Process Instrumentation : Flow Measurement

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Maintaining proper flow of fluids in a process system is essential to maintain correct supply of raw materials to reactors, correct supply of water or steam for cooling or heating purpose etc. Flow meters sense the amount of flow passing through a particular pipe and sends this information to process controller which then applies process logic and sends control information to control valves or pump control unit.

A variety of flow meters are used in process industry depending on type of fluid, operating temperatures and pressures, required flow accuracy and economy. Most of the flow meters require proper flow pattern established in pipe. Flow turbulence can affect accuracy of readings. Hence they have straight pipe requirements without any branches upstream and downstream of them. These are provided by instrument department and instrument vendor. They must be strictly adhered to.

Here is a brief description of all these flow meters sufficient from piping engineering point of view.

Orifice Flow Meter

This is the most common type of flow meter used in process industry. It consists of an orifice plate with a pair of orifice flanges, which are tapped to receive sensing connections. Complete assembly consists of an orifice plate, orifice flanges, bolts and gaskets along with valved tapping connections to differential pressure transmitter.

Tapping connections are normally 1/2″. The transmitter is located adjacent to the assembly before it is connected to the remote panel and controlling device (e.g. a control valve or vessel level controller).

Orifice Flow Meter works on the principal of reduced pressure just downstream of orifice plate due to reduced flow diameter for the fluid. Increase in velocity due to reduced cross section results in reduced pressure. Differential pressure is proportional to the flow velocity and density of the fluid.

Installation in Piping Network

  1. Orifice flanges need not be accessible from grade or a platform but should be positioned at a location and elevation that can be reached by a portable ladder.
  2. Care should be taken at horizontal banks of lines to allow for adequate space for side mounted taps and when required, close-coupled transmitters.
  3. Depending on service, orifice flanges can be mounted in the vertical and the horizontal sections of piping as per instrument hookup drawing.
  4. Orifice flanges shall be located adjacent to walkways, platforms, or structures.
  5. Orifice Flanges shall be installed in horizontal pipe runs, wherever possible.
  6. Orifice flanges shall have threaded taps.
  7. Orifice flanges with a centerline elevation 4500 mm above the high point of finish surface, except pipeways, shall be accessible from a platform or permanent ladder.
  8. Straightening Vanes shall not be used.
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Straight Run Requirements

  1. Orifice meter runs require straight runs upstream and dowstream so that flow passing through it is laminar flow instead of turbulent flow.
  2. Most of the time, downstream straight length required is 5 times Diameter of the pipe.
  3. Upstream straight length requirement varies based on type of components immediately upstream of the orifice meter.
  4. Straight run between orifice meter and a upstream throttling valve is usually 39D.
  5. Straight run between orifice meter and a upstream fully open gate valve is usually 14D.
  6. Straight run between orifice meter and a upstream Tee or Cross is usually 31D.
  7. Straight run between orifice meter and a upstream reducer is usually 14D.Straight run between orifice meter and a upstream throttling valve is usually 39D.
  8. Straight run between orifice meter and a upstream elbow depends on the relative angle of next elbow upstream of the immediate elbow. Usually 14D to 31D.

Orifice Flange Tap Orientations.

  1. For clean liquid, Orifice flanges can be in horizontal or vertical (Upward Flow) and tap locations Horizontal.
  2. For Liquids with solids, Orifice flanges should be in vertical pipe with downward flow and tap locations Horizontal.
  3. For steam, orifice flanges can be in horizontal or vertical (Downward Flow Only) and tap locations horizontal.
  4. For dry gas or air, orifice flanges can be in horizontal or vertical pipe with taps in vertical position.
  5. For wet gas or air, orifice flanges can be in horizontal or vertical (Downward flow only) with tap locations vertical.
Orifice Flow Meters

Orifice Flow Meters

Check out these youtube videos to better understand them.

The Differential Pressure Flow Measuring Principle
Orifice Plate Theory
Orifice Flow Meter Assembly

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Variable Area Flow Meter : Rotameter

Rotameter is also known as Variable Area Flow Meter due to variable cross section of this meter. This flow meter consists of a tapered tube, usually made of glass, with a float inside that is pushed up by the fluid flow and pulled down by the gravity.

