level technology

8/8/2019 Level Technology

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LEVEL

MEASUREMENT

By: Ramir M. Salenga

Bataan Peninsula State University


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Level Measurement


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What?

Level measurementis the determinationof the linear vertical distance between areference point (usually the base of aholding container) and the surface of aliquid or the top of a solid.

Precise control of the level of liquid or drymatter in a tank, reactor, or other vessel isimportant in many process applications. Toprovide good control, accurate

measurement is essential.


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Level Measurement


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The following five sections are included:

Why Measure Level?

Level Terminology

Device Selection

Classifying Level Technologies

Level Measurement Technologies

Level Measurement and Control


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Inventory

Custody transfer

Efficiency

Safety

Consistent supply

WHY?


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Level measurements are typically

expressed in terms of feet or meters.

Level may also be given in terms of

percent fullorpercent of measured

span.

Level Terminologies


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Level Terminologies


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INTERFACE

An interface is the boundary between

two immiscible (incapable of being

mixed) fluids with different densities

(e.g., oil and water).

Level Terminologies


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Level Terminologies


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DENSITY

Densityis the mass of a material per unit

of volume. Density is often expressed in

terms of grams per cubic centimeter

(g/cm3) or pounds per cubic foot (lb/ft3).

Specific gravity is often used to describethe density of a material compared to the

density of water.

Level Terminologies


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MASS

Mass, the amount of matter an object

contains, is often equivocated to weight.

Mass is typically expressed in terms ofkilograms, grams, tons, or pounds. Mass is

unaffected by temperature. Thus, 60 lb

(27.2 kg) of oil at 50 F (10 C) is still 60 lbat 86 F (30 C)however, the overall

volume of the oil may change due to

expansion.

Level Terminologies


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VOLUME

Volume is the space occupied by a

quantity of material. Volume is typicallyexpressed in gallons, liters, cubic

centimeters, cubic feet, or barrels.

Volume is the measurement most

commonly derived from level.

Level Terminologies


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Atop-down measurementposes less

potential for leakage and enables level

measurement devices to be installed or

removed without emptying the tank

(e.g., dipstick).

Device Characteristics


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Top-down measurements may or may not

contact the process fluid. Abottom-up

measurementtypically contacts the

process fluid (e.g., weigh scale). Level

devices that use pressure transmitters arebottom-up measurement systems.



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Directmeasurement indicates that level

is measured directly.

For example, when you use a dipstick to

check the oil level in your car, you are

making a direct measurement. A direct

measurement is independent of any

other process parameters.



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Indirectmeasurement, also known

as inferredmeasurement,indicatesthat a variable other than level is

first measured and then used to

determine a level measurement.



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A continuous level-measurement system

monitors the height of product within a

range of all possible levels at all times.

Continuous measurement is used for

precise control, to maintain the level of amaterial at a particular point, and to

ensure a consistent supply.



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Single-point measurementindicates

whether a product is at least as high or

low as a certain point, usually the high- orlow-level limit.

In multipoint measurements, level

indication is observed at two or more

discrete points in the tank. Two single-point measurement devices may sound

alarms or operate equipment at high and

low limits.



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Single Point Level Measurement

The vibration/tuning fork is

piezoelectrically energised and

vibrates at a frequency ofapproximately 1,200 Hz. When

the fork is covered in material,

the frequency shifts. The

frequency shift is detected by

the internal oscillator and

converted into a switching

command.


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Level measurementindicators enable an

on-site level check.

Indicators require the operator to interpret

the measurement and take the

appropriate action. Systems with level

measurement indicators are referred to

as open-loop controlsystems.



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Automatic control systems, or closed-

loop systems, are able to electronically

control level in a vessel. A level-

measurement device, combined with a

transmitter, generates either a pneumatic

or electronic control signal that is

proportional to the level in the vessel.



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In a contactingmeasurement, some part

of the measurement system is in direct

contact with the contents of the vessel.Examples of contacting measurement

techniques include floats and dipsticks.

In a noncontacting measurement, nopart of the measurement system directly

contacts the contents of the vessel.



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Manual/mechanical

Electromechanical

Electronic contacting

Electronic noncontacting



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MANUAL/MECHANICAL

Devices in the manual/mechanical category

have no electronic output. The operatoruses the device to get a visual indication

of the amount of material in the vessel.

