressure tap.
- Provision of the process pipe
- Competence of straight pipe runs
- Gasket intervention
- Misalignment of pipe and orifice bores
- Lead line design
Extra detrimental conditions consist of
- Dulling of the sharp edge or nicks caused due to corrosion or erosion.
- Warpage of the plate because of waterhammer and dirt.
- Grease or secondary phase deposits on any of the orifice surface.
Any of the above said conditions has the tendency to affect the discharge coefficient of an orifice plate to a large extent.
Also Read : Flow Measurement Technologies
What is a Venturi Tube?
The venturi tube, illustrated in Figure, is the most accurate flow-sensing element when properly calibrated.
The venturi tube has a converging conical inlet, a cylindrical throat, and a diverging recovery cone.
It has no projections into the fluid, no sharp corners, and no sudden changes in contour.
The inlet section decreases the area of the fluid stream, causing the velocity to increase and the pressure to decrease.
The low pressure is measured in the center of the cylindrical throat since the pressure will be at its lowest value, and neither the pressure nor the velocity is changing.
The recovery cone allows for the recovery of pressure such that total pressure loss is only 10% to 25%. The high pressure is measured upstream of the entrance cone.
The major disadvantages of this type of flow detection are the high initial costs for installation and difficulty in installation and inspection.
Explain Concentric, Segmental, and Eccentric Orifice Plates.
The concentric orifice plate is the most common of the three types. As shown, the orifice is equidistant (concentric) to the inside diameter of the pipe.
Flow through a sharp-edged orifice plate is characterized by a change in velocity. As the fluid passes through the orifice, the fluid converges, and the velocity of the fluid increases to a maximum value.
At this point, the pressure is at a minimum value. As the fluid diverges to fill the entire pipe area, the velocity decreases back to the original value. The pressure increases to about 60% to 80% of the original input value.
The pressure loss is irrecoverable; therefore, the output pressure will always be less than the input pressure. The pressures on both sides of the orifice are measured, resulting in a differential pressure that is proportional to the flow rate.
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Segmental and eccentric orifice plates are functionally identical to the concentric orifice. The circular section of the segmental orifice is concentric with the pipe.
The segmental portion of the orifice eliminates damming of foreign materials on the upstream side of the orifice when mounted in a horizontal pipe.
Depending on the type of fluid, the segmental section is placed on either the top or bottom of the horizontal pipe to increase the accuracy of the measurement.
Eccentric orifice plates shift the edge of the orifice to the inside of the pipe wall.
This design also prevents upstream damming and is used in the same way as the segmental orifice plate.
Orifice plates have two distinct disadvantages; they cause a high permanent pressure drop (outlet pressure will be 60% to 80% of inlet pressure), and they are subject to erosion, which will eventually cause inaccuracies in the measured differential pressure.
What are the selection criteria for Magnetic flowmeters?
The key questions which need to be answered before selecting a magnetic flowmeter are:
- Is the fluid conductive or water based?
- Is the fluid or slurry abrasive?
- Do you require an integral display or remote display?
- Do you require an analog output?
- What is the minimum and maximum flow rate for the flow meter?
- What is the minimum and maximum process pressure?
- What is the minimum and maximum process temperature?
- Is the fluid chemically compatible with the flow meter wetted parts?
- What is the size of the pipe?
- Is the pipe always full?
What are the Head types of flow meters?
In these head type flowmeters, some devices is inserted into a pipe carrying fluid.
It obstructs the flow of fluid and creates a pressure difference on either side of the device.
The most commonly used devices are as follows:
- Orifice plate.
- Venture plate.
- Flow nozzle.
- Doll flow tube.
- Pilot tube.
The basic principle of all such devices is that due to obstruction, the velocity of the fluid increases and the pressure decreases.
Then the volume flow rate is proportional to the square root of pressure difference across the obstruction. To measure pressure difference, diaphragm based differential pressure transducer is used.
What is a Pitot Tube or Total Pressure Probe?
A probe is a device used for point pressure measurement in a flowing fluid. This point measurement of pressure is done to determine fluid flow rate.
The most popular probe is the &#;PITOT TUBE&#; which is one of the total pressure probes. The Pitot tube measures the combined pressure (static pressure + impact pressure).
The pitot tube has one impact opening and eight static openings. The impact opening is provided to sense impact pressure and the static opening are provide to sense static pressure.
The pitot tube is introduced in the fluid flow area where point pressure details is required (which is an indirect measure of flow rate).
The pressure in the outer tube is the static pressure in the line. The total pressure in the inner tube is greater than static pressure. That is, total pressure is the static pressure plus the impact pressure.
The differential pressure (P1-P2) is measured using a differential pressure sensor. This differential pressure becomes a measure of flow rate at that point where the pitot tube is present in the flowing fluid.
