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LEVEL MEASUREMENT CHAPTER 5

chap 5 - level measurement.pdf

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Page 1: chap 5 - level measurement.pdf

LEVEL MEASUREMENT CHAPTER 5

Page 2: chap 5 - level measurement.pdf

Level measurement

¨  Measure the height of a fluid or solid within a containing vessel. ¨  The method used will be widely depending on the nature of the

industry, the process, and the application. ¨  Inventory

¤  A constant supply or storage of material

¨  Control ¤  Continuous, batch, blending and mixing control. ¤  Stabilize flow to the next process

¨  Alarming ¤  High/Low limits, safety shut down

¨  Data logging ¤  Material quantities for inventory and billing purposes and where

regulatory requirements are necessary

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What is measured?

The measured medium can be liquid, gas or solid and stored in vessels (open/closed tanks), silos, bins and hoppers

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Selection criteria

When determining the type of level sensor that should be used for a given application, there are a series of questions that must be answered

¤ Open tank or closed tank ¤ Can the level sensor be inserted into the tank or should be

completely external? Contact or No-contact? ¤ Continuous measurement or point measurement ¤ Direct or indirect measurement? ¤ What type of material is being measured? Liquid or Solid?

Clean or Slurry?

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Methods – Direct or Indirect?

¨  Hydrostatic head ¨  Float ¨  Load cells ¨  Magnetic level gauge ¨  Capacitance transmitters ¨  Magnetostrictive ¨  Ultrasonic ¨  Microwave ¨  Laser ¨  Radar ¨  Guided wave radar ¨  Dip stick ¨  Vibration

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Direct method

¨  Dip stick ¨  Sight glass ¨  Hook gauge ¨  Float gauge ¨  Resistance tapes

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Indirect method

¨  Hydrostatic pressure device ¨  Bubbler or purge technique ¨  Ultrasonic level gauge ¨  Nucleonic gauge ¨  Magnetoresistive ¨  D/P cell transmitter

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Dipstick method

Dipstick is used to measure the level of liquid in an inaccessible space, by inserting and removing the graduated rod and then checking the extent of it covered by the liquid.

¤ Economical ¤ Common method ¤ Not very accurate ¤ Not possible to get continuous on-line observation

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Sight glass method

Simple sight glasses may be just a plastic or glass tube connected to the bottom of the tank at one end and the top of the tank at the other. The level of liquid in the sight glass will be the same as the level of liquid in the tank.

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¨  Made from toughened glass & provided with protecting cover.

¨  Graduated glass tube mounted on the side of the liquid containing vessel.

¨  Operator required to record the liquid level. ¨  Unsuitable for dirty, viscous and corrosive liquids. ¨  Simple, direct for clean and colored liquid.

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Hook gauge method

n  The device consists of a sharp hook suspended from a micrometer cylinder, with the body of the device having arms which rest on the rim of a still well inside the pan.

n  The still well serves to isolate the device from any ripples that might be present in the sample being measured, while allowing the water level to equalize.

n  The measurement is taken by turning the knob to lower the hook through the surface of the water until capillary action causes a small depression to form around the tip of the hook. The knob is then turned slowly until the depression "pops," with the measurement showing on the micrometer scale.

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¨  Accurately measuring very small changes in liquid level in open/closed containers

¨  Accurate upto ±0.1mm. ¨  precision instrument used to measure changes in

water levels due to evaporation. FAIRMOUNT

WEATHER SYSTEMS

DATA SHEET

DS9000 28-04-03

UNIT 4, WHITECROFT ROAD,MELDRETH, HERTFORDSHIRE, SG8 6NE ENGLANDTel: (0) 1763 263415 Fax: (0) 1763 261414

email: [email protected]

VAT No 637 0065 55

Evaporation Hook Gauge

The Fairmount Evaporation HookGauge has the following features:

Material : Brass tripod frame withprecision machined & engraved brasscomponents

Specification : Graduated from 0 to 10mmand subdivided to 0.02mm by means ofan engraved micrometer dial

Ordering Information:

FM9000 Hook Gauge

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Float gauge

¨  The float (hollow metal sphere) is attached to a cable which is wound around a pulley to which the indicating pointer is attached. The movement of pointer is read on a scale which is calibrated in terms of level of the tank.

