Types of Level Gauge, Working Principle and different applications

TRANSPARENT GLASS LEVEL GAUGES

This instrument consists of a metal body, machined to have an internal chamber and one or more front windows (on each side of the gauge).
On each window a special high resistance plate transparent glass is applied with sealing joint and metal cover plate hold by bolts and nuts.
The chamber is connected to vessel with cross fittings and flanged, threaded or welded ends. Usually, between the instrument and its connecting ends, valves are fitted to consent shut-off piping and to disassemble the level gauge without to empty the vessel. Drain valves can also be fitted to cross fittings device.
To avoid leakage in case of glass breakage, safety ball-check device can be provided in cross-fittings or shut-off valves.

To protect glass surfaces from corrosive action of the process fluid, Transparent Level Gauges can be fitted with Mica shields or Polytrifluorochloroethilene shields.
This kind of indicator is suitable for liquids colorless and very fluid.

A variation of transparent level gauges, however equipped with a special back illuminator, as described in Bicolor level Gauges, gives a sharp reading made by contrast between bright colors: green in lower part (liquid) and red in upper part (steam) of visible window. This kind of indicator is suitable for water/steam.

WORKING PRINCIPLE: Fluid level indication is the result of the different transparency property of the two media (i.e. liquid and air or vapor).

In some case (i.e. for water / steam) the best reading is obtained by conveying upwards on the surface of separation (liquid/steam or vapor interface), a source of light, located on the back of the gauge, the rays of which are totally reflected down to the observer.

APPLICATIONS:
Transparent level gauges are suitable for all application. In fact they permit:

• the use of mica shields or Polytrifluorochloroethylene shields to protect the glass from the corrosive action of the process fluid;
• the observation of interface;
• the observation of the liquid color.

PRODUCT RANGE
Our standard range of transparent level gauges covers from PN 16 up to PN 420/ ASME 150 up to 2500, and can be manufactured, depending on model & type, in different materials, as ASTM A105, ASTM A350 LF2, ASTM A182 F304 / F316, Monel, Hastelloy, Incoloy, Carpenter, Nickel, Titanium, Zirconium, Tantalum, Ebonite, PVC, Polypropropilene, PTFE etc.

REFLEX GLASS LEVEL GAUGES

This instrument consists of a metal body, machined to have an internal chamber and one or more front windows

(on one side only of the gauge). On each window a special high resistance plate reflex glass is fitted with sealing joint and metal cover plates hold

by bolts and nuts.

The chamber is connected to vessel with cross fittings and flanged, threaded or welded ends. Usually, between the instrument and its connecting ends, valves are fitted to consent shut-off piping and to disassemble the level gauge without to empty the vessel. Drain valves can also be fitted to cross fittings device.

To avoid leakage in case of glass breakage, safety ball-check device can be provided.

WORKING PRINCIPLE:
Reflex level gauges working principle is based on the light refraction and reflection laws.
Reflex level gauges use glasses having the face fitted towards the chamber shaped to have prismatic grooves with section angle of 90°. When in operation, the chamber is filled with liquid in the lower zone and gases or vapors in the upper zone; the liquid level is distinguished by different brightness of the glass in the liquid and in the gas/vapor zone. The reflex level gauges do not need a specific illumination: the day environmental light is enough. Only during the night an artificial light must be provided.
The different brightness in the two zones is obtained as explained below:


LIQUID ZONE
This zone appears quite dark when the gauge is in operation and lighted as above said.
Given the construction, most of the environmental light rays incident on the external face of the glass are quite perpendicular to said face and, therefore, not deviated by the glass. These rays reach the glass/liquid interface with an inclination of approx. 45°. The critical angle glass/liquid is always superior to 45°. Therefore the rays incident within the critical angle (practically the totality) are refracted within the liquid and, since the internal walls of the gauge chamber are not reflecting, the rays cannot be seen from the outside. In fact the zone will appear dark, nearly black, to the observer.

