LNG Industry - July 2016 - page 59

57
K
nowing the level of a tank and its content could be helpful, or even
mandatory, in emergency situations. In fail safe systems, automated
actuators commonly include a manually actuated function. However,
how can an operator decide whether to open or close a valve without any
information about whether a tank is empty or full?
In the event of a power shutdown or failure, the process control system is
no longer available; all information from electrical driven sensors is down;
the electrical power to control the actuators is missing; and the main safety
actions have to be performed manually.
The use of a direct and power-independent sensor, such as the WEKA
Visual Level Indicator (VLI), provides an alternative option.
How it works
The non-electric indicator is based on a magnetic float principal. The VLI
consists of a float chamber that uses non-magnetic stainless steel, or other
materials, such as titanium, higher graded nickel alloys or even plastics. The
chamber is mounted on the outer side of a tank, such as along the vertical
side or the top, in case there are no other connections available. The float,
incorporating a bar magnet, will indicate the current level of the liquid inside
of the tank. The magnet attracts the flaps of an indication rail mounted to the
float chamber and turns them from white to red.
VLI are available for a pressure range from vacuum to 63 MPa (630 bar),
temperature range from -196°C to 400°C and for densities starting from
0.27 g/cm
3
.
The tank level reading does not use any electronics or calculations, and
does not carry the risk of information failure in case of emergencies.
In addition to pure visual indication, both magnetic switches and
transmitters can be added to create signals for the process control system or
other remote controls.
WHEN THE
POWER IS
DOWN
Stefan Otto, WEKA AG, Switzerland,
presents a non-electrical solution for
measuring LNG tank levels in the event of
a power failure or emergency.
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