What is a &#;k&#; factor on a turbine meter?
Each turbine meter is specified with a &#;k&#; factor which represents the number of pulses produced per a known quantity of liquid.
Example: k = 265 pulsed/gallon
Generally, the &#;k&#; factor is provided by the manufacturer.
What are the Types of Thermal mass flow meters?
Thermal mass flow meter types
- Constant Current Method
- Constant Temperature Method
What is a Flow profile or velocity profile?
The relative velocities of a fluid as it moves through a pipe, the velocity at the center being greater than the velocity at the pipe wall.
Laminar flow is characterized by large differences in velocity along the profile, while turbulent flow exhibits a &#;flatter&#; profile with more consistent velocity across the pipe diameter.
Relevant to insertion type flowmeters such as Pitot tubes where the flowing velocity is sampled at only one point in the flow stream.
How do you convert NM3/hr to MMSCFD?
Assume this is a gas. Different industries use different temperatures for &#;standard,&#; including but not limited to 15 °C, 60 °F, 20 °C, and 25 °C.
Unless you know which &#;standard&#; is meant, the final step is not possible.
Nm³ means the gas is measured at 0 °C and 1 atm (101.325 kPa).
Step (1): Multiply by 24 h/day to get Nm³/day.
Step (2): Multiply by 35. ft³/m³ to get cubic feet/day at 0 °C and 1 atm.
Step (3): Determine &#;standard&#; temperature in °C, and multiply by temperature correction factor (273.15 + T)/273.15 to get SCFD.
Step (4) divide by 1 million to get MMSCFD.
What are the Types of Flow?
In general, we come across two types of flow in liquid flow Measurement operations.
Laminar flow:
This type of flow occurs at very low velocities or high viscosities.
In this, the liquid flows in smooth layers with the highest velocity at the center of the pipe and low velocities at the boundary (wall) of the pipe where the viscous forces hold it back.
Turbulent flow:
It takes place at high velocities or low viscosities.
In this, the liquid flow breaks up into turbulent eddies which flow through the pipe with the identical average velocity.
In this type of flow, fluid velocity is not much significant, and the velocity profile is a lot more uniform in shape.
What are the Factors Affecting Flow Rate?
From the basic relationship, we deduce that factors affecting liquid flow rate comprises average velocity of the flow and cross sectional area of the pipe.
Apart from these, other factors which can influence liquid flow rate are:
- Liquid&#;s viscosity
- Density
- Friction of the liquid in contact with the pipe
Also Read : Capillary Tubing Pressure Sensor Temperature & Elevation Problems
What are the Types of Ultrasonic Flowmeters?
Ultrasonic flowmeters are two types:
Doppler meters:
Doppler meters measure the frequency shifts caused by liquid flow. In this, two transducers are mounted in a case attached to one side of the pipe.
A signal of known frequency is transmitted into the liquid to be measured.
Solids, bubbles, or any other discontinuity in the liquid, cause the signal to be reflected to the receiver element.
Since the liquid causing the reflection is moving, the frequency of the returned pulse is shifted. This frequency shift is proportional to the liquid&#;s velocity or flow rate.
A portable Doppler meter which is competent enough of being operated on AC power or from a rechargeable power pack has lately been introduced.
A typical Doppler meter using sound pulse reflection principle.
Time-of-travel meters:
These are also known as Transit meters. They have transducers installed on each side of the pipe.
They use the transit time principle for flow measurement. In this, opposite sending and receiving transducers are employed to transmit signals through the flow.
The signal travels faster when moving with the flow stream rather than against the flow stream. The difference between the two transit times is used to determine the flow rate.
As per configuration, the sound waves travel between the devices at a 45 degree angle to the fluid flow direction.
The speed of the signal traveling between the transducers depends upon (increases or decreases) the direction of transmission and the velocity of the liquid being measured.
A time-differential relationship proportional to the flow can be acquired by transmitting the signal alternately in both directions.
A major limitation of time-of-travel meters is that the liquids being measured must be moderately free of entrained gas or solids. This is crucial for minimizing signal scattering and absorption.
What are the Types of Variable Area Flowmeters?
Variable area flowmeters are available in following variety of designs
- Rotameter (a float in a tapered tube)
- Orifice/Rotameter combination i.e. Bypass Rotameter
- Open-channel variable gate,
- Tapered plug, and
- Vane or Piston designs
What are the Advantages and disadvantages of Vortex-Shedding Flow Meters?
Advantages
- Accurate regardless of temperature, pressure, density and viscosity when flow is turbulent.
- Suitable for measuring liquids, gases and steam.
- Excellent for metering steam flow.
Disadvantages
- Flow must be turbulent.
- Ineffective for slurries and viscous flow.
How to convert SCFM to ACFM Conversion?
