Flow measurement Flashcards
Pipe flow.
The pipe is completely filled with fluid/slurry
Driving force is likely to be a pressure gradient along the pipe
Velocity distribution is usually symmetrical about pipe axis
Closed channel flow
Open channel or partially filled pipe
Usually a large scale flow
Gravity driven flow
Open channel flow
Situations where measurement of flow will be difficult
- flow is laminar or transitional
- fluids can be corrosive or toxic
- Multiphase systems
- Where flow obstructions cannot be tolerated
- Temporary indication ofor investigative work
Used to measure a flowrate or flow velocity
4-20mA
Quantification
Flowmeter
Used to detect a flow or absence of a flow within a pipe/channel (ON/OFF) digital
Flow switch
- fluid flow moves a mechanism placed in the flow path
- movement is proportional to flow rate of liquid
- integration - indication is given of the total quanityt of fluid passing in a given time interval
- Instrument is only accurate within a specified flowrate
- Gear, paddle, impeller rotated by fixed amount depebding on the amount of fluid that flows through
Mechanical integrating flowmeters
Advantages of mechanical integrating flowmeters
- good reproducibility and responsiveness
- simple low-cost structure
- compact and can perform large capacity measurement
Disadvantages of mechanical integrating flowmeters
- sensitive to forein objects (clogging)
- because the paddle wheel spins at high speeds, periodic maintenance is needed to deal with axial wear
- needs enough flow velocity to spin the paddle
Do not measure flow directly but infer it based on pressure drop over a constriction
Differential pressure flow meters
Three basic parts of differential pressure flow meters
- Device inserted in the flow line to produce a pressure difference
- Transmission lines which carry pressure information
- Secondary device which translates pressure information to flowrate
Advantages of differential pressure flow meters
- suitable for any clean fluid
- no moving parts in contact with medium
- can be indicating, recording, integrating
- no limit on pipe size or maximum flow
- can add a transmitting device
- easy to install. Some types inexpensive
- traditional well accepted technology
Disadvantages of differential pressure flow meters
- Square relationship between pressure drop and flow
- Permanent pressure drop
- Needs a developed flow proile
- Apparent flow rate is affected by temperature and density variations
- Can only be used with clean fluids
- Multicomponent@ needs pressure measurement
Plate with a hole in the centre is placed between two flanges.
Fluid flows through this orifice plate and loses pressure
The pressure drop across the plate is used to calculate flow rate
Orifice meter
Advantage of orifice meter
- SImplest and cheapest flowmeter
Orifice meter disadvantages
- unrecoverable pressure drop
- plate erosion will affect accuracy
Convergent - divergent nozzle system of two truncated cones
Fluid is accelerated through the constriction
This gives a pressure difference between upstream end and the throat
Venturi Meter
Venturi meter advantages
- higher energy recovery
- Secondary use: can draw in fluid at thr throat, for example to aerate the fluid
Venturi meter disadvantages
Most expensive meter of this type
Longer pipe section needed than orifice plate (12 times pipe diameter - this can be shorted by using higher outlet cone angles but at the expense of pressure drop
Nozzle meter advantages
- Compromise design
- Gradual approach (as with venturi, high discharge coefficient)
- No downstream section (as with orifice plate
- Difficult to manufacture
The actual flow divided by the theoretical flow
The discharge coefficient Cd
Problems with Q=Cd * A2 * sqrt(2 delta P/ density)
- cross sectional area of the orifice is not the cross sectional area of the fluid at P2
- It assumes frictionless flow
Volumetric flowrate
Q=Cd * A2 * sqrt(2 delta P/ density(1-beta^4)
beta = flowmeter diameter/pipe diameter
takes into account the additional narrowing that occurs after the plate
coeffecient of contraction
narrow point
vena contracta
The drop in pressure is constant and the flow rate is a function of the area of the constriction
Tapered tube with freely moving float
Fluid flow causes float to rise
Float weight is balanced by up thrust of the fluid
Variable area flowmeters
Advantages of variable area flowmeters
- easy to use
- cheap
- fairly accurate
- good direct visual indication of the flow
- available in a large range of sizes and flow ranges
Disadvantages of variable area flowmeters
- not suitable for unclear/dirty media as the float cannot be seen
- limited range
- only accurate at a given density
- display only (cannot communicate flow to a control system and can be made to output but ths is costly)
- have to be installed in a vertical pipe section (upward flow)
- relatively low pressure
requires a notch (weir)
rectangular or trigangular obstruction placed across the open channel (liquid flows through it then down)
the depth of waater behind the weir can be used to calculate flow rate
Open channel flow measurement
Types of electronic flowmeter
- electromagnetic
- ultrasonic
- vortex
- coriolis
- thermal mass
Based on induction principle
Alternative magnetic field
Flow generates a voltage proportional to velocity of liquid
Sensors in pipe (flush)
Electromagnetic flow meter
