Level Measurement (Tanks)

8/13/2019 Level Measurement (Tanks)

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Level Measurement

To assure the safety and profitability of processes, it is often essential to be equipped

with instruments providing reliable and precise measurements of level.

A level measurement can be categorized in :

- A top-down or bottom up measurement

A top-down measurementsmay or may not contact the process fluid. A top-down measurement poses

less potential for leakage

A bottom-up measurementtypically contacts the process fluid . Level devices that use pressure

transmitters are bottom-up measurement systems.

Direct and indirect measurement

- Direct measurementindicates 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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- Indirect measurement, also known as inferred measurement, indicates that a

variable other than level is first measured and then used to determine a level

measurement. For example, pressure transmitters use mass and the fluids specific

gravity to calculate level

Continuous ,Single-Point or Multipoint Measurement:

A con t inuou s level-measurementsystem monitors the height of product within a

range of points within the tank at all times. Continuous measurement is used for precise

control, to maintain the level of a material at a particular point, and to ensure a

consistent supply, like in a batch reactor.

Single-point measurementindicates whether a product is at least as high or low as a

certain point, usually the high- or low-level limit. They are typically used to prevent

overflow. A common example is a toilet tank float.

In mult ipo int 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. Several single-point devices located throughout the

vessel could approximate a continuous level-measurement system

CONTACTING VS. NONCONTACTING

In a contact ing measurement, 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 non contact ing measurement, no part of the measurement system directly

contacts the contents of the vessel.


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There are several types of level measurement technologies

- point level

- pressure-based

- ultrasonic non-contact

- and radar based.

I will start to explain in detail how each device works.

POINT LEVEL DETECTION

Point level detection can either be single point or multipoint measurement and is

normally used where either high or low alarm levels are to be indicated. The most

common application is for over-fill protection, and is frequently used in addition to

continuous level measurement systems where additional back up or high integrity alarm

signals are required. Multipoint measurement is commonly used for automatic pump

control for filling or emptying vessels, where separate start and stop levels are required

for the pump.

ULTRASONIC LEVEL SWITCHES

Ultrasonic level switches are used in most industrial processes applications to

detect high and low levels. Operation is achieved using the time-proven principle of

ultrasonic transmission between two crystals. Liquid presence is detected by virtue of its

bulk. Liquid droplets, condensation, or foaming are ignored.

Typically an ultrasonic gap sensor is operated at a nominal frequency of 1 MHz.

Sensor electronics are set to respond to the gain or the attenuation due to the lack of

liquid in the sensor gap


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Generally, gap sensors are designed for fail safe low level duty. A special Hi-sens type sensor is

used for fail safe high level duty.

Advantages of ultrasonic gap switches:No moving parts, no maintenance

Simple installation

Hazardous area use

Immune to foam

Unaffected by:RF interferenceConductivityDropletsMost coatings

Liquid color/opacity

MAGNETIC FLOAT-OPERATED SWITCHES

Magnetic float-operated switches usually fall into two categories,

- horizontal

- vertical-operated configurations.

In most applications it is common to have the float switch directly mounted to the tank

or vessel via a stand-off nozzle and flanged connection. In certain cases, especially in

the process industries where vessels may be at high

temperatures and pressures, the float switch device may be

installed in a separate chamber or bridle as shown in

Figure below.


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Horizontal Type Level Switches

The horizontal level switch typically employs a two magnet principle to provide the switching action. The

main advantage of this design is that the internal float is mechanically isolated from the electrical

switching mechanism via the magnetic coupling through the non-magnetic flange of the switch body. In

this figure it is shown how the horizontal magnetic two-magnet principle operates.

One permanent magnet forms part of a float assembly which rises and falls with

changing liquid level. A second permanent magnet is positioned within the switch so

that the adjacent poles of the magnet repel each other through a non-magnetic

diaphragm.

A change of liquid level which moves the float through its permissible travel causes

the float magnet to move and repel the switch magnet to give the snap action operation.


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Vertical Type Level Switch es

The float in a vertical type level switch carries a stainless steel sheathed permanent magnet which rises

and falls in the glandless pressure tube with changing liquid level. A switch mechanism is mounted inside

the enclosure adjacent to the pressure tube. Switching is achieved with a unique three-magnet system,giving snap action latch-on switching.

