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EPIC Lunch and Learn Series: Flow Meters

EPIC Modular Process Systems designs, fabricates and installs complete modular process systems. Almost always, these modular process systems involve the use of flow meters. EPIC Modular Process hosted a lunch and learn on flow meters to refresh and expand our process engineers knowledge of this key piece of process equipment.

Measuring flow is nothing new to the engineers at EPIC or to humanity. The first attempts at flow measurement were made over 8000 years ago by the Romans, who wanted to charge money for the use of bath water, according to the Encyclopedia of Water Science. The Romans didn’t quite get it right though, neglecting to take into account the velocity of the water they were measuring. It wasn’t until the Hero of Alexandria, who worked out an equation that correctly measured fluid flow, that time, volume, velocity and area were all taken into account for measuring flow.webflowmeterworkshoppic copy_1

Sadly, the Hero’s breakthrough was largely ignored for 1500 years. It wasn’t until 1690 that correct measurement of flow became well known, after the concept was discovered in Leonardo Da Vinci’s and Catelli’s manuscript on hydraulics, which extended flow measurement into the continuity equation.

Thankfully, today the correct way to measure flow is known and there are many technologies and methods for measuring flow accurately in your process system. Wayne Brinkman and Brian Lux from Emerson provided information on the various types of flow meters and the best practices for applying them in today’s manufacturing environment.

Flow is measured for many different reasons. Perhaps you need to know how much fluid is passing from supplier to customer, for example, gas going from the station tank into the your car. Maybe you need to know your total throughput or you want to check your products integrity. Whatever the reason, flow is a common thing that needs to be accurately measured.

Basic Guidelines for Flow Meters

There are some basic things to remember when selecting the correct flow meter for you application and using a flow meter in your process system. They may seem simple, but neglecting any of these can cause your flow meter to work incorrectly or not at all. These basic guidelines include:

    • Size around capacity
    • Be accurate regarding required turn-down and performance
    • Select the right technology based on fluid expected flow rates
    • Install the technology using the manufacturers recommendations
    • Insure you have the required amount of straight pipe run
    • Use proper valves for isolation
    • Install impulse line correctly
    • Configure the meter correctly
    • Maintain the system properly once it is installed
    • Utilize your suppliers for support

 

If you have an existing flow measurement system, making sure it is measuring accurately is important. There are three types of tests that can performed on existing flow meters, depending on the desired result at the end of the test.

    • Calibration – involves establishing a relationship between flow and signal produced by the sensor. This is not something that should normally be done in the field because it is hard to control the situational elements in the field.
    • Validation – confirming flow performance by primary flow standard to sensor readings
    • Verification – establishing confidence in performance by analysis of secondary variable associated with flow.

 

Basic Flow Principles

Some basic flow principles have been established that apply to all flow measurement applications. Velocity based flow meters require a minimum

Reynolds number, which gives a measure of the ratio of inertial forces to viscous forces. The Reynolds number can be used to distinguish between the two

major types of flow:

    1. Laminar flow – 2000 or less Rd – consists of streaming layers sliding over one another in a fluid motion
    2. Turbulent flow – 4000 Rd and up – which is characterized by unstable flow and fluid that is susceptible to inertial forceswebflowmeteremerson

 

The type of flow being measured is important because certain flow meters require turbulent flow and cannot measure laminar flow.

An important installation consideration for velocity based flow meters is the number of straight runs on either side of the flowmeter. The correct amount of straight run before and after your flow meter is vital to getting accurate measurements. Fluid going around a corner gets thrown to the outside of th

e pipe, but as it goes

down a straight run it becomes more and more uniform.

Specifying Flow Meters

The upfront information needed to specify a flow meter includes the following:

    • Flow rate (minimum, normal and maximum)
    • Fluid state (gas, liquid..)
    • Fluid temperature, pressure, density and viscosity (minimum, normal, and maximum)
    • Acceptable maximum pressure drop
    • Accuracy of measurement desired

 

There are three major classes of flow meters. The rest of this blog post will go through these three catagories of flow meters and provide basic guidelines for use.

 

Differential Pressure Flow Meters

The concept behind differential flow meters comes from three sources, the theory of continuity (thanks Da Vinci!), Bernoulli’s equation, and correction factors.