GAS/VAPOR ZONE

This zone appears almost silver bright to the observer.
As for the liquid zone, the light rays reach the glass/gas-vapor interface with an angle around 45°. Since this angle is greater than glass/gas-vapor critical angle, the rays are not refracted , but totally reflected making 90° turn, thus reaching the nearest glass/gas-vapor interface again with angle of 45°. For same reason they will be reflected and turned by 90° towards the observer, to whom the zone will appear silver bright.

APPLICATIONS:

Reflex Level Gauges can be used in most of the cases and offer great advantages in terms of: low initial cost, low operating cost, easy level reading.
Reflex Level Gauges cannot be used in certain cases (where Transparent Level Gauges must be used) as for example:

• when the separation level between two liquids has to be read (interface),
• when, besides the level indication, observation of the liquid color is required,
• when the process fluid is high pressure water/steam (since in this case the glass must be protected from the chemical attack of the boiler basic water by using mica shields),
• when the process fluid is such that can corrode the glass (e.g. high temperature alkaline solutions or hydrofluoric acid) since mica shields or Polytrifluorochloroethylene shields must be used to protect the glass.

PRODUCT RANGE

Our standard range of Reflex Level Gauges, starting from PN 16 up to PN 420/ ASME Class150 up to 2500, and can be manufactured, depending on model & type, in different materials, as ASTM A105, ASTM A350 LF2, ASTM A182 F304 / F316, Monel, Hastelloy, Incoloy, Carpenter, Nickel, Titanium, Zirconium, Tantalum, Ebonite, PVC,

BICOLOR GLASS LEVEL GAUGE

Max. operating conditions: p

ressure: 21,5 MPa (3000 psi) - temperature: 370 °C (700 °F)

Left drawing shows a typical Bicolor Glass Level Gauge, used for measurement of water level in high pressure boiler steam drums. This instrument consists of a stainless steel body having trapezoid cross section and machined to have a longitudinal chamber and front holes uniformly distributed on the two not parallel faces.
On each hole a special high temperature resistant aluminium silicate transparent cylindrical glass is tightly pressed between a metallic cover and the body by means of bolts and nuts. A mica shield protect the glass from the corrosive attack of the boiler basic water.
Belleville washers fitted between cover plates and glasses provide compensation of their different thermal expansion, giving guarantee of tightness at any operating condition.
The chamber is connected to vessel with cross fittings and flanged, threaded or welded ends.
For severe conditions (over 8MPa - 1116 psi water/steam pressure) water column and expansion joint are recommended.
Each Bicolor glass level gauge shall be fitted with top, bottom and drain shut-off valve. Automatic shut-off ball check can be supplied.
A special back illuminator (suitable also explosion proof) gives a sharp level reading by means of the contrast between bright color: green in lower part (liquid) and red in upper part (steam) of level gauge.
Bicolor level gauges can be equipped with long glasses instead of cylindrical glasses (as in Transparent Level Gauges).
Nevertheless we do not recommend bicolor long glass level gauges for steam drums over 10 MPa (1450 psi).
BONETTI Bicolor Glass Level Gauges are designed and manufactured in accordance with ASME BOILER AND PRESSURE VESSEL CODE - SEC. I.

WORKING PRINCIPLE:  

When in operation, bicolor level gauge shows to the observer port holes lighted green in the water zone and port holes lighted red in the steam zone. In case of long glass gauge, the window appears part green and part red. This result is obtained exploiting the following optical principle: different colors (in this case: red and green) have a different index of refraction when passing obliquely through different media (in this case: glass, water and steam).

For the purpose, the gauge body has a trapezoid section with glasses placed on the non parallel faces. An illuminator with special red and a green filters is fitted on the gauge at the opposite side with respect to the observer. This special illuminator conveys light through the filters obliquely to the back glasses of the level gauge.   Said filters allow crossing only to red and green rays. Such colored rays reach, through the back glass, the media inside level body. When the gauge contains steam, green rays are considerably deviated and prevented from emerging by the observer side; then only red light, whose rays are smoothly deviated by steam, passes through the whole internal hole, reaching the observer. Conversely when rays find water, red rays are considerably deviated and lost inside the internal part of level gauge, green rays can reach the front glass and seen by the observer.

