Top mounted means the sensor or measuring element of level instrument is
inserted into the vessel or tank from the top. Side mounted with external cage
means sensor or measuring element is installed integrally in
manufacturer prefabricated external cage.
The price of level
instrument with external cage is higher than that of top mounted type. This is
due to the cost for the cage, moreover for special material such as Duplex
Stainless Steel. However, external cage level instrument can have isolation
valve installed in between vessel and cage. This arrangement allows instrument
maintenance without the need of process shutdown. Therefore external cage level
instrument is preferred in most application.
Top mounted level instrument is utilized in some cases such as:
§ Buried
vessel, since it has no side access.
§ Semi-submersible
tank such as caisson.
§ Liquid
which may change properties/form due to temperature change and the use of steam
or electrical heat tracing is not feasible.
The picture shows the arrangement of top mounted level
transmitter and side mounted level transmitter. The isolation valve(s) can only
be installed on the arrangement of side mounted with .
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