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Level Measurement - Information Resource Center
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file:///E:/project%20study/IPC%20notes/Level_Measurement
Level Measurement
From Information Resource Center
Objectives:
Upon completion of this lesson, the student/trainee must be able to:
Differentiate between point level and continuous level measurement;
Discuss how a differential pressure transmitter can be used as primary device to measure liquid level both in an
open and pressurized vessel;
Discuss the ways of using capacitive level probe in conductive and nonconductive media;
Discuss the ways of handling corrosive fluid and interface problems related to liquid level measurement and
List the parameters that can be measured using HTG
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A wide variety of
options are available, some more universal than the
others, and each has its own advantages and
limitations
...
The last part of the material
offers a rough guide on how to go about choosing the
right technology for a specific liquid level
measurement from Fisher-Rosemount Instruments
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The first one works basically like a
switch which gives a discrete signal for a pre-set
liquid level, examples are: vessel alarm functions and
in pump controllers
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Bouyancy Based Systems
Floats
Float is the simplest, albeit mechanical liquid
level measurement device
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Floats can be
mechanically designed to fit point level and
continuous level measurement applications
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Floats are more tolerant to turbulence on the
surface of the liquid (pumping into vessel) and can be
calibrated without actually moving the liquid level
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Displacers
Displacers can provide either continuous level,
point level and density measurements for liquids
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The vertical movement of the displacer is
transferred to an angular movement through a level
assembly
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Displacers are generally used on cleaner fluids
over a wide temperature and pressure range, and can
provide an economical way to measure short span
levels, or between 14 and 36 in
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They are often used for
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liquid-liquid interface measurements, or in a
continuous process vessel
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To
maximize stability, the displacer is usually mounted in
a cage outside of the tank
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Float Switch
Pressure Type Devices
Pressure, measured at the bottom side of a vessel will increase proportionately as you pour-in more fluid into the
vessel
...
Not surprisingly, pressure transmitters are the most common devices for continuously measuring basic liquid levels
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An example is paper pulp stock, which become more like a solid than a liquid as it nears the
13-15% consistency level, changing its pressure characteristics
...
They also allow the devices to be used on the underground tanks, where access is limited
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Because the pipe is immersed throughout the full level range, the bubbler system is immune to turbulence, foam
and viscous products as long as it stays open
...
They can't be used with fluids that solidify,
since this can plug the pipe
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Generally, bubblers are
used with open or vented vessels, but they can also be used in closed systems
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5%, far less precise than most pressure transmitters
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If the product tends to coat or
solidify, then the pipe may become plugged and artificially high level reading
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Thus, for
water, a reading of 60 indicates a level of 60 in
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9, pressure reading of
54 would mean a level of 60 in
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Gage pressure transmitters are
essentially differential transmitters which record
pressure changes against the reference of the
atmospheric pressure
...
Pressurized tanks, however, require a
differential pressure transmitter whose low-pressure
side is referenced to the pressure above the fluid
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First is the fluid's density
...
In reality, however, many fluids change
densities as their temperatures or concentrations
change
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When this
measurement is made, the taps for the transmitter
must be placed so that the upper tap remains flooded
by the process fluid
...
,
the upper tap for the density transmitter must be at or
below the 20-in
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Once the change in specific
gravity is measured, that value is used to correct the
level equation
...
If the liquid is stratified,
then its measured density may not represent the whole fluid
...
Pressure transmitters are usually mounted on the side wall
near the bottom of the tank, but sometimes they can also be placed on the bottom of a suspended tank
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Bottom-mounted pressure transmitter work well in vessels that are subject to turbulence, as long as they are
protected from the major impacts of the agitation
...
However, all pressures transmitters must be placed so
that process materials cannot build up on their surfaces
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They allow
transmitters to work with fluids of much higher temperatures, and are available in a wider range of materials than are the
transmitters
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Differential Gage with two remote
seals
However, the entire sealing assembly-seal, capillary, and fill fluid-must be chosen carefully and mounted correctly
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optimize performance
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These seals are used with long capillaries with big internal diameters, because they hold a large volume of fill
fluid, which expands and contracts as ambient temperatures rise and fall
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In addition, many widely used fill fluids have large coefficients of
thermal expansions, which contribute to the temperature error
...
Not only must they be compatible with the
process, but they must also withstand the temperature extremes of both the process and environment
...
A fluid that is rated to 400°F at
atmospheric pressure may only be good to 200°F under full vacuum
...
