LVL MSRMNT

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is the determination of the linear vertical distance between a reference point and the surface of aliquid or the top of a solid or the interface of two liquids.

Level measurement

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Level gauges (sightglasses)

are perhaps the simplest indicatinginstrument for liquid level in avessel.

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Interface problems

Level gauges(sightglasses)

One such circumstance is in the presence of a lighterliquid layer existing between the connection ports of

the gauge. If a lighter (less dense) liquid exists above aheavier (denser) liquid in the process vessel, the levelgauge may not show the proper interface.

1000kg/m

900kg/m

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The only way to ensure proper two-part liquidinterface level indication in a sightglass is tokeep both ports (nozzles) submerged.

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Temperature problems


Temp (C) Density(kg/m)

100 958.4

80 971.8

60 983.2

40 992.2

20 998.2071

10 999.7026

4 999.9720

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FLOAT

a device that rides on the surface of the fluid or solidwithin the storage vessel. The float itself must be ofsubstantially lesser density than the substance ofinterest, and it must not corrode or otherwise reactwith the substance.

Ullage- being the distance from the top of the vessel to

the surface of the process material.

Ullage = Total height Fillage

Fillage- may be determined by subtracting this ullage

measurement from the known height of the vessel.

Fillage= Total height Ullage

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FLOAT

The spring reels angular position may be measured by a multi-turn potentiometer or a rotary encoder (located inside

the head unit), then converted to an electronic signal for transmission to a remote display, control, and/or recordingsystem. Such systems are used extensively for measurement of water and fuel in storage tanks.

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FLOAT

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Avariation on the theme of f loat level

measurement is to place a small float insidethe tube of a sightglass-style level gauge

FLOAT

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Two major advantages of a magnetically-coupled float

Increased pressure rating and safety(since the float tube need not be constructed of clear materialsuch as plastic or glass)

Increased readability (since the viewing tube will never getdirty with process fluid residue, and the float may be brightly

colored).

FLOAT

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Magnetic and mechanical float

The principle behind magnetic,mechanical, cable, and other float

level sensors involves the opening or

closing of a mechanical switch, either

through direct contact with the

switch, or magnetic operation of a

reed.

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Hydrostatic pressure

Avertical column of fluid generates a pressureat the bottom of the column owing to theaction of gravity on that fluid.The greater thevertical height of the fluid, the greater thepressure, all other factors being equal.

Pressure of a f luid column

The relationship between column height andfluid pressure at the bottom of the column isconstant for any particular fluid (density)regardless of vessel width or shape.

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The mathematical relationship between liquid column height

and pressure is as follows:

P = g h P = h

Where,

P = Hydrostatic pressure

= Mass density of fluid in kilograms per cubic meter (metric)or slugs per cubic foot (British)

g =Acceleration of gravity

=Weight density of fluid in newtons per cubic meter (metric)or pounds per cubic foot (British)

h = Height of vertical fluid column above point of pressuremeasurement


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The pressure generated by a column of oil 12 feet high having a weight density of 40 pounds per cubic foot?


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Differential pressure transmitters

are the most common pressure-sensing deviceused in this capacity to infer liquid level within avessel.

ADP is used to transmit the head pressure thatthe diaphragm senses due to the height of thematerial in the vessel multiplied by a densityvariable.

Advantages

The primary benefit of DPs is that it can beexternally installed or retrofitted to an existingvessel.

It can also be isolated safely from the processusing block valves for maintenance and testing.

Disadvantages

DPs are mainly intended for clean liquids and

require two vessel penetrations.D/P transmitters are subject to errors due tochanges in liquid density.

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Some pressure-sensing instruments arebuilt specifically for hydrostatic

measurement of liquidlevel in vessels, doing away with impulsetubing altogether in favor of a specialkind of sealing

diaphragm extending slightly into thevessel through a flanged pipe entry(commonly called a nozzle).


