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The Impact of Roof Configuration on Tank Overfill Protection: Level Instrumentation Considerations

The API RP 2350 4th Edition Update provides details about the type of storage tank, class of liquid, level of concern and attendance category that are affected by the new recommended practices. Additionally, it requires sensors on tanks with floating roof configurations to detect both the roof and the liquid level, should the liquid cover the roof.

Beyond this recommendation, however, the API 2350 does not detail level instrumentation standards for external versus internal floating roofs – with or without an instrument well – or for fixed roofs. Yet, certain level instrumentation technologies work better than others, depending on the roof configuration.

External or Internal Floating Roofs Without an Instrument Well

As mentioned above, for external or internal floating roofs without an instrument well, tank facility operators need to deploy High-High sensors that can detect both the liquid level and the roof, if it becomes submerged in the liquid media. For Category III tanks, defined as “unattended” (personnel are not required to be present during receiving operations but are remotely located at a control center), API 2350 recommends using two independent sensors.

An effective, reliable instrumentation configuration under these roof conditions can include a top-mounted, point-level buoyancy displacer switch combined with a continuous non-contact (through-air) radar transmitter. Both technologies can detect the roof or liquid level, whichever is higher, without damage to the roof or control device if the roof should sink.

Fixed Roofs or Floating Roofs with an Instrument Well

In fixed roof or floating roof with instrument well conditions, the combination of a contact ultrasound switch with a guided wave radar transmitter provides reliable overfill prevention. In this configuration, the ultrasonic switch serves as the point level sensor triggering the High and High-High alarms. The guided wave radar provides continuous level sensing over the tank height and may be used as the level sensor or the High-High alarm sensor.