ACFM = SCFM x 14.696 / (Pa + 14.696) x degrees F / 530
Where as:
ACFM = Actual Cubic Feet per Minute measured gas flow
SCFM = Standard Cubic Feet per Minute gas flow
Pa = Operating Pressure in (PSIA) = PSIG + 14.696
Ta = Temperature in degrees Rankine = F + 460
Note: All calculations are at sea level.
What is Cryogenics and which Flow Meter Works with Cryogenic?
Cryogenics is the study of low temperatures.
The turbine flow meter is a very good choice of measurement due to the material and its ability to hold up to cryogenic temperatures and to accurately repeat at any flow rate.
The turbine creates minimal pressure drop or flow constriction in a catastrophic failure.
Flows can be maintained under any circumstance.
The turbine is also the most accurate and durable for use in a transport due to vibration resistance.
Why Generally a Flow Transmitter Installed Upstream of a Flow Control Valve?
A flow transmitter is always installed on the upstream of the flow control valve in order to maintain the operating pressure across the flow transmitter sensors.
Downstream of the control valve the pressure changes as the control valve open or closes.
What is the Purpose of Orifice Vent Hole?
Vent hole is a small hole which is provided in the upper region of orifice plate. Vent hole is required in liquid flow service where gas entrainment may occur.
Vent hole size could affect the accuracy of flow measurement. However, if the diameter of the vent hole is less than 10% of the orifice bore, then the unmeasured flow is less than 1% of the total flow.
Vent hole is not recommended in dirty fluid service or slurries as the hole could be plugged. In this application, the use of eccentric orifice plate becomes alternative.
What is the Purpose of Orifice Drain Hole?
Drain hole is a small hole which is provided in the lower region of orifice plate. Drain hole is required in gas flow service where liquid entrainment may occur.
Drain hole size could affect the accuracy of flow measurement. However, if the diameter of the drain hole is less than 10% of the orifice bore, then the unmeasured flow is less than 1% of the total flow.
Drain hole is not recommended in dirty fluid service or slurries as the hole could be plugged. In this application, the use of eccentric orifice plate becomes alternative.
Also Read : Flow Meter Installation Guidelines
Explain about Restrictive Flow Orifice.
A Restrictive Flow Orifice (RFO) is a type of orifice plate. They are used to limit the potential danger of an uncontrolled flow from, for example, a compressed gas cylinder by:
- limiting the accidental release of a hazardous gas (flammable, toxic, etc.) resulting from regulator or other component failure,
- restricting flow in a system in order to assure adequate pressure relief valve sizing and system over pressure protection, or
- restricting flow from bulk sources
What are the Types of Positive displacement meters?
The following are the Positive displacement flowmeters
- Reciprocating Piston type.
- Rotating or Oscillating Piston type.
- Nutating Disc type.
- Fluted Spiral Rotor type.
- Sliding vane type.
- Rotating vane type.
- Oval Gear type.
Which cable should be used to connect the Transmitter?
Generally three-core, twisted and shielded, 19/36 gauge cable.
In applications requiring mechanical ruggedness, we recommend three-core 1.5 mm² armored (shielded) cable.
Can we install flow meter in a vertical/inclined pipeline?
Yes, the flow meter can be installed in a vertical, inclined or in any angular position of pipeline for compressible fluids like low pressure air/gas, compressed air/gas.
For liquid applications in a vertical pipeline, the direction of flow should necessarily be from bottom to top, so as to avoid flow separation phenomenon.
Why is a ground strap required?
Ground strap is required for proper grounding (Earth) of meter body, i.e. both pipe and meter body should be at the same voltage potential.
Which gaskets are recommended?
For general applications asbestos grade or any non-yielding gaskets are required (Champion #20 or equivalent). NEVER use rubber gaskets!!!
Gasket I.D. must be 5mm larger than the meter I.D. Gaskets must be non-yielding so that they do not protrude and obstruct the flow.
Where should we place the taps for pressure gauges and temperature gauges?
Pressure gauge should be located within a distance of 4D to 7D on either upstream or downstream of the meter body.
Pressure tap fittings must not protrude in the pipe. Tap holes must be burr free.
RTD Temperature tap must be placed on the downstream side of the flow meter and within a distance of 5D to 10D from the flow meter body (since a temperature measuring probe /thermo-well can interfere with vortex generation)
Is the Flow Meter multi-variable?
No, the flow meter measures only the actual flow at operating pressure and temperature conditions i.e. AM³/HR or ACFM.
How do we pack the flow meter to send for servicing if required?
The Flow Meter system should be packed using adequate (2&#;-3&#; thick) thermocol on all 6 sides inside a plywood box to avoid any transit damages.
Flow Meter and Flow Indicator should be packed in separate boxes.
&#;Used Goods / Goods for Repairs&#; Declaration should be accompanied with the consignment.
Reference: inconel flow meters
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