Examples of level measurement devices in

this category are sight glasses or rodgauging systems. These devices are low

cost but have no automation


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ELECTROMECHANICAL

Devices in the electromechanical category

are mechanical assemblies with a number ofmoving parts that produce an electronic

output for control. Unlike manual/mechanical

devices, electromechanical devices provide

an automated measurement that can be

read remotely.


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ELECTRONIC CONTACTING

Devices in the electronic contacting category

have no moving parts. While they are not

immune to problems with coating or corrosion,

electronic contacting devices tend to be more

robust and thus require less maintenance than

electromechanical devices. Examples of level-measurement devices in this category are

capacitance probes and pressure-based level

transmitters.


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ELECTRONIC NONCONTACTING

Devices in the electronic noncontacting

category provide sophisticated level

measurement without ever touching the

product.

An example of a level-measurement devicein this category is a radar measurement

device


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Level Technologies

Pressure transmitters

Bubbler systems

HTG

Radar

Hybrid inventory systems


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Level Technologies

Pressure transmitters are the most

commonly used level measurement

devices for liquids. In general, pressuretransmitters measure level by measuring

pressure at two taps, a high-pressure tap

(near the base of the tank) and a low-

pressure tap (near the top of the tank), and

manipulating the results to find level.


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Level Technologies

There are two types of pressure transmitters:

Gauge pressure (GP) transmitters

Differential pressure (DP) transmitters


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Level Technologies

OPEN-VESSELLEVELMEASUREMENT

Simple head measurementis used to

measure the level ofliquids in open vessels.Any column of liquid exerts a force at the

base of the column because of its own

weight. This force, called hydrostatic

pressure orheadpressure, can be

measuredin pressure units


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Level Technologies


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Level Technologies

CLOSED-TANK LEVELMEASUREMENT

If a vessel is pressurized, the pressure

gauge measures not only hydrostaticpressure resulting from the height of the

liquid column, but also the pressure above

the stored liquid (P). To solve this problem,

a DP transmitter is used.


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Level Technologies

Depending on the atmosphere inside

the vessel, one of two types of

measurement methods is used:

Dry leg system

Wet leg system


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Level Technologies

Dry Leg System

The dry leg system is typically used when the

gas above th

e liquid cannot condense (e.g.,nitrogen). The dry leg system provides

compensation for the pressure in the vapor

space, whether positive (above atmospheric)

or negative (vacuum). The output of the DPtransmitter is proportional to liquid level only.


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Level Technologies

Wet Leg System

In applications where the gas above the

liquid can condense (e.g., water intosteam), condensation will gradually fill

the low-pressure piping and cause

measurement error.


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REMOTE SEALS

Remote seals are transmitter extensions

that enable the transmitter to avoid direct

contact with the process liquid


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A bubbler system consists of three main

components:

Dip tube (vertical pipe extending theheight of the liquid to be measured)

Air supply and regulator

Pressure transmitter


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Hydrostatic tank gauging (HTG) uses a

multipoint system to measure mass,

volume, density, level, and temperaturefor liquid inventory and process

applications


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Radar (radio detection and ranging)

technologies transmit a continuous

microwave signal from a radar gauge

mounted on top of a holding vessel to the

surface of the material held inside. The

transmitted signal is reflected back to the

radar gauge, and the gauge measures thedistance (and thus level) to the surface of the

material by differentiating the received

signals from the transmitted signals.


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Hybrid inventory systems combine the

advantages of an HTG system with those of

a radar system to provide a completesystem for precise measurements of level,

volume, density, and mass.

An average temperature measurement can

also be made.


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Ultrasonic

It consists of anultrasonic generator oroscillator operating at a

frequency ofapproximately 20 KHzand a receiver.

The time required for the

sound waves to travel tothe liquid and back to thereceiver is carefullymeasured. The time is ameasure of level.


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Radiation

A radiation level

measurement

generally

consists of a

radioactive

source on one

side of the tankand a suitable

detector on the

other side.


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Capacitance

Application

Used best tononconductive

liquids, since itis best toavoid theproblemsgenerated byconductingmaterials like

acids.


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I/P

(Current to Pressure)

Transducer


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