With increasing flow, the fluid viscosity and pressure forces on the float increases, making it rise until it becomes stationary at an elevation where these forces balance out against gravity.

Shapes of floats are mostly spheres and spherical ellipse type. Some of these floats are designed to spin so that operator can guess if float is stuck at the location or working properly.

Rotameters are used for wide range of liquids but are most commonly used for water or air. They can be made to reliably measure flow down to 1% accuracy.

Rotameter

Rotameter

Check out these youtube videos to better understand them.

Variable Area Flowmeter – Rotameter

Rotameter

Measuring Principle of Variable Area Flowmeters (by KROHNE)

Variable Area Flow Meters

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Coriolis Mass Flow Meter

This type of flow meter uses the Coriolis Effect. When a tube is vibrating laterally and fluid is passed through it, it starts to distort due to weight and flow of fluid.

This distortion can be measured and mass flow can be inferred directly from it, including density of the fluid.

Coriolis meter can be very accurate irrespective of the type of gas or liquid that is being measured. Same measurement tube can be used for hydrogen gas and bitumen without recalibration. They can be used for the measurement of natural gas flow.

Most common configuration includes one or two U-shaped flow tube with inlet on one side and outlet on the other enclosed in a sensor housing connected to an electronics unit. The flow is guided into the U-shaped tube. When an oscillating excitation force is applied to the tube causing it to vibrate, the fluid flowing through the tube will induce a rotation or twist to the tube because of the Coriolis acceleration acting in opposite directions on either side of the applied force. Vibration of Coriolis flowmeters has very small amplitude, usually less than 2.5 mm (0.1 in), and the frequency is near the natural frequency of the device, usually around 80 Hz.

Finally, the vibration is commonly introduced by electric coils and measured by magnetic sensors. This twist results in a phase difference (time lag) between the inlet side and the outlet side and this phase difference is directly affected by the mass passing through the tube.

Recently, single straight tube designs are available to measure some dirty and/or abrasive liquids that may clog the older U-shaped design.

An advantage of Coriolis flowmeters is that it measures the mass flow rate directly which eliminates the need to compensate for changing temperature, viscosity, and pressure conditions.

Coriolis Flow Meter

Coriolis Flow Meter

Check out these youtube videos to better understand them.

The Coriolis Flow Measuring Principle

Siemens FC430 Coriolis Meter – How it Works

Introduction to Coriolis Flowmeters and Density Flow Measurement Technology

Coriolis Flow Meter

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Electromagnetic Flow Meter

Electromagnetic flow meter, also called magnetic flow meter, is one of the most common flow meter in process industry.

This flow meter works on the principal of electromagnetic induction. When an electric conductor moves in a magnetic field, potential difference is induced across the conductor. This potential difference can be measured to infer the speed of conductor movement.

In this case, a conducting fluid such as water acts as a electric conductor. Water contains ions which help it conduct electricity. Magnetic field is applied to the stainless steel metering tube, lined with insulating material like plastic. This results in potential difference across fluid which is proportional to the flow velocity perpendicular to the magnetic flux lines.

Usually electrochemical and other effects at the electrodes make the potential difference drift up and down, making it hard to determine the fluid flow induced potential difference.

To mitigate this, the magnetic field is constantly reversed, cancelling out the static potential difference. This however impedes the use of permanent magnets for magnetic flow meters.

Electromagnetic Flow Meter

Electromagnetic Flow Meter

Check out these youtube videos to better understand them.

The Electromagnetic Flow Measuring Principal

EMFlow Meters

Working of EMFlow Meter

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Single Jet Flow Meter

A single jet meter consists of a simple impeller. This impeller has radial vanes. A Single jet of fluid rotates the impeller. Rotations of the impeller are counted mechanically or electronically to get the cumulative flow. They are increasingly being used in UK for larger sizes are commonplace in the European Union.

Check out these youtube videos to better understand them.