Vertical movement of the float magnet in the pressure tube simultaneously actuates the secondary and

tertiary magnets in the switch mechanism to operate the contacts


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DISPLACERS

The displacer element, typically made of stainless steel, is suspended on a stainless steel cable from a

spring. The element is always heavier than its equivalent volume of the liquid in which it operates, so it

always creates tension on the spring. In free air, the spring will be extended to a known length,

controlled by a mechanical stop to prevent overstressing. Fixed to the spring is the float rod and magnet

assembly, free to move up and down as the spring extends or contracts, andt the switch mechanism is

outside the pressure tube in the usual manner.

As liquid rises to cover the displacer element, a force is created equal to the weight of the liquid

displaced. This force is sensed by the spring as a reduction in weight, causing the spring to contract,

moving the magnet upwards inside the pressure tube, and actuating the switch mechanism


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TUNING FORK LEVEL SWITCHES

The tuning fork type of level switch typically comprises a sensor consisting of a pair of tines that act

like a tuning fork plus associated electronics that provide either a solid state electronic output or simple

relay contact.

Tuning fork tines are oscillated at their natural resonant frequency of typically 1300 Hz by a

piezoelectric crystal located near the head of the fork. When the sensor is in the vapor space, the

natural resonant frequency is maintained at 1300 Hz.

When the sensor tines become immersed in liquid, the sensors natural frequency is reduced. Typically

electronics are set to respond when the natural frequency drops by approximately 200Hz to 300Hz

ADVANTAGES - Temperature range40 F to +300 F (-40 C to 150 C)

- High pressures up to 1500 psi (100 bar~)


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Ultrasound

ULTRASONIC SIGNALS

An ultrasonic signal is generated by driving a piezo-electric crystal with a high voltage AC signal. The

crystal tries to oscillate but is unable to because it is bonded to the inside face of the transmitter. As a

result, the whole assembly oscillates at the crystals natural frequency and an ultrasonic signal is

transmitted.

Ultrasonic level transmitters are non-contacting instruments installed over a liquid that may be in a tank,

wet-well, or open air reservoir. An ultrasonic pulse is emitted by the transmitter toward the liquid

surface.

Ultrasonic level transmitters typically send a signal directed toward the liquid surface about once every

second. The signal travels at the speed of sound and is reflected back as an echo towards the

transmitter when it hits the liquid surface.

The transmitter knows the instant in time when the signal was sent and also the instant in time when the

echo is received back, so the overall journey time is known.


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Because the transmitter knows both the speed of sound and time taken, the distance to the target is

calculated using the basic equation:

Distance = Speed x Time

BLANKING DISTANCE & RING-DOWN TIME

All transmitters have a blanking distance, sometimes called a blocking distance or dead zone, in which no

measurements can be made.

The time it takes for the oscillation to spread is known as the ring-down time.The ring-down time can

be used to calculate the distance to the surface, since in ultrasonics time equals distance.

If the liquid surface is too close to the transmitter face, an echo is received before the transmitter

oscillation decays, which makes it extremely difficult to detect the echo among other noise. To avoid

this situation, manufacturers stipulate a minimum blanking distance based on the ring-down time of the

transmitter.

ATTENUATION OF ULTRASONIC SIGNALS

Ultrasonic signals can be affected by vapors, condensation, foam, and various other factors


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Vapors This error can usually be overcome by programming a speed of sound correction factor.

Condensation Heavy condensation is best avoided.

Foam Correct positioning of the transmitter above this foam-free area usually solves the problem.

Turbulence Turbulent liquid surfaces can also be problematical but a stilling tube can minimize

excessive agitation.

Use of Pressure Transmittersis a another way o level measurement

when

-The process is corrosive and requires frequent transmitter replacement

-the placement of the tank doesnt allow to put the pressure transmiters directly on the tankin contact

with the process fluid

Pressure transmitters with remote seals are used so this kind of system allow transmitter to be removed

from direct contact with the process fluid. Seals function act as an extension of the transmitter.


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So in a open OPEN TANK - SINGLE SEAL SYSTEMwith transmitter above the tap : the head pressure

of the liquid is measured to infer a level measurement.

Remote seal systemsconsist of external sensing diaphragm seals that are connected to the transmitter

with oil-filled capillaries. The oil used in the capillaries is not compressible.

