Level Measurement
P=S x H
P = Pressure (Pa)
S =
Weight density of the liquid (N/m)= pg
H = Height
of Liquid column (m)
p = Density (kg/m')
g = Acceleration due to gravity (9.81 m/s)
The level of
liquid inside a tank can be determined from the pressure reading if the weight
density of the liquid is constant.
Differential Pressure (DP) capsules are the most commonly used devices to measure the pressure at the base of a tank.
When a DP transmitter is
used for the purpose of measuring a level, it will be called a level
transmitter. To obtain maximum sensitivity, a pressure capsule has to be used,
that has a sensitivity range that closely matches the anticipated pressure of
the measured liquid. However, system pressures are often much higher than the
actual hydro-static pressure that is to be measured.
If the process pressure is accidentally applied to only one side of the DP capsule during installation or removal of the DP cell from service, over ranging of the capsule would occur and the capsule could be damaged causing erroneous indications.
Point sensing level probes only sense tank level at a discrete level. Point sensing probes are typically used for high-high or low-low level sensing to prevent plant personnel and/or process equipment from being exposed to harmful conditions. Point level probes are also used in pairs in processes in which we do not particularly care what the exact level in a tank is, only that it is between two points. Continuous level probes sense the tank level as a percent of span of the probes capabilities.
Continuous level probes are typically used where we need some type of inventory control, where we need to know with some degree of confidence what the particular level in a tank is.
Non-Contact Level Measurement
Non-contact level measurement, as the name implies, requires that the sensing element not be in contact with material being measured. The use of non-contact level sensors eliminates agitator interference. Non- contact level sensing is primarily either ultrasonic or radar/microwave.
1-Ultrasonic Measurement
Ultrasonic makes use of sound waves in the 20 - 200 kHz range (above the range for human hearing). A transducer mounted in the top of a tank transmits sound waves in bursts onto the surface of the material to be measured. Echoes are reflected back from the surface of the material to the transducer and the distance to the surface is calculated from the burst-echo timing.
The key points in applying an ultrasonic transducer are:
• The speed of sound varies with temperature. If the transducer
does not use temperature compensation and the temperature of
the air space in the vessel varies your level readings will not be
correct.
• Heavy foam on the surface of the material can absorb the sound
wave bursts resulting in no echo or an echo that is too weak to
process.
• An irregular material surface can cause false echoes resulting in irregular readings.
• Heavy vapor in the air space can distort the sound waves resulting in false reading.
2-Radar / Microwave Level Measurement
Radar, or microwave level measurement operates on similar principles to ultrasonic level probes, but instead of sound waves electromagnetic waves in the 10GHz range are used. When properly selected, radar can overcome many of the limitations of ultrasonic level probes.
Radar can:
• Be unaffected by temperature changes in the tank air space.
• See through heavy foam to detect the true material level.
• See through heavy vapor in the tank air space to detect true material level.
Nuclear/Radiation Level Sensor :-
Continuous nuclear level detection is typically used where most other technologies are unsuccessful.Different radioactive isotopes are used, based on the penetrating power needed to pass through the tank.Radiation from the source is detected on the other side of the tank. Its strength indicates the level of the fluid. Point, continuous, and interface measurements can be made.
As no penetration of the vessel is needed there are anumber of situations that cause nucleonic transmitters to be considered over other technologies. These applications generally involve high temperatures / pressures or where toxic or corrosive materials are within the vessel. Placing the source and / or detector in wells within the vessel can reduce source sizes. An extension of this is to use a moving source within the vessel; this facilitates the unique ability to combine density profiling with accurate tracking of a moving interface.
Nuclear level detection has some drawbacks. One is high cost, up to four times that of other technologies. Others are the probable requirement for licenses, approvals, and periodic inspections; and the difficulty and expense of disposing of spent radiation materials. Another factor to consider is that the radiation symbol found on these devices can cause concern to plantpersonnel. From a psychological standpoint, the radiation symbol found on these controls is frequently the cause of unfounded concern with uninitiated plant personnel. Plant Management is usually required to ensure that appropriate education is given to any staff likely to be involved with this measurement technology.
Contact Level Measurement
Contact level measurement, as the name implies, requires that the sensing element be in contact with material being measured. Continuous level measurements can either be direct or indirect with contact level sensors. With direct level measurement the sensor is in contact with the material over the entire span that we wish to measure. Direct contact continuous level measurement is commonly used in bulk powder storage silos and un-agitated tanks. RF capacitance/resistance level sensors are the typical direct contact level sensors for both the pointand continuous level variety. With indirect level measurement the sensor is in contact with the material at a single point and the tank level is inferred from this point measurement. Pressure transmitters measuring hydrostatic head are typically used for indirect contact continuous level measurement. For agitated tanks indirect contact continuous level measurement is advantageous
Pressure Measurement
Measurement of level by pressure relies on hydrostatic principles.