Basically, in dP flow meters a restriction is introduced into the flow of the pipe and flow rate is measured. Flow rate is proportional to the square root of the differential pressure. dP meters are most accurate at low flow rates when a multi-variable compensating meter characterized at low dP’s is used. Differential flow meters consist of three major parts:

    • Primary element – orifice plate, conditioning plate, or annubar
    • Secondary element – transmitter
    • Connection system – manifold, valves, ext…

 

Things to keep in mind when specifying a dP meter include:

    • They are suitable for gas, liquids and steam
    • Calculated mass flow and standard volume can be measured
    • They have moderate turn-down
    • Some installations will have long, straight pipe requirements
    • There is a potential to plug impulse lines
    • Potential leak points must be identified

 

Best practices for dP flow meter management focus on minimizing leak points. The best way to do this is to eliminate the use of impulse lines in favor of direct mount installations. Some dP flow meters are available with a diagnostic feature built-in. This diagnostic feature runs a test periodically to take statistical measurements and alert operators when the meter is getting out of range.

The major dP flow meter types include:

 

Orifice Plates

The orifice plate was first introduced in 1915. Flow rate is proportional to the square root of the measured pressure drop across the plate.

Advantages:

    • Well understood
    • Known Cd, no need to wet for calibration
    • Flexible
    • Excellent response time
    • Work well in tight spaces

 

Disadvantages:

    • Pressure loss
    • Potential for plugs

 

A special type of orifice plate, the integral orifice assembly is an orifice meter designed for small lines – lines 1.5 inches in diameter or less.

Annubar Flow Meter

Annubar flow meters have holes on both sides of the annubar and measure pressure on each side of the disturbance. Flow is still related to the square root of differential pressure. Annubar flow meters experience a relatively low pressure loss and provide a high measure of accuracy. However, they are most suitable for clean liquid and gasses.

Conditioning Plates

Conditioning plates measure dP across a “flow straightener” which consists of 4 holes through which the process fluid passes. This type of flow meter requires little upstream and downstream straight runs of piping to meet accuracy specification, however it performs better than an orifice plate because it is virtually unaffected by upstream disturbances.

Volumetric Flow Meters

dP flow meters can measure on volume as well, but volumetric flow meters here refers to meters that only measure based on volume.

Types of volumetric meters include:

 

Turbine meters

Accurate measurement is achieved by using a bladed rotor that turns at a speed proportional to rate of flow. The rotation of the rotor is sensed by electrical pick-offs mounted on the meter body, generating a pulsing voltage. The total number of pulses collected over a period of time represents the metered volume. Accuracy is density and viscosity dependant, and they are most accurate when first installed, because they wear over time. They are very cost effective to install

Ultrasonic flow meters

An ultrasonic beam is shot across the stream to measure flow in transit time ultrasonic flow meters. They can only be used in clear fluids because and bubbles or debris will absorb the ultrasonic energy and the beam will not go all the way across flow. These have 1-2 % installed accuracy, obstruct less flow, are velocity and application sensitive, and are best used on large lines. They can also be bi-directional.

Positive Displacement flow meters

PD flow meters require the fluid being measured to mechanically displace components in the meter in order for any fluid flow measurement to occur. These should be used only with clean lubricating fluids. If the temperature or the fluid changes this can cause the calibration to be off, causing in accurate measurements. These meters also have lots of moving parts, which means many potential wear points and they can clog easily. However, no power is required to run a positive displacement flow meter.

Vortex meters

The operation of vortex meters is based on the von Karman effect. Imagine a flag flapping in the wind. It flaps because of the von Karman effect. The flag pole is acting a as a shedder bar, which alternately separates fluid on either side of the bar face, causing vortices’s, and causing alternating differential pressures around the back of the bar (or flag). The frequency of the alternating vortex development is linearly proportional to flow rate. Alternating DP flexes a portion of the shedder bar, and the flexing motion is detected by a sensor outside of the flow line, which converts the alternating forces to an electrical signal.

Vortex meters are also available in a plug free design, where the sensor is not directly in the process flow. Vortex meters are moderately accurate and are good for utility applications, gases and liquids. A high Reynolds number is very important for proper operation of Vortex flow meters. Usually the best way to install a Vortex meter is reduce down to a smaller meter, for example a 2 inch flange to 1.5 inch vortex meter.

Multi Variable (MV) vortex design – These meters measure both flow and temperature.  The temperature sensor is integrated in the shedder bar and accurately measures the temperature of the flow stream.