Any column of liquid exerts a force at the base of the column because of its own weight. This force,

called hydrostatic pressure or head pressure, can be measured in pressure units. Hydrostatic pressure is

determined by the following equation:

Hydrostatic Pressure = Height x Specifics Gravity


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in a open OPEN TANK - SINGLE SEAL SYSTEMwith transmitter below the tap The difference is that

the distance between the tap and the transmitter must be calculated with the specific gravity (S) of the

fill fluid instead of the process fluid. Note this is the vertical distance, not the capillary length


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CLOSED-TANK LEVEL MEASURement

In closed systems, the transmitter location is restricted by the maximum allowable distance above the

lower tap. In pressurized systems, this is the same as the 1 atmosphere equivalent seen previously. In

sub-atmospheric systems (vacuum systems), the transmitter should be mounted at or below the lower

tap. This ensures the transmitter always sees a positive pressure on both the measurement and the

reference sides.

Because Differentel Pressure transmitteris used, changes in the overall vessel pressure affect the

high- and low-pressure taps of the transmitter equally, so the effects of pressure variation are canceled

out.

When using DP level-based technology for a closed-vessel application, customers have traditionally used

one of this methods:


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Dry leg system

Wet leg system

Remote seal/capillary system

Wet and Dry Leg Systems

In a wet/dry leg configuration, impulse piping is used to connect the DP transmitter to the high and

low pressure taps on the vessel. The user then must fill the low-side impulse piping with a suitable gas

(dry leg) or liquid (wet leg) to endure that a suitable reference pressure is applied on the low side of the

DP transmitter sensor. Dry leg configurations are used when the gas in the vapor space of the vessel

cannot condense (e.g., nitrogen). Wet leg configurations are used when the vapor gas can condense,

such as steam.

Remote seal systemsconsist of external sensing diaphragm seals that are connected to the

transmitter with oil-filled capillaries. The oil used in the capillaries is not compressible, and thus they

offer significantly better performance than wet/ dry leg configurations.

In The most used two seal systems, the distance between the taps becomes the reference offset from

zero. The calculations are the same regardless of where the transmitter is mounted.


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Hydrostatic Tank Gauging

Hydrostatic tank gauging (HTG) uses a multipoint system to measure mass, volume, density, level, and

temperature for liquid inventory and process applications .


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Hydrostatic pressure is the pressure created by a height of liquid above a given point. HTG works on the

principle that the hydrostatic head pressure of a column of liquid is directly proportional to the height of

that column.

Massis equal to the pressure difference between the bottom and top transmitters multiplied by the

average area of the tank. The tank area is based on the current product level (determined by the

pressure difference) and strapping table data.

Densityis equal to the pressure difference between the middle and bottom transmitters divided by the

distance between them. Density cannot be calculated when the product volume is below the middle

transmitter

Levelis equal to the difference between the bottom and top pressure measurements divided by the

density plus the heel. The heelis the height of the process fluid from the bottom transmitter to the floor

of the tank

A temperature measurementis taken between the bottom and middle pressure transmitters. The

temperature measurement, combined with the products measured density and density correction

factors, is used to calculate standard density and standard volume values.

Level Measurement using RADAR technology

Radar (radio detection and ranging) technologies transmit a continuous microwave signal from a radar

device mounted on top of a vessel to the surface of the material held inside. The transmitted signal is

reflected back to the device and the gauge measures the distance (and determines the level) bydifferentiating the transmitted and returned signals. It is similar like in ultrasound measurement.

The level measurement is determinedby using the reference height of the gauge minus the distance to

the surface.


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Radar level devices are available in two basic versions: free radiating and guided wave. Both free

radiating and guided wave radar provide a top-down direct measurement where they measure the

distance to the surface.

Free radiating radar sends a signal through the vapor space that bounces off the surface and returns to

the gauge. Free radiating radar can frequently be used in vessels with agitators.

Guided wave radarsends a low energy pulse down a probe or cable that bounces off the surface and

back to the device

It should not be used in applications with sticky fluids.the main advantage of the GWR is that can be

used in tight vessels due to bypass.


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A key advantage of radar is that no compensation is necessary for changes in density, dielectric, or

conductivity of the fluid.

Changes in pressure, temperature, and vapor space conditions have no impact on the accuracy of

radar measurements

The most important differences between Guided-wave radar and ultrasonic are

1. Measurement Principle point of view

Ultrasonic systems are using sound waves and Guided-wave radar High-frequency radar (radio)impulses

2. From Operating Limits point of view

Ultrasonic systems have Limited operating pressures and temperatures values but Guided-waveradar can operate in High temperatures and pressures environment.

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Level Measurement (Tanks)