What is the consideration on deciding whether level instrument (transmitter or switch) to be top mounted type or side mounted with external cage type?
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.
Above 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 external cage type.