Level Measurement Flashcards
Why measure level?
- determing the position of a surface inside a tank, reactor or other vessel
- inventory management
- safety
- process efficiency
- maintaining a consistent supply
Basic operating principles level measurement
- continuous 4-20 mA or point level
- Simple techniques based on specific gravity
- Conductance & capacitance
- Advanced techniques
Considerations about level measurement
- level cannot be always be simply used a measure of volume
- fluid density can affect some methods
- mass measurements are not affected by temperature
- complex vessel geometries
- might needs a height/volume calibration based on vessel geometry
do not measure level but indicate the presence of absence of a media at a certain point only
level switches
- less risk of leakage
- enables level measurement devices to be installed/removed without emptying the tank
- may or may not contact the process fluid
- can use cabes or rods to extend through the vessel
Top down measurement
- contacts the process fluid
- greater risk of leakage
Bottom up measurement
Simplest methods for liquid level measurement is to use this; this is an external clear tabe connected to the lowest point on the tank. The level in the tube is then a direct indication of the level in the vessel. A coloured float within the tube makes it easier to see the level
Sight glass
Disadvantages of sight glass
- not suitable for toxic or flammable fluids as the tube could be damaged
- weak spot of the instllation
- replaced by more advanced technologies
Advantages of direct level measurement techniques
- indicators easy to read from a distance
- customisable for different applications
- low cost
- easy to install
- simple to operate
- don’t need power supply
Disadvantages of direct level measurement techniques
- prone to corrosion if liquids and materials are not compatible
- does not work properly if placed in a slurry or liquid solution with surface particles
- build up of material on the float causes changes in weight displacement
- can only be used with non freezing fluids
- indication only
Used for alarm and control functions.
The indication of low or high levels to maintain between these two points
Level switches
- simplest and least expensive form of level sensor
- can be as little as £20
- typically made of steel or chemically inert plastics such as nylon,polypropylene or polyphenyl sulphide
- can act as low level or high level switches
- used for small vessels but not for key process vessels
Float switches
More expensive float switches are available but the increase in price results in these being in competition which more sophisticated electronic devices
Disadvantages of float switches
- moving parts so limited to clean process materials
- not readily cleaned
- need to match float density to process fluid density
- suitable where fluids contain solid matter
- float contains a micro switch
- or float can be suspended from a conventional float switch
Suspended float switch
Tuning fork in vessel is excited at its resonant frequency. When fork in submerged the fork changes its oscillating frequency
Vibronic point level detection
AKA Liquiphants
Vibronic level sensors advantages
- not affected by media properties
- no calibration needed
- resilient and unlikely to wear
- does not take up much space in the vessel
- self monitoring
- hygienic design and materials
Vibronic level sensors disadvantages
- reliant on contact with the material
- low level switches have to be installed at the base of the vessel
- can be affected by material build up
- AC voltage between probe rods
- Liquids creates a connection and current dlows
- Allows multiple point level detection in conductive liquids
Conductive point level switches
Conductive point level switches advantages
- can be installed at top of vessel
- multipoint detection with a single entry point
- no moving parts
- no calibration needed
- resilient and unlikely to wear
Conductive point level switches disadvantages
- only works on conductive liquids
- instrument can be big and cumbersome
- reliant on contact with the material
Paddle type switches are the most common. The paddle is driven by a motor. When material contacts the paddle, rotation stops
Level switches for solid
- vibrating rod
- rod is driven at resonant frequency
- if medium covers the rod, amplitude changes
soliphants
transmitter output for level measurement
4-20mA
Technologies used to measure level
- hydrostatic pressure
- air bubbler
- load cells
- ultrasonic
- radar
- nucleonic
- capacitance
- electromechanical
- weight of liquid creates hydrostatic pressure
- wall mounted, rod or cable
- pressure transducers
- housed in stainless steel
- pressure sensors as described in previous section
Hydrostatic level measurement
- Can be used in pressurised vessels
- Top and bottom of vessel are connected to either side of a differential pressure measuring device
Differential pressure
If density varies then liquid level measurement will not be accurate
Advantages of differential pressure
- hygienic
- pressure transducers can work with almost any tank geometry
- wide pressure and temperature range
- unaffected by foaming, conductivity, dielectric constant
- well known easy to use technology
- low cost option
Disadvantages of differential pressure
- less accurate than other level measurement methods
- indirect measure of height
- can requires multiple detection points
- DP transmitters are subject to errors due to density variations of the liquid
- DP’s are primarily useful for clean liquids
- flow of air is passed through a dip tube in the vessel
- measured pressure divided by liquid density gives the depth of the liquid
Air bubblers
Advantages of air bubblers
- low cost
- no moving parts
- no electrical parts in contact with media
Disadvantages of air bubblers
- Requires constant supply of air
- Better suited for open vessels
- large vessels that have three support legs can have the load cells as an integral part of the legs
- this allows easy access to the instruments, manway, pipework etc on top of the vessel which is about 1m height from the floor
- the floor below then allows access to the discharge valves, instruments, discharge pump etc.
