Hydrocarbon Processing - November 2021 - 38

Engineering and Construction
Magnetic level gauges. This type of
level sensor is used to measure the level of
fluid in a vessel or tank and includes a floatable
device that can float both in high- and
low-density fluid. The measuring chamber
is fitted with a float with a magnet inside,
which will float on the medium; the magnet
in the float will turn the flaps of the
indicating rail. Some manufacturers use
a magnet bar, while others use a toroidal
magnet on a float for the guidance.
As shown in FIG. 2,2
a typical magnetic
level gauge contains the following
components:
* Chamber: The chamber is
directly connected to a vessel or
tank to measure the exact fluid
level. Chamber configurations
depend on the application and
project requirements. A chamber
may be side-mounted when
both process connections are
connected to the side of a vessel
(FIG. 2) or top-mounted (FIG. 3).3
Most projects require that the
chamber be designed and tested
as per Boiler and Pressure Vessel
Code, ASME section VIII, Div. 1.
They should also have ASME U
stamping. Chambers are available
in a variety of materials, depending
on process fluid properties and
pressure and temperature ratings.
Chambers should be constructed
out of non-ferrous material to
avoid erroneous movement of
the magnetic float, which leads to
an inaccurate level indication.
* Float: The float is designed
based on the specific gravity of
the process fluid and rests on the
fluid's surface (FIG. 4). As the float
moves up or down, the magnet
assembly rotates a series of bi-color
magnetic flags or flaps, changing
the visual indicators mounted
just outside the chamber from
one color to the other, as shown
in FIG. 5.4
The gauge pressure and
temperature ratings also depend on
the float. Floats are available in a
variety of materials, depending on
fluid properties and the capability
to withstand the temperature
and pressure of process fluid.
* Indicator housing: A visual
indicator housing is clamped to the
piping column in total isolation
from the process liquid. It contains
the choice of indicator, either a
series of flags or a follower (also
known as shuttle). The individual
flags or the follower contain an
alignment magnet that couples
with the float magnets as the float
moves up or down within the liquid
chamber.5
As shown in FIG. 6, liquid
level is indicated when flags flip
and the color of the flag changes.
The indicator housing is directly
connected to the scale, hermetically
sealed, and should be made of
aluminium or stainless-steel.
* Process connection (upper and
lower): Center-to-center (c-c)
length is the difference between
the minimum and maximum level
in the vessel to be measured. Based
on the height of the fluid to be
measured, multiple level gauges
can be mounted on a vessel or
bridle. Process connections are
FIG. 2. Side-mounted magnetic level gauge.2
FIG. 4. The float moves up based on
the level in the liquid chamber. Source:
InstrumentationTools.
FIG. 3. Top-mounted magnetic level gauge.3
38 NOVEMBER 2021 | HydrocarbonProcessing.com
FIG. 5. Float movement affecting the
movement of magnetically coupled flags.4
FIG. 6. Indicator housing. Source: Magnetrol.
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Hydrocarbon Processing - November 2021

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