For vacuum applications, the
transmitter should always be mounted below the level of the bottom tap
...
Both of these measures help prevent damage to the
seal, and assure proper functions of the entire assembly
...
H2O for an ambient temperature change of 25°F
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level reading, this equates to a 10-20% error! Therefore, if you want reasonably good performance from your
transmitter and seal assembly, choose components carefully
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Seals with wide-diameter measuring surfaces, around 3 in
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Bubbler System
Hydrostatic Tank Gauge
Combining pressure, temperature, mass, density, volume, and level, Hydrostatic Tank Gauging (HTG) offers a
versatile measurement system for liquid inventory and process measurements
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The length of an arrowhead is usually 3 mm, but it may be drawn up to 6 mm long on larger drawings
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All arrowheads on a drawing should be of the
same style and size
...
Because HTG devices can be
placed near the bottom of the tank, they eliminate
errors that occur with top down measurement systems
when the tank roof moves during filling and emptying
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While HTG
systems are typically used in storage tanks where
temperatures and pressures are not excessive, they are
capable of handling up to 375°F and 3,000 psi
...
The outputs from those
devices are sent to a tank side interface unit, where
data are compiled and calculations completed
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The average area of the tank is based on the
current product level, determined by the pressure
difference, and "strapping table data," indicating the
level-to-volume profile unique to that particular tank
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The tank volume is
determined by the mass measurement divided by the
density measurement
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A
temperature measurement, taken between the bottom
and middle pressure transmitters, is combine with the
product's measured density and correction factors to
calculate standard density and volume
...
For a closed tank, the following relations
are involved:
Resistive Probe Type
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For point level measurement of conductive
fluids, a simple conductive probe is recommended for
its simplicity
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In a typical application like pump
controllers, as the liquid is being discharged, at some
point the liquid level clears the probe tip
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Mineral plating in probes will decrease
sensitivity
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Resistive probes are prone to liquid splash
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They can provide either
point or continuous level measurement, and may be
used on either liquids or solids
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Capacitance
instrumentation can also be set up to measure the
interface of two fluids with different dielectric values
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However, the technique is not
well suited to processes in which the dielectric
property of the process medium changes, or where
nonconductive products, such as grain slurries, build
up
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Point level devices may be mounted on the
tank's side, with the probe extending horizontally into
the fluid or at a slight angle
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Insulated probes work
with both conductive and nonconductive applications
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In nonconductive applications,
the measuring circuit applies a high-frequency signal
to the probe, which acts as one side of a capacitance
plate
...
Capacitance devices work with slurries, foam-filled and turbulent fluids, as well as those whose density changes
...
For conductive applications, with water-based liquids, the insulation of the probes provides
the dielectric and the liquid itself, the second dielectric
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For tanks
with curved walls, a reference must be used to create a constant distance between capacitance plates
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C = k (A/d)
The area of the plates covered by process material depends on the fluid's, so measured capacitance is proportional to
the process level
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Metallic tanks with straight walls can serve
as one of the capacitance plates
...
Occasionally, there may not be
sufficient change in capacitance over the desired level span
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With conductive applications, the fluid becomes the second dielectric plate and the insulation on the probes
becomes the dielectric
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Capacitance is still proportional to level
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When both the level and the dielectric change, each must be
considered as a variable; systems are available that compensate for these variations, but they work best with homogenous
fluids
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The level will be read, up to where the coating starts
...
If the coating is even, a coating rejection circuit may be set
up by driving the probe at higher frequencies or incorporating a phase detection circuit
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If the tank or holding vessel is nonmetallic, a process ground must be added
...
Incorporating a concentric metal shield into the probe's construction solves both problems, providing grounding,
and acting as a constant reference source
...
Other options are adding a reference probe parallel to the device probe, or incorporating a "stilling well," a pipe
traveling the length of the level measurement
...
depth, a cable is used in place of the probe
...
They
work well in clean, solids-laden viscous or corrosive
fluids, and some slurries and aerated liquids
...
Continuous ultrasonic level devices are
noncontact instruments, attractive for corrosive
process or slurries that would foul or corrode a
standard process-contacting device
...
To protect them from corrosive vapors, some devices
can operate through a Teflon protective covering
...
Continuous ultrasonic devices are limited to
pressure from slightly negative to about 100 psi and
temperatures below 200°F
...