Considerations when using a vortex meter:

    • Steam – is it saturated and superheated?
    • Multi-variable mass flow of steam
    • Can measure any liquid with low viscosity
    • Vortex meters cannot measure down to zero – stops measuring when flow goes from turbulent to laminar
    • Suitable for liquid, gas or steam- generally have good accuracy, are cost effective, and loop powered
    • Integrated flow meter
    • Multi-variable measuring option
    • Lower cost of installation – no mechanical moving parts
    • Offered in stainless steel, Hastelloy, and carbon steel
    • Pipe installation – must ensure the pipe stays full
    • Vertical upwards flow is best for liquids
    • Vertical downwards flow may be acceptable for gas or steam
    • Horizontal flow is acceptable for most applications
    • Make sure the arrow on the meter body is pointing in flow direction
    • Connections – flanged, wafer and weld-end
    • Thum option – can add a Thum, which will turn the vortex flow meter into a wireless transmitter. Thums can be used with high power or legacy devices – scavanges power off the loop or transmitter and transmits the signal wirelessly.

 

MAG meters (Magnetic Flow Meters)

MAG meters come in three types: flanged, wafer and sanitary style. Magnetic flow meters are based on Faraday’s law, which says that if you move a conductor through a magnetic field a voltage will be generated. These flow meters require a conductive process fluid, and cannot be used for gases, fluids with a lot of entrained air, foam or two-phase flow.

MAG meters used to only be available in alternating current (AC) versions, but most used today are direct current (DC) meters. AC meters work well in high-noise applications, but DC meters are more accurate, provide continuous automatic zeroing, are immune to power line disruptions and are available in high signal versions that work well in noisy applications.

Electrodes on the MAG meter sense the voltage generated. These electrodes come in platinum, stainless steel or Hastelloy and a number of other materials.

Vertical flow up is best practice for MAG meter installation. Horizontal flow can only be used if you can assure the liquid level will continuously cover the electrodes. Vertical down is a definite no-no with this type of flow meter.

MAG flow meters do have straight run requirements; however they are less susceptible to uneven flow profiles than other types of flow meters. Non-uniform flow profiles usually only cause a 1-3 % shift in measured flow rate.

Line sized MAG meters are not always advisable. Smaller-than-line-size magmeters may be suitable for the application, will be more accurate and less expensive. Gaskets should always be used with MAG flow meters, but not metallic or spiral wound gaskets, because these can damage the flow meter. MAG meters have a low pressure drop, can handle bi-directional flow and are immune to changes in fluid properties.

A simply calibration verification test can be performed on MAG flow meters. A voltage is passed through the transmitter to represent a velocity of 30 ft/sec, which the meter should read back at 30 ft/sec. If it doesn’t, your meter is not working correctly. Verification will also measure electrode resistance and coil inductance.

Coriolis Meters

Coriolis meters have a wide range of flow rates that they can handle. Anything from 0.5 gal/min up to 35 million grams/min. Typical applications for larger flow meters are in loading tankers with oil or liquefied natural gas. High temperature coriolis meters are made of Hastelloy or super duplex. Super duplex can handle high pressures and is resistant to chlorides.

Mass flow meters can measure two distinct things:

  • Direct flow mass measurement – as the mass flows through two vibrating tubes inside the meter, the tubes start to twist. Higher mass flow = more twist. Sensors generate an electrical signal based on the shift in the tube, which is sent to the transmitter for measurement. The shift in the phase of the signals is proportional to the mass flow rate.
  • Direct density measurement – based on the natural frequency of the vibration of the tubes. As the mass of the system (ex. a high density fluid in the tubes) increases the natural frequency of the system decreases, and as mass of the system (ex. a low density fluid in tubes) decreases, the natural frequency of the system increases.

 

Coriolis meters can also be used to measure gas. In this instance they are measuring standard volume and acting as a molecule counter. Gases are compressible and their density changes with pressure and temperature, but their mass does not change. A standard cubic foot of a substance always has the same number of molecules. With a Coriolis meter you can get a standard gas flow measurement that is immune to changes in temperature or density.

Benefits to using Coriolis meters include:

    • Direct mass and density measurement
    • High accuracy
    • Wide turn-down
    • No moving mechanical parts
    • Easy installation and start-up
    • No flow conditioning or straight-run piping required
    • Bi-directional measurement is possible

 

Applications for Coriolis meters include:

    • Measuring density
    • Measuring temperature
    • Measuring specific gravity
    • Leak detection
    • Measuring mass flow of gases
    • Testing interface direction
    • Finding the % concentration
    • Custody transfer
    • Finding net solids
    • Blending
    • Measuring net volume
    • Batching % of solids
    • Finding % HFCS

 

To verify your Micro Motion Coriolis meter is working correctly, Micro Motion uses a structural integrity test along with electronics diagnostics checks to verify performance. Field verifications are compared to baseline information that was collected at the factory before the meter was installed on location.

If a coriolis meter has stopped working correctly, the most common causes are cracked sensor tubes, physically altered tubes through corrosion/erosion or exposure of the tubes to high temperatures.

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