Pressure is a unit force over a unit area. A cubic foot (12"L x 12"W x 12"H) of water weighs 62.4796 pounds. The area that a cubic foot of water occupies is 144 square inches (12"L x 12"H); therefore a cubicfoot of water exerts a force of 62.4796 pounds over 144 square inches,or 0.4339 psig for a 12" water column.
It would not matter how many cubic feet of water were placed side byside, the pressure would still be 0.4399 psig. Hydrostatic pressure is only dependent on the height of the fluid, not the area that it covers.Going back to the cubic foot of water, every inch of water we add or subtract to the level will change the pressure measurement by 0.03616 psig. The relationship between psig and level is linear so pressure is easily converted to a level process variable.
1 inch H20 = 0.0316 psig
While this is all well and good for water, how would we handle measuring other fluids using pressure - compare the densities.
For instance, chocolate weighs somewhere around 80 pounds per cubic foot, so 80 divided 62.5 times 0.0316 = 0.0404 psig. For a change in level of one inch in a tank filled with liquid chocolate the pressure measurement will change by 0.0404 psig.This also points to one of the constraints of level measurement by pressure; the material requiresa constant density for accurate measurements.Another consideration for using pressure to infer level is the pressure found in the head space ofthe tank. As we are using gauge pressure our reference is atmospheric pressure, we have an open
RF Capacitance / Resistance level measurement
RF (radio frequency) Capacitance level sensors make use of electrical characteristics of a capacitor to infer the level in a vessel. A capacitor is essentially two metal plates separated by a dielectric (insulator) and acts as a storage vessel for electrons.
The size of the metal plates, the distance between the plates and dielectric filling the space all determine the capacitance value, or how many electrons can be stored. In an RF capacitance probe one of the metal plates is the tank wall, the other metal plate is the sensor probe and the dielectric is either air (where the tank is empty) or the material in the tank.
C= E Ka/d
C=Capacitance , E= dielectric constant , A=surface Area of plate, D= Distance between plate, K=Dielectric
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b)Sight Glass:- This consists of a graduated glass tube mounted on the side of the vessel. As the level of the liquid in the vessel change, so does the level of the liquid in the glass tube.
Indirect level measurement:
(a) Pressure gauge:
This is the simplest method,
for pressure gauge is located at the zero level of the liquid in the vessel.
Any rise in level causes an increase of pressure, which can be measured by a
gauge.
b) Purge system:
In this method a pipe is installed vertically with
the open and at zero level. The other end of the pipe is connected to a
regulated air r supply and to a pressure gauge. To make a level measurement the
air supply is adjusted so that pressure is slightly higher than the pressure
due to height of the liquid. This is accomplished by regulating the air
pressure until bubbles cab be seen slowly leaving the open end of the pipe.The
air pressure to the bubbler pipe is minutely in excess
of the liquid pressure in the vessel, so that excess
of the liquid pressure in the vessel, so that air
pressure indicated is a measure of the level in the Tank. 
The methods above are suitable for open tank applications. When a liquid is in a pressure vessel, the liquid column pressure can't be used unless the vessel pressure is balanced out. This is done through the use of different pressure meters.
Purge level system
This method is also known as bubbler method of level measurement. A pipe is installed vertically with its open end at the zero level. The other end of the pipe is connected to a regulated air supply and to a pressure gauge or to ^P transmitter. To make a level measurement the air supply is adjusted so that pressure is slightly higher than the pressure due to the height of the liquid. This is accomplished by regulating the air pressure until bubbles can be seen slowly leaving the open end of the pipe. The gage then measures the air pressure needed to over come the pressure of the liquid.
/\ P = H X D
USE: On for corrosive liquids where the transmitter cannot be directly connected to process eg... Acids, Some organic liquids.
(c) Differential pressure meter:
Connections are made at the vessel top and bottom,
and to the two columns of the D.P. meter. The top connection is made to the
L.P. column of the transmitter and the bottom to H.P. column of the
transmitter. The difference in pressure in the vessel is balanced out, since it
is fed to both the column of the meter. The difference in pressure deducted by
the meter will be due only to the changing, level of the liquid.