This pulse is reflected off the
process surface and returns to the sensor as an echo
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The most recent
device feature's microprocessor capabilities, providing
echo-processing algorithms and higher energy sensors
that make them easy to setup and use, and more
accurate than other models
...
Today's include microprocessor capabilities and
sophisticated echo-processing algorithms, as well as higher energy sensors
...
In other cases, the foam can absorb the waves so that no signal is returned
...
Turbulent surfaces can have a similar effect and can absorb the sound wave
...
Vortexes caused by agitation can also misdirect the echo
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Otherwise, if product flowing into the tank is in the path of the sound wave, the wave will reflect off
the product flow
...
This is specially true if the product level drops below any of these devices and that point
will be interpreted as the level measurement
...
Baffles or stilling wells can be installed to limit this problem
...
In these
devices, a sound wave is passed between a transmitter
and a receiver, which are fixed distance away from
each other Point level Ultrasonic Sensor
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To provide a general idea of level within a tank,
ultrasonic point level devices can handle pressures up
to 1,000 psi and temperatures up to 320°F
...
The devices are actuated only by the presence or
absence of liquid in the sensing gap
...
Like continuous models, they have no moving
parts
...
They are unaffected by
foam, coating and droplets
...
Devices are available in a variety of lengths for
top-down installations
...
The probes can
also be mounted horizontally, with the sensor facing
sideways, to allow fluid to drain from the sensor area
...
Radar Type
Although radar technology has been around since WWII, it has only recently been applied to level measurement
...
Although its primary use have been in inventory and custody transfer applications,
where accuracy is crucial, it is rapidly becoming more affordable for process level applications
...
Radar devices measure level independently of changes in
density and dielectric constant
...
In addition, most radar devices cut through foam layers, and are unaffected by changes in the vapor space
...
Radar or microwave
signals are sent from the transmitter to the surface of the material and reflected back to a receiver
...
The
velocity of the microwave signal, though, is the speed of light, divided by the square root of the dielectric constant of the
gas or vapor through which it travels
...
The accuracy of a radar device can be as good as 1 mm
...
Radar can work from -40 to 800°F , and from vacuum to 800 psi, and
devices can also see through foam layers to the more-reflective product surfaces, so that measurements are only
minimally affected by changes in air temperatures, by dust, dust particles
...
These devices are
noninvasive, making them ideal for use with corrosive
or viscous liquids or at extreme process conditions
...
Nucleonic instruments measure the intensity of
nuclear radiation as it passes through a vessel
...
They are commonly used to measure the
interface between vapor, foam, and coke on coke
drums
...
Nucleonic devices measure the intensity of the
nuclear radiation as it passes through a process fluid
...
In
addition, periodic 'wipe checks' are needed to check
the surface and housing integrity
...
The effective exposure is
typically less than the radiation from a single chest
x-ray
...
Why is the measurement needed? Do you need a rough indication of fluid level, or do you want to know exactly
how much product is in the tank? For example, if you only want to keep liquid in a tank from a spilling over, or to
know when to refill a tank, then a point level device may suffice
...
What are the conditions in or on the vessel? High temperatures and pressures may not affect the way that some
level measurement devices perform, but they may affect other's performance
...
3
...
4
...
5
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It may also be difficult to install some devices on double-walled or smaller tanks
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6
...
Vibration and electromagnetic interference
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may also have an impact
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What are the process fluid's characteristics? Corrosive processes may require devices made from exotic materials,
so it may be better to consider a device that does not have come in contact with the fluid
...
In addition, if the
fluid's density level changes, the level reading from a pressure device will be affected and should be compensated
...
If the
process fluid coats an instrument's sensing surfaces, it will affect the sensitivity of the most devices
...
What are the instrument requirements? What level of accuracy is needed? A device with 0
...
06 in
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5mm, on a 5-ft level, in a stable environment
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The impact of temperature and other parameters
should also be considered
...
How much does the device cost, per unit? Would a lower-grade, less-expensive model to the job?
10
...
Explosion Proofing
may be sufficient for many devices, but some plants or applications may require additional steps
...
11
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In some cases, an alarm or control relay may be also needed
...
What power is available? Most devices require 12-24 V DC , although some run on 110 or 220V, and a few may
operate in a lower power
...
What will be device's total cost? While list price is important, installation and startup costs, should also be
considered
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Instrumentation and Process Control | Back: Flow Measurement | Next: Type of Process Loop Control System
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