Nozzle

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OpenTankMeasurement

If the tank is open to atmosphere,the high-pressure side of the level

transmitter will be connected tothe base of the tank while the low-pressure side will be vented toatmosphere.

We have:

Phigh = Patm + SH

Plow = Patm

Differential pressure

P = Phigh - Plow = SH


LT

HeightLiquid of Weight Density (S)

Vented to

Atmosphere

H L

Isolating Valve

Atmospheric Pressure Patm

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ClosedTankMeasurement


We have:

Phigh = Pgas + SH

Plow = Pgas

P = Phigh - Plow = S

H

LT

Pgas

Isolating Valve

Isolating Valve

H L

Low Pressure

Impulse Line

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Wet Leg

We have:

Pgas + 1h1) (Pgas + 2h2)

= 1h1 2h2


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Transmitter suppression and elevation


37.2InH20

181.2InH20

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75InH20

-57InH20

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Displacement

Displacer level instruments exploit

Archimedes Principle to detect liquid

level by continuously measuring the

weight of an object (called the displacer )

immersed in the process liquid.

As liquid level increases, the displacer

experiences a greater buoyant force,

making it appear lighter to the sensinginstrument, which interprets the loss of

weight as an increase in level and

transmits a proportional output signal.

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If the weight density of the process fluid is 57.3 pounds per cubic foot and the displacer is a cylinder measuring 3 inches

in diameter and 24 inches in length,

Displacement

Fbuoyant = V Fbuoyant = r2l

Where:

V= the displaced volume of fluid

= weight of density fluid

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Displacer instrument appear here, showing how the displacer fits inside the cage pipe:

Displacement

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Torque tubes

An interesting design problem for

displacement-type level transmitters

is how to transfer the sensed weight

of the displacer to the transmitter

mechanism while positively sealing

process vapor pressure from that

same mechanism. The most common

solution to this problem is an

ingenious mechanism called a torquetube.

Displacement

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Torque tube serves three distinctpurposes when applied to adisplacer-type level measurementapplication;

(1) to serve as a torsional spring

suspending the weight of thedisplacer.

(2) to seal off process f luidpressure from the position-sensingmechanism.

(3) to transfer motion from the farend of the torque tube into thesensing mechanism.

Displacement

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Displacement

Looking closer at each end of the torque tube reveals the open end where the small-diameter rod protrudes (left) and the

blind end of the tube where it attaches to the lever (right):

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In pneumatic level transmitters, the

sensing mechanism used to convertthe torque tubes twisting motion

into a pneumatic (air pressure) signal

is typically of the motion-balance

design.

uses a C-shaped bourdon tube with a

nozzle at the end to follow

a baffle attached to the small rod.

The center of the bourdon tube is

aligned with the center of

the torque tube. As the rod rotates,

the baffle advances toward the

nozzle at the bourdon tube tip,

causing backpressure to rise, which

in turn causes the bourdon tube toflex. This flexing draws the

nozzle away from the advancing

baffle until a balanced condition

exists.

Displacement

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Magnetostrictive level measurement

liquid level is sensed by a lightweight, donut-shaped float containing a magnet.

a mechanical wave (pulse) is generated at thelocation of a magnetic float in response to anelectrical pulse.

Aprecision electronic timing circuit measuresthe time elapsed between the electric currentpulse (called the interrogation pulse) and thereceived mechanical pulse. So long as thespeed of sound through the metal waveguiderod remains fixed, the time delay is strictly afunction of distance

between the float and the sensor,

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Ultrasonic Level measurement

Ultrasonic transmitters work on the

principle of sending a sound wave

from a peizo electric transducer to

the contents of the vessel. The

device measures the length of time it

takes for the reflected sound wave to

return to the transducer. The sensorsemit high frequency (20 kHz to

200 kHz) acoustic waves that are

reflected back to and detected by

the emitting transducer.

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Ultrasonic Level

LVL MSRMNT

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