- Volume based measurement
- Works on movement of conductive liquid through pipe (does not work for non conductive liquids or gases)
- Seals for electrodes on pipe walls which are hygienic and can degrade and need replacement
Electromagnetic flowmeter
Electromagnetic flowmeter advantages
- measuring principle is independent of pressure, density, temperature and viscosity
- wide range of nomial diameters
- free pipe cross section: CIP/SIP cleanable, hygienic, piggable
- no moving parts, low maintenance an upkeep
- cheaper than ultrasonic
Electromagnetic flowmeter disadvantages
- electrodes on pipe walls can be daaged by heat even though they are coated
- Expensive
- Use is limited to conductive fluids
- does not work on gas or vapour
- needs to be installed with straight sections before and after for undisturbed flow
Acoustic signal sent from one sensor to another in both directions
Different transit times
Propagation velocity is greater in direction of flow than against direction of flow
Ultrasonic flow meter
- volume based measurement
- ultrasound can travel through the pipe walls
- readings affected by transit of ultrasound through material
- will be affected by interruptions to passage of ultrasound signal
Ultrasonic flowmeters
Ultrasonic flowmeters advantages
- best meter for non invasive applications as pipe can be completely free
- free pipe cross section: CIP/SIP cleanable, hygienic and piggable
- in line or clamp on design for stationary to temporary flow measurements
- long service life, no abrasion or corrosion from the fluid
- no moving parts minimum maintenance and upkeep
- measurement independent of pressure, density, temperature, conductivity and viscosity
- good fo large pipe diameters
Ultrasonic flowmeters - disadvantages
- relatively high cost due to close tolerances and high accuracies required for mechanics, acoustic and electronic portions of the system
- best for large lines and expensive when used with small pipes
- a long section of straight pipe is required
- Not good for certain media such as changing media, liquids with solid content, lots of air bubbles, the majority of gases
- build up on pipe walls can affect accuracy
Vortex flow meter
- when a fluid flows part a bluff body, vortices are alternatately formed shed on both sides, with oppositvve senses of rotation
- these vortices each generate a local low pressure. The pressure fluctuations are recorded by the sensor and converted to electrical pulses
- the vortices develop very regularly within the application limits of the device
- the frequenxy of vortex shedding is directly proportional to the volume flow
Vortex flow meter
Vortex flowmeter advantages
- suitable for liquids, gases and vapours
- high temperatures can be used (steam)
- measurement is independent of the fluid viscosity and density
- good range and accuracy
- no electrodes, better for chemical resistance
Vortex flowmeters disadvantages
- less well suited for slurries
- range limitations at high end and low flow ends
- requires a fully developed flow
- some pressure drop due to flow interruption
- not piggale or hygienic
- sensitive to pipe vibtration
what thermal mass flowmeters measures
mass
heat is drawn from a heated body when a fluid flows past
thermal mas flowmeter
two temperature sensors
second sensor held at constant differential temperature to the first
greater flow causes a greater cooling effect
the electric current required to maintain this differential is increases
well suited for gases
need to know thermal characteristics of material
has to penetrate into flow path for best accuracy
Thermal Mass Flow meter
Thermal mass flowmeter
- multivariable - direct measurement and display of mass flow and fluid temperature
- no pressure or temperature compensation required
- due to small size of the probe, it does not cause much disturbance or pressure loss to the flow
- high turndown
- excellent low end sensitivity
- quick reaction to fluctuations in flow
- good for gases
Thermal mass flow meters disadvantages
- better suited for gases than liquid
- relatively low maximum temperature
- reduced accuracy compared with coriolis meters
- has to penetrate into centre of tube for best accuracy
- operator has to set-up specifications that match the type and composition of the fluid
- sensitive to flow disturbances
- errors occur when there are deposits/build up on the temperature sensors
- fragile can be used only with high reliability in clean gas flows
- needs to be recalibrated frequently due to dust/dirt accumulation
- operating characteristics are non linear
- it does not sense the flow direction
Vibrating U shape pipe sections
when fluid flows the coriolis effect causes twisting of the pipe section
amount of twist is proportional to mass flow
direct mass based measurement
Coriolis flow measurement
Advantages of coriolis
- direct measure of mass flow
- very accurate direct measure of mass flow
- sanitary applications
- good in small pipes
- no lead up pipes required (undeveloped flow OK)
- unaffected by air bubbles so long as not too high
Disadvantages of coriolis
- most expensive
- not suitable for larger pipes
- obstruction to pipe flow - pressure drop for bent tube tupes, not piggable and can cause cavitation
Problems for flowmeters
- limescale
- sludge
- rust
- slime
- slurries
- air bubbles
- undeveloped flow
- piping vibration