MAGNETIC LEVEL GAUGE

The Magnetic Level Gauge is the instrument to read a level indication in whatever plant or operating conditions giving free maintenance, preventive security against leakage, environmental safety, sure and trouble free application with chemically aggressive, pollutant, harmful or poisonous, inflammable or explosive, optically similar fluid interface.
Our very wide range of Magnetic Level Gauges allows to have the proper instrument for any kind of plant or specific purpose. Our world patented system includes a float able to float both in high density fluid or in low density fluid up to 0.4 specific gravity. But more! You can have low specific gravity and high temperature and pressure together (i.e. water/steam of H.P. vessels). Well! We manufacture a float able to float and to resist with external pressure of 210 bar at 370 ºC!
And more! Do you need Remote Direct Reading? We have it, as a completion of Magnetic or Glass level Gauge. Are you looking for a Transmitted Remote Signal? We can supply a sensing element able to transduce the signal to LED, Analogical or Recording Instruments that you can have in Control Room.

DESIGN

In a vertical chamber (tube of suitable diameter connected by stub ends to the vessel) there is a float containing a permanent magnet -fixed by means of special patented device that permit to the magnet a 360º rotation- placed exactly on liquid level line.
Magnetic action of permanent magnet in the float generates rotation of a set of small permanent magnets enclosed in small cylinders -contained in indicating scale- which cylinders, having different colours in the North and South pole, can rotate on their horizontal axis.
So the scale appears of one colour (i.e. white) over chamber area taken by gas and of another colour (i.e. red) over chamber area taken up by liquid phase.
The chambers of the Magnetic Level Gauge are dimensioned according to ASME B31.1 and B31.3.
The chamber is connected to vessel with flanged, threaded or welded ends. Usually, between the instrument and its connecting ends valves are fitted to consent shut-off piping and to disassemble the level gauge without empting the vessel or shutting off the plant. Drain valves can also be fitted on the top and bottom of level gauge.

MAGNETIC LEVEL GAUGE OPERATING PRINCIPLE

Operation of BONT Magnetic Level Gauge is based on some elementary physical principles: The principle whereby liquid in communicating vessels is always at same level; Archimede's principle according to which a body immersed in a liquid receives a buoyancy equal to the weight of displaced liquid; The principle of attraction between North and South poles of two permanent magnets and that of repulsion between like poles. This principle has two applications in the BONT magnetic level gauge: first between the magnet in the chamber float and every single magnet of the indicating scale: second between the magnets of the indicating scale.

The cylindrical magnet "A" in the float is able to turn horizontally on the axis "B" like a compass needle, so that magnetic force is always free to best orientate itself towards the magnetic scale, in which small cylinders "C", rotating vertically on their axis "D", contain smaller cylindrical magnets fitted in orthogonal position to rotation axis. Cylinders No. 1 & 2 are in stable position, as cylinders 4 & 5, because the magnetic chain between their magnets is well oriented "N-S-N-S"; cylinder No. 3 is in not stable position because North Pole "N" of the bigger magnet "A" inside the float attracts the South Pole "S" of the smaller magnet inside the cylinder. The same cylinder tries to rotate clockwise because its "N" pole would reach the "S" pole of cylinder 2. Imagine now that fluid, growing in the level gauge, pushes up the float. The float magnet, going upside, will approach the cylinder 4, attracting its "S" pole, and so, after clockwise rotation, it will be in the same position as now the cylinder 3. Same cylinder 3 will be in that case free to complete rotation clockwise, and its position will became stable as cylinder 1 & 2. And so and so with growing of the liquid level. The same, with inverse rotation of small cylinders, happen when the float goes down.