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¨  Continuous measurement. ¨  Sufficient accurate – change in liquid density due to

change in temperature. ¨  The advantages of this method are that is relatively

simple, suitable for various products. ¨  The disadvantages are that it requires a certain

amount of mechanical equipments, especially in pressure vessels.

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Float & Chain liquid level gauge

¨  Measuring level and it is employed in open vessel ¨  Float, counter weight & flexible connection (chain,

metallic perforated tape) ¨  The counter weight keep the chain/ tape taut as

liquid rises/fall. ¨  The chain/tape link wound on a great/spocket

wheel to which the pointer is attached ¨  Any movement if the wheel would indicate on a

suitability calibrated scale the liquid in the tank.

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Float & Shaft level gauge

¨  The motion of the float on the surface of the liquid is transferred to the shaft.

¨  The level is indicated by the pointer on the dial ¨  It is employed either in open vessel or in pressure

vessels having upto 1000 psi internal pressure. ¨  Temperature in the vessel may be upto 750 oF. ¨  The float and the shaft level is suitable for wide

range of liquids and semi liquids. ¨  Float gauge - steel to cast iron. ¨  Float - copper, stainless steel, nickel or aluminium.

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¨  Float operated schemes with electrical read outs. ¨  The float act as primary transducer – convert liquid

level variation into suitable displacement.

Float-actuated rheostatic (resistive) device

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Resistance tape method

¨  The pressure of the fluid in the tank causes the tape to short-circuit, thus changing the total resistance in the measuring tape. An electronic circuit measures the resistance; it is directly related to the level in the tank. When the sensor is installed, the outer envelope jacket (1) is compressed by the hydrostatic pressure of the liquid (2). This causes the gold wire winding to contact the gold plated base strip. The resulting change in the resistance of the gold wire (3) indicates the length of active helix and the distance from sensor top to liquid surface.

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Indirect level measurements

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Hydrostatic head level sensors

¨  This method infer level by measuring the hydrostatic head produced by the liquid column.

¨  A pressure sensing element is installed at the bottom of the tank and pressure is converted to level.

¨  Different liquid densities or clsed tank applications must be accounted for.

¨  The accuracy will be affected for e.g., temperature variations is sufficient to cause changes in the density of the liquid.

¨  Since these sensors sense increasing pressure with depth and because the specific gravities of liquids are different, the sensor must be properly calibrated for each application

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Example:

¨  A dipstick measurement of the level of 2 tanks indicates 30 feet of liquid in both tanks. Calculate the pressure that each gauge will read if the tank 1 contains water (S.G = 1)and tank 2 contains oil (S.G = 0.85).

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Closed tank applications

¨  Closed tanks are not vented to atmosphere, so the pressure at the bottom of the tank is due to thehead pressure of the liquid and the vapor pressure above the surface.

¨  P indicated = P head + V.P ¨  P indicated = h ρ g+ V.P ¨  h = (P indicated - V.P)/ (ρg)

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Bubbler or Purge technique

¨  A bubble tube is inserted into a tank, and a fixed flow of air is forced through the tube such that bubbles emerge from the end of the tube.

¨  The hydrostatic pressure of the liquid in effect acts as a seal, which restricts the escape of, purge gas from the bubbler tube.

¨  As a result, the gas pressure in the bubbler tube will continue to increase until it just balances the hydrostatic pressure of the liquid.

¨  At this point the backpressure in the bubbler tube is exactly the same as the hydrostatic pressure of the liquid.

¨  Any excess air supply will escape as bubbles through the liquid. ¨  As the liquid level rises, the backpressure in the bubbler tube

increases

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•  Usage: for corrosive liquids, liquids that contain suspended objects, high viscosity liquids, and underground tanks.

•  Cannot be used in closed tanks or where purging a liquid is not allowed

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D/P cell transmitter

¨  Differential pressure level measurement technique makes use of a differential pressure detector which is installed at the bottom of the tank whose level is to be detected.