Elster’s S150 single-jet water meter for Europe and Latin America

Flow Simulation of a single jet impeller meter

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Rotating Piston Flow Meter

This flow meter has a disc with an annular groove on its underside. This groove is capable of holding the fluid and transporting it from the chamber inlet to the chamber outlet. Some fluid also gets transported in a cavity formed between rotor outside wall and the chamber inside wall.

The centre shaft under the rotor runs in a circular groove in the body. A wall in the body is engaged with a slot in the rotor. This changes rotation into an oscillation movement which compartmentalizes the fluid into positively displaced pockets.

A magnetic sensor or a mechanical gear train measure the fluid flow. Metering repeatability is better than 0.2% and meter accuracy of 1%. Accuracy can be improved using electronics circuit as difference between actual flow rate and measured flow rate is consistent at low flow rates.

Rotary Piston Flow Meter

Rotary Piston Flow Meter

Check out following youtube video to better understand them.

Rotary Piston Flowmeter

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Multiple Jet Flow Meter

This is a velocity type flow meter. It includes a horizontal impeller mounted on a vertical shaft. Impeller is installed in a housing with multiple inlet ports all of which direct fluid onto the impleller, and thus rotating it.

Rotation of impeller is proportional to the velocity of the flowing fluid.

Impingement of fluid on the impeller via several ports ensures even wearing of the impeller and shaft and thus reduces early damage.

Multiple Jet Flow Meter

Multiple Jet Flow Meter

Check out following youtube video to better understand them.

Fluid mechanics in a multi jet impeller water meter

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Turbine Flow Meter

This flow meter consists of a turbine, mounted axial to the pipe, and rotates due to flow of fluid. Number of rotations per unit of time are counted to get the flow rate. Initially, turbine speed is not proportional to flow rate. But as steady state is reached, speed of turbine becomes proportional to fluid velocity.

Turbine flow meters are used for the measurement of natural gas and liquid flow. Although turbine flow meters are relatively less accurate than displacement type and jet type flow meters, they occupy very less space inside the fluid path and thus do not restrict the flow severely. Pressure drop is very less.

They are mostly used by large commercial users and as master meters for the water distribution system. Strainers are generally required to be installed in front of the meter to protect the measuring element from gravel or other debris that could enter the water distribution system.

Turbine meters are generally available for 1-1/2″ to 12″ or higher pipe sizes.  Body of a turbine meter is commonly made of bronze, cast Iron, or ductile iron. Internals can be plastic or non-corrosive metal alloys.

Turbine Flow Meter

Turbine Flow Meter

Check out these youtube videos to better understand them.

MV Series Turbine Meter

G Series Turbine Meter

Hoffer Turbine Flowmeter Technology

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Pelton Wheel Flow Meter

It consists of a horizontally mounted pelton when on a vertical shaft. Fluid impinges upon the pelton wheel and rotates it. Rotations are counted per time period and translated into user readable flow rates such as gallons per minute, liters per minute etc.

Tendency of pelton wheel is to have all the flow travelling around it with the inlet flow focused on the blades by a jet.

Originally, pelton wheels were used for power generation.

Pelton Wheel Flow Meter

Pelton Wheel Flow Meter

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Helical Gear Flow Meter

This type of flow meter consists of helical shaped gears or rotors. These rotors are in the shape of a helix, a spiral shaped structure. When fluid enters the meters, it flows through the compartments in the rotors, causing it to rotate. Speed of rotation can be measured to calculate the flow rate.

Helical Gear Flow Meter

Helical Gear Flow Meter

Check out this youtube video to better understand it.

What is a flow meter?: Helical Flow Meter, Positive Displacement

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Ultrasonic Flow Meter

This flow meter uses the principal of ultrasound to measure the velocity of gas or liquid. It uses multiple ultrasonic transducers, installed at an inclined angle to the pipe. One pair of ultrasonic transmitter and receiver is installed opposite to each other with transmitter at downstream flow direction and receiver at upstream flow direction. Other pair of transmitter and receiver is installed with transmitter at upstream direction and receiver at downstream direction.