(d)
Displacer type level measurement:
The leveltrol is one of the most common instruments
used measuring level in closed tanks. This instrument works of Archimedes
principle. The displacer in immersed in the liquid due to which there is loss
of weight depending on the specified gravity of the liquid. This displacer
hangs freely on a knife transmitted to the pneumatic or electronic counterpart
at the other end.
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Calibrate a leveltrol in the field:-
Wt.
test calibration method:
- Remove the displacer from the torque arm.
- Apply equivalent weight on the torque arm that is equal to the wt.
of the displacer. Adjust zero % output.
- For Span: V = πr2h
Loss in weight = Wt. of
float - wt. of the float immersed in liquid
Loss in weight = [ wt. of float - Vol. x d ]
Span wt. = (wt. of float - Loss in wt.)
r = radius of the displacer.
h = ht. of displacer.
- Apply equivalent wt. equal to the (Wt. of float - Loss in weight).
Adjust Span to get 100 % out put.
- To check linearity apply average of the two weights.
Explain different pressure transmitter.
The bottom connection of the vessel is
connected to high-pressure side of the transmitter.
Different Pressure = H X D
This difference pressure is applied to H.P. side of
the transmitted and calibrated.
Commission D.P. transmitter in field in pressurized
vessel :-
1. Close both the isolation
valves, Vent the H.P. side.
2. Fill it with the sealing
liquid.
3. Open the L.P. side vent
valve.
4. Adjust zero with suppression
spring.
5. Close the L.P. side vent
valve.
6. Open both the isolation valves.
zero Check of a level D.P. transmitter while is line:-
1. Close both the isolation
valves.
2. Open the vent valve on L.P.
leg and H.P. leg drain.
3. Check and adjust zero if
necessary.
D.P. transmitter applied to a close tank
In close tank the bottom of the tank is connected to
the high-pressure side of the transmitter and top of the tank in connected to
L.P. side of the transmitter. In this way the vessel pressure is balanced.
D.P transmitter applied to a close tank & open
tank with Dry leg
Span = (X) (GL)
HW at minimum level = (Z) (GS)
+ (Y) (GL)
HW at maximum level = ( Z ) ( GS ) + ( X
+ Y ) ( GL )
Where:
GL = Specific gravity of tank liquid.
GS = Specific gravity of seal liquid.
HW =
Equivalent head of water.
X, Y & Z are shown fig
Example:
Open tank with X = 300
inches
Y = 50
inches
Z = 10
inches
GL = 0.8
GS = 0.9
Span =
(300) (0.8) = 240 inches
HW
at minimum level = (10) (0.9) + (50)
(0.8) = 49 inches
HW
at maximum level = (10) (0.9) + (50
+ 300) (0.8) = 289 inches
Calibrated range = 49 to 289 inches
head of water
Close
tank with wet leg:
Span = (X)
(GL)
HW
at minimum level = (Y) (GL) –
(d)(GS)
HW
at maximum level = (X + Y) (GL) – (d)
(GS)
Where: GL = Specific gravity of tank liquid
GS = Specific gravity of tank liquid
HW = Equivalent head of water
X,
Y and Z are shown in fig.
Example:
X = 300 inches
Y = 50 inches
d = 500
inches
GL = 0.8
GS = 0.9
Span = (300)
(0.8) = 240 inches
HW minimum level = (50)
(0.8) -
(500) (0.9) = - 410 inches
HW maximum level = (300 +
50) (0.8) – (500) (0.9) = - 170 inches
Calibrated range
= - 410 to –170 inches head of
water.
(Minus sings indicate that the higher pressure is
applied to the low pressure side of the transmitter)
Explain the working of an Enraf level gauge:-
Principle:
A displacer with a relative density higher than that of the product to be measured is suspended from a stainless steel wire B, which is attached to a measuring drum. A two phase servo meter controlled by a capacitive balance system winds or unwinds the measuring wire until the tension in the weighing springs is in balance with the weight of the displacer partly immersed in the liquid. The sensing system in principle measures the two capacitance formed by the moving center sensing rod E provided with two capacitor plates and the side plates. In balance position the capacitance are of equal value. Level variations will a difference in buoyancy of the displacer. The center sensing rod will move in the direction of one of the side capacitor plates. This causes a difference in value of these capacitance. By an electronic circuit this change is detected and integrated. During the rotation of the servomotor the cam driven transmitter continuously change the voltage pattern to a remote indicator of which the receiver motor drives a counter indicating level variation.
ENRAF Servo Level Gauge
- Level accuracy < 0.4 mm
- Interface accuracy < 2 mm
- Density accuracy < 3 kg/m3
- Temperature accuracy < 0.1 °C
Reading
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