¨  Low pressure side is open to atmosphere. ¨  High pressure side measures the hydrostatic

pressure which is proportional to the height of the liquid

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Open tank

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Closed tank

In a closed tank, the low side of the d/p cell is connected to the top of the tank and will cancel the effects of the vapor pressure above the surface.

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Capacitance level gauge

¨  Capacitance is the property of a circuit that stores electrons and thus opposes a change in voltage in the circuit.

¨  A capacitor is an electrical component that consists of two conductors separated by a dielectric or insulator.

¨  The capacitance value of a capacitor is measured in Farads (F).

¨  The value is determined by the area of the conductors (usually called plates ), the distance between the plates, and the dielectric constant of the insulator between the plates.

¨  one plate is a probe while the other plate is comprised of the tank of the level vessel.

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¨  The dielectric is the material in the vessel which will determined the capacitance value when it rises and falls.

¨  The liquid in the tank will be measured by the capacitance measurement meter.

¨  Sensing the presence of a wide variety of solids, aqueous and organic liquids, and slurries.

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Ultrasonic

¨  Ultrasonic level instruments operate on the basic principle of using sound waves to determine liquid/solid/slurries level.

¨  The theory is based on a sound wave emission source (transmitter) and the reflection of a sound wave pulse (echo) to receiver. Measurement of the transit time of this pulse provides a means for level detection and measurement.

¨  A piezoelectric crystal inside the transducer converts electrical pulses into sound energy that travels in the form of a wave at the established frequency and at a constant speed in a given medium.

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¨  Transducer mounted on the top of the tank directs waves downward in bursts onto the surface of the material whose level is to be measured.

¨  The device measures the length of time it takes for the reflected sound wave (echo) to return to the transducer

¨  The longer the interval, the farther away is the liquid surface which in turn is a measure/indication of the level.

¨  Distance=(Velocity of Sound x Time)/2 ¨  The frequency range for ultrasonic methods is in the range

of 15 -200 kHz. ¨  The lower frequency instruments are used for more

difficult applications; such as longer distances and solid level measurements and those with higher frequency are used for shorter liquid level measurements.

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Minimum measuring distance (Xm ): (also known as the “Dead Band”) is a feature common to all m ultrasonic level meters. This is a short range in front of the sensor within which the ultrasonic device can not measure.

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¨  Advantages: ¤ Wide range applications (liquid/ solid/slurries, small to

large reservoirs) ¤ Non-contacting device ¤ No moving parts

¨  Limitation: ¤ Various influences such as powders, heavy vapors, surface

turbulence, foam and ambient sound, that affect the return signal.

¤ Temperature can also be a limiting factor

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Nucleonic gauge

¨  A nuclear level gauge or gamma ray gauge measures level by the attenuation of gamma rays passing through a process vessel.

¨  A narrow fan of radiation from a source, such as Cobalt-60 or Cesium-137, is directed through the vessel to a detector mounted on the opposite side.

¨  As the process level rises, the electromagnetic energy from the source is progressively shielded from the detector by the process medium.

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Can be used to regulate the level of molten steel in a continuous casting process of steel making.

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¨  The output is measured & correated with the level of liquid using the equation:

I = Io e(-μρx)

I = Intensity of radiation falling in the detector. Io = Intensity of radiation at the detector with absorbing material is not present. μ = mass absorption coefficient, m3/kg. ρ = density of the material kg/m3. X = the thickness of absorbing material, m.

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¨  Advantages: ¤ Wide range applications (liquid/ solid/slurries, small to

large reservoirs) ¤ Non-contacting device ¤ Quit rugged ¤ Withstand severe operating conditions

¨  Limitation: ¤ Risk involved due to radiation (Adequate shielding has to

be provided)

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Magnetoresistive

¨  Consists of a magnetoresistive wire in a probe and a donut shaped float containing a permanent magnet. The float is the only moving part and travels up and down the probe.

¨  As the float moves up the motion and location are transmitted as the angular position of the magnetic field.

¨  The choice of float and stem materials depends on the liquid in terms of chemical compatibility, specific gravity and other factors that affect buoyancy of the float.

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¨  Advantages: ¤ High accuracy ¤  Independence from the material characteristics such as

conductivity, dielectric constant.

¨  Limitations: ¤ Application limited to only clear liquids

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