The average velocity of fluid along the path of ultrasound is measured by averaging the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow.

Readings are affected by the conditions of the fluid such as temperature, density and viscosity. But they are expensive to use due to lack of moving parts like mechanical flow meters.

There are three types of ultrasonic flow meters. Transmission flow meters, inline(intrusive, wetted) flow meters and clamp-on(non-intrusive) flow meters. Ultrasonic flow meters that use the Doppler shift are called Reflection or Doppler flow meters. One more type  is Open-Channel flow meter.

Check out these youtube videos to better understand them.

Ultrasonic Flow Meter

Ultrasonic Flow Meter

Check out these youtube videos to better understand it.

The Ultrasonic Flow Measuring Principle

Siemens SITRANS FUS1010 clamp-on flowmeter installation guideline

Ultrasonic flow measurement in the chemical industry

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Ventury Flow Meter

Ventury flow meter works on the principle of Ventury Effect. A reduction is fluid pressure occures when a fluid flows through a constricted section of pipe. This happens mainly due to increased flow velocity because of reduced cross section.

Reduction is pressure is related to actual fluid velocity as well as density of the fluid. Pressure is measured at normal cross section as well as constricted cross section, to get the differential pressure. Based on this differential pressure, fluid velocity and hence volumetric flow rate can be determined.

Ventury Flow Meter

Ventury Flow Meter

Check out these youtube videos to understand them better.

Ventury Flow Meter

Ventury Flow Meter

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Nutating Disc Flow Meter

Nutating disc flow meter is used for measuring water supply. It is a reliable flow measurement device within 1 percent. This flow meter is used since last 100 years and it is highly reliable.Its function is unaffected by viscosity of the fluid and any other flow profile disturbances. It can handle suspended solids where other types of flow meters can malfunction.

This flow meter works both with a mechanical register as well as electronic registers. It can operate upto 150 PSI pressure and temperatures of upto 250 degree F.

This device consists of an eccentrically mounted nutating disc or wobbling disc which rotates when the fluid passes through it. Its rotations are counted by the mechanical or electronic register and thus gives direct indication of amount of fluid passing through it.

Nutating Disc Flow Meter

Nutating Disc Flow Meter

Check out this youtube video to understand its function.

Badger Nutating Disk

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Oval Gear Flow Meter

The oval gear flow meter is a kind of Positive Displacement Flow Meter. As the fluid being measured passes thru the meter, it rotates 2 oval gears in measuring cavity to displace a precise volume of fluid.

A sensor detects the gear rotation to determine displaced volume and flow rate. Fluid pressure rotates the oval gear, Figure 1. In position (a), the fluid exerts a clockwise driving force on gear A. there is no driving force on gear B. It is perpendicular to the flow so the fluid forces are balanced around the shaft. As the gear rotate to position (b), the fluid being to exert a force on gear B. At position (c), all the driving force is on the gear B. This alternating driving force provides a smooth rotation of almost constant torque.

The meter design minimizes the slippage between the gears and the measuring cavity wall. As a result, the oval meter is less affected than other designs by the liquid’s viscosity and lubricity.

Applications

Oval gear flowmeter is mainly used for high value liquids such as oil, chemical etc. Main applications include unloading, transfer and consumption monitoring.

Features

  1. Only Two Moving Parts
  2. Higher Accuracy, 0.5% or 0.2%.
  3. Standard Viscosity up to 5,000 cP(Centipoises = mPa.s).
  4. Size of wide flow ranges 0.04-340 M3/h (18-37,396GPM).
  5. Wide range of working environment –20 ~ +280 o C
Oval Gear Flow Meter

Oval Gear Flow Meter

Check out these youtube videos to understand them better.

Oval Gear Flowmeters for Viscous Fluids GPI GM Series Meters

Turbine Flow Meters vs Oval Gear Flow Meters

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Jitendra Surve About the author : Jitendra Surve is a graduate in petrochemical engineering, programmer, web designer, blogger, piping engineer, a geek as well as a good friend...!! If you would like to connect with him, follow him on Twitter.
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