Load cells
Load cell advantages
- key advantage is that load cells are non-invasive
- no openings in the vessel are required
- direct measure of the mass of material (but not level)
Load cell disadvantages
- load cells are expensive
- very sensitive
- the vessel has to be able to move
- cannot be retrofitted
- they require on site calibration using certified weights on small vessels or calibration using set volumes of a fluid
- they also require a control unit which takes the signal from the loads cells and converts this to a measurement of weight
- not suitable for very large vessels
Tricky scenarios for direct measurement of level
- measurement of an oil-water interface
- measurement of level in a vessel that cannot have any process connections
- direct level measurement where the headspace contains dust and vapour and is under vacuum
Pulse is emitted, reflected from the material surface and then detected by the sensor
Time of flight principle
Time of flight is proportional to the distance travelled
Height not volume measured
Can also be used for bulk solids
Have to consider how the materials reflect sound/radiation
Types of electronic level measurement
- Time of flight
Ultrasonic, non contact radar, guided wave radar - Gamma (nuclear)
- Electromechanical
A sensor emits ultrasonic pulses, the surface of the media reflects the signal and the sensors detects it again
The time of flight of the reflected ultrasonic signal is directly proportional to the distance travelled
Ultrasonic
Time of flight principle
L = E - D
D = C * T/2
Ultrasonic advantages
- lowest cost TOF system
- Unaffected by material properties such as density, dielectric constant, viscosity
- Transducer does not come into contact with the process material and does not contain any moving parts
- A single top side tank installation makes leaks improbable
Ultrasonic disadvantages
- sucessful measurement depends on reflection from the - process material in a straight line back to the transducer
- care must be taken to avoid interference with the vessel
- a minimum liquid density is required to reflect back the ultrasound
- contruction materials limit temperature and pressure compared with other systems
- can measure solids but not as good as radar or nuclear
- ## has to be a certain height above the liquid - blocking distance
Microwave signal bounced off product surface
Signal strength depends on dielectric constant of material
Non-contact radar
Non contact radar advantages
- liquids or solids
- higher accuracy than ultrasonic
- radiation - so not affected by vacuum above liquid
- not affected by vapours
Non contact radar disadvantages
- successful measurement depends on the transmitter being mounted in the correct position so that the internal structure of the vessel will not interfere with the signal path
- relatively high cost compared with ultrasonic
- minimum dielectric constant is required for material to reflect microwaves
Positioning of TOF systems - Ultrasonic
- TOF systems will not work properly if they are too close to the liquid surface
- false echoes should be avoided
Positioning of TOF systems - Radar
- consider the beam angle
- stationary surfaces should be kept outside the beam
- avoid inlets
- do not install in tank centre
- Mounted on the top of the rank or chamber
- Probe usually extends to the full depth of the vessel
- Pulse of microwaves is sent down the probe
- At the interface microwave is reflected back up the probe
- Secondary echo in liquid/liquid interfaces
Guided wave radar
Guided wave radar advantages
- liquid or solid
- height accuracy
- unlike other TOF techniques, interfaces between two liquids can be measured
- not affected by foaming, vapour etc. in the same way as the non-contact radar
Guided wave radar disadvantages
- Contact measurement : rods extending through vessel, less suited for agitated vessels, less suited for slurries or viscous/sticky materials and can be coated by media
- Minimum dielectric constant required
- Gamma radiation source placed in a lead container on one side of a tank
- Gamma detector placed on other side of tank
- If tank is empty detector receives most of the radiation
- as tank is filled the liquid level will attenuate the radiation
- radiation passes through vessel walls as well as process fluid
Gamma (nuclear) level measurement
For gamma level measurement, can be used to indicate a specific level
point detector
For gamma level measurement, will detect the range of heights
rod detector
Gamma advantages
- liquid or solid
- non-invasive
- good for extreme conditions
Gamma disadvantages
- most expensive level device
- requires extra documents/paperwork, safety measures and regulations dependent on country
- radiation sources are expensive and difficult to dispose of
- Process fluids have different capacitance to air
- Probe and tank wall form a capacitor
- Capacitance is dependent on the level of fluid in the tank (air has lower capacitance than process fluid)
- Capacitor is two electric plates separaed by a dielectric
Capacitance level measurement
Capacitance formula
Capacitance = constant * relative permittivity * area/ distance between electrode plates
Capacitance advantages
- solids and liquids
- high temperatures and pressures possible
- cost effective
- fast speed of response
- interface measurement possible for combinations of low and high conductivity
Capacitance disadvantages
- intrusive technology since the probe needs to pass vertically through the tank
- Must be re-calibrated with different media
- needs a minimum dielectric constant to work
- not as good for solids as radar or nuclear
- tank wall materials have to be considered
- chemical compatibility issues
For … the metal capacitance probe is one electrode, the metal tank wall acts as the second electrode and the liquid is dielectric
non- conductive liquids
For … (which do not act as dielectrics) the metal probe is one electrode and is insulated with a dielectric. The liquid in the tank coupled with the metal tank wall is the second electrode
conductive liquids
- measuring “tape” loaded with a sensing weight, is lowered into the vessel
- As weight meets the surface of the product, the tension on the measuring tape is relaxed
- This is detected by the device electronics
- after measurement the weight is retracted
- measured value retained until next measurement is taken
- ## Height is transmitted at 4-20mA current
Electromechanical
Electromagnetic advantages
- weight or float can come in a variety of shapes sixes and materials
- can be used in solid. liquids and slurries
- can take individual or periodic measurements
Electromagnetic disadvantages
- slow and